Magellan's primary instrument was a multimode Radar Mapper (2.385 Ghz, or 12.6 cm wavelength). In the SAR imaging mode, looking between 18° and 50° off-nadir, it could capture scenes with resolutions between 360 m and 120 m (1181-394 ft), depending on its position within its elliptical, near-polar orbit (at altitudes between 275 and 8,443 km (171 to 5,246 mi above a mean venusian radius of 6,051 km [3,760 mi] ). It first established orbit on August 10, 1990 after 1 1/2 loops around the Sun. Its altimeter mode achieved a vertical accuracy of better than 50 m (164 ft) within a ground cell of 10 km (6.2 mi) diameter. Operating in a radiometer mode, the radar could sense surface radio-emission, whose signals can be converted to brightness temperatures with an absolute accuracy of ±20° K. Investigators gleaned information on mass distribution (causing gravitational anomalies) from Doppler frequency variations due to gravity effects that varied orbital speeds. Even as Venus rotated slowly beneath (one complete day every 243 Earth days), during stages of its orbit closer to the planet, the radar imaged surface swaths between 17 and 28 km (10.6-17.4 mi) wide. Through the first cycle lasting 8 months, it mapped 84% of the surface. In the next 16 months, that percentage rose to 98%. Additional coverage provided repeat looks in search for possible transient or short-term changes. After several adjustments to lower orbits, the spacecraft finally burned up in the venusian atmosphere, in mid-October, 1994.

Image strips covering thousands of kilometers, especially after being joined as mosaics, provide stunning views of a fascinating venusian surface that are still undergoing thoughtful interpretations. Although Venus no doubt formed concurrently with Earth, its surface today is largely younger than one half billion years (Earth has some surficial regions older than 2 billion years). Planetologists base this estimate on venusian crater frequencies. Even though not uncommon, the numbers of resolvable impact structures are consistent with 1) destruction of the much larger numbers from the first 4 billion years, most probably by active processes that removed them by lava overplating (resurfacing) and by still arcane erosional actions, and 2) asteroidal flux rates for the last 5 hundred million years, in line with estimates from other planetary surfaces. Effects of volcanism are conspicuous, with thousands of small volcanoes detected, along with many lava channels. Although fracture zones and sets of close-spaced ridges are evident, no direct indications of terrestrial-like plate tectonics are discernible. Surface water, if ever present, left no signs of stream or ocean activities, and would have escaped from the planet (traces are present in its atmosphere) as Venus heated up, until a massive "runaway greenhouse" warming effect overwhelmed the planet. The slowly rotating atmosphere seems to have caused some wind streaks and dune-like deposits on the surface.

The gallery of Magellan images is vast. We show only a select few here but you can access more at JPL's Magellan Home Page (http://www.jpl.nasa.gov/magellan/).

To familiarize you with some of the major features and their locations on Venus, look at this shadowed relief map of the non-polar regions of the planet with the key geomorphic features labelled, as prepared by the U.S. Geological Survey:

Next, consider this color-coded relief map of nearly all of Venus, on a Mercator-like projection, derived by integrating imaging and altimeter data.

Blues represent the lowest surfaces followed by greens, then yellows and oranges with red being highest. The greatest elevations are within Maxwell Montes (top left), that includes the high point of the uplands known as Ishtar Terra. Another high region, near the equatorial center, is called Aphrodite Terra. Beta Regio, near the central left, is also elevated. Two blue regions in the north are low plains, called Sedna Planitia, below Ishtar, and Atalanta Planitia, well to its east. A large curved channel south of Aphrodite is known as Artemis Chasma.

Now look at a hemispherical projection that lies within this full map. It was made by the U.S. Geological Survey using Magellan topographic data. A vertical line through the center lies at 180 degrees east. Try to identify the high central region (in pink); refer to the first relief map above (hint: think of a lovely goddess).

19-28: Try to identify the high central region (in pink), enlarged from the first map (hint: think of a lovely goddess). ANSWER

Here are four other topographic maps located as indicated in the captions:

We can also display (below) this same hemispherical segment as a quasi-natural color image of a mosaic of rectified Magellan scenes. There is no direct proof that Venus has this much red (the choice of assigned colors was a rather arbitrary, best guess) but, if so, the presence of oxidized iron could account for such tones. The dark, blackish low areas are presumably basalts.

19-29: Once again, try to orient yourself in this image relative to the shaded relief map and localities described on that map. ANSWER

Magellan carried a microwave experiment (managed at MIT) from which a map of thermal emissivity (see Section 8) could be derived, as shown here. Note that the lowest emissivities (in blue) are found in the highest parts of the venusian surface, implying that the rock types there were other than basalt.

Thermal emissivities help to pinpoint "hot spots" and "cold sinks". Here is an emissivity map of the Atla Regio region on Venus which indicates the details procured by the radiometer:

Magellan's greatest revelations were a wide variety of volcanic features. It is not an exaggeration to refer to Venus as the "Volcanic Planet". Most of the planetary surface has been judged by geoscientists to be less than a half billion years old. Some volcanism appears to be recent and the possibility that there is even now some activity cannot be dismissed (although no changes were observed during the mission which last until 1992). Impact craters (next page), while present, are uncommon, with perhaps 1000 large enough to be resolved by Magellan's radar; this is consistent with the presumption that the present venusian surface is relatively youthful, as the crater flux by then would have greatly diminished (as extrapolated from terrestrial crater frequency since the end of the Precambrian). There have been older surfaces, some parts of which may still persist at the surface, but these are largely "paved over" (resurfacing) by the continuing activity.

This (cluttered) map shows the major volcanic features and their locations on the venusian surface

The first volcanic feature we will look at is characteristic of the plains regions. Here at Lakshmi Planum are several light and dark surfaces that are interpreted to be equivalent to the basalt flow types known as pahoehoe (smooth lavas; somewhat specular surfaces) and aa (chunky lavas, better backscatterers [lighter tones]) - counterparts to common Hawaiian lavas.

A series of Lava flows emanate from the Sils Mons volcanic source:

This radar image shows a long channel filled with volcanic flow material, over which a younger flow has straddled; note volcanic material on the right. This is the Ammavaru flow sequence in the Lada region. The scene's dimensions are 450 by 630 km:

One of the longest flows (1000 km long) occurs as Myletta Fluctus in Lavinia Planitia.

Like the Moon, thin channels and sinuous rilles have been found on Venus. Here are three examples (check captions for description):

Flows can sometimes be traced to shield volcanoes (with central calderas) as exemplified by Theia Mons, 4 km high, with a central caldera measuring 75 by 50 km and surrounded by a lava field reaching 800 km in maximum dimension.

Another major volcano, seen in this colorized rendition, is Sapus Mons (1.5 km high; 120 km at its base, on an upwelled domical surface 1000 km in diameter) in the Alta Regio region.

Perspective color views of this type of volcanic structure, made by applying altimetry data to the radar image, show it to be much like a shield volcano, with a broad base and often a central depression. This rendition of Sif Mons, about 2 km (1.2 miles) high and covering an area of nearly 300 km (200 miles) in diameter, illustrates this:

One of the highest mountains on Venus is Maats Mons which reaches to 8 km (5 miles) above the mean venusian elevation; its shape is transitional to a stratocone, suggesting its lava may have differentiated into that of an intermediate silica content. A younger lava flow from the main volcano appears as bright flow:

Topographic maps made from Mariner data can bring out structures that are large volcanoes:

Some of the larger volcanoes have summit calderas. The largest on Venus is Sacajawea Patera (140 km [89 miles] in long dimension):

Similar to that is this 30 km wide caldera

Such calderas often look almost identical to large impact structures (discussed below). A case in point is the circular depression below which could have been identified as such except for the prominent lava flow emanating from its side. (Note: another interpretation considers this to be a genuine impact crater filled post-impact with shock-generated melt that leaked out.)

A variant of the caldera type has been given the descriptive name of "tick volcano" because of its resemblance to the insect of that name. Emanating radially from the crater walls are ridges that form the "tick legs".

While these large shield volcanoes are uncommon on Venus, there is a much larger number of smaller volcanic structures (those that rise up from the surface). This map locates most of these by category, and also includes the principal volcanic fields which contain features like those shone above:

Cinder cones and stratocones (Vesuvius-like) are rare on Venus. Here is one example of a swarm of cones (each about 2 km wide) on the plains that are larger than terrestrial cinder cones but not typical of stratocones.

19-30: If large volcanic edifices like, say, Mount Rainier or Fujiyama were to occur on Venus, what would that imply? ANSWER

Typical small shield volcanoes occur in swarms such as is evident in these views:

Irregular flattened shield types, called fan volcanoes, are built up by several overlapping outpourings are illustrated by this example:

Domical hills (Tholi), shieldlike in structure, as much as 25 km (15.5 mi) wide and up to 750 m (2,460 ft) high, dot the plains of Alpha Regio. Astrogeologists believe these pancake-shaped features result from upwelling at tubular vents of lavas that spread uniformly in all directions.

Here is a closer look at two such domes in Tinatin Planitia; the larger is 65 km wide:

Using laser data, a pancake dome in Alpha Regio is shown in a perspective view, being colored such that it reminds the writer of the Devil's Platform in Hell (or more realistic like an upwelled lava dome in a lava lake).

Some of the small domes develop distinctive flows around them that have reminded some venutian planetologists of "sea anemones", to which that name is colloquially applied.

For a good summary of venusian volcanic activity, check out the Volcanoes on Venus produced by Oregon State University.

On the next page, we finish our tour of Venus now with examples of some other landforms, some having a volcanic connections, others of different nature and origin.

 

 

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Primary Author: Nicholas M. Short, Sr.