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Astronomy Picture of the Day |
APOD: 2007 August 1- Unusual Cratering on Saturns Dione
Explanation:
Why does one half of Dione have more craters than the other?
Start with the fact that
Saturn's moon Dione
always has one side that faces Saturn, and always has one side that faces away.
This is similar to
Earth's Moon.
This tidal locking
means that one side of
Dione always leads as the moon progresses in its orbit,
while the other side always trails.
Dione
should therefore have undergone a significant amount of impacts on its leading half.
But the current leading half of
Dione
is less cratered than the trailing half!
A possible explanation is that some impacts were so large they spun
Dione,
sometimes changing the part that suffered the highest impact rate before the
moon's spin again became locked.
Pictured above,
it is the top part of Dione that appears significantly more cratered than the bottom half.
APOD: 2006 September 5 - Bright Cliffs Across Saturns Moon Dione
Explanation:
What causes the bright streaks on Dione?
Recent images of this unusual moon by the
robot Cassini spacecraft now orbiting
Saturn
are helping to crack the mystery.
Close inspection of Dione's trailing hemisphere,
pictured above, indicates that the white wisps are composed of deep
ice cliffs dropping hundreds of meters.
The cliffs may indicate that Dione has undergone some sort of
tectonic surface displacements in its past.
The bright ice-cliffs run across some of
Dione's many craters, indicating that the
process that created them occurred later than the
impacts that created those craters.
Dione
is made of mostly water ice but its relatively high density
indicates that it contains much
rock inside.
Giovanni Cassini discovered Dione in 1684.
The above image was taken at the end of July from a distance of about 263,000 kilometers.
Other high resolution images of
Dione were taken by the passing
Voyager spacecraft in 1980.
APOD: 2005 December 31 - A Year at Saturn
Explanation:
Arriving at Saturn in July of 2004, the
Cassini spacecraft
has now spent a year and a half exploring the
magnificent rings and moons of the distant gas giant.
The year 2005 began with Cassini's
Huygens probe landing
on Saturn's large moon Titan.
Cassini's continuing series of close flybys
also revealed
details and discoveries
across the surface of the smog shrouded moon.
In fact, with a ringside
seat throughout 2005, Cassini's cameras
have made spectacular pictures of Titan along with
Saturn's
other moons and rings
almost
common place.
But often, Saturn itself provided the most dramatic backdrop.
In this
view, Saturn's moon Dione lies in front of edge-on
rings and the gas giant's cloud tops draped with broad
ring shadows.
Dione is 1,118 kilometers across
and lies about 300,000 kilometers from the ring's edge.
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.
APOD: 2005 October 21 - Ringside
Explanation:
Orbiting in the plane of
Saturn's rings,
Dione and the other icy saturnian moons have a perpetual
ringside view of the
gorgeous gas giant
planet.
Of course, while passing through the ring plane
the Cassini spacecraft also shares
their stunning
perspective.
The rings themselves can be seen slicing across the bottom of
this Cassini snapshot.
Remarkably thin,
the bright rings still cast arcing shadows
across the planet's cloud tops.
Pale
Dione, in the foreground, is
about 1,100 kilometers
across and orbits over 300,000 kilometers from the
visible outer edge of the A ring.
APOD: 2005 March 29 - Crescents of Titan and Dione
Explanation:
What would it be like to see a sky with many moons?
Such is the sky above
Saturn.
When appearing close to each other, moons will show a similar
phase.
A view with two of the more famous moons of
Saturn in crescent
phase was captured last month by the
robot spacecraft
Cassini now orbiting Saturn.
Titan,
on the lower left, is among the largest moons in the
Solar System and is perpetually shrouded in clouds.
Recently, the Huygens probe
landed on Titan
and gave humanity its first view of its unusual surface.
Dione,
on the upper right, has less than a quarter of
Titan's
diameter and has no significant atmosphere.
Dione,
although appearing smaller, was only half the distance to
Titan when the
above image was taken.
APOD: 2005 February 15 - Saturns Moon Rhea from Cassini
Explanation:
Each moon of Saturn seems to come with its own
mystery.
Rhea, Saturn's second largest moon behind
Titan, shows unusual wisps, visible above as light colored streaks.
Higher resolution images of similar wisps on Dione indicate that they
might be made of long braided fractures.
Rhea is composed mostly of
water ice,
but likely has a
small rocky core.
Rhea's rotation and orbit are locked together, just like
Earth's Moon, so that one
side always faces Saturn.
A consequence of this is that one side always leads the other.
Rhea's leading surface is much more heavily
cratered than the trailing surface, pictured above.
The above image in natural color was taken last month by the
Cassini robot spacecraft in orbit around Saturn.
APOD: 2004 December 1 - Saturn's Moon Dione from Cassini
Explanation:
What causes the bright streaks on Dione?
Recent and likely future images of this unusual moon by the
robot Cassini spacecraft now orbiting
Saturn
might help us find out.
The above image was taken at the end of October from a distance of about one million kilometers.
The bright streaks run across some of Dione's many craters, indicating that the
process that created them occurred later than the
impacts that created those craters.
Dione
is made of mostly water ice but its relatively high density
indicates that it contains much rock inside.
Giovanni Cassini discovered Dione in 1684.
The Cassini spacecraft is scheduled to photograph Dione
at higher resolution in mid-December.
Currently, the
highest resolution images of
Dione remain those taken by the passing
Voyager spacecraft in 1980.
APOD: 2003 February 22 - Infrared Saturn
Explanation:
This delightfully detailed
false-color
image of Saturn was taken in January 1998 by the
orbiting Hubble Space Telescope.
The picture is a combination of three images from Hubble's
NICMOS instrument and shows the lovely ringed planet in reflected
infrared
sunlight.
Different colors indicated varying heights and compositions of cloud
layers generally thought to consist of ammonia ice crystals.
The eye-catching rings cast a shadow
on
Saturn's upper hemisphere.
The bright stripe seen within the left portion of the shadow
is infrared sunlight streaming through the large
gap
in the rings known
as the Cassini Division.
Two of
Saturn's many moons have also put in an appearance,
Tethys just
beyond the planet's disk at the upper right, and Dione at the lower left.
Presently, Saturn shines brightly in evening skies as a
pale yellow "star" near the constellation
Orion.
APOD: July 24, 1999 - Infrared Saturn
Explanation:
This delightfully detailed
false color image of Saturn
was earmarked to
celebrate the 8th anniversary of the orbiting Hubble Space Telescope.
The picture is a combination of three images taken in January 1998
and shows the lovely ringed planet in reflected
infrared light.
Different colors indicated varying heights and compositions of cloud
layers generally thought to consist of ammonia ice crystals.
The eye-catching rings cast a shadow
on Saturn's upper hemisphere,
while the bright stripe seen within the left portion of the shadow
is infrared
sunlight streaming through the large
gap in the rings known
as the Cassini Division.
Two of
Saturn's many moons have also put in an appearance,
Tethys just
beyond the planet's disk at the upper right, and Dione at the lower left.
APOD: April 24, 1998 - Infrared Saturn
Explanation:
This delightfully detailed
false color image of Saturn
has been earmarked to
celebrate the 8th anniversary of the orbiting Hubble Space Telescope.
The picture is a combination of three images taken in January of this year
with the Hubble's new NICMOS instrument and shows the lovely ringed planet in reflected
infrared light.
Different colors indicated varying heights and compositions of cloud
layers generally thought to consist of ammonia ice crystals.
The eye-catching rings cast a shadow
on Saturn's upper hemisphere,
while the bright stripe seen within the left portion of the shadow
is infrared
sunlight streaming through the large
gap in the rings known
as the
Cassini Division.
Two of
Saturn's many moons have also put in an appearance,
Tethys just
beyond the planet's disk at the upper right, and Dione at the lower left.
APOD: January 18, 1998 - Saturn, Rings, and Two Moons
Explanation:
NASA's robot spacecraft
Voyager 2 made this image of Saturn as it
began to explore the Saturn system in 1981.
Saturn's famous rings are visible along with two of
its moons,
Rhea and
Dione
which appear as faint dots on the right
and lower right part of the picture.
Astronomers believe that Saturn's moons play a
fundamental role in sculpting its elaborate ring system.
A robot spacecraft named
Cassini was
launched last October and is expected to
rendezvous with the giant gas planet in 2004.
APOD: May 31, 1997 - Saturn with Moons Tethys and Dione
Explanation:
Saturn and two of its larger moons -
Tethys and
Dione - were photographed by the
Voyager 1
spacecraft which flew by the planet in November of 1980.
This picture gives an indication of
Saturn's extensive ring system, which
can be seen casting a shadow on the planet, as does Tethys.
Saturn's rings are composed of many chunks of ice ranging in size
from a pebble to a car. The rings have several large gaps, the largest of
which is clearly visible in the picture and is named the
Cassini
Division, after its
discoverer.
Saturn
appears brighter than most stars in the sky,
and its rings can be discerned with a small telescope.
A new spacecraft -
Cassini - will visit
Saturn
and is currently scheduled for launch later in 1997.
APOD: May 12, 1996 - Tracking Saturn's Moons
Explanation:
These five pairs of
Hubble Space Telescope images track some of
Saturn's moons as they orbit the ringed planet.
A pair of images was taken every 97 minutes on November 21, 1995 with
the Wide Field Planetary Camera-2, the normally bright ring system
appearing nearly edge-on. In the top pair, the large
bright moon Dione hangs above center while the smaller moons
Pandora, Prometheus,
and Mimas (top right image)
appear near the planet's disk close to the outer ring. By the second and third
pair of images, moons Rhea
and Epimetheus have joined the dance.
During the Saturn ring plane crossings, the reduction in light from
the edge-on rings provided an opportunity for astronomer's to
explore Saturn's complex moon system and search for elusive
undiscovered satellites.
APOD: March 18, 1996 - Saturn with Moons Tethys and Dione
Explanation:
Saturn and two of its larger moons -
Tethys and
Dione - were photographed by the
Voyager 1
spacecraft which flew by the planet in November of 1980.
This picture gives an indication of
Saturn's extensive ring system, which
can be seen casting a shadow on the planet, as does Tethys.
Saturn's rings are composed of many chunks of ice ranging in size
from a pebble to a car. The rings have several large gaps, the largest of
which is clearly visible in the picture and is named the
Cassini
Division, after its
discoverer.
Saturn
appears brighter than most stars in the sky,
and its rings can be discerned with a small telescope.
A new spacecraft -
Cassini - will visit
Saturn
and is currently scheduled for launch in 1997.
APOD: October 10, 1995 - Dione's Lagrange Moon Helene
Explanation:
Saturn's moon
Helene
is very unusual in that it circles
Saturn near the
orbit of a bigger moon:
Dione.
Helene is situated in what is called a
"Lagrange point" of Dione - a place of stability created by Dione's
gravity. Were
Helene to stray slightly from its orbit 1/6 ahead of Dione,
the larger moon's gravity would cause Helene to move back toward the
Lagrange point. Many massive orbital bodies have stable Lagrange points,
including the Earth and Moon. Helene was discovered from the ground by P.
Laques & J. Lecacheux in 1980. The photograph above was taken by Voyager 2
as it passed Saturn in 1981. NASA's
Cassini
mission to Saturn is currently
scheduled for launch in October 1997.
APOD: October 9, 1995 - Saturn's Moon Dione
Explanation:
Dione, one of
Saturn's larger moons, is remarkable for its
bright surface streaks. These streaks run across some of Dione's many
craters, which indicate that the process which created the streaks occurred
later than the process which created the craters.
Dione
is made of mostly water ice but its relatively high density indicates that
it contains much rock inside. Dione was discovered by Giovanni Cassini in
1684. NASA's
Cassini
mission to Saturn is currently scheduled for launch in
October 1997. Dione's orbit is remarkable it that it also houses the much
smaller moon
Helene.
This moon, once designated "Dione B", precedes Dione by about 1/6th of an
orbit.
APOD: July 6, 1995 - Saturn, Rings, and Two Moons
Explanation:
This image of Saturn was made by NASA's robot spacecraft
Voyager
2 as it began to explore the Saturn system in 1981.
Saturn's famous rings are visible along with two of its moons, Rhea and
Dione which appear as faint dots in the right and lower right
part of the picture.
Astronomers believe that Saturn's moons play a fundamental
role in sculpting its elaborate ring system.