Astronomy Picture of the Day |
APOD: 2006 February 15 - Rotating Titan in Infrared Light
Explanation:
Titan is one of the strangest places in
our Solar System.
The only moon known with
thick clouds, this unusual satellite of
Saturn
shows evidence of evaporating lakes created by methane rain.
The clouds that make
Titan featureless in
visible light
have now been pierced several times in
infrared light by the
robot Cassini spacecraft
currently orbiting Saturn.
These images have been compiled into the
above time-lapse movie.
Like Earth's Moon,
Titan always shows the
same face toward its central planet.
It therefore takes Titan about 16 days to complete one rotation.
Titan has numerous areas of
light terrain with some large areas of
dark terrain
visible near the equator.
Small areas of brightest terrain might arise from
ice-volcanoes
and have a high amount of reflective frozen
water-ice.
Titan's surface was imaged for the first time early last year by the
Huygens probe, which survived for three hours on a
cold and sandy dark region.
APOD: 2005 March 8 - Crater on Mimas
Explanation:
Whatever hit Mimas nearly destroyed it.
What remains is one of the
largest impact craters on one of
Saturn's smallest moons.
The crater, named Herschel after the 1789 discoverer of Mimas,
Sir William Herschel, spans about 130 kilometers and is
pictured above in the dramatic light of its
terminator.
Mimas'
low mass produces a surface gravity just strong enough
to create a spherical body but weak enough to allow
such relatively large surface features.
Mimas is made
of mostly water ice with a smattering of rock - so it is
accurately described as a big dirty snowball.
The above image was taken during the 2005 January flyby of the
robot spacecraft Cassini now in
orbit
around Saturn.
APOD: 2005 September 6 - Fresh Tiger Stripes on Saturns Enceladus
Explanation:
The tiger stripes on Saturn's moon Enceladus might be active.
Even today, they
may be spewing ice from the moon's icy interior into space,
creating a cloud of fine ice particles over the moon's South Pole
and creating Saturn's mysterious E-ring.
Recent evidence for this has come from the
robot Cassini spacecraft now orbiting
Saturn.
Cassini detected a marked increase in particle collisions
during its July flyby only 270 kilometers over a South Polar region of
Enceladus.
Pictured above,
a high resolution image of Enceladus is shown from the close flyby.
The unusual surface features dubbed
tiger stripes are visible on the left in false-color blue.
Why
Enceladus is active remains a mystery, as the neighboring moon
Mimas,
approximately the same size, appears
quite dead.
APOD: 2006 May 30 - Ancient Craters on Saturn's Rhea
Explanation:
Saturn's ragged moon Rhea has one of the oldest surfaces known.
Estimated as changing little in the past billion years,
Rhea shows
craters
so old they no longer appear round – their
edges have become compromised by more recent cratering.
Like Earth's Moon,
Rhea's rotation is locked on Saturn, and the
above image shows part of
Rhea's surface that always faces Saturn.
Rhea's leading surface is more highly cratered than its trailing surface.
Rhea is composed mostly of water-ice but is thought to have a
small rocky core.
The above image was taken by the
robot Cassini spacecraft now orbiting Saturn.
Cassini swooped past Rhea two months ago and captured the
above image from about 100,000 kilometers away.
Rhea
spans 1,500 kilometers making it Saturn's second largest moon after
Titan.
Several
surface features on Rhea
remain unexplained including large light patches.
APOD: 2006 January 3 - Dark Terrain on Saturns Iapetus
Explanation:
Why are vast sections of Iapetus as dark as
coal?
No one knows for sure.
Iapetus, the
third largest moon of
Saturn,
was inspected again as the Saturn-orbiting robot
Cassini spacecraft swooped past the
enigmatic world again late last year.
The dark material covers most of the surface visible in the
above image,
while the small portion near the top that appears almost white is of a
color and reflectance more typical of Saturn's other moons.
The unknown material covers about half of the 1,500 kilometer
wide moon.
The material is
so dark
that it reflects less than five percent of incident sunlight,
yet overlays craters indicating that it was spread after the
craters were formed.
Iapetus
has other unexplained features.
The bright part of
Iapetus
is covered with unexplained long thin streaks.
The orbit of
Iapetus is also unusual, being tilted to the plane of Saturn's
orbit by an unusually high fifteen degrees.
A strange ridge
about 13 kilometers high crosses much of Iapetus near the
equator and is visible
near
the bottom.
Oddly, this ridge is almost exactly parallel with Iapetus' equator.
The exact shape of Iapetus remains undetermined,
but images indicate that it is quite strange --
something like a
walnut.
Research into the formation and history of
mysterious Iapetus
is active and ongoing.
APOD: 2005 October 12 - Cratered Cliffs of Ice on Saturns Tethys
Explanation:
The surface of
Saturn's
moon Tethys is riddled with icy cliffs and craters.
The most detailed images ever taken of
Tethys
were captured late last month as the robot
Cassini spacecraft
swooped past the frozen ice moon.
The above image was taken from about 32,000 kilometers distant and
shows a jagged landscape of long cliffs covered with craters.
At the bottom of many craters appears some sort of unknown light-colored substance,
in contrast to the unknown dark substance that appears at the bottom of Saturn's moon
Hyperion.
Tethys is one of the
larger moons of Saturn, spanning about 1,000 kilometers across.
The density of Tethys indicates a composition
almost entirely of
water ice.
Tethys
is thought to have been predominantly liquid sometime in its distant past,
creating some of its long ice-cliffs
as it cracked during
freezing.
APOD: 2005 October 26 - 4500 Kilometers Above Dione
Explanation:
What does the surface of Saturn's moon Dione look like?
To find out, the
robot Cassini spacecraft
currently orbiting
Saturn
flew right past the fourth largest moon of the giant planet earlier this month.
Pictured above
is an image taken about 4,500 kilometers above
Dione's icy surface,
spanning about 23 kilometers.
Fractures, grooves, and craters in
Dione's ice and rock are visible.
In many cases, surface features are caused by unknown processes and can only be described.
Many of the
craters have bright walls but
dark floors, indicating that fresher ice is brighter.
Nearly parallel grooves run from the upper right to the lower left.
Fractures sometimes across the bottom of craters, indicating a relatively recent formation.
The lip of a 60-kilometer wide crater runs from the middle left to the upper
center of the image, while the crater's center is visible on the lower right.
Images like this
will continue to be
studied to better understand
Dione as well as
Saturn's complex system of
rings and
moons.
Authors & editors:
Robert Nemiroff
(MTU) &
Jerry Bonnell
(USRA)
NASA Web Site Statements, Warnings, and
Disclaimers
NASA Official: Jay Norris.
Specific rights apply.
A service of:
EUD at
NASA /
GSFC
& Michigan Tech. U.