The story is different for the Meridiani Planum site, where more specific evidence for a once extensive water presence has been found. Spirit's early operational setback (now overcome) was offset by the successful landing of the Rover "Opportunity" at 9:05 PST on January 24, 2004. A map of the Meridiani region is shown below (the touchdown site is about 1 inch to the right of the blue-filled crater shown at the left center edge; look carefully for an elongate ellipse shown in red that defines the aiming point for the landing):
The touchdown site was well within the planned target ellipse (blue): The specific location of Opportunity has been worked out by triangulating on three landmarks seen from the rover's video camera. Here is a picture taken from the descending rover at an altitude of about 1400 m, indicating that the surface at the landing site was smooth, had few large rocks, and contained a moderate-sized crater (East, renamed Endurance) which exposed very light material. (Opportunity itself landed in a small crater, now named Eagle, about 20 m wide and 3 m deep): With its position known, the rover's location was then picked out using MOC imagery from Odyssey, shown here (the arrow points to the small crater, named Eagle, within which the lander fortuitously set down. A processed enlargement of a Mars Reconnaissance Orbiter image shows the small Eagle crater in which Opportunity landed, with its mother platform and the rover itself deployed near the white outcrop (see below)
Meridiani Planum was chosen because MGS Thermal Emission Spectroscopy (TES) data indicated it to have an unusually high amount of iron oxide. The thermal spectrum for hematite is this:
A general distribution rock type map of non-polar Mars was made from MGD TES data, in which this hematite-enriched area (the largest, and one of only three with this map) is shown in red:
A large-scale map specifically of the Meridiani region pinpoints the zones of iron enrichment:
Most hematite found on Mars is the more common red variety, which accounts for that being the prevailing color of dust and many of the rocks (at the least, their exteriors). Terrestrial red hematite is seldom in distinct crystals, being found instead in earthy to granular form. A second form of hematite, called "gray hematite" (sometimes referred to as specular hematite because the crystals are platy hexagons that reflect light in the specular mode, giving them a dark steely gray color), develops on Earth around thermal hot springs or in other water-rich environments. Here is a photo of a specimen of terrestrial gray hematite:
Shortly after deploying onto the surface on January 30, the mini-TES made this analysis of the soil beneath,in which gray granules (consistent with hematite) have been visually noted. The spectral signature indeed confirmed that some of the material is hematite:
If this environmental factor holds true for Mars, then the Meridiani site will prove to be a strong candidate for containing rocks formed under hydrous conditions. Assuming that recent evidence of water deposits holds up, the site should also be optimal for the search for some form of primitive life. Fortuitously, Opportunity set down in a crater estimated to be about 20 meters in diameter. This color image taken from Opportunity reveals a darker than average surface, a notable paucity of rocks, wind blown ripples or mini-dunes, and lighter material in the background that appears to be "beds" or strata of some sort.
The image beneath, displayed in black and white, shows the strong contrast (lighter) of the beds relative to the general surface veneer:
A view taken closer to Opportunity brings out the striking differences of the scattered exposures of these light rocks compared with the darker background materials; note the pronounced sparcity of larger rock blocks both within and beyond the crater outcrop: A panoramic (mosaic) view taken from Opportunity's base shows the full extent of the outcrop, which extends for about half the crater's circumference: The Opportunity geology team did not at first take a firm stand on what these rocks are lithologically and therefore no certain origin has been proposed. At the outset, the two most widely suggested rock types are volcanic ash and lake beds (perhaps CaMgCO3). In the writer's opinion, ash is unlikely since it doesn't seem widespread in the scene or neighboring area. Water filling the crater might then produce some kind of sedimentary rock (magnesian-limestone by direct precipitation; this should be a good possibility since CO2 is present in the atmosphere). This accounts for the apparent confinement of the light layers to the crater (and the larger nearby one). It is doubtful whether this rock, if proved sedimentary, contains any life indicators since the total time the water would be present is probably short (ephemeral lake). Conceivably, the whitish material is much more widespread and is now covered by hematitic wind blown deposits everywhere except where the light material is exposed in the craters. Reflectance spectra taken from the MER base are directed towards some of the main features in the Meridiani crater area. They show mainly relative brightness and do not contain any diagnostic absorption troughs. These spectra are plotted here with the colors tied to the scene shown. As expected the brightest spectrum associates with the light-colored layered rocks. Similar photometric spectra were acquired for individual small rock fragments on the floor of the crater. Again, diversity of color and brightness is the hallmark: A broad-brush compositional map of the scene that includes the white outcrop shows hematite-rich surfaces in red and orange and hematite-poor (but possibly gray hematite) in blues and greens. The white outcrop does not seem to have any associated hematite on its surface: A Mossbauer spectroscopic analysis (which is especially sensitive to iron content and type) of this soil area indicates that olivine is present, confirming basalt as the likely rock material in the granules, and suggests one or more types of hematite are present, but seemingly in low concentrations. Here is the first view of this soil area released by the Opportunity team, showing granular material (identified as basaltic with iron coating) and one of a number of spherical bodies (less than 1 cm diameter) seen here and areas in close proximity. This image shows more of these spherical bodies (a few millimeters in diameter), and indicates that they depart somewhat from perfect sphericity (either as formed or by later erosion). In the black and white image, they are gray in color. Their origin when first found was not specified: candidates under consideration were glass spheres formed either during strong volcanic eruptions or as impact glass); concretions; oolites (ooids could be carbonate or hematite). An especially large number of these spherules, which were nicknamed "blueberries" is present at a place called "the blueberry bowl". Here is a color image of this site: Careful reprocessing to approach "true color" reveals these spherules to be dark gray, with a reddish coating: Using the Mossbauer spectrometer, these spherules have now been shown to consist mainly of hematite (the gray variety). In this graph, two sets of curves appear. The first is of the soil by itself; the second consists of soil with several spherules. By differencing, the deduction is made of the hematitic composition of these spherules.
Other evidence now suggests the spherules to be concretions formed in a water environment, possibly by rolling on a water-covered floor, but the current consensus is that they form in place within the layered rock as concretionary growths during deposition or later, as water percolates through the rock. Those loose in the soil of Eagle crater have likely been freed by weathering and rolled into the crater or were moved by wind. Except for color, they are similar to the red iron oxide pisolites found in the Silurian Clinton iron ore (New York) on Earth. Unlike some concretions found on Earth, the spherules lack any concentric layering in their internal structure, as is clear from this image of a spherule interior made when the RAT cut through a single "blueberry" in the rock "Pilbara": The blueberry concretions on Mars seem to have a counterpart on Earth. Geologists at the University of Utah have described small concretions lying within cracks and scattered elsewhere on surfaces of Navajo Sandstone in the Escalante National Monument in southern Utah. Here is an example: Seen closeup in the left of this next image, the Utah concretions are similar to the martian "blueberries" shown on the right: The Utah concretions contain varying amount of hematite, from as little as 3% to much higher. They appear to form by precipitation from fluids, such as groundwater, that leach out iron and other elements and reprecipitate these at chemically favorable locations within the sandstone. This origin is similar to the one thought most likely for the martian concretions. Over the weekend of January 7-8, 2005, Opportunity rolled up to the first of the light-colored outcrops, an isolated rock exposure named Stone Mountain. Here is its appearance from nearby: The Microscope Imager has now looked closely at the surface, seeing (in black and white) a jagged set of whitish rock separated by dark shadows where erosion has cut into the layering. A single sphere is attached: When examined from afar, Opportunity team geologists think they see cross-bedding in the layers. If so, this could be produced by water action, by wind, or much less likely by ash flow deposition. Here is a closeup which shows this cross-bedding: And here is a color version of another block, nicknamed "Last Chance", in the Meridiani crater that shows obvious cross-bedding: Following the rather rapid transit past individual parts of the white outcrop, during which the only compositional analysis reported was a high sulphur content, MER-2 returned to the soil and tried a notable trenching experiment. This is done by spinning the right front wheel into the soil and moving the rover in and out of the deepening trench. This view shows the IDD (Instrument Deployment Device) arm in place over the trench taking Mossbauer data: Here is a closer look showing the trench in which a depth of about 10 cm has been attained: Of special interest is the light-colored material found at the bottom and also along a trench slope. It's composition is being determined but the possibility of a relation with the white rock outcrop is one consideration.
The team geologists, during a press conference, commented that the several spherules evident had a shiny surface. So far, the nature of these spherules is speculative. An eventual traverse outside the crater could answer the question - if found beyond the bedrock sources in the crater, they would likely be related to whatever causes gray hematite deposits. Meanwhile, back at Gusev Crater Spirit dug a revealing trench on February 20th, 2004. Here it is: Overall, the trench contents, from a distance, looks remarkably like the Opportunity trench. The mystery here is the same white patches seen in both trenches which are separated by large distances on the martian surface. Three explanations seem obvious at first thought: 1) the white material is some kind of artifact caused by the digging (unlikely); 2) compositionally, there are two different materials, coincidentally light-toned; and 3) they are the same material and perhaps represent to regional event (for instance, a volcanic eruption of massive scale) that has laid down this layer over much of the planet. The key discovery from both of these Mars rovers is likely to be the nature and composition of the white rock. The mini-TES instrument on the rovers can also be used to determine temperatures in the martian atmosphere up to several kilometers. Here is a temperature-time plot for the near surface around the Opportunity site: As expected, the atmosphere at 500 meters is colder and more stable than the gases just 30 meters above the surface. These temperatures vary not just vertically but to different extents as the martian day warms up, much as they do in a desert clime. This is illustrated by the next diagram: The Opportunity scientists have performed an interesting experiment with remarkable results: As the Thermal Emission Spectrometer (TES) on Mars Global Surveyor orbited directly over the Meridiani site, it took measurements looking straight down. The mini-TES on Opportunity simultaneously looked straight up as it took temperature readings. This plot resulted: The near-perfect join of the two data sets is testimony to the excellent calibration of the two instruments. Note the rapid warming as the surface is approached. Other meteorological imagery above the Meridiani site has documented distinct cloud types. On April 10, 2004 another MOC image from MGS includes wispy clouds that are analogous to cirrus clouds on Earth: Distinct cirrus clouds were observed over the Meridiani site in mid-December 2004: An image from Opportunity shows a martian sunset, with the Sun itself appearing as light yellow-white through the atmospheric dust. Beyond the lower atmosphere dust layer the sky appears blue, owing to the same scattering phenomenon that gives Earth its blue skies: But in view of new revelations, perhaps a sunrise scene (portending something new and favorable is happening) would be more appropriate. On March 2, 2004 at a press conference at NASA Headquarters, the science teams announced confirmation of the presence of materials on Mars that have been definitively affected by water sometime in the past. Thus, the primary goal of the MER missions has been accomplished. We will take sufficient time/space on this page to give the details of the measurements because of the importance of this discovery. As was said above, the whitish rocks at Opportunity's Meridiani site visually are strongly suggestive of layered rocks, which may have been water-laid sediments, although altered basalt or even volcanic ash deposits were plausible alternatives. These rocks, in Eagle crater, have been termed "Etched Rocks" because of "differential erosion" and close-up indications that material within has been dissolved out. At the moment each hypothesis has merit and thus both are still valid, but the activity of water causing or influencing the rocks is now definitively confirmed. Opportunity's sensors and tools have engaged in some detailed analysis of this and other outcrops: Here is a closer look at the El Capitan segment. The block that has been studied most extensively with the Microscope Imager and the compositional instruments - El Capitan itself - actually appears to have several different textures, as shown here, with the location of the two drilled RAT holes: Seen close-up by the Microscope Imager, the rock texture in part of El Capitan appears finely crystalline. Notable are several thin elongate lenticular (cigar-shaped) voids that assume various orientations. These are tentatively interpreted as sites where soluble mineral species (one or more) have been removed (etched out) by a solution process. An even closer look at a part of El Capitan in which the RAT has exposed a fresh surface shows one of the "blueberry" spherules. Opinion as to their nature is shifting now in favor of concretions formed as described below When shown in color, one of the RAT holes, in Guadalupe, has a brownish red color. This looks like hematite, and has been tentatively identified as such: A closer look at Guadalupe shows an even better developed set of thin layers, with the lenticular cavities, and more spherules: The best image to date of the thin (1-2 mm) equi-thick layers in the white rock group (again, this group is not truly white but is light-colored in tone) comes from a block called Flat Rock. This is a typical pattern found in evaporites (see below): Some of the individual rocks in the white layer group possess inclined layers which has almost universally been accepted as evidence for cross-bedding, a structural feature that is usually attributed to either water or wind action. Prevailing opinions now favor water currents as the cause of this cross-bedding, reenforcing the opinion that the layers were laid down in a widespread shallow water body (a large lake?) that covered the Meridiani area. Look at the close-up view of a part of the rock known as Upper Dells. Another example is this Microscope Imager view of inclined bedding in the rock called Lower Last Chance: Cross-bedding on a larger scale has been observed along Endurance crater walls (see below): Intriguing and suggestive as these textural displays are, the real excitement has come from the Mossbauer and APXS (the Alpha Particle X-ray Spectrometer that uses radioactive Curium-244 as the source of the bombarding alpha [helium ions] particles) instrument analyses of the surface of these rocks. The first two show plots of element detection and abundance at a rock that is part of El Capitan, dubbed "McKittrick" and its adjacent soil, named "Tarmack". The data indicate that there is a rock type that could be volcanic in nature but one that also contains high sulphur and bromine, as well as chlorine. A Mossbauer spectral plot of El Capitan itself indicates presence of a mineral called Jarosite, and other (not yet specified) minerals containing Fe++ and Fe+++ (a candidate could be Goethite, a hydrous iron oxide, that can revert to hematite). The mini-TES sheds more light on the rock composition. The top curve is representative of sulphate minerals; the bottom curve is typical of iron-bearing silicate minerals. Note that the actual curve for El Capitan is between these two, suggesting both silicates and sulphate minerals are present. The next diagram plots as histogram bars the relative abundances of the sulphate component in two of the rocks analyzed. Chlorine (and bromine), while present, remain in steady, low amounts but the sulphate mineral(s) vary considerably. So what do these analyses mean? How are they being interpreted? There is consensus that the mineral Jarosite is definitely present, but is mixed with other minerals rather than occurring as a mono-mineralic deposit. On Earth, Jarosite, K2Fe6(OH)12(SO4)4, is a yellowish-brown mineral that usually develops from alteration of volcanic rocks that are subjected to high hydrous vapors (solfataric deposits). It appears both as hexagonal crystals (top below) and in an earthy form.
(Jarosite, along with Alunite, is present at the White Mountain mineralized zone in southwest Utah, as discussed in Section 5). A second mineral tentatively thought present in these martian rocks is Kieserite (MgSO4.H2O), which is more familiar to us as "Epsom Salt". It occurs in bedded saline deposits on Earth (such as at Salzburg, Austria). Some evidence (not conclusive) for Gypsum (CaSO4.2H2) exists in the compositional analysis reported for the White Rocks. Clearly absent are any carbonates (these would be stable only in much less acid conditions).
The rover team has proposed that these sulphate minerals, plus the presence of Cl and Br, bespeak of evaporite deposits. These, if indeed they are formed from evaporation processes, could mean that the crater and perhaps the whole region was at some (unknown) time covered with water (to an unknown extent) that gradually evaporated leaving these minerals. They probably mixed with either volcanic material (likely, ash) or silicates from some other origin. An alternate explanation proposes these sulphate and other saline indicators formed by groundwater seeping into the rocks and altering them or carrying dissolved saline ions that precipitated within the host rocks, giving the mixture of silicate-sulphate suggested by the second diagram upwards. The one salient fact and conclusion that seems firm: water has played one or more roles in affecting some of the rocks at the martian surface. The present writer (NMS), who began his career as a mineralogist (age 12), at first questioned the interpretation that Jarosite indicates true evaporite deposits (those that form when a body of water gradually dries up and facilitates deposition of various mineral salts [controlled by the solubility constant of each under given T and P conditions). He conducted a thorough search of textbooks and Internet sites for mention of Jarosite as a bona fide evaporite mineral, and at first came up empty. The usual conditions for Jarosite formation are evident in this phase diagram that pertains to the Fe-O-S-H2O system. The stability field for Jarosite is fairly small, occuring where water present is moderately acid and mildly oxidizing. This occurs most commonly when hydrothermal solutions pass through rock bodies that contain pyrite which may then alter to Jarosite or to Limonite (gossan). Evaporites, such as those found at Strassfurt, Germany, generally form under higher pH conditions. Kieserite, if verified for Meridiani, supports some evaporite activity. Iron-bearing evaporite minerals are rare; those that form use Ca, Mg, Na, and K cations and Cl, Br, SO4 and CO3, and borate anions. Some mineral phases contain H2O as bound water. However, after further Net search the writer found reports of Jarosite occuring in very acid saline lakes that, when they evaporate, form both common and unusual evaporite minerals as end products along with red mineral phases. Examples of such lakes are found in Australia; they are characterized by a lack of carbonate minerals, often associated with saline deposits. A brief write-up of this special environment, as they expected it to apply to Mars, is on the Net as authored by K. Benison and D. LaClair. This prescient study seems especially relevant to Meridiani. But as is usual when evidence is incomplete, alternate hypotheses will eventually show up. A team led by a Dr. McCollum at the University of Colorado claims that the low quantities of Mg, Ca, and other elements found in evaporitic precipitates mitigate against the origin of these deposits in lake beds or shallow seas. Instead, they find the data consistent with alteration of volcanic rocks by gases associated with their emplacement. Thus, the mineralogy in their view is typical of solfataras (emanations of hydrous fluids and sulphurous gases such as occur around Mt. Vesuvius) developed during volcanism. These interact with the recently laid deposits, altering them by replacement. A test of this hypothesis would be analysis of the Jarosite beds elsewhere on Mars, since the alteration by volcanic effluents would probably be uneven. The Opportunity team has also now reported on rock types within the small crater. Its floor is basalt, with a dusty hematite coating. Near its south rim, hematite from beyond the crater has blown into the outer edge of the crater floor to form a thin veneer probably centimeters thick. Many of these conclusions are really hypotheses - not finalized as yet. More data may change opinions and generate new hypotheses. It is interesting that the MOC image shown next for a wider region not far from Meridiani shows both dark rocks (hematitic?) and light rocks (analogs or matches to the Meridiani light-colored rocks) interspersed over the surface. It is instructive at this point to speculate on the nature of the gray hematite and how it may have formed. Articles in 2000 and 2001 by P.R. Christensen et al in the J. of Geophysical Research circumscribe 5 plausible modes of origin for this form of hematite: 1) low temperature precipitation of iron oxides/iron hydroxides in oxygenated Fe-rich standing water, followed by conversion to gray hematite; 2) low temperature leaching of iron (lateritic weathering) from iron-bearing minerals, with subsequent conversion to gray hematite; 3) direct precipitation of gray hematite from Fe-rich circulating hydrothermal fluids; 4) gray hematitic surface coatings by direct weathering; and 5) thermal oxidation of lavas rich in magnetite.
A phase diagram for the Fe-O-OH system sheds some light on the possibilities. The ferrous form of iron (Fe+2) has a stability field that tends to be stable from pH = 8 to more acid conditions (low pH) and exists in moderately oxidizing conditions. (Fe+3 is most stable under acid conditions and higher oxidation state [Eh more positive]) Thus, it can develop in the gray form under pH-Eh conditions not too different from Jarosite. In a published paper by Catling and Moore, a flow diagram of chemical reactions considered possible for Mars, shown below, indicates how both gray hematite and red hematite may form: The starting material is volcanic - probably basalt but andesite can contain iron-bearing minerals (for example, biotite). Temperatures for hematite formation in an aqueous environment should be at least 100° C. This is warmer than the present surface temperatures on Mars; hydrothermal activity could achieve these temperatures but this seems implausible in an evaporite lake environment. The geochemistry of gray hematite on Mars is still to be worked out as more MER data are acquired. Suffice for now to consider this model proposed (as an educated "guess") by the writer: An ancient basalt surface existed in Meridiani Planum; water collected in the past and held dissolved ions that crystallized the iron and sulphur families of minerals as the lake dried up. Iron sulphates formed first, then as sulphur was depleted gray hematite formed; the thickness of the evaporite deposites (including hematite which is occasionally found in evaporite sequence on Earth) probably was less than a few meters. The Opportunity MER before leaving Eagle crater took a series of MI images of particles on Eagle's floor, of which this is an example:
Using the Mossbauer Spectrometer, Opportunity examined the floor composition as it traversed it and then moved onto the plains beyond the crater rim: Visual evidence shows the plains to be covered by black sands, which reminded several MER scientists of the black basalt sand found at Punaluu beach in Hawaii. However, on Meridiani this sand is largely coated with hematite. As exploration by MER-2 continued, opinion shifted towards the ubiquity of coated basalt sand mixed with gray hematite as the prevalent surface material. Opportunity, after several tries to leave Eagle crater (wheel slippage on the 16° slope caused temporary problems), succeeded in escaping and has reached its next objective - Endurance crater. As it looked back at Eagle crater and elsewhere, it spotted on the otherwise smooth, black sand-covered surface low structures that superficially resembled mini-dunes but some geologists have interpreted these as "ripples" that are more likely to have a water origin: As the Opportunity pan camera scanned these smooth plains after leaving Eagle crater, one of the very few rocks in the foreground caught their attention, such that it was soon visited by the rover. The rock, named Bounce, because during the descent the airbag had hit it, breaking it, then bounced away. Here is a view of this rock: The RAT cut into it, exposing fresh undersurface material. Reflectance spectra show these differences: The green and red spectra represent the cuttings (blue in the inset) from the rock and the rock interior respectively. The red refers to the bright red soil next to Bounce rock and the teal color ties to the darker soil. The purple relates to larger grains in the soil. Mossbauer spectra of the rock and soil appear below. All of the soils contain peaks related to Fe+3. The fresh rock however contains no Fe+3 but the pair of peaks relate to Fe+2. Using both mini-TES and APXS data, this diagram gives a close approximation to the mineralogy of Bounce rock: The rock belongs to the broader range of basaltic rocks but is enriched in orthopyroxenes and with lower plagioclase content. It is not typical of the more general basaltic composition for Mars surficial rocks. Part of the Bounce rock has a texture that is typical of the special basaltic type Lherzolite. But the really intriguing thing about this rock is its close similarity to a meteorite group (SNC) found on Earth that has been claimed (and largely accepted by geoscientists) as coming from Mars, being ejected by impact with a few individuals (perhaps 20 now) eventually having fallen onto terrestrial surfaces. The best known of these meteorites is Shergotty, pictured here (for a review of the martian meteorite group, check this Internet site, referenced on the preceding page). The Mossbauer spectrum of Bounce rock is almost identical to that for the Shergotty meteorite (which reveals its shock origin from impact by the presence of maskelynite [glass pseudomorphs of plagioclase[). The composition is actually a bit closer to the Mars meteorite EETA79001, pictured here (see also page 19-13, in the Life subsection): This next chart shows compositions of several martian rocks in terms of MgO/FeO and Al2O3 ratios. Bounce rock is on the left; followed by EETA79001; a Mars rock at the Pathfinder site; the Shergotty meteorite (light blue); the two bars on the right are for Adirondack and Humphrey, rocks at the Spirit site. Speculation has Bounce possibly coming as ejecta from a large crater (black arrow) about 50 km to the south, shown in this nightime MGS thermal image. The importance of this characterization of the Bounce rock is that it appears to be a counterpart of one or more terrestrial SNC meteorites, thus helping to confirm the identification of that group as derived from Mars. As MER-2 proceeded to rove forward at rates of 50 to 100+ meters/day enroute to Endurance crater, it encountered another crater, named Fram, which contains more of the white rock than Eagle. Here is a view of this intermediate stop where new measurements have been made: Opportunity reached Endurance's rim on May 2, 2005 and peered in to reveal the distribution of the white rock units. These are now undergoing analysis. The image below is purposely made larger to show details: Endurance crater is 130 meters (430 feet) across from rim to rim. It is about 20 meters (66 feet) from rim to the base. Slopes as steep as 20° have been measured and in the above scene the slope locally is much steeper. The anomalous material in the bottom center of the crater has, on closer look with the Navigation Pan Camera, been identified as small dunes (or large ripples) in sand accumulated there and affected by the strong winds that prevail on Mars. These minidunes, seen close up as Opportunity moved well into the crater, look like this in color; the top is in natural color, the bottom in false color which emphasizes the blue: Larger dunes have also been encountered. This view shows some of these, with blocky rocks in interdune swales: Opportunity traversed about one half the distance along the Endurance crater rim, as indicated in the image below. The ongoing strategy had it backtrack from Pan Position 2 to an entry pointed dubbed Karatepe where it could take measurements with the MiniTES and other instruments and then possibly be sent partway down the crater slope to look at the layered units described below. Based on slope estimates, the Karatepe incline, seen in this panoramic view, seems favorable for the incursion of the rover. This near true color view of a part of the crater wall named Burns Cliff shows an excellent exposure of several types of layering within a steeper sloped area: This view seems to display most of the layered units in Endurance. The total thickness may be at least 10 meters (33 feet). The blockier white rock units (appearing red because of the iron oxide coating) that correlate with the Etched rocks seen at Eagle and Fram craters are just below the rim. Beneath these there seems to be a sequence of more massive layers (without notable joints that produce the blocks) whose nature is yet to be determined. There is a hint of more layering in the lower slopes. On the walls below are apparently loose blocks which are probably pieces of the upper layer sequence that have slid down under gravity as detached float. This same scene has been examined by the Panoramic Camera using combinations of its 13 color filters. The result for one such combination is seen below. The blue color is correlated with a basalt signature, the red with the sulphate units found at Eagle Crater, and the green seems to be thin layers of basalt coated with iron oxides. By June 10, 2004 Opportunity had picked its entry point at Karatepe, retraced its steps from Pan 2 to get there, and looked down at what it will traverse. This pair of images summarizes the plan. Slopes are 18° maximum; gentler inclines will be followed with compositional measurements made as the rover proceeds downward. Entry has occurred on June 10, 2005. By June 24 Opportunity had descended 5 meters into Endurance, turned around, moved partially outward to test its return capability, and has started taking readings with mini-TES, Mossbauer Spectrometer, and the APXS instruments. Here is a traverse line showing stopping points where measurements have been made:
There seems to be three distinct units, each to be studied, as seen in this view looking down into the crater. A closeup shows still another division of the possible units: Since geologists normally view outcrops from the bottom up, this next view shows the stratigraphy from within the older laminated units looking upward towards the rim. The main mystery is the nature of these lower units - are they basaltic layers, a different assemblage of sedimentary salt beds, a mix of both, or something different altogether? One now popular interpretation considers these beds to be analogous to basaltic sand, formed by mechanical disaggregation of basalt bedrock elsewher and blown over long periods to form the layers. (Note: on Earth basalt sand and sandstone are rare but the sedimentary rock type called "graywacke" [derived largely by weathering of basalt] is fairly common.) Also just beyond Endurance loose grains of basalt have been found. An APXS profile down this sequence shows that magnesium and sulphur decrease systematically from top to bottom whereas chlorine increases somewhat erratically going down section.
Mossbauer and APXS spectra from rocks above and below the A-B boundary in the side of Endurance Crater disclose salts, mainly Jarosite, almost identical to that at Eagle crater. There is one important difference. Bromine in the upper unit varies considerable but is much more uniform in the lower unit. This suggests that earlier units were reworked by wind and/or water, homogenizing the bromine, but such action was more limited in the last unit deposited. Sulphur content is similar at both craters. Of particular significance is the increase in chlorine at Opportunity relative to Eagle and as shown above and here moves downslope into the crater. The APXS measurements show that chloride minerals (whose identities have not been established) seem to increase - there may be a mixture of chloride and sulphate minerals in the D and E rocks. This is similar to a sequence observed at some terrestrial evaporite bedding sequences. The "Jury is still out" on the full nature of the depositional environment. Considerable water was needed to produce all the units. Interpretation is leaning towards acid lakes (perhaps with playa conditions) rather than a broad, continuous ocean. But more recently, another hypothesis has been considered: subsurface acid waters have altered the layered sequence converting basaltic sand components to the sulphates as secondary minerals. A RAT coring in Unit D (in a rock labeled London - see second figure up) has exposed two more features indicative of sedimentary deposition. The fresh surface shows fine (1 mm) layers which may correlate also to clay-silt sized particles that may be fine deposits of disaggregated basalt. Also present are voids that may have been site of crystals of (most likely) evaporite mineral(s). Another feature observed in the Karapete descent is ridges made of materials with differing textures. A tentative interpretation holds these to be filled cracks in the bedrock penetrated by the impact excavation. At the base of the layered sequence, numerous blueberries and gray to yellow nodules abound. These are similar to those found at Eagle and Fram craters, indicating that they are a ubiquitous constituent of the sedimentary units, at least in the vicinity of the landing site. A rock exposure named Escher, on the SW slope of Endurance crater, shows both open linear fractures and closed fracture patterns that seem polygonal (this might indicate a mudlike depositional environment since on Earth polygons cracks may form during drying and lithification [usually filled during the process] of sediments). An APXS analysis of Escher indicates it to contain CL, Br, and Na (but a deficiency of SO3), placing it in the class of Meridiani sedimentary rocks of evaporitic nature (or, of secondary alteration mineralogy of igneous rock resulting from acidic hydrothermal water from the interior), but with less or no Jarosite: Clarification of some of the remaining uncertainties may ensue as the rover continues to perform, having reached the base of the Endurance crater. It is now proceeding onto the plains beyond. But, as this tour of Endurance, which lasted until the end of 2004, so aptly illustrated, 'tis true to say "Opportunity Knocks!" A 70 m wide crater, named "Viking" was encountered in April, 2005. As seen here, the crater is similar to Fram (above) in having numerous blocks of light-toned rock lying haphazardly in and around the depression. There are hints of thin source layers exposed near the rim. One can speculate that these have been covered by martian surface "fines" - the small particles from pea to dust in size. As Opportunity resumed its traverse across the plains beyond Endurance, it encountered a small rock the size of a potato - thus it was named Russet. It appears to be an ejecta fragment from a nearby crater. Chemical analysis shows it to be infused with magnesium sulfate in a form suggesting the chemistry is that of salt deposition in a rock of another composition resulting from hydrate solution that permeated from subsurface or groundwater. It once again raises the question: Are there both primary depositional salt units and secondary replacement salts within pre-existing rocks? In a strange but plausible twist, Opportunity has found evidence of iron of another kind: As it traversed the plains beyond Endurance, it discovered an odd-looking rock in its path. So unusual, that it took a close-up look and found the object was a nickel-iron meteorite. This constitutes a "first" - the first time that an extraterrestrial body has been found on another extraterrestrial body. Here is Opportunity's view of this meteorite: 
This one acquired on May 1, 2004 shows gray clouds of water-ice that resemble cumulo-nimbus clouds on Earth:
Opportunity headed for a very large crater (750 meters; 150 m depth, exposing a thicker section of layers) named Victoria. Whether it would function long enough to reach Victoria, especially since this Mars site is involved with the extended martian winter in 2006-07, was the underlying question - but it did. As of September, 2006 Opportunity was at the edge of the crater.
A favored interpretation is that this is one of the light-toned sedimentary layers that is being removed by some type of erosion, leaving individual separated surface outcroppings.
This MGS-MOC image shows the Victoria Crater, where the white rim material consists of a thicker exposure (~40 meters; more than 125 feet) of layered rocks.
A Mars Orbiter image provided this detail:
Opportunity provided this image of Victoria crater in the distance (the foreground "hills" are actually low sand dunes made to appear higher by vertical exaggeration. The lower strip shows another view in a different direction:
In a triumph of persistence and good luck, Opportunity reached the rim of Victoria Crater on September 27, 2006 and looked in - here is the first panoramic view:
For the rest of 2006 and beyond, MERS Opportunity studied the outcrops at Victoria Crater. This promises to shed much new light on its local sedimentary history. Here is a view of the light-colored outcrop rocks below the rim, made in September of 2007 (after a long hiatus when no data were acquired during a dust storm):
This page will be kept current as more MER results are reported. In mid-July, 2006, NASA gave a "go" for another 180 days of operation of both rovers. That was then extended into 2008, as Opportunity continues to explore inside Victoria crater. This traverse map shows the history of Opportunity from touchdown into April of 2008 (the movable arm on Opportunity began to malfunction then):
A good summary of the two MER findings through summer 2004 appears on this Space Daily site. Here is the writer's summary of what the prime significance of the MER findings appears to be, especially in terms of their bearing on possibilites for life.
First, the Spirit site in Gusev Crater has so far revealed almost nothing that would prompt one to hope for life evidence. The rock types there are limited to basalt and to coatings and soil enriched in hematite. No obvious sedimentary layering is present. However, strong arguments for layering are the observations in the Columbia Hills. Whether these are volcanic or sedimentary in origin is not yet clear. Signs of water are restricted to an assumption that the thin cracks filled with unknown material(s) could be veins that contain hydrous phases, to detection of magnesium and sulphur in a few rocks, and to the iron mineral Goethite which can form through the agency of water. The most likely cause of these hints at water action in the past is either hydrothermal activity related to dewatering as the basalts were emplaced or possibly later igneous hydrothermal expulsions of water or some sort of ground water activity. The site appears not to have been occupied by any standing water (crater-filling lake) for any significant duration sufficient to produce sedimentary beds and hence the most common hosts for life forms of any kind.
In contrast, the Meridiani region seems to contain sedimentary strata, which may be widespread (beyond the immediate MER site). So far two rather exotic evaporite minerals have been identified but the more common minerals from that depositional environment - salt (NaCl), gypsum, anhydrite, and carbonates - have not been detected. The present sequence of rock materials appears to be pyroxene-rich basalts (regional bedrock constituting the last major lava emplacement in this part of Mars), evaporites, hematitic deposits and soils. This suggests that at one time there was a shallow "sea" or lake (disconnected individuals or a single larger water body) in the region. Its chemistry was that of slightly oxidizing, strongly acidic water. The thickness of the evaporate layers indicates a relatively short life span for the water (unless there are more evaporite beds below the basalt flow rock - a condition similar to one in southern Idaho that the writer studied as a possible site for a nuclear cratering experiment; there the basalts flowed in successive outpouring between which lacustrine (lake) beds formed layers a few meters thick).
The question of iron minerals on Mars deserves special comment. No strata which would be called "iron formation" on Earth has been observed to date. The iron is present as granules and flakes and "dust" (finer sized, in the clay size range) in unconsolidated surface deposits and as coatings up to millimeters thick on basalt and evaporite rocks (loose and layered). The hematite then appears to be a weathering product that can be removed by wind ablation and transported by wind such that the fines are repeatedly relocated. The larger particles probably are lag sands. The very production of hematite implies that at some time(s) in the past, and possibly even now, oxidation of basalts and other igneous rocks (ranging from pyroxenites to andesites) have freed the Fe2+ from minerals such as pyroxenes and olivines and oxidized the iron to Fe3+, facilitating direct combination with available oxygen. The amount of oxygen in today's martian atmosphere is very low but over time that small amount must have been capable of producing the hematite (this is also a way to lower the atmospheric oxygen by tying it up in Fe2O3). Also formed is magnetite, not found in notable quantities in the soil or coating but now a significant presence in the transported dust (as proved by its accumulation on the magnets on each rover). Conspicuous by its absence so far is the mineraloid "limonite" - a hydrous phase of iron oxide (the rust on Earth), suggesting that in the hematite-forming process water is sparse to absent. Jarosite, however, does contain water as well as iron, so that in the evaporites a water-containing iron mineral did form.
A group at Virginia Tech has reported results, based on a Ph.D. thesis, that offer another perspective on the Jarosite (and perhaps Gypsum) deposits. They found that basalt when subjected to acidic oxidizing water will alter to these minerals. But for Jarosite to survive after formation, continued exposure to hydrous solutions of higher alkalinity would destroy this mineral by solution. They tentatively have speculated that the Jarosite layers are actually basaltic flow beds that were subjected to acid waters (hydrothermal rather than lacustrine or marine) for a period of time, which they surmise to have been relatively short geologically, during which the sulphate phases were produced on exposed surfaces. The depth of alteration is unknown but could have been in the multi-centimeter range. After this hydrous activity, water became scarce as it was lost to space and since then hydrous activity has been sporadic to absent, thus preserving the sulphate alteration veneer. This is quite different from the widespread "water bodies" models. This is a plausible hypothesis but probably will be untestable during the current Rover missions.
The writer (NMS) is skeptical of one aspect of the VTech model: they postulate the water involved to have been hydrothermal solutions from the dewatering process in the martian crust; while such solutions produce chemical/mineralogical changes in the Earth's outer crust, the patterns are localized, variable in their effects, and usually not widespread. The numerous examples of layering on Mars seems to indicate that one or more depositional processes operated over time. It is hard to conceive that at least some of these layers were altered hydrothermal in such a widespread scale and uniformity. However, if hydrothermal solutions emerged in large quantities and formed lakes or shallow seas, the solutions could have altered basaltic layers already emplaced, accounting for some of the white (actually light-toned with reddish surficial "staining") layers observed in many parts of Mars.
Now, as to "life" or at the least, organic matter on Mars. The one hopeful chance is the presence of the white rock evaporites. On Earth, fossils are often in evaporite deposits associated with lake beds. Most such beds are modern but evaporite deposits deeply buried that are millions to billion years old are known. Those lakes that are acidic (Jarosite is an indicator) usually are fossil-sparse and the life forms present are primarily those that can live in such environments. Algae, ostracods, gastropods, and primitive unicellular plant/animal forms are most typical. But they occur (generally in beds deposited from water with a pH of 6 to 8; less acid than the pHs of martian waters) because these are carried into the lake environment from outside sources - all but the most primitive life forms have already evolved over billions of years. On Earth, hese creatures do not spontaneously arise within the lake just because water is present for short to long time periods. If present, they must just be redistributed samples that bespeak of a long evolutionary history. Although there have been good studies of terrestrial evaporite beds that contain fossils, renewed studies are called for to better characterize the conditions that sponsor their presence. The Jarosite beds in Australian lacustrine evaporites would be a good place to resume this study; any forms on Mars would not be the same phylogenetically but should possess some similarities. Perhaps samples from the Meridiani beds could still reveal life forms by a RAT process such as is being done by the MERs to gain an insight as to how these life forms appear.
However, in early Earth history, where ancient evaporites are known, primitive life forms can be expected and have been observed. These are mainly microfossils (prokaryotic) and algae layers that are representative of beginnings of life in the evolutionary process. They may have come into existence over relatively short time spans. A generalized diagram shows the evolutionary tree for most microorganisms developed on Earth:
In recent years studies of many of these life forms have determined that some can exist under very unusual, seemingly adverse environmental conditions. Such organisms have ben called Extremophiles. One subclass of this group is known as Acidophilic Microorganisms. The Internet now has many references to these categories; rather than cite any one here, for those interested, use your search engine employing either of the italicized terms as a starting point. Cases involving terrestrial extremophiles and those speculated as possibly being on other planetary bodies will be found. Most likely, martian organisms will fall into the Extremophile category; you can narrow the search by adding acid saline lakes to the entry. Thus, the possibility of "spontaneous" life development in a harsh evaporitic environment is not zero but probably low; realistically, this could occur in the first billion or so years of Earth history if the essentials for organic development - water, carbon sources, appropriate energy, and sufficient time - were present. The Meridiani site might have duplicated such a situation - life forms might be present in the white layers. But, sampling of fresh surfaces (mainly exposed by the RAT) has been so limited (probably less that a combined square meter has been exposed) that only pure luck would have found a "candidate" for a life form. As Opportunity explores more white rock in Endurance Crater, that luck might just produce a discovery. This is likely only if spontaneous creation (in the Meridiani "Sea") actually operated - unless life had already formed on Mars earlier and was carried to the local lake. Unless there was a general and long-lasting environment capable of producing life (for which evidence is still conjectural), this infusion of life from extra-Meridiani sources is of low probability. But, if that generation of earlier life (almost certainly of primitive organisms) turns out to be the case, the most efficient means of transporting life forms during the later history of Mars would be the wind. If this is so, then another environment to look for life would be the polar regions. Dark layers are interspersed with carbon dioxide ice and water ice. These layers are probably dust deposits. Life may exist within them and/or within the water ice itself (the CO2 ice would provide a source of carbon for on the spot generation). No one really expected to find actual life forms from the Mars Exploration Rover activities - such a discovery would be pure serendipity. An Internet site involving an interview with Prof. Andrew Knoll of Harvard University speculates on possibilities at the MER sites. But, the results so far can be rated as "encouraging" and add justification for continued search. The detection of methane (CH4) by Mars Express is a further inducement to a continuing search program. Probably no reasonable conclusion that lifes DOES NOT exist on Mars can be made until robots or humans have returned a wide variety of samples devoid of any evidence for life from areas of favorable likelihood over many parts of Mars. The corollary: Mars meteorites are insufficent as samples that might contain life - they are all igneous bedrock in nature and salt-rich layers would not survive the impact that launches the meteorite. An onsite Mars exploration program is almost mandatory to collect samples that have the potential for having preserved life. Even then, if no life forms are discovered, the scientific community and the general population of Mankind will continue to hope that the next visit will finally find the proof. The writer firmly believes that life exists elsewhere in the Universe. It may be on Europa or Callisto or Triton or Titan, satellites among the Giant Planets that we will describe shortly. It may even be extra-solar. But it sure would be nice if it does indeed occur on Mars. Reports are coming in of sedimentary rocks in other areas in the general vicinity of the Meridiani site. These may be related to the Meridiani sedimentary rocks, implying a widespread occurrence of a "sea" that may have covered this broader region. Part of a 43 km (66 mile) wide crater is shown in this processed THEMIS image, in which the interior is covered by yellow and red material identified as sedimentary rocks: As indicated on page 19-13, Day-Nite IR color composites can bring out distinctions between bedrock and loose material. The crater Holden, in Meridiani Planum, appears "ordinary" in the MOC image below but when THEMIS data are used, its interior shows considerable variation in the nature of materials filling it:
In the course of searching the Web for material for this page, the writer found another area at 20.5° W and 2.5° N., about 20° W of the Meridiani site, that has white rocks interpreted as sedimentary. The sedimentary features occur within Aram Chaos, the name applied to a structure which appears to be a crater 380 km wide. While it may be impact in origin, the structure also looks like a dome with a central crater and an outer ring of lower elevation, suggesting some arching of the rim during a subsequent uplift. This crater interior was a leading candidate for one of the MER sites but had to be rejected because the terrain was too rough and irregular to provide a safe landing area that optimized a successful touchdown. Here is an MGS image of the entire crater followed by a perspective view made from MOC data, seen from different directions:
This perspective view provides a feel for what is meant by chaotic terrain - jumbled areas of fractures, grabens, and irregular blocks.
A geomorphic units map, based on albedo and shape of features, indicates the variety of landforms present in this crater:
Investigators have found good evidence that the darker areas (green low albedo material in the above map) are gray hematite. The lighter rocks' composition (brownish-orange in the map) is presently unknown but it may be similar to the Opportunity site rocks. Notice how widespread that unit is. The crater is presumed to have been filled at one time (during the Hesperian Era) with lake water (probably ice-covered at least part of the martian season then).
Above the hematite, lighter materials have a distinct pitted, sculpted surface texture, as shown in this image. The writer is reminded of similar looking surfaces in carbonates in Jamaica as seen by radar.
This next MOC image really singles out the terrain type that gives rise to the term "Chaos". Various detached units and younger light-colored sedimentary layers are evident:
A pithy discussion of how decisions were made during the earlier months spent at the Meridiani site has been put on the Net by SpaceDaily. 'Tis worth reading.
As areas such as Aram are re-examined in light of the Meridiani discoveries, one naturally wishes that the rovers could somehow be transported to the Aram site to check out the speculations just described in the captions. For a future mission, perhaps?
As we near the end of our treatment of Mars, this is a good place to summarize from all the missions what investigators believe to be the status of water still on this planet. Small amounts of water are locked up in certain minerals that require H2O in their formulae; this may in toto be a large amount but being bound in the crystalline structure this water is "unavailable" for direct use by life. Some water is within the active atmosphere as it is transported between the martial polar regions; the amount of this migrating water seems larger than first suggested in Mariner days and may in fact constitute significant present-day reservoirs in the frozen state present either as distinct water ice or mixed with carbon dioxide ice with the polar cap deposits. Mars Express radar data indicate water-rich layers kilometers thick occur in these deposits. Some water is almost certainly present in the upper meter or so just below the surface over much of non-polar Mars, in amounts approaching 50% by volume, distributed in a manner similar to "permafrost" water in high latitudes on Earth; this water may be in ice layers or may act as a cement.
In the past, water seems likely to have played a much greater role than is indicated by atmospheric transport and by possible gullying in parts of today's Mars. Most investigators now conclude that Mars was a "once wet" planet. Water could have accumulated in time periods of varying duration within lakes, shallow seas, or glaciers. Evidence for this includes the widespread distribution of clay minerals and the sulphate and other evaporite minerals identified at both the Spirit and Opporunity sites. There could even have been ice cycles in which glaciation on a large scale took place. Evidence of snow or frost in the present supports this past assumption. Some landforms suggest glaciation, including valley shapes and outflow channels. Rainfall may have occurred in earlier times and flooding is indicated by many of the channels found on Mars and by the Eberswalde fan and deltas. Mars scientists have proposed brief periods when the martian climate was wet and warm (relative to present temperatures). How recent this may have been is still uncertain but there is evidence for young volcanism which would have released subsurface water from the martian crust. Today, however, the most active surface-modifying process on Mars is atmospheric circulation.
The ultimate question remains unanswered: Does this history of Mars, suggesting water was once more ubiquitous, also mean that life may have formed? Maybe! Depends on how long this water remained; on the degree to which it was present in discrete bodies; on whether the temperatures were favorable; on the extent to which carbon and other ingredients were available; on the conditions that led to some evolution; and on the prospects for successful preservation. Andrew Kroll of Harvard University reported in February 2008 at a AAAS meeting that he and his group of MER investigators have concluded that, even though the sulphate deposits could have resulted from precipitation from extensive water bodies, the geochemical conditions in the water environment were too acid (as indicated by the Jarosite) and too oxidizing (suggested by the hematite) to have allowed life to even start. In other words, the most primitive organic molecules needed to begin the long and complicated process of life form evolution simply couldn't have begun to form nor couldn't have survived if the first stages were to have initiated.
While the unmanned orbiters and rovers provide some new evidence, probably only sample returns (actually several, from different localities) - by unmanned two-way landers and by astronaut visitors - will either confirm life's presence or offer strong reasons that rule out its absence. So, onward to Mars, as will be previewed later on this page, after a brief detour to consider the two moons of Mars. Now, the (semi-)final topic: the Moons of Mars. It has been known since 1877 that
Mars has two small satellites - Phobos (mean diameter = 21 km; [13 miles]) and
Deimos (17 km [10.6 miles]). Mariner 9 captured the first detailed views of
these tiny bodies. Their irregular shapes are similar to asteroids that have
been imaged by radar and their dark surfaces are suggestive of the carbonaceous
chondrite matter thought to comprise these bodies; these insinuate that the
martian satellites are captured strays from the asteroidal belt that extends
between Mars and Jupiter. Here is an early view of Phobos:
MRO has returned better images of Phobos, including this one in color:Phobos and Deimos: The two Martian Satellites
Small craters dot its surface but one large one, named Stickney, came close to disrupting Phobos. This cratering is consistent with the likelihood of collisions with smaller objects during a period of residence in the asteroid belt. The grooves may relate to this crater.
Phobos, large as it seems in the above images, is very tiny when compared to Mars as a whole. This image shows a small part of the red planet with Phobos at its actual relative size. This clearly supports the argument that the two satellites are just captured asteroids.
The Soviet Union sent a probe called Phobos to Mars in 1988 and sent back images including this view of its cratered namesake:
The pan camera on the Opportunity rover has captured an eclipse of the Sun by Phobos, shown in this sequence (a first from another planet):
Deimos has fewer craters and a smoother surface:
The most versatile and largest Mars orbiting satellite, Mars Reconnaissance Orbiter (MRO), was launched on August 12, 2005. Its main purpose is to search for a variety of direct and indirect evidence relevant to the presence of water (in the past and perhaps even today). It arrived at the planet on March 10, 2006 and was placed successfully into an elliptical orbit (which will use Mars gravity to gradually convert into a lower circular orbit).
The MRO imaging camera, with the largest optical telescope yet flown on a planetary probe, will be able to examine surfaces at less than 2 meters resolution. It will also carry more sophisticated mineral identification sensors, including CRISM (Compact Reconnaissance Imaging Spectrometer for Mars), a narrow band imaging spectrometer, that can differentiate compositions at a 100 meter scale. SHARAD (Shallow Radar) is a radar that can penetrate the martian surface to shallow depths in search for water or water ice. MRO also has instruments to examine Mars' atmoshere. Here is an artist's sketch of the MRO:
Here is the spacecraft in its fabrication setting showing its main components before covered by its protective shroud:
Science instruments onboard MRO emphasize the spacecraft’s roster of key jobs at Mars: the High Resolution Imaging Science Experiment (HiRISE), Context Camera (CTX), Mars Color Imager (MARCI), Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), Mars Climate Sounder (MCS), and the Shallow Radar (SHARAD).
HiRISE provides the best resolution images of any of the martian orbiters - about 70 cm. An example of what this can do is this picture of Opportunity seen from MRO's orbit:
From its initial orbit, on March 24 the first pictures by its HiRISE instrument produced this view, and an enlarged subscene, of a region centered near 31° S - 305° E, from an altitude of 2489 km (1547 miles). From that elevation, the resolution of the enlargment (an area 4.5 km by 2.1 km; 1.6 x 1.3 miles) is 2.5 m (98 inches). As the spacecraft was lowered by a combination of thrust burns and gravity, its final orbit in Fall 2006 was near 280 km (174 miles), allowing images to discern objects as small as 28 cm (11 inches)
Here is the first "true" color image made by HiRISE:
Another example: this view inside Becquerel crater:
By September, 2006 MRO had routinely sent a considerable number of high-resolution images of the martian surface, such as this one of the Valles Marineris region:
The HiRISE also took this image of offset along a strike-slip fault; note that drag folds are display on either side of the fault plane, indicating that this is a right-lateral wrench fault:
In 2008, the HiRISE almost fortuitously took a "picture" of an avalanche actually happening at that moment:
An indication of the benefit of high resolution in the MRO imager is afforded by this view of ice features in the northern polar region. Resolution here is 0.67 meters:
Even more striking is this color view of north polar layers (those with reddish tints are enriched with iron oxide dust):
MRO has been used to locate the optimum touchdown site for the Phoenix Lander (see below). Here is one promising candidate:
A MRO color image of the Mawrth Valley shows patterns attributed to clay minerals in the surface rocks:
The CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument on the MRO is capable of sensing the composition of different phyllosilicates, as clay minerals. This image shows clays in the Mawrth Valley, with Fe-Mg clays in red, Al clays in blue, and hydrated Fe+2 silica (iron-rich opal) in green:
Clay minerals also are found in a stream delta on Mars:
An interesting region on Mars is the Nili Valley (Nils Fossae; about 20° north of the martian equator), shown below. Beneath it is an image of the area with identified minerals, in which clay minerals show up green and olivine is rendered in red:
The MRO CRISM spectrometer provides information on both clay mineral and iron variations in this region:
CRISM can also detect gypsum (calcium sulphate) which makes up the light tones in this image of high latitude dunes:
By December of 2006 some interesting subsurface information that points to thick layers had been acquired by SHARAD (shallow radar, built by the Italian Space Agency) in both polar regions:
In November, 2008, announcement was made of a major discovery by SHARAD. Several areas around the Hellas basin contain large area water-ice glaciers up to 500 meters thick. These are covered by rock debris analogous to morainal areas found in the Antarctic. One importance of this discovery is that the buried ice is a source of water that could be used for several purposes during a future manned mission to Mars. Here are two images that show the rock apron on subsurface ice:
In October, 2008 NASA-funded scientists released information about the presence at several localities on Mars of the mineral Opal - a hydrated form of silica - as imaged by MRO. This is further confirmation, along with the clay mineral and iron sulphate discoveries, that water had an active role in the martian past.
As MRO continues to acquire high resolution images, a website sponsored by the HiRISE Operations Center of the University of Arizona shows many select examples of the imaged surface of Mars.
One mission series now actively underway is the Mars Scout Program. Not all of these have received final approval and replacement missions may result from the "new vision" of space exploration proposed by President George W. Bush. This next list includes both approved and possible candidates for the Scout program that would consist of small but well-equipped spacecraft:
Phoenix: In August 2003, NASA announced the funding of a versatile lander, named Phoenix, programmed to concentrate on detection of organic material and of water - in response to the continuing rancorous disputes that life should exist in the upper layers of martian materials, especially where water as ice is believed to be abundant. The Lunar and Planetary Laboratory at the University of Arizona is managing this (Scout) program. For more information, check the JPL Phoenix and University of Arizona websites.
Phoenix was successfully launched on August 4, 2007. The landing site is in the north polar region. The Phoenix Lander touched down flawlessly exactly as planned at 7:37 PM, Sunday, May 25, 2008 and began sending back pictures of the surface within a few hours. It landed within the chosen site shown in this MGS image as a target ellipse:
With a robot arm and scoop surficial samples will be dug out in progressive steps to a depth of nearly a meter. The samples are then examined by TEGA (the Thermal Evolved Gas Analyzer) which analyzes gases (including the expected water vapor) released by incremental heating up to about 1000 degrees centigrade. Other instruments include a mass spectrometer and a wet chemical lab for soil analysis. Here is an artist's view of Phoenix on the polar surface, with its major components labeled:
In a remarkable bit of planned serendipity, the Mars Global Surveyor was in a position to look for - and FIND - the Phoenix spacecraft as it descended by parachute just before jettisoning the chute and firing its twelve rocket engines in burst whose timing is controlled by a surface-seeking radar:
As expected, the local landscape contained mostly small rocks. Also forecast was a polygonal pattern that is characteristic of permafrost terrain on Earth. As we saw earlier, polygonal ground had been found in the martian polar regions, such as seen here:
One of the first panoramic views looking outward to the horizon shows the polygonal features on the rock-strewn terrain:
These polygons are remarkably similar to ones found on Earth. Compare this scene from the Phoenix site with a photo below it that shows polygons in the Arctic:
Close-up images of the surface and an individual ice-wedge polygon:
The prime task of Phoenix is to dig into the soil and analyze the material, searching for ice, water, and possible organic molecules. The scoop and long movable arm of Phoenix is shown here:
The scoop worked well and retrieved top soil which was delivered to the first of 8 small furnaces.
Here is a drawing of one cell of the furnace, called TEGA for Thermal and Evolved Gas Analyzer:
Difficulty was encountered in getting the soil into the furnace because it was notably clumped. The clumps were finally broken down (by shaking it remotely) and material entered the furnace. This is an actual image taken of dispersed soil after a shaking test:
Soil was also delivered also to a small microscope. Here is the first view of this soil:
White material was exposed during the first scoop dig. It disappeared after a few martian days, suggesting it was ice that had sublimed after exposure to the Sun's rays. This image shows this likely ice mass, about a centimeter or two in size:
The results from the first heating experiment proved rather unexciting. No water was found. The first wet chemistry experiment (water is added from a supply carried by Phoenix) found the soil to have a pH between 8 and 9 - alkaline, similar to many terrestrial soils that can support life.
Other digs are underway. As new results from sample analysis come in, they will be reported in the future. As the "Return to Mars" program continues, other potential missions are: Mars Science Laboratory: Now under development, the Mars Science Laboratory, a long-range rover with a variety of instruments, has been rescheduled for launch sometime in 2011. A prime goal will be to search for signs of organic active - past and present. This is a sketch of the MSL vehicle:
The MSL is notably larger than the tiny Sojourner and the two MERs we've previously encountered:
As of mid-November, 2008 no specific landing site for the MSL has been chosen. From a larger list of 32, 4 sites are prime candidates. Here is a map showing each site (identified in the caption); below it is a panorama of the 7 semi-final sites as imaged by the Mars Reconaissance Orbiter:
Two of the candidate sites for the MSL landing site, chosen because of their abundance of clay minerals (suggesting possible water) are Mawrth Valley and Nili Fossae. Each is shown here with a pair of images - the first an MRO overview, the second a CRISM image with clay minerals colorized in blue:
There is a variety of instruments on MSL. Science instruments are state-of-the-art tools for acquiring information about the geology, atmosphere, environmental conditions, and potential biosignatures on Mars. Mars Science Laboratory will carry:
Mast Camera (MastCam)
Mars Hand Lens Imager (MAHLI)
Mars Descent Imager (MARDI)
Alpha Particle X-Ray Spectrometer (APXS)
Chemistry & Camera (ChemCam)
Chemistry & Mineralogy X-Ray Diffraction/X-Ray Fluorescence Instrument (CheMin)
Sample Analysis at Mars Instrument Suite with Gas Chromatograph, Mass Spectrometer, and Tunable Laser Spectrometer (SAM)
Radiation Assessment Detector (RAD>
Dynamic Albedo of Neutrons (DAN)
Rover Environmental Monitoring Station (REMS)
Two more Scout missions are "in the works". One is MAVIS - Mars Atmosphere and Volatile Evolution Mission - whose name reveals its intent to better understand the gaseous envelope around Mars. The second (more tentative) is ExoMars, a joint venture with ESA (which has several of its own missions on the drawing-boards), that will focus on a further search for life.
Mars Sample Return Lander: This is an ambitious but as yet unapproved lander that would be launched in 2014 or later, put down on Mars, gather samples and return these to Earth.
The next group are "longshots" and may already have been removed from contention, depending on how NASA plans to implement Pres. Bush's call for a long-term Mars program.
Artemis: Three small landers and microrovers on the Martian surface, with two directed to the polar regions, to explore the surface and shallow subsurface for water, organic materials and climate;
Mars Environmental Observer: An orbiter to investigate the role of water, dust, ice and other materials within the Martian atmosphere to understand parts of the hydrologic cycle;
Mars Scout Radar: Orbiter mission that radar maps the surface geomorphology and very shallow subsurface -- down to about 10 to 16 feet (three to five meters) deep -- to detect buried water channels and other features;
Naiades: A suite of tiny landers that explore for subsurface liquid water using a novel low-frequency sounding method; and
CryoScout: Designed to use heated water jets to descend through Martian polar ice caps. It would be equipped with a device which could potentially probe to depths of tens to hundreds of meters, or yards, while measuring composition and searching for organic compounds.
NASA's home page on Solar System Missions has a section on future Mars missions that describes several of the above.
Mars, the outermost of the terrestrial
planets, remains an exciting and diverse place to visit (perhaps by humans) in
the foreseeable future. NASA, and much of the scientific world, continues to
promote the idea of a Mars Manned mission, sometime after the year 2010, and after more unmanned probes visit the planet. The International Space Station,
now under construction, and scheduled for completion perhaps by 2012, would be a favorable
launch pad for Man's great adventure to the Red Planet. President George W. Bush has put his prestige behind a manned mission sometimes in the 2020s. 19-51: Assuming
enough fuel to get home, besides food what essential products would you need
to bring with you if you go to Mars to explore for a week or so? ANSWER
In preparing this subsection on Mars, the writer has been fully converted into a proponent of "full-speed ahead" exploration of this extremely varied planet. I am struck by this analogy: In the 15th Century, the great achievement of mankind was exploration - culminating in the discovery of the western hemisphere. In the 21st Century, such revelatory exploration is no longer possible for, and on, Earth. Instead, mankind is choosing to explore the planets and the Universe. A highpoint in this quest must be a manned trip to Mars. I propose that the spacecraft carrying the astronauts be dubbed "Santa Maria II". We close our visit to Mars with two remarkable images:
The first image answers the question: "What would the Sun look like from Mars as it sets near close of the martian day?" Well, as seen by Opportunity the answer is a cross between a smaller "emaciated" Sun and a large star. Look: 
A worthy goal for the future of space exploration is to have the astronauts themselves take a photo of this vista from the surface of Mars. The discovery that water has played asignificant role - in the past and apparently even now - provides strong motivation to ultimately send humans to likely sites to seek out evidence for that supreme goal - proof of life elsewhere in the Universe, even if it is only the next planet beyond us. If the problems of weightlessness and cosmic radiation can be minimized to allow safe travel to and from Mars (at best a six months journey each way), then on-site human exploration of the planet could come as early as the third decade of the 21st millenium. The first human trip to Mars will probably involve a lander similar to the Apollo spacecraft but improved from the experiences gained in the Bush initiative return to the Moon. In time, a more elaborate space facility will be landed to house the travelers and conduct onsite science experiments. Here is one suggested habitation:
Many imaginative scientists have proposed eventual life-sustaining techniques for long stays or even settlement on Mars. The concepts are included in the term "Terraforming", which in essence is a conversion of the martian atmosphere to one that can provide sufficient oxygen, hold water, and provide nitrogen for plant growth. The prospects of creating an environment with standing water and plant life are realistic, if also difficult. Check Google to learn more about Terraforming, and as a start check out this PBS website. One of the illustrations from that site is reproduced here - it is a fanciful depiction of a colony on Mars after the planet has been converted to conditions that allow inhabitation without elaborate spacesuits; if this ever happens, it will provide a "second world" for a likely overcrowded Earth in which the population overflow can be resettled:
Primary Author: Nicholas M.
Short, Sr.