Icy Bodies in the Outer Solar System:
From Pinpoints of Light to Enigmatic Worlds
Anne J. Verbiscer
Mission Planner, Cassini Mission to Saturn Research Associate Professor, University of Virginia
Astronomy 1210 University of Virginia 9 April 2012
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
Discovered by William Herschel
28 August 1789
40­foot telescope, Slough, UK
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:
Earth's Moon
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)
Icy bodies in the outer Solar System exhibit remarkable diversity: Size, shape
100­m moonlets to 10s­km potato­shaped to >1000­km spherical
Visible Albedo (reflectance)
Comet nuclei, some transNeptunian objects (TNOs) reflect only a few %
Enceladus reflects nearly 100% of the visible light from the Sun
Orbit
Satellites orbit their primary (planet or TNO) at distances as large as several hundred times the radius of the planet.
Orbits can be prograde (counterclockwise) or retrograde (clockwise) at various inclinations, relative to the planet or TNO equator
Many are tidally locked ­ in synchronous rotation
Geologic Activity
Inert bodies, cratered to saturation (new craters erase old ones)
Currently active worlds: Enceladus, Titan, Triton..... more?
There are >160 natural satellites (moons) of giant planets and >1300 TNOs in the outer Solar System; some (3­10%) TNOs have their own moons
In the outer Solar System, Ice Geology Reigns
Density determines internal composition and availability of heat source
from radioactive decay of rock (closer to water ice, 1 g/cm3, less rock, heat); Temperature determines type of surface geology:
Morphology of impact craters
Cryo­volcanic features (flows, calderas)
Ice tectonics (folds, grooves, ridges, fractures)
Relaxed (softened) impact craters, ridges, and fractures on Saturn's Enceladus
Grooves, and ice cliffs 5 km tall on Miranda, a 230­km moon of Uranus
In the outer Solar System, Ice Rocks!
5152 km
504 km
Charon
Active
5152 km
504 km
Active
Active
Pluto
Charon
Active
Weywot
Vanth
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
Small Moons or Large Ring Particles?
Saturn's A Ring has giant propeller­like structures created
by gravitational interactions with (unresolved) small (100m­1km) “moonlets”
“Propeller” in Saturn's A Ring
Named for Amelia Earhart
Saturn's Flying­Saucer Shaped Ring Moons
Atlas
45 km
Pan 35 km
in the Encke Gap
9 km
Daphnis
in the Keeler Gap
just outside the A Ring
Two Saturnian Ice Balls:
Impact Craters
Central peaks
Large Basins
Concentric rings
1080 km
400 km
MIMAS
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
1121 km
'Wispy' Terrain = fractures
Only on trailing hemispheres
(remember these are tidally
locked and always keep the
same side toward Saturn)
The trailing hemisphere is centered on the moon's anti­apex of motion
RHEA
Density 1.2 g/cm3
1534 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
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,beyond Titan,
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 Back to Enceladus....
March 1980
The Grand Tour
1981 Voyager 2
ENCELADUS
Geologically young terrain,
But just how young?
Density ~1 g/cm3 ?
Diameter 504 km
Saturn Orbit Insertion (SOI) 1 July 2004
Launch 15 October 1997
2005 Cassini view of
ENCELADUS Revised
Density = 1.6 !
More rock in core?
Does this mean more
heating from radioactive decay
of that rock?
February 2005 Cassini ISS NAC
Density 1.6 g/cm3
More relaxed craters...
Fresh fractures (sharp edges)
Cassini Magnetometer
February, March, July 2005
November
2005
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?
(False color) Plumes at High 'Phase' (Sun­Enceladus­Cassini) Angle
Particles are very forward­scattering (small size)
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
Cassini's Enceladus
Cassini Spacecraft Trajectories
Flybys Near
in the Prime and Enceladus
2005­2010
Equinox Missions
You are here
Earth
ENCELADUS' 'TIGER' STRIPES
Damascus
Baghdad
South Pole
Cairo
Alexandria
Plume eruption sites are found along
or near these four prominent fractures
Alexandria
Cairo
Damascus
Baghdad
November 2009
Cross­Eye
Stereo of
Cairo Sulcus
8 Jets identified by triangulation
South Polar
Terrain
A­G “hotspots”
August 2008
Cassini Skeet Shoots Enceladust August 2008
Damascus Sulcus
10 km
Eruption site
Eruption site
Damascus Sulcus
Enceladan 'Fog'? ­ no direct solar illumination of surface.
13 August 2010 16 m/pixel
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)
Damascus Sulcus
Blue, purple: cooler temperatures (60 K)
Yellow: hotter temperature (190 K)
Warm “distal” ends of Cairo and Alexandria Sulci
Encleadus' Plume Composition
90% water, 10% organics
Measured by Cassini's Ion and Neutral Mass Spectrometer (INMS)
Is Enceladus Like A Comet?
Comet Hartley 2
Hartley 2 is MUCH smaller....
And much, much darker.
Robotic spacecraft have transformed our views of icy bodies in the outer solar system
from pinpoints of light to enigmatic worlds...
How will our views of these pinpoints of light be transformed
in 2015 when the New Horizons spacecraft flies by Pluto?
Yuri Gagarin's View
12 April 1961
Where are we now, 50 years later???
ANNOUNCEMENTS
Exam THIS Friday 13 April
Review Session Wednesday, 11 April 4:30pm – Wilson
Details given in class on Wednesday and on home page.