Artwork by Ron Miller; “8 Wonders of the Solar System”
Fall 2011
Probing Our Planets
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http://www.scientificamerican.com/article.cfm?id=8-wonders
“You are cruising in the troposphere of Saturn under the most
magnificent ring structure in the solar system. Few sights are
more astounding. The white, icy rings soar 75,000 kilometers
above your head. Ringshine illuminates everything around
you. No fewer than six crescent moons rise in the sky. The
light from the setting sun scatters against a mist of ammonia
crystals, forming a beautiful sun dog. You are buffeted by
ammonia clouds that stream by you at speeds greater than
1,500 kilometers an hour. These are some of the fastest
winds in the solar system. More than 30,000 kilometers
below you, with pressures no human-made thing could
survive, is a global ocean of liquid metallic hydrogen. There
will be no landing on this planet.” Caption by Ed Bell
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Cassini to Saturn
Impacts recorded while
crossing the Ring Plane
 Rocket science and “Gravitational Slingshots.”
Slingshots.”
Fall 2011
Probing Our Planets
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Cassini JPL Home Page: http://saturn.jpl.nasa.gov/index.cfm
Cassine NASA Home Page: http://www.nasa.gov/mission_pages/cassini/main/
Trajectory video from: http://saturn.jpl.nasa.gov/video/videodetails/?videoID=18
Rocket Science--Gravity Assist--In orbital mechanics, a gravity
assist maneuver or gravitational slingshot uses the relative
movement and mutual gravity between a planet and spacecraft
to alter the path and speed of a spacecraft at some small cost to
the planet’s orbital motion. It is used in order to save propellant,
time, and expense. Gravity assistance can be used to accelerate,
decelerate and/or re-direct the path of a spacecraft.
The "assist" is provided by an exchange of orbital angular
momentum between the two objects. From the perspective of
the planet, the spacecraft’s velocity is symmetric but not from the
perspective of the Sun!
The “trick” has been used from Mariner 10 onwards, including
Voyagers’, etc.
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Cassini Eclipsed
Fall 2011
Probing Our Planets
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Image Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA
http://apod.nasa.gov/apod/ap110904.html
In the shadow of Saturn, unexpected wonders appear.
The robotic Cassini spacecraft now orbiting Saturn
drifted in giant planet's shadow for about 12 hours in
2006 and looked back toward the eclipsed Sun. Cassini
saw a view unlike any other. First, the night side of
Saturn is seen to be partly lit by light reflected from its
own majestic ring system. Next, the rings themselves
appear dark when silhouetted against Saturn, but quite
bright when viewed away from Saturn, slightly scattering
sunlight, in this exaggerated color image. Saturn's rings
light up so much that new rings were discovered,
although they are hard to see in the image. Seen in
spectacular detail, however, is Saturn's E ring, the ring
created by the newly discovered ice-fountains of the
moon Enceladus and the outermost ring visible above.
Far in the distance, at the left, just above the bright main
rings, is the almost ignorable pale blue dot of Earth.
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Survey of Spectra
 Sir Isaac Newton
(1642-1726)
 Somewhat fanciful
view of actual
experiments in 1660’
1660’s
 “Opticks,
Opticks,” 1704
Fall 2011
Probing Our Planets
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Isaac Newton & Spectra
Through a brilliant series of experiments, Newton
showed that the colors displayed in a rainbow are
inherent in white light itself.
Later physicists showed that visible light was just a
human being’s eye’s physical response to one tiny part
of a larger electromagnetic wave spectrum.
The visible portion of the spectrum extends on both red
and blue sides into invisible forms of radiation.
Red light has longer wavelengths than blue light and
Infrared light has longer wavelengths still. Ultraviolet or
“black light” has wavelengths shorter than blues and
violet.
Infrared radiation is perceived by our body as heat rays;
ultraviolet is not directly perceived but causes sunburn
and skin cancer.
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Electromagnetic Spectrum
Fall 2011
Probing Our Planets
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Image © UC Regents from UC Berkeley “Living with a star.”
http://ds9.ssl.berkeley.edu/LWS_GEMS/2/espec.htm
Images taken using wavelengths of radiation invisible to the
human eye obviously must be somehow adjusted so humans
can interpret them.
This is usually done by representing the variations of intensity
in the invisible wavelengths through “pseudo color” or “false
color” techniques.
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Hurricane Irene
in Visible Light & in Pseudo-color
Left: 27 Aug 2011
by GOES-East
Fall 2011
Right: 26 Aug 2011
by Aqua
Probing Our Planets
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US landfall on August 27, 2011, on the outer banks of
North Carolina. Visible light image shows landfall.
Left: Visible light view by GOES-East satellite in
geosynchronous orbit.
Right: Infrared view by Aqua satellite in high orbit.
Here the IR brightness (think shades of “grey”) is
replaced with an ordinary spectral color bar.
Purple and blue are colder; oranges and reds
warmer.
For the hurricane we are seeing cloud tops but in
other regions we are probably seeing surface land or
water temperature.
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Ground Ivy
Photoart by Bjorn Rorslett
Visible
IR
intensity
UV filter
Lit by
Black Light
“false colored”
Fall 2011
Probing Our Planets
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http://www.naturfotograf.com/index2.html
Upper Left: Illumination--white light; normal color photo;
Upper Right: Illumination--white light; Infrared filter shows almost
uniform IR reflectivity, that is to say, the flower and leaves are at
essentially the same temperature.
Lower Left: Illumination--white light; Ultraviolet filtered image has been
“false-colored” to emphasize contrast.
Lower Right: Illumination--black light; but normal color image showing
the fluorescence in visible light of the flower’s anthers.
“It should be realised that there cannot possibly be any "true" colour
rendition of the irradiation outside the spectral band visible to our eyes.
We are entirely free to select any colour scheme of our own liking. Thus,
my images should not be compared directly to other people's pictures as
far as colours are concerned. I have to stress this often neglected fact
because many believe UV or IR has just one "correct" rendition, while in
reality there are as many possibilities as you can imagine. Thus, seeing
my pictures and stating "UV is rich in red" is meaningless (but people do
this).”
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Image (PIA13405) Credit: NASA/JPL/ASI/University of Arizona
http://photojournal.jpl.nasa.gov/catalog/PIA13405
This false-color composite image, constructed from data obtained by NASA's
Cassini spacecraft, shows Saturn's rings and southern hemisphere. The
composite image was made from 65 individual observations by Cassini's visual
and infrared mapping spectrometer in the near-infrared portion of the light
spectrum on Nov. 1, 2008. The observations were each six minutes long.
In this image constructed from data collected in the near-infrared wavelengths of
light, scientists designated blue to indicate sunlight reflected at a wavelength of 2
microns, green to indicate sunlight reflected at 3 microns and red to indicate
thermal emission at 5 microns.
Saturn's rings reflect sunlight at 2 microns, but not at 3 and 5 microns, so they
appear deep blue.
Saturn's high altitude haze reflects sunlight at both 2 and 3 microns, but not at 5
microns, and so it appears green to blue-green.
The heat emission from the interior of Saturn is only seen at 5 microns
wavelength in the spectrometer data, and thus appears red. The dark spots and
banded features in the image are clouds and small storms that outline the deeper
weather systems and circulation patterns of the planet. They are illuminated from
underneath by Saturn's thermal emission, and thus appear in silhouette.
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Fall 2011
Probing Our Planets
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Image (PIA09212) Credit: NASA/JPL/University of Arizona
This striking false-color mosaic was created from 25 images taken by Cassini's
visual and infrared mapping spectrometer over a period of 13 hours, on Feb. 24,
2007, and captures Saturn in nighttime and daytime conditions. The visual and
infrared mapping spectrometer acquires data simultaneously at 352 different
wavelengths, or spectral channels. Data at wavelengths of 2.3, 3.0 and 5.1 microns
were combined in the blue, green and red channels.
At 2.3 microns (shown in blue), the icy ring particles are highly reflecting, while
methane gas in Saturn's atmosphere strongly absorbs sunlight and renders the
planet very dark. At 3.0 microns (shown in green), the situation is reversed: water ice
in the rings is strongly absorbing, while the planet's sunlit hemisphere is bright. Thus
the rings appear blue in this representation, while the sunlit side of Saturn is
greenish-yellow in color. Within the rings, the most opaque parts appear dark, while
the more translucent regions are brighter. In particular, the opaque, normally-bright B
ring appears here as a broad, dark band separating the brighter A (outer) and C
(inner) rings.
At 5.1 microns (shown in red), reflected sunlight is weak and thus light from the
planet is dominated by thermal (i.e., heat) radiation that wells up from the planet's
deep atmosphere. This thermal emission dominates Saturn's dark side as well as the
north polar region (where the hexagon is again visible) and the shadow cast by the A
and B rings.
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Saturn’s Auroras
Fall 2011
Probing Our Planets
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Image (PIA06436) Credit: NASA/JPL/University of Colorado
http://photojournal.jpl.nasa.gov/catalog/PIA06436
Images taken on June 21, 2005, with Cassini's ultraviolet imaging
spectrograph are the first from the mission to capture the entire "oval"
of the auroral emissions at Saturn's south pole.
In the side-by-side, false-color images, taken about one hour apart,
blue represents aurora emissions from hydrogen gas excited by
electron bombardment, while red-orange represents reflected sunlight.
The images show that the aurora lights at the polar regions respond
rapidly to changes in the solar wind.
The same process produces auroras on both planets: variations in the
plasma environment release trapped electrons, which stream along the
magnetic field lines into the upper atmosphere. There, they collide with
atoms and molecules, exciting them to higher energies. The atoms and
molecules release this added energy by radiating light at particular
characteristic colors and wavelengths. On Earth, this light is mostly
from oxygen atoms and nitrogen molecules. On Saturn, it is from
emissions of molecular and atomic hydrogen.
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Lightning Storms
Fall 2011
Probing Our Planets
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Credit: NASA/JPL/University of Iowa
http://saturn.jpl.nasa.gov/video/videodetails/?videoID=111
Lightning Sounds from Saturn
This audio clip was created from radio signals
received by the radio and plasma wave science
instrument on the Cassini spacecraft. The bursty
radio emissions were generated by lightning flashes
on Saturn and are similar to the crackles and pops
one hears on an AM radio during a thunderstorm on
Earth. This storm on Saturn occurred on January 23
and 24, 2006. The clip compresses two hours of
observations into about 28 seconds. Therefore,
every second of the audio clip corresponds to
about 4 minutes, 18 seconds.
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Fall 2011
Probing Our Planets
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Image (PIA08388) Credit: NASA/JPL/Space Science Institute
http://photojournal.jpl.nasa.gov/catalog/PIA08388
Little light makes its way through the rings to be scattered in
Cassini's direction in this viewing geometry, making the rings
appear somewhat dark compared to the reflective planet. .
This mosaic was constructed from wide-angle camera images
taken just before the narrow-angle camera mosaic PIA08389.
The view combines 45 images -- 15 separate sets of red, green
and blue images--taken over the course of about two hours, as
Cassini scanned across the entire main ring system.
The images in this view were obtained on May 9, 2007, at a
distance of approximately 1.1 million kilometers (700,000 miles)
from Saturn. Image scale is about 62 kilometers (39 miles) per
pixel.
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Saturn’s A Ring in the UV
Fall 2011
Probing Our Planets
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Image (PIA05075) Credit: NASA/JPL/University of Colorado
http://www.nasa.gov/mission_pages/cassini/multimedia/gallery/gallery-index.html
This ultraviolet view indicates there is more ice toward the outer
part of the rings, than in the inner part, hinting at the origins of the
rings and their evolution.
Images taken during the Cassini spacecraft's orbital insertion on
June 30 show compositional variation in the A, B and C rings.
From the inside out, the "Cassini Division" in faint red at left is
followed by the A ring in its entirety. The Cassini Division at left
contains thinner, dirtier rings than the turquoise A ring, indicating
a more icy composition. The red band roughly three-fourths of the
way outward in the A ring is known as the Encke gap.
The ring system begins from the inside out with the D, C, B and A
rings followed by the F, G and E rings. The red in the image
indicates sparser ringlets likely made of "dirty," and possibly
smaller, particles than in the icier turquoise ringlets.
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High Resolution View of Rings
 Rings consist of countless small particles, ranging in size from


micrometers to meters, that form clumps that in turn individually orbit
about Saturn.
The ring particles are made of 99.9% of water ice, with some
contamination from dust and other chemicals.
The system is perhaps 75,000 km wide but only 10 meters thick.
Fall 2011
Probing Our Planets
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Image (PIA08389) Credit: NASA/JPL/Space Science Institute
http://photojournal.jpl.nasa.gov/catalog/PIA08389
Incredible complexity but precision and order. Saturn
is to the left.
What is the composition? Dirty ice, water ice, in
forms probably down to the size of snowflakes,
clumps of ice the size of snowballs and maybe even
dump truck sized bolders of ice, mixed with crushed
rock and organic goo.
The view combines 45 narrow-angle images -- 15
separate sets of red, green and blue images -- taken
over the course of about 2.5 hours, as Cassini
scanned across the rings.
The images in this view were obtained on May 9,
2007, at a distance of approximately 1.1 million
kilometers (700,000 miles) from Saturn.
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Shepherds Prometheus & Pandora
Fall 2011
Probing Our Planets
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Image (PIA 07712) Credit: NASA/JPL/Space Science Institute
http://ciclops.org/view.php?id=1361&js=1
This movie sequence from Cassini shows dark drapes in the inner
strands of the F ring caused by the gravitational influence of the
shepherd moon Prometheus (86 kilometers, 53 miles across).
Prometheus appears first in the sequence, interior to the F ring, and
Pandora (81 kilometers, 50 miles across) follows along outside of
the ring.
Prometheus orbits closer to Saturn, and thus faster, than the icy
particles that make up the F ring. Each apoapse passage causes
greatest effect. On the next apoapse passage of Prometheus, a
new gore in the inner ring material is made. The gores, together with
the sheared-out material due to differential orbital motion, create the
dark, diagonal drapes.
The visible light images in this sequence were acquired using the
narrow angle camera on April 13, 2005 from a distance of
approximately 1.2 million kilometers (700,000 miles) from Saturn.
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Titan
Fall 2011
Probing Our Planets
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Image (PIA09833) Credit: NASA/JPL/Space Science Institute
http://photojournal.jpl.nasa.gov/catalog/PIA09833
Titan's atmosphere is the only dense, nitrogen-rich
atmosphere in the Solar System aside from the Earth's. The
atmospheric composition in the stratosphere is 98.4%
nitrogen with the remaining 1.6% composed mostly of
methane (1.4%) and hydrogen (0.1–0.2%). There are trace
amounts of other hydrocarbons, such as ethane, acetylene
and propane, hydrogen cyanide, carbon dioxide, carbon
monoxide, cyanogen, argon and helium.
The orange color as seen from space must be produced by
other more complex chemicals in small quantities, possibly
tholins, tar-like organic precipitates. The hydrocarbons are
thought to form in Titan's upper atmosphere in reactions
resulting from the breakup of methane by the Sun's ultraviolet
light, producing a thick orange smog.
Titan has no magnetic field,
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Huygens Descent
without bells & whistles
Fall 2011
Probing Our Planets
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Huygens Descent Data Movie without Bells and Whistles
Credit: ESA/NASA/JPL/University of Arizona
http://saturn.jpl.nasa.gov/video/videodetails/?videoID=127
This movie was built with data collected during the 147-minute
plunge through Titan's thick orange-brown atmosphere to a soft
sandy riverbed by the European Space Agency's Huygens Descent
Imager/Spectral Radiometer on Jan. 14, 2005,
In 4 minutes and 40 seconds, the movie shows what the probe 'saw'
within the few hours of the descent and the landing. On approach,
Titan appeared as just a little disk in the sky among the stars, but
after landing, the probe's camera resolved little grains of sand
millions of times smaller than Titan.
At first, the Huygens camera just saw fog over the distant surface.
The fog started to clear only at about 60 kilometers (37 miles) altitude,
making it possible to resolve surface features as large as 100 meters
(328 feet). Only after landing could the probe's camera resolve the
little grains of sand. The movie provides a glimpse of such a huge
change of scale.
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Huygens Descent
with bells & whistles
Fall 2011
Probing Our Planets
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Huygens Descent Data Movie with Bells and Whistles
Credit: ESA/NASA/JPL/University of Arizona
http://saturn.jpl.nasa.gov/video/videodetails/?videoID=126
The almost four-hour-long operation of the camera is shown in less than
five minutes. That's 40 times the actual speed up to landing and 100 times
the actual speed thereafter.
The first part of the movie shows how Titan looked to the camera as it
acquired more and more images during the probe's descent. Each image
has a small field of view, and dozens of images were made into mosaics of
the whole scene.
The scientists analyzed Huygens' speed, direction of motion, rotation and
swinging during the descent.
For details on the graphics displayed go to the web site.
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Titan’s North Polar Surface
Fall 2011
Probing Our Planets
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Image (PIA10008) Credit: NASA/JPL/USGS
http://photojournal.jpl.nasa.gov/catalog/PIA10008
This Cassini false-color mosaic shows radar images of Titan's north
polar region.. About 14 percent of the mapped region is covered by
liquid hydrocarbon lakes.
Features thought to be liquid are shown in blue and black, and the
areas likely to be solid surface are tinted brown. These seas are most
likely filled with liquid ethane, methane and dissolved nitrogen. Many
bays, islands and tributary networks are associated with the seas.
The large feature in the upper right center of this image is at least
100,000 square kilometers (40,000 square miles) in area, greater in
extent than Lake Superior (82,000 square kilometers or 32,000 square
miles), one of Earth's largest lakes. This Titan feature covers a greater
fraction of the surface, at least 0.12 percent, than the Black Sea,
Earth's largest terrestrial inland sea, at 0.085 percent.
Of the 400 observed lakes and seas, 70 percent of their area is taken
up by large "seas" greater than 26,000 square kilometers (10,000
square miles).
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Titan’s Lakes
of light liquefied petroleum gases
Fall 2011
Probing Our Planets
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Image (PIA09102) Credit: NASA/JPL/USGS
http://photojournal.jpl.nasa.gov/catalog/PIA09102
The existence of oceans or lakes of liquid methane on Titan was
predicted more than 20 years ago. But unconfirmed until the Cassini
flyby of July 22, 2006.
This false color radar image, used on the journal Nature’s cover, gives
a taste of what Cassini saw. Intensity is proportional to how much
radar brightness is returned.
The lakes, darker than the surrounding terrain, are emphasized here
by tinting regions of low backscatter in blue. Radar-brighter regions
are shown in tan. The strip of radar imagery is foreshortened to
simulate an oblique view of the highest latitude region, seen from a
point to its west.
This radar image was acquired by the Cassini radar instrument in
synthetic aperture mode on July 22, 2006. The image is centered near
80 degrees north, 35 degrees west and is about 140 kilometers (84
miles) across. Smallest details in this image are about 500 meters
(1,640 feet) across.
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Artwork by Ron Miller; “8 Wonders of the Solar System”
Fall 2011
Probing Our Planets
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http://www.scientificamerican.com/article.cfm?id=8-wonders
The Geysers of Enceladus
You feel it before you see it: an ominous rumble,
reverberating deep in your chest and up from your feet.
There is no sound here. And then the eruption comes: two
huge ice plumes explode through the surface of Enceladus,
spewing ice crystals into space at more than 1,000 miles per
hour. The silent violence is lit by our distant sun. With just
1/16 of our own moon's gravity, Enceladus will not be an
easy world to tread on; hikers may need to strap on jetpacks
and take care to avoid the valleys that give birth to the
powerful geysers.
Caption by Ed Bell
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Ice Fountains on Enceladus
(2005)
Fall 2011
Probing Our Planets
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Image (PIA07758) Credit: NASA/JPL/Space Science Institute
http://photojournal.jpl.nasa.gov/catalog/PIA07758
Recent Cassini images of Saturn's moon Enceladus
backlit by the sun show the fountain-like sources of
the fine spray of material that towers over the south
polar region. This image was acquired on Nov. 27,
2005.
Imaging scientists, as reported in the journal Science
on March 10, 2006, believe that the jets are geysers
erupting from pressurized subsurface reservoirs of
liquid water above 273 degrees Kelvin (0 degrees
Celsius).
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Ice Plumes of Enceladus
(2005)
Fall 2011
Probing Our Planets
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Image (PIA08386) Credit: NASA/JPL/Space Science Inst.
Cassini imaging scientists used views like this one to help
them identify the source locations for individual jets spurting
ice particles, water vapor and trace organic compounds from
the surface of Saturn's moon Enceladus.
This false-color view was created by combining three clear
filter images taken at nearly the same time as PIA07759. This
image product was then specially processed to enhance the
individual jets that compose the plume. Some artifacts due to
the processing are present in the image. The final product
was colored as blue for dramatic effect.
The images were acquired with the Cassini spacecraft
narrow-angle camera on Nov. 27, 2005 at a distance of
approximately 148,000 kilometers (92,000 miles) from
Enceladus and at a sun-Enceladus-spacecraft, or phase,
angle of 161 degrees. Scale in the original images is about
880 meters (0.5 mile) per pixel. This view has been magnified
by a factor of two from the original images.
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Tiger Stripes
at Enceladus’
Enceladus’ South Pole
Fall 2011
Probing Our Planets
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Image (PIA12448) Credit: NASA/JPL/GSFC/SWRI/SSI
The right-hand image shows an improved close-up view of heat radiation from a
warm fissure near the south pole of Saturn's icy moon Enceladus. It was obtained
by NASA's Cassini spacecraft during its Nov. 21, 2009, flyby. The fissure, named
Baghdad Sulcus, is one of four so-called "tiger stripe" features that emit jets of water
vapor and ice particles. The left image was obtained in March 2008.
The new data show that the heat sources previously detected are confined to a
narrow, intense region no more than a kilometer (half a mile) wide along the fracture.
The thermal image also reveals that the strength of the signal varies considerably
along the length of this fissure segment. The composite infrared spectrometer data
indicate that the temperature along Baghdad Sulcus reached more than 180 Kelvin
(about minus 140 degrees Fahrenheit).
The intensity of heat radiation increases as the color shades from violet to red to
orange to yellow. No internal heat was detected in the darkest violet regions, and
uncolored regions were not mapped by Cassini's composite infrared spectrometer.
The background image is derived from visible-light images taken by Cassini's
imaging science subsystem cameras in July 2005: alignment of the infrared data
with the visible-light map is approximate.
The fractures themselves may be even warmer than the 180 Kelvin (minus 140
degrees Fahrenheit) detected -- perhaps warm enough for liquid water to exist
under the surface.
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Ice Plumes of Enceladus
(2009)
http://saturn
Enceladus//enceladus.html
http://saturn..jpl.
jpl.nasa.
nasa.gov/multimedia/flash/
gov/multimedia/flash/Enceladus
enceladus.html
Fall 2011
Probing Our Planets
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Image Credit: NASA/JPL/Space Science Institute
Dramatic plumes, both large and small, spray water ice out from many
locations along the famed "tiger stripes" near the south pole of Saturn's
moon Enceladus. The tiger stripes are fissures that spray icy particles,
water vapor and organic compounds.
More than 30 individual jets of different sizes can be seen in this
image and more than 20 of them had not been identified before. At
least one jet spouting prominently in previous images now appears less
powerful.
This mosaic was created from two high-resolution images that were
captured by the narrow-angle camera when NASA's Cassini spacecraft
flew past Enceladus and through the jets on Nov. 21, 2009. (For other
images captured during the same flyby, see PIA11686 and PIA11687).
Imaging the jets over time will allow Cassini scientists to study the
consistency of their activity.
The south pole of the moon lies near the limb in the top left quadrant
of the mosaic, near the large jet that is second from left. Lit terrain seen
here is on the leading hemisphere of Enceladus (504 kilometers, 313
miles across).
Cassini scientists continue to study the question of whether reservoirs
of liquid water exist beneath the surface of the moon.
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Cassini steps into the shadow and
looks back
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http://photojournal.jpl.nasa.gov/catalog/?IDNumber=PIA08324
Earth is captured here in a natural color portrait made possible by the passing of
Saturn directly in front of the sun from Cassini's point of view. At the distance of
Saturn's orbit, Earth is too narrowly separated from the sun for the spacecraft to
safely point its cameras and other instruments toward its birthplace without
protection from the sun's glare.The Earth-and-moon system is visible as a bright
blue point on the right side of the image above center.
Here, Cassini is looking down on the Atlantic Ocean and the western coast of
north Africa.
The Saturnian moon, Enceladus, is among them, and is also captured on the
left in this image, with its plume of water ice particles and swathed in the blue
E ring which it creates. Delicate fingers of material extend from the active
moon into the E ring.
The narrow tenuous G ring and the main rings are seen at the right.
The view looks down from about 15 degrees above the un-illuminated side of the
rings.Images taken using red, green and blue spectral filters were combined to
create this view. The image was taken by the Cassini spacecraft wide-angle camera
on Sept. 15, 2006, at a distance of approximately 2.1 million kilometers (1.3 million
miles) from Saturn and at a sun-Saturn-spacecraft angle of almost 179 degrees.
Image scale is approximately 250 kilometers (155 miles) per pixel.At this time,
Cassini was nearly 1.5 billion kilometers (930 million miles) from Earth.
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Enceladus
& the E Ring
Fall 2011
Probing Our Planets
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Image (PIA08321) Credit: NASA/JPL/Space Science Institute
http://photojournal.jpl.nasa.gov/catalog/?IDNumber=PIA08321
Wispy fingers of bright, icy material reach tens of thousands of
kilometers outward from Saturn's moon Enceladus into the E ring, while
the moon's active south polar jets continue to fire away.
The sun-Enceladus-spacecraft angle here is 175 degrees, a viewing
geometry in which structures made of tiny particles brighten
substantially.
These features are very likely the result of particles injected into Saturn
orbit by the Enceladus geysers. In addition to the wisps, another
unexpected detail is the dark gore in the center of the ring, following the
moon in its orbit, likely brought about by the sweeping action of
Enceladus as it orbits in the center of the E ring.
The view looks down onto Enceladus (505 kilometers, or 314 miles
across) from about 15 degrees above the ringplane. Tethys (1,071
kilometers, or 665 miles across) is visible to the left of Enceladus.
The image was taken in visible light on Sept. 15, 2006, at a distance of
approximately 2.1 million kilometers (1.3 million miles) from Enceladus.
Image scale is 128 kilometers (80 miles) per pixel.
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Varied Moons
Icy Helene
(~25 miles across
Co-orbital with Dione
& Polydeuces)
Phoebe
(~135 miles across,
dark as lampblack)
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Probing Our Planets
Hyperion
(~225 miles long,
spongy)
Dark Dione
(~700 miles across
Co-orbital with
Telesto & Calypso)
&
Bright Tethys
(~660 miles across)
28
To date, 62 moons have been discovered, 53 moons have been
officially named. (Pre-launch Cassini Mission handout says 18
moons.)
The dozens of icy moons orbiting Saturn vary drastically in shape,
size, surface age and origin.
Some of these worlds have hard, rough surfaces, while others are
porous bodies coated in a fine blanket of icy particles.
All have greater or smaller numbers of craters, and many have
ridges and valleys. Some, like Dione and Tethys, show evidence of
tectonic activity, where forces from within ripped apart their surfaces.
Many, like Rhea and Tethys, appear to have formed billions of years
ago, while others, like Janus and Epimetheus, could have originally
been part of larger bodies that broke up.
The study and comparison of these moons tells us a great deal
about the history of the Saturn System and of the solar system at
large.
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Mimas, Saturn’s “Death Star”
Fall 2011
Probing Our Planets
29
Mimas is a moon of Saturn which was discovered in 1789 by William
Herschel.
It is named after Mimas, a son of Gaia in Greek mythology, and is also
designated Saturn I.
By diameter (~250 miles/396 kilometers) the twentieth-largest moon in the
Solar System, Mimas is the smallest known astronomical body that is
thought to be rounded in shape due to self-gravitation
The low density of Mimas, 1.15 g/cm3, indicates that it is composed
mostly of water ice with only a small amount of rock.
Mimas' most distinctive feature is a colossal impact crater 130 kilometres
(81 mi) across, named Herschel after the moon's discoverer. Herschel's
diameter is almost a third of the moon's own diameter; its walls are
approximately 5 kilometres (3.1 mi) high, parts of its floor measure 10
kilometres (6.2 mi) deep, and its central peak rises 6 kilometres (3.7 mi)
above the crater floor.
If there were a crater of an equivalent scale on Earth it would be over
4,000 kilometres (2,500 mi) in diameter, wider than Australia. The impact
that made this crater must have nearly shattered Mimas: fractures can be
seen on the opposite side of Mimas that may have been created by shock
waves from the impact travelling through the moon's body.
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Herschel Crater on Mimas
Fall 2011
Probing Our Planets
30
http://apod.nasa.gov/apod/ap100511.html
APOD caption: Why is this giant crater on Mimas oddly
colored?
Mimas, one of the smaller round moons of Saturn, sports
Herschel crater, one of the larger impact craters in the entire
Solar System. The robotic Cassini spacecraft now orbiting
Saturn took the above image of Herschel crater in
unprecedented detail while making a 10,000-kilometer record
close pass by the icy world just over one month ago. Shown
in contrast-enhanced false color, the above image includes
color information from older Mimas images that together
show more clearly that Herschel's landscape is colored
slightly differently from more heavily cratered terrain nearby.
The color difference could yield surface composition clues to
the violent history of Mimas.
An impact on Mimas much larger than the one that created
the 130-kilometer Herschel would likely have destroyed the
entire world.
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