Astronomy Picture of the Day |
APOD: 2004 December 9 - Jupiter and the Moon's Shadowed Horizon
Explanation:
Early Tuesday
morning, December 7th, June Croft thought the
southeastern sky above Atmore, Alabama, USA was beautiful.
Watching
the Moon rise through gossamer clouds, she
noted, " ... the crescent Moon looked like it was held in
the sky by a star just off its shadowed horizon."
What was that star?
Bright Jupiter of course, and
some watched as the Moon actually
occulted or passed in front of
the Solar System's reigning gas
giant planet.
For astronomer Jimmy Westlake in Colorado, Jupiter was
already hidden at moonrise that morning,
but later he was able to record this lovely image,
not unlike the view that inspired Croft.
Seen through gossamer clouds, Jupiter along with large
Jovian satellites
Ganymede and Callisto (bottom to top) has emerged from
behind the crescent Moon's
shadowed horizon.
APOD: 2004 November 11 - Pastel Planet, Triple Eclipse
Explanation:
This false-color
image of banded gas giant Jupiter
shows a triple eclipse in progress on March 28 - a relatively
rare
event, even for a large planet with many moons.
Captured by the Hubble Space Telescope's near-infrared camera are
shadows of
Jupiter's moons Ganymede (left edge), Callisto (right
edge) and Io, three black spots crossing the sunlit Jovian cloud tops.
In fact, Io itself is
visible as a white spot
near picture center with a bluish Ganymede above and to the right,
but Callisto is off the right hand edge of the scene.
Viewed from Jupiter's perspective, these
shadow crossings would be
seen as solar eclipses, analogous to the
Moon's shadow crossing
the sunlit face of planet Earth.
Historically,
timing the eclipses of Jupiter's moons allowed
astronomer Ole Roemer
to make the first accurate
measurement of
the speed of light in 1676.
APOD: 2003 December 4 - New Horizons at Jupiter
Explanation:
Headed for the first close-up exploration of the
Pluto-Charon
system and the icy denizens of
the
Kuiper belt, NASA's
New Horizons
spacecraft is pictured here in an artist's vision of the
robot probe outward bound.
The dramatic scene
depicts the 465 kilogram spacecraft about
one year after
a planned 2006 launch, following a flyby of
gas giant Jupiter.
While the Jupiter flyby
will be used as a
gravity
assist maneuver to
save fuel and cut travel time to the outer reaches of
the Solar System,
it will also provide an opportunity to test
instruments and study the giant planet, its moons, and magnetic
fields.
The Sun is seen from eight hundred million kilometers away,
with inner planets Earth, Venus, and Mercury
aligned on the left.
A dim crescent of outermost Galilean moon
Callisto, orbiting Jupiter
just inside of the spacecraft's trajectory, appears to the
upper right of the fading Sun.
Left of Jupiter itself is Europa and
in the distant background are the faint, unresolved stars and
dust clouds of the
Milky Way.
New Horizons' planned arrival at
Pluto-Charon is in
the summer of 2015.
APOD: 2003 February 27 - When Moons and Shadows Dance
Explanation:
It's no wonder Jupiter is a favorite
target
for even modest earthbound telescopes.
The most massive planet
in the solar system with
four of the largest moons also boasts the famous
Great Red Spot,
a giant hurricane-like storm system over three hundred years old.
Recorded on December 15, 2002 between 7:19 and 8:40 UT,
over a thousand digital images were processed and stacked to
create this spectacular 21 frame animation of the
Jovian system.
South is up and as the Great Red Spot tracks across the face of Jupiter,
innermost Galilean
moon Io enters the scene at the far right.
Io occults (passes in front of) the edge of the more
sedately orbiting Ganymede with
Io's shadow moving quickly across the gas giant's
cloud tops, just below the Red Spot.
While the moon Callisto is outside the field of view, its large,
dark shadow is also
visible crossing the Jovian disk at the upper left.
Viewed from Earth, the orbits of the Galilean moons presently
lie nearly edge-on, offering many chances to observe similar
dances of Jupiter's moons.
APOD: 2002 January 20 - Callisto Full Face
Explanation:
Callisto's surface shows its age.
While probably formed at the same time as
Io, the difference between the
surfaces of these two moons of
Jupiter could hardly be greater.
Io's surface is young,
shows practically no impact craters,
and is continually being repaved by the
lava exploding from its many
large volcanoes.
Callisto's surface is old, shows the highest density of
impact craters in the
Solar System, and harbors no
volcanoes or even any large mountains.
Callisto's surface
is one large ice-field, laced with
cracks and
craters
from billions of years of collisions with
interplanetary debris.
The above image was taken in 2001 May and is, so far,
the only complete global color image taken by the Jupiter-orbiting
Galileo spacecraft.
APOD: 2001 August 28 - Jagged Hills on Jupiters Callisto
Explanation:
Why does Jupiter's moon Callisto
have unusual jagged hills?
This mystery came to light after the
robot spacecraft
Galileo, in orbit around
Jupiter since 1995, swooped past the
dark moon in May.
The
resulting pictures were the highest resolution yet obtained for a
Jovian moon: objects as small as 3 meters across are discernable.
The strange landscapes
pictured above show areas rich in bright sharp mounds about 100 meters tall.
A likely
formation hypothesis holds that these hills are the
result of ejecta thrown billions of years ago during a
violent impact.
The lower inset region apparently has
undergone an epoch of relatively high
ice-erosion, where dark rock has filled in
some of the inter-hill regions.
NASA has
recently cleared Galileo to continue swooping
Jupiter's moons until 2003, when it will
end its journey with a
spectacular dive into
Jupiter's atmosphere.
APOD: 2001 August 1 - Young Martian Terrain
Explanation:
What caused the pits, ridges, and gullies on
otherwise smooth Martian terrain?
One hypothesis is water.
The lack of
craters at this
mid-latitude location indicates that the terrain
is quite young by geological standards,
perhaps only 100,000 years old.
Were the terrain since saturated by water ice,
that ice would soon evaporate into the
thin Martian air.
Left over, however, might be fragile
cake-like sand that can be broken up by wind into pits and ridges.
Consequences of
this hypothesis include that even the
Martian equator
undergoes epochs of relative wet and dry,
and that future spacefarers might be able to find
water (ice) in a relatively
mild climate near the
Martian equator.
Pictured above is young-ridged terrain that
also shows evidence of a
downhill flow.
APOD: 2001 July 31 - Oceans Under Jupiters Callisto
Explanation:
Why does
Jupiter's moon
Callisto alter the
magnetic field of Jupiter in its vicinity?
Callisto itself does not have a strong magnetic field.
One possible answer is that
Callisto harbors sub-surface oceans of
electrically conducting salt-water.
This hypothesis was
bolstered recently by a new analysis of how
Callisto creates and dissipates heat.
Callisto is thought to create heat by the
radioactive decay of internal rock -- a process that keeps the
Earth's mantle molten.
Callisto may not be able to dissipate this heat very efficiently,
however, as it has thick layers of ice and rock on its surface.
Perhaps this heat is enough to keep sub-surface water from
freezing into ice.
With this hypothesis, Callisto joins two other of
Jupiter's moons,
Europa and
Ganymede,
in candidates for sub-surface oceans.
Callisto's oceans, however,
might prove too hostile to support
Earth-like life.
APOD: 2001 January 16 - Europa Rotating
Explanation:
Evidence has been mounting that beneath the
vast planes of ice that cover
Europa lies water --
liquid oceans that might be home to
alien life.
The smallest of
Jupiter's
Galilean Moons
(which include
Io,
Ganymede, and
Callisto), Europa's deep interior is composed of mostly of
silicate rock.
Upon close inspection, many
surface cracks
stop abruptly only to continue on somewhere else -- indicating
surface plates that might be sliding.
The
above time-lapse sequence is a composite of images
taken during the
Voyager spacecraft flyby of the moon twenty years ago.
Not all regions are resolved in high detail.
The movie shows
Europa during a complete rotation,
which corresponds to a complete revolution around
Jupiter since
Europa always keeps the same face toward the giant planet.
The cause of many of the surface colors on
Europa also remains a
topic of research.
APOD: 2001 January 2 - Jupiter, Europa, and Callisto
Explanation:
As the
robot Cassini spacecraft rounds
Jupiter on its way toward
Saturn, it has taken a
sequence of images
of the gas giant with its
four largest moons.
Previously released images have highlighted
Ganymede and
Io.
Pictured above are the two remaining
Galilean satellites:
Europa and
Callisto.
Europa is the bright moon superposed near
Jupiter's Great Red Spot,
while Callisto is the dark moon near the frame edge.
Callisto is so dark that it would be hard to see
here if its brightness was not digitally enhanced.
Recent evidence indicates that both moons hold salt-water seas under surface ice that might be home to extra-terrestrial life.
By noting the times that moons disappeared and
reappeared behind Jupiter in 1676,
Ole Roemer was able to make the
first accurate estimation of the speed of light.
APOD: 2000 December 18 - Oceans Under Jupiter's Ganymede
Explanation:
The
search for extraterrestrial life
came back into our own
Solar System last week with the
announcement that there may be
liquid oceans under the surface of
Jupiter's moon
Ganymede.
Ganymede now joins
Callisto and
Europa as moons of
Jupiter that may harbor seas of liquid water under
layers of surface ice.
The ocean hypothesis surfaced as an explanation for
Ganymede's unusually strong magnetic field.
Ganymede, the largest moon in the Solar System,
also has the largest measured
magnetic field of any moon.
Some
exobiologists hypothesize that
life may be able to emerge in such an ocean,
much as it did in the
oceans of ancient Earth.
Above, a frame from a
computer simulation shows what it would look like to
fly over the surface of Ganymede,
as extrapolated from photographs of the
grooved moon taken by the
robot spacecraft Galileo currently orbiting Jupiter.
APOD: 2000 November 18 - Jupiter And Family
Explanation:
This composite image features classic portraits of members of one of
the Solar System's
most prominent families -
Jupiter and its four large
"Galilean" moons.
Starting from the top the moons are
Io,
Europa,
Ganymede, and
Callisto.
The top-to-bottom order is also the order of increasing distance from
Jupiter.
These are big moons indeed which attend
the largest planet.
The smallest of the lot, Europa, is the size of
Earth's moon while
Ganymede is the largest moon in the Solar System.
In fact, Ganymede with a diameter of 3,100 miles, is larger
than the planets Mercury and
Pluto.
The swirling
Great Red Spot appears at the edge of Jupiter.
A hurricane-like storm system that has persisted for over 300 years,
two to three earths could fit inside it.
Battered Callisto's
image was recorded during the 1979 flyby
of Voyager.
The other portraits were taken by
the
Galileo spacecraft which began
exploring the Jovian system in 1995.
APOD: 2000 October 24 - Io Rotating
Explanation:
The surface of Io is continually changing.
Jupiter's moon
is the home to many powerful
volcanoes so active
they are effectively turning the moon inside out.
The above time-lapse sequence is a composite of images
taken during two space missions that approached the violent moon:
Voyager and
Galileo.
The sequence shows
Io during a complete rotation,
which corresponds to a complete revolution around
Jupiter since
Io always keeps the same face toward the giant planet.
The rampant volcanism is thought to be caused by
Jupiter's more distant
Galilean Moons
(Europa,
Ganymede, and
Callisto) pulling on Io and continually distorting its shape,
causing internal
frictional heating.
Io is composed mostly of rock, with the yellow color originating from
sulfur.
The causes of many of the other colors remain a
topic of research.
APOD: 2000 May 8 - Jupiters Moons Thebe, Amalthea, and Metis
Explanation:
The robot
spacecraft Galileo in orbit around
Jupiter has recently photographed the
inner moons of Jupiter in greater detail than ever before.
These pictures of
Thebe,
Amalthea,
and
Metis
are shown to scale, and reveal details as
small as three kilometers across.
Amalthea, by contrast, has a
total length of about 200 kilometers.
The
moons are composed
mostly of ice, are much smaller than Jupiter's
more famous
Galilean satellites
(Io,
Europa,
Ganymede, and
Callisto), and orbit between Io and
Jupiter's rings.
Thebe appears dominated by a huge
impact crater
40 kilometers across.
Astronomers are uncertain of the
origin of the unusual white gash at the bottom of
Amalthea.
APOD: November 7, 1998 - Globular Cluster 47 Tucanae
Explanation:
Stars come in bunches.
Of the over 200
globular star clusters
that orbit the center of our
Milky Way Galaxy,
47 Tucanae is the second brightest
globular cluster
(behind Omega Centauri).
Known to some affectionately as
47 Tuc or NGC 104, it is only visible from the
Southern Hemisphere.
Light takes about 20,000 years to reach us from 47 Tuc
which can be seen near the
SMC in the constellation of
Tucana.
Red Giant stars
are particularly easy to see
in this picture.
The dynamics of stars near the center of 47 Tuc are not well understood,
particularly why there are so few
binary systems there.
APOD: November 6, 1998 - Cutaway Callisto: Ice, Rock, and Ocean
Explanation:
Cruising past the moons of reigning gas giant Jupiter,
Voyager and
Galileo have returned tantalizing evidence for a
liquid water ocean
beneath the surface of Europa.
Now researchers are reporting telltale indications that the
battered Jovian moon Callisto may also harbor
a subsurface ocean.
This cutaway view of Callisto shows
a whitish 200 kilometer thick band of
ice just beneath the moon's surface.
The hypothetical ocean - indicated by the underlying light blue stripe -
is potentially a salty layer of liquid water up to 10 kilometers thick,
while the rest of the interior is seen as a jumble of rock and ice.
Why a salty subsurface ocean?
Magnetic measurements made during Galileo flybys so far indicate
Callisto's magnetic field is variable, analogous to
results during Europa passes, and a plausible explanation
is that Callisto too has a subsurface liquid layer.
If the liquid were salt water it could easily carry
electrical currents
and produce the
changing magnetic field.
APOD: May 12, 1998 - Callisto Enhanced
Explanation:
Callisto
is half rock and half ice. This moon of
Jupiter is approximately the size of the planet Mercury, making it the third largest moon in the
Solar System, after
Ganymede and Titan.
Callisto's icy surface is
billions of years old, lacks any sign of
volcanic activity,
and is densely covered with rifts and craters.
These features are particularly apparent in this contrast-enhanced image
taken by the
Galileo spacecraft, and released last week.
Visible near the image center is Valhalla, one of the largest impact craters in the
Solar System, measuring about 4,000 kilometers across.
The rings and size of Valhalla make its appearance similar to the
Caloris Impact Basin on Mercury.
APOD: May 11, 1998 - Callisto in True Color
APOD: February 2, 1998 - A Triple Eclipse on Jupiter
APOD: September 29, 1997 - Jupiter And Family
APOD: June 12, 1997 - Jupiter's Dry Spots
APOD: February 18, 1997 - A Big Cliff on Jupiter's Callisto
APOD: December 9, 1996 - Callisto Full Face
APOD: October 25, 1996 - A Flyby View of Ganymede
APOD: October 22, 1996 - The Cracked Ice Plains of Europa
APOD: July 12, 1996 - Ancient Cratered Plains on Ganymede
APOD: July 10, 1996 - Galileo Photographs Ganymede
APOD: November 3, 1995 - Jupiter's Moon Amalthea
APOD: September 6, 1995 - Callisto: Dark Smashed Iceball
APOD: July 15, 1995 - The Crater Chain
Explanation:
Callisto's surface has many stories to tell.
The most distant of Jupiter's Galilean Moons, Callisto shows the highest density of
impact craters in the
Solar System, but harbors no volcanoes
or even any large mountains.
Callisto's surface is laced with
cracks and
craters from billions of years of collisions with
interplanetary debris. This image shows
Explanation:
Part of Jupiter is missing.
Actually, three parts appear to be missing.
In reality though, the three dark spots seen in the above photograph are only shadows. The
unusual alignment of three of Jupiter's moons between the
Jovian giant and the Sun was imaged last November 10th.
The shadows of
Io,
Callisto, and
Ganymede move across
Jupiter as these moons progress in their orbits.
It was by noting the times of eclipse
of Jupiter's moons in 1675 that Ole Roemer became the first person to measure the
speed of light.
When a shadow from
Earth's Moon
crosses the Earth's surface, the people inside the shadow see an
eclipse of the Sun.
Explanation:
This composite image features classic portraits of members of one of
the Solar System's most prominent families -
Jupiter and its four large
"Galilean" moons.
Starting from the top the moons are
Io,
Europa,
Ganymede, and
Callisto.
The top-to-bottom order is also the order of increasing distance from
Jupiter.
These are big moons indeed which attend
the largest planet.
The smallest of the lot, Europa, is the size of
Earth's moon while
Ganymede is the largest moon in the Solar System.
In fact, Ganymede with a diameter of 3,100 miles, is larger
than the planets Mercury and
Pluto.
The swirling
Great Red Spot appears at the edge of Jupiter.
A hurricane-like storm system that has persisted for over 300 years,
two to three earths could fit inside it.
Battered Callisto's
image was recorded during the 1979 flyby
of Voyager.
The other portraits were taken by
the Galileo spacecraft which began
exploring the Jovian system in 1995.
Explanation:
Known for its spectacular images of Jupiter's moons,
Io,
Ganymede,
Callisto,
and Europa,
the robot spacecraft Galileo has also aggressively explored the Jovian
atmosphere.
In December of 1995,
Galileo's atmospheric probe descended into
Jupiter's clouds and reported
a surprising absence of water.
It is now believed that the probe entered through one of
Jupiter's dry spots,
similar to the dark region in
this image of the swirling Jovian cloud deck.
The smallest features visible here are tens of miles in size.
These dry regions appear to correspond to locations where
winds converge creating downdrafts. The downdrafts generate
local cloudless clearings through which
Jupiter's deeper warmer layers can be glimpsed.
Just as the dark areas are extremely dry, the surroundings are
full of moisture. The contrast is analogous to
the desert and tropics of Earth.
Explanation: Callisto's surface
is not without fault. In fact, an explorer crossing the surface
of this large moon of Jupiter
would need climbing equipment to pass this large, recently discovered
fault. The above picture
was released last week and was taken in November 1996 by the robot spacecraft Galileo
currently orbiting Jupiter.
As the Sun illuminates Callisto's
surface from the left, the unusual cliff or scarp
stands out by the shadow it casts to the right. This cliff and
others were probably formed when a large object collided with
Callisto
early in its history. Of the many visible craters
in the above photograph, the smallest visible is about a football
field across, while the largest is more than a kilometer.
Explanation: Callisto's surface shows its age. While probably
formed at the same time as Io, the
difference between the surfaces of these two moons of Jupiter
could hardly be greater. Io's
surface is young, shows practically no impact craters, and is
continually being repaved by the lava
exploding from its many large volcanoes.
Callisto's
surface is old, shows the highest density of impact craters in
the Solar System, and harbors no
volcanoes or even any large mountains. Callisto's
surface is one large ice-field, laced with cracks and craters
from billions of years of collisions with interplanetary debris.
The high-resolution vertical band in the above mosaic
was taken by the robot spacecraft Galileo
currently orbiting Jupiter. The
rest of the mosaic was compiled from pictures taken by the Voyager spacecraft
which passed the Callisto
in 1979.
Explanation: This is what it would look like to fly over
the surface of Jupiter's moon Ganymede. NASA's robot spacecraft Galileo
recently approached only 6000 miles from this frozen ice-ball
of a moon. The above image
is a digital reconstruction from two images taken during this
flyby. The blue color of the sky is artificial. Deep furrows
cover Ganymede's surface, and several
impact craters are also present. Galileo continues to orbit Jupiter
and send back breathtaking photos of Ganymede,
Io, Europa,
Callisto, and, of course, Jupiter
itself.
Explanation: What caused the cracks in this giant ice-ball?
Jupiter's moon Europa
has smoothest surface in the solar system and is composed mostly
of cracked water-ice. In the above false-colored picture
released last week by the NASA team
in charge of the Galileo mission,
blue hues represent ice plains divided by dirty red and brown
bands of mottled terrain. As the robot Galileo spacecraft orbits
Jupiter, it sends back revealing pictures of Jupiter and
its large moons
including Europa, Io,
Ganymede, and Callisto.
The region of Europa
highlighted above is known as Minos Linea. The cause for many
of the cracks remains unknown but may involve shifting stresses
from gravity and temperature variations. The new Galileo pictures
have increased evidence that liquid oceans may indeed exist
under these giant ice-sheets, a place possibly ripe for the development of life.
Explanation:
The largest moon in the Solar System shows regions that are ancient and
battered. The high density of craters demonstrate that patches of
Ganymede
are indeed billions of years old.
This photo is
one of a series
released by
NASA two days ago from the
Galileo spacecraft orbiting
Jupiter.
The large impact crater on the left is 19 kilometers across, while dark
indentations in
Ganymede's crusty surface-ice run diagonally.
Ganymede is composed of
half rock and half water-ice. The
Galileo
spacecraft will continue to orbit
Jupiter
over the next 16 months and send back data about
Jupiter and its
four largest moons:
Io,
Europa,
Ganymede, and
Callisto.
Explanation:
Ganymede's surface is slowly being pulled apart.
This photo of Ganymede was
released earlier today by the Galileo team at NASA. The
Galileo Spacecraft arrived at
Jupiter
in December 1995. In late June, the spacecraft passed
within 10,000 kilometers of
Ganymede's icy surface, and took pictures
showing complex surface details for the first time. The
line-like features in this photo are sunlit ridges rising above
Ganymede's ice-plains.
The circular features are impact craters.
Ganymede
is the largest moon of
Jupiter and hence the largest of
the four Galilean satellites:
Io,
Europa,
Ganymede, and
Callisto.
Explanation:
Amalthea is
Jupiter's fifth largest moon, much smaller than
the four
Galilean
satellites
Io,
Europa,
Ganymede, and
Callisto.
The orbit of
Amalthea is inside of these moons, and with its long axis always
pointing toward
Jupiter. It's dark surface color is
probably due to sulfur being
expelled from Io.
Amalthea is not massive enough for its internal
gravity to make it spherical.
Amalthea was discovered in 1892 by Edward Barnard.
Explanation:
Callisto is a dirty battered world, showing the most beaten surface of
Jupiter's major moons. Made of a rocky core
covered by fractured ice,
Callisto's past collisions with large meteors are evident as large
craters surrounded by concentric rings. The four largest moons of Jupiter:
Io,
Europa,
Ganymede, and
Callisto were all discovered by
Galileo
and
Marius in 1610 with early telescopes and are now known as the
Galilean satellites. The
NASA spacecraft Galileo
is scheduled to arrive at Jupiter is December of 1995.
Explanation:
NASA's robot spaceprobe
Voyager 1,
took this closeup image of the surface of Jupiter's crater scarred moon
Callisto in 1979. A mysterious chain of craters is seen to extend diagonally
across the image (upper left to lower right). What could cause the craters
to line up in such a regular fashion? Scientists were at a loss to explain
this crater chain along with several other chain like features observed
on Callisto's surface. Fifteen years later, with the discovery of
Comet Shoemaker-Levy 9,
also known as the
"string of pearls" comet,
the mystery was solved. Comets whose orbits stray too close to
Jupiter are torn apart by the strong gravity.
When the individual pieces, strung
out along the orbital path of the comet hit an object like Callisto,
the sequence of impacts produces a crater chain.
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Today's Astronomy Picture of the Day