Icy Satellites in the Outer Solar System:
From Pinpoints of Light to Enigmatic Worlds
Anne J. Verbiscer
Astronomy 1210 University of Virginia 4 November 2009
Our view of icy satellites has dramatically changed... To this!
From this...
SATURN
View of Saturn (bright center) and
several satellites from the 60­inch
telescope on Mt. Palomar
Enceladus
False color mosaic of images taken by the Cassini­Huygens spacecraft in 2005.
Prior to the 1970s, all we knew about outer planet moons (satellites) came from studying timy pinpoints of light seen through ground­based telescopes:
Callisto
Europa
Io
JUPITER
Ganymede
We thought that these moons might resemble our own..... with surfaces dominated
by impact craters:
As an example, observe how our view of Jupiter's moon Europa improves in images
acquired by the spacecraft Pioneer 10, Voyager, and Galileo:
Pioneer 10 December 1973
Voyager 1979
Galileo 1995­1998 And at even higher resolution.......
Very few impact craters!
30 km
Europa as seen by the Galileo Spacecraft in the 1990s
Ice Cliffs on
Europa!
1.7 km
Landslides
Clearly, this
does not look
like our Moon...
Aerial view of Providence, RI
at the same
scale
(900 m/pixel)
There are >160 natural satellites (moons) in the outer solar system!
All Major satellites:
size: > 300 km, generally spherical
orbit: make one rotation with each revolution around their planet
(synchronous rotation; tidally locked)
composition: densities nearly match that of water ice, 1 g/cm3
In outer solar system, ice geology reigns: Ice is a Rock!
Temperature determines type of surface geology:
Morphology of impact craters
Volcanic features (flows, calderas)
Ice tectonics (folds, grooves, ridges, fractures)
Active
5152 km
504 km
Active
Active
Pluto
Charon
Active
Weywot
Jupiter's Galilean Satellites and our own Moon to scale
Density  (g/cm3)
3.5
3642
3.0
3038
3.3
3474
Diameter (km)
1.9
5268
1.8
4820
Jupiter's Io ­ the most geologically active satellite
Volcanic plume
Volcanoes powered by tidal heating
Density 3.5 g/cm­3
Infrared view
shows hot plumes on nightside of Io
EUROPA
Europa has a subsurface ocean.
Europa's surface is mostly (white.blue) water ice, with darker, redder linea (lines)
composed of hydrated salts and carbonates, possibly extruded onto the
surface through these fractures from the salty ocean below.
Ice 'rafts' on Europa's surface broke free when layer beneath
them melted.... then it all refroze
GANYMEDE is the largest moon in the Solar System
(larger than Mercury)
Has its own magnetic field! (Iron core)
Surface features on Ganymede illustrate extensional tectonics.... Ganymede has expanded since its formation, and the grooves
seen in detail below are evidence of this expansion Grooved, bright terrain
(fewer craters, younger)
Dark terrain
(more craters, older)
Most of the surface is heavily cratered
as seen here:
CALLISTO
Outermost Galilean moon
May have subsurface ocean
Knobby Terrain
TITAN
Second largest moon in Solar System
(larger than Mercury)
Only one with thick, nitrogen atmosphere
Density 1.9 g/cm3
Diameter 5152 km Surface of Titan from the Huygens lander
14 January 2005
Rocks are ~20 cm across
A cryovolcano?
Near­infrared image from Cassini
Changes in appearance of surface features indicate current geologic activity
Cassin RADAR imagery
Lakes of liquid hydrocarbons (methane and ethane) near north pole
Ontario Lacus
lake near south pole Dunes sweep across equatorial region (no lakes)
Two Saturnian Ice Balls:
Impact Craters
Central peaks
Large Basins
Conentric rings
MMAS
aka 'Death Star'
Density 1.15 g/cm3 TETHYS
Density 0.97 g/cm3
Tethys would float!
Two more Saturnian Icy Moons
DIONE
Density 1.5 g/cm3
RHEA
Density 1.2 g/cm3
'Wispy' Terrain = fractures
Only on trailing hemispheres
(remember these are tidally locked
and always keep the same face toward Saturn)
Some of Saturn's Smaller Satellites
Why do their surfaces look so different?
Location, location, location
Sharply defined craters, pits
Crater rims are softened and muted
HYPERION
 = 0.6 g/cm3
360 x 280 x 225 km
Craters have been filled in by dust and ejecta
TELESTO
 <<1 ?
29 x 22 x 20 km
JANUS
 = 0.6 g/cm3
193 x 173 x 137 km
Some of Saturn's Smaller Satellites
Why do their surfaces look so different?
Location, location, location
Sharply defined craters, pits
Crater rims are softened and muted
HYPERION
 = 0.6 g/cm3
360 x 280 x 225 km
Craters have been filled in by dust and ejecta
TELESTO
 <<1 ?
29 x 22 x 20 km
JANUS
 = 0.6 g/cm3
193 x 173 x 137 km
JANUS and TELESTO orbit within Saturnian rings,
subjecting their surfaces to repeated impacts, blasts
HYPERION is further from Saturn, outside such a productive impact zone
Equatorial Ridge
Voyager Mountains
IAPETUS
Density 1.1 g/cm3
Two of Uranus' icy satellites which show
evidence of geologic activity:
Cryovolcanic flows
MIRANDA
 = 1.2 g/cm3
472 km
ARIEL
 = 1.7 g/cm3
1158 km NEPTUNE'S TRITON
Geysers driven by sublimation
Lava lakes
Has a thin, nitrogen
atmosphere
Density 2.1 g/cm3
2706 km 1981 Voyager 2
ENCELADUS
Geologically young terrain,
But just how young?
Density ~1 ?
504 km
Enceladus within Saturn's E ring
Before 2005, the relationship between
Enceladus and the E ring was only a theory.
Enceladus
(tiny black dot)
In 2005, the Cassini Spacecraft observes active eruptions from the south pole of Enceladus, which feed the E ring
2005 Cassini view of
ENCELADUS Revised
Density = 1.6 !
More rock in core?
Does this mean more
heating from radioactive decay
of that rock?
ENCELADUS is a (relatively) tiny ACTIVE moon
with plumes of water ice and vapor erupting from four fractures near its south pole
Temperatures (Kelvins)
Heat Source?
Tides?
Radioactivity?
Ammonia may provide a clue....
The Cassini spacecraft sees water plumes
erupting from these four warm fractures
Heat emanating from these four
fractures is
Localized – as high
as ~ 200 K !
(White = hot;
Grey = cool)
ENCELADUS' 'TIGER' STRIPES
Damascus
Baghdad
South Pole
Cairo
Alexandria
Plume eruption sites are found along
or near these four prominent fractures
Damascus Sulcus
10 km
Eruption site
Eruption site
Raw, unprocessed image of ENCELADUS acquired on 2 November 2009
shows transition region near south pole from 'ropy' terrain to cratered terrain
Cratered terrain
(older)
Ropy terrain
(younger)
To south pole
(jet source region)
View from Cassini's cameras on Nov. 2, 2009, looking nearly directly back
at the Sun. Why are there bright and dark features?
ENCELADUS
Main rings (dark)
jets
F ring
Pandora
(small, irregularly­shaped moon,
orbits just outside F ring)
Saturn's F ring and Enceladus' jets are bright because they scatter light efficiently in the 'forward' direction, just like dust on a car windshield. Enceladus itself is dark because it is behind Saturn, so there is no Saturn­shine or Sun­shine on its surface.
Stay tuned for much more close­up imaging!
ENCELADUS flyby 21 November 2009
http://saturn.jpl.nasa.gov/photos/raw/
How will our views of these pinpoints of light be transformed
in 2015 when the New Horizons spacecraft flies by Pluto?