Transcript ASTR 330: The Solar System Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Announcements
•
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 23:
Small Satellites
Picture credit: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
From Moons to Rings
• You may have thought that moons and rings were quite different. This
was the view of scientists until spacecraft reached the outer planets, and
discovered small moons orbiting within the rings.
• We will see that rings are themselves composed of millions and billions
of small chunks each orbiting the parent planet and obeying Kepler’s
laws of orbital motion - each piece in effect a tiny moon.
• We will also see that rings may in fact be created by the catastrophic
break-up of a moon or moons, so the relationship is even more direct.
• For today, we will begin by examining the smaller satellites of the
planetary system: those smaller than Pluto.
• In the next class we will go on to consider rings and shepherd moons.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Types of Satellites
• Satellites of the outer solar system are conveniently divided into two
types, depending on orbit:
• REGULAR SATELLITES: have low eccentricity orbits and/or low
inclination orbits (i.e. their orbits are close to the equatorial plane of the
planet). These moons are analogous to the planets orbiting the Sun.
• IRREGULAR SATELLITES: have orbits of high eccentricity, inclination
or both. These resemble short-period comets or near-earth asteroids
orbiting the Sun.
• Do these orbital differences tell us anything about the origin of the
satellites?
• Yes, it is quite obvious that the regular satellites probably formed from a
mini-disk, called a sub-nebula, around the planets. The irregular
satellites are mostly captured objects.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Satellite Systems
• The number of irregular satellites known is constantly increasing, as we
find more and more small objects far out from the planets.
• For example, 21 new Jovian irregular satellites were found in 2003
alone, all 4 km or less in size, and 17-28 million km from Jupiter.
• Compare to the Galilean satellites: 3000 km or more in size, and less
than 2 million km from Jupiter.
Planet
Jupiter
Saturn
Uranus
Neptune
Regular Irregular Ring Radius
(Rplanet )
Satellites Satellites
8
21
15
6
55
35
12
7
Table info: http://www.ifa.hawaii.edu/~sheppard/satellites/
1.8
2.3
2.2
2.5
Ring Mass
(kg)
101 0
101 8
101 4
101 2
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Medium-sized moons of Saturn
• In the Jovian system, after Europa, the next largest moons are less
than 200 km across, so we will look first at the medium-sized moons of
Saturn.
• The largest of these six moons, Rhea is 1500 km across: less than a
third of Titan’s diameter, whereas Mimas is just 390 km across.
Image: Cassini VIMS Team Website
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
From Mimas to Iapetus
Satellite
Period
Diameter
Mass
Density
(days)
(Moon=1)
(Moon=1)
(g/cm3)
Mimas
0.94
0.11
0.0005
1.2
Enceladus
1.37
0.14
0.0011
1.1
Tethys
1.89
0.30
0.0100
1.2
Dione
2.74
0.32
0.0150
1.4
Rhea
4.52
0.44
0.0340
1.3
Iapetus
79.30
0.41
0.0260
1.2
• Rhea, Dione, Tethys and Mimas form a fairly self-similar group.
• Rhea’s density of 1.3 g/cm3 is lower than Titan, Ganymede etc, but only
because its interior is less compressed. Its composition is likely to be
about 2/3 water ice, 1/3 rock and metal.
Table after: Morrison and Owen, The Planetary System
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Rhea
• The image (left) is
from Cassini. Rhea
is highly reflective
(60% reflection).
• Its spectrum
shows the features
of water ice on the
surface, as we
might expect.
• Does this mean
that Rhea has been
frequently resurfaced, like
Europa? Is the
surface young?
Image credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Rhea’s surface
• The answer is no: Rhea’s surface is
very heavily cratered, as the close-up
image (left) shows. The crater density of
1000 10-km craters per million km2 is
comparable to the lunar highlands.
•
• At the low temperatures (100 K) this far
from the Sun, ice behaves almost like
rock during impacts.
• There is little indication of geologic
activity - perhaps evidence of one resurfacing event in the polar regions,
which must have occurred early on.
• Rhea is tidally locked to Saturn:
synchronously rotating like the Moon.
Image credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Dione
• Dione (1126 km) is the densest Saturnian satellite apart from Titan, with
a density of 1.4 g/cm3. The surface of Dione shows light, medium and
heavily cratered terrain.
• Dione is tidally locked to Saturn, and yet the leading hemisphere shows
a lesser crater density than the trailing hemisphere.
• This may indicate that Dione faced
the opposite direction in the past,
and was spun around by a large
impact, before settling into its
current orientation about 2 Gyr ago.
• The most interesting riddle is the
bright, wispy terrain, which is now
believed to be ice cliffs due to
tectonic fractures. Rhea has similar
markings.
Image credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tethys
• Tethys (1071 km diameter) has several interesting surface features.
Visible in the image (below) is the massive crater Odysseus, 450 km
across. When formed, Odysseus must have had a high wall and towering
central peak.
• Over time, the
crater has
collapsed and
softened its
shape. This tells
us that Tethys
must have been
warmer when
the impact
occurred: the
ice is too cold
and hard now to
lose shape.
Image credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tethys: Ithaca Chasma
• Tethys is also notable for a
huge trench, the Ithaca
Chasma, which stretches 3/4
of the way around the moon.
• This huge rift is up to 100
km wide, 2-3 km deep, and
over 2000 km long.
• Calculations suggest that
Ithaca formed when Tethys
was fluid, and the crust
hardened before the interior.
• Also note the smoother
terrain in the lower right of the
image, evidence of long-ago
internal activity.
Image: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Dr Conor Nixon Fall 2006
Mimas (no it’s not the Death Star!)
• Mimas was a mythical giant who was slain by Hercules: a very
appropriate name for this moon. The moon Mimas (397 km) has suffered a
very large impact relative to its size: the crater Herschel is 140 km wide, 10
km deep, and has a central peak 6 km high!
• The Herschel impact must have nearly
destroyed this small moon.
• This crater has retained its sharp-edged
original shape, unlike Odysseus. Mimas
would have become cold much more
quickly than the larger Tethys.
• We can also see how impacts appear to
have become more intense as we have
traversed the system inwards, from Rhea
through to Mimas, due to gravitational
focusing of Saturn.
Image: NASA/JPL/Space Science Institute
ASTR 330: The Solar System
Enceladus
• Enceladus is a remarkable
object. Its surface has an
albedo of nearly 100%
(meaning what?) the highest of
any object in the solar system
(even higher than Europa).
This causes the surface
temperature to be very cold, a
chilly 55 K at the equator.
• Some areas are heavily
cratered (right), but especially
in the south (left) the terrain is
wrinkled and shows few
craters. The surface has
tentatively been dated at
several hundred million years
old - very recent in planetary
terms.
Image: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Image credit: NASA/JPL/Space Science Institute
‘Tiger Stripes’
• This false color image (right)
dramatically highlights the
fractured tectonic ridges in the
south, dubbed ‘tiger stripes’.
Below is a close-up.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Enceladus Atmosphere
• The mystery of the south pole
deepened when the Cassini
magnetometer detected a bending of
Saturn’s magnetic field near the south
pole.
• This was the first firm evidence of a
neutral gas cloud over the south pole.
• Confirmation came from the
Ultraviolet spectrograph, which
observed gradual dimming as the
moon occulted a star.
• Changes in the spectrum indicated
the presence of water vapor.
Image credit: NASA/JPL/University of Colorado
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tiger stripes are warm!
• Further news came from the Cassini infrared spectrometer which
announced that the south pole was much warmer that the rest of planet, not
colder as expected.
• Moreover, the warmth was coming from the ‘stripes’ or ‘cracks’ especially.
Image credit: NASA/JPL/GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Plume spotted!
• At last Cassini’s cameras were able to view the plume directly, by
looking back at the Sun from behind Enceladus. In the south, a smaller
plume is seen about 100 km away from the main plume.
Image credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Enceladus cryovolcanoes
• It is now clear that some sort of cryo-eruptions are occuring in the south
polar stripes, blasting huge amounts of water vapor into space. This
material is the source of Saturn’s E-ring.
• The diagram (right) shows one
possible mechanism for the
eruptions to occur.
• Enceladus is heated by a
combination of tidal forces, and
radiogenic heating from rocks.
• Underground reservoirs of
liquid are the source of the
eruptions, periodically venting
through the tiger stripe fractures.
Graphic: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Active Moons: Io and Enceladus
• In some respects, Enceladus is like a colder version of Io: it is an active
planet which has been recently re-surfaced, and has ‘volcanic eruptions’
of a sort: except cold rather than hot.
• For Io to remain warm and active, there had to be a source of heating.
We found this to be the tidal effect of Jupiter, combined with the noncircular orbit Io is forced into by its neighbours.
• Enceladus’ heating is more perplexing however, as its orbital
eccentricity is low. Radiogenic heating is one possible explanation.
• Alternatively, we may be seeing it at a unique point in its history (after a
giant impact, for example?).
• No-one knows for sure at the present time, but Cassini will try to find
out during the course of the mission.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Iapetus
• Iapetus is even stranger
than Enceladus. Iapetus is
like the Chinese yin-yang
symbol, having a dark and a
bright side.
• The leading hemisphere is
dark, and the trailing
hemisphere bright, so we
see a ‘police light’ effect as
as it goes round its orbit,
watched from the Earth.
• The bright side is water ice
with a 50% albedo: the dark
side has an albedo of only
3%, like a carbonaceous
asteroid.
Image Credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Domino Effect
• The reddish-black substance covering the leading side of Iapteus
makes it one of the darkest surfaces in the solar system.
• Study seems to indicate that the dark surface coloring can be matching
well with a carbon-nitrogen-hydrogen organic compound, with a frosting
of ice.
• The organic substance may be akin to the dark surfaces of Pholus (a
centaur comet), some comets and asteroids, and perhaps even the dark
ring material of Uranus and Neptune.
• The fact that the dark surface material is on the leading side of Iapetus
itself may be a clue: it seems to have been ‘swept up’ by the motion of
Iapetus. (Iapetus is a similar composition to the other satellites internally,
so the dark material does not seem to come from within).
• But why only Iapetus, and not any of the other moons? No-one knows
for now…
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Medium-size Satellites Of Uranus
• Uranus has 5 satellites in the same general size range as Saturn’s 6
medium-size satellites.
Satellite
Period
Diameter
Mass
(days)
(Moon=1)
(Moon=1)
(g/cm3)
Miranda
1.41
0.14
0.0011
1.3
Ariel
2.52
0.33
0.0180
1.6
Umbriel
4.14
0.34
0.0180
1.4
Titania
8.71
0.46
0.0480
1.6
Oberon
13.50
0.45
0.0390
1.5
Table after: Morrison and Owen, The Planetary System
Density
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Uranus and Family
• A Voyager 2 montage of the Uranian system: clockwise from bottom left: Ariel,
Umbriel, Oberon, Titania, Miranda and Puck.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Miranda to Oberon
• Four of these five moons are named for Shakespearean characters
(Umbriel is the exception): with many of the tinier moons following the
same trend (Juliet, Portia etc).
• Most of our knowledge about them comes from the swift Voyager 2 flyby of 1986.
• The sizes are similar to the medium satellites of Saturn, although the
densities are somewhat greater (1.3-1.6, rather than 1.1 to 1.4 g/cm3),
indicating a higher rock to ice fraction.
• Titania and Oberon therefore are substantially heavier than the Rhea,
although much the same size.
• The surface albedos are around 20-30%, indicating a dirty water ice
composition (water ice has been detected spectroscopically).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
King of the Faeries
• Oberon was discovered in 1787 at the same time as Titania, and by the
same gentleman who 6 years earlier had discovered Uranus itself! (Who?)
• Oberon is the outermost of the
large Uranian satellites.
• The surface is old and heavily
cratered, with scant evidence of
internal activity.
• Oberon shows large, rayed
craters similar to Callisto. The
crater floors seem to be flooded
by an unknown, dark material.
• On the limb (horizon) a 6-km
high mountain is seen.
Image: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Queen of the Faeries
• Titania shows a few large impact basins, but mostly small craters and
boulders on the surface.
• A large double-walled crater is
also visible at the top of the image.
• We also see a 1600-km long
trench. There is an extensive
system of interlocking rifts all over
the surface.
• Scientists believe that Titania’s
surface cooled before the interior.
As the interior cooled it expanded
and caused massive rifts.
Image: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Umbriel
• Umbriel and Ariel are near twins in size. Umbriel however is the darkest
satellite of Uranus. The surface shows a lot of large craters, and is
apparently old.
• The picture below shows an original Voyager 2 image (right) and a falsecolor version which has been contrast enhanced. The bright ring at the top
is called the ‘fluorescent cheerio’!
• The two very bright
regions appear to be
ice from fresh (recent)
craters.
• Scientists have
suggested that the dark
material originated from
a long-ago re-surfacing
event.
Image: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Ariel
• Ariel is the brightest moon of Uranus, and shows a surface pock-marked
with craters and criss-crossed with huge canyons, like those of Mars.
• The canyons are thought to be
caused by down-dropped fault blocks
(graben), due to expansion of the
moon.
• The canyon floors have been
smoothed by some fluid, which cannot
have been water. Water is hard as
steel at Uranus. Ammonia, methane or
CO ice is possible.
• The newer floors of the old valleys in
turn are carved with younger valleys
and scarps, either by more faulting, or
perhaps fluid flow.
Image: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Miranda
• Miranda was expected to
be the least interesting of the
Uranian five mid-sized
satellites, being smallest.
• However, being the
innermost, Voyager 2 was
compelled to get close
(28000 km) as it made its
gravity assist maneuver to
get to Neptune.
• Rather than being
disappointing, Miranda
turned out to be the most
interesting of the five!
Image: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Miranda Surface
• The surface of Miranda is like a jigsaw puzzle (remember the chaos
terrain of Europa?). The surface shows massive cliffs and valleys 10 to 20
km deep.
• The surface also shows oval and
trapezoidal mountain ranges, and a
bewildering array of old and young
surfaces.
• One theory is that the moon
incompletely differentiated, and froze
part-way through the process.
• Another theory is that the moon was
shattered apart and fell back together
again multiple times, exposed portions
of the core and burying parts of the
surface.
Image: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Small Satellites of Saturn
• Taking another step down in size, we come to the small satellites of
Uranus. Below, clockwise from top left are: Helene, Epimetheus,
Calypso, Janus, Telesto, Pandora and Prometheus.
Image: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Co-orbital Satellites Of Saturn
• Janus and Epimetheus form one of the strangest pairings in the solar
system.
• First spotted from the Earth in 1966, their existence was confirmed in
1980 by Voyager 1. They are 13,000 km beyond the main rings.
• Janus and Epimetheus have orbital distances which differ by only 50
km in radius, which means that the inner one orbits faster, and catches
up with the outer one every 4 years.
• The satellites are 200 km and 150 km across, so there is no room for
them to pass one another! What happens?
• Incredibly, the two satellites attract each other gravitationally, and
exchange orbits. Then in four years time, the cycle repeats.
•This is the only known example of such co-orbital satellites, in the whole
solar system.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Image: James Schombert, U. Oregon
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Janus and Epimetheus
• Both satellites have extensive cratering,
indicating that their surfaces are old.
• Both have densities around 0.6 g/cm3,
indicating that they must be porous icy bodies.
• Craters as large as 30 km are seen, and
Epimetheus is additionally traversed by large
and small grooves, valleys and ridges.
• The most popular theory for this strange
partnership is that the two were once joined as a
single parent body, which was fractured by a
long-ago impact.
• Remember that the inner Saturnian system
was heavily bombarded (e.g. Mimas).
Image: nineplanets.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
1. What are the main differences between regular and irregular satellites?
How did each group probably form?
2. Which planet has the most moons discovered (so far)? Is this what you
would expect? Is the distribution of sizes evenly spread?
3. What is thee main component of Saturn’s medium-sized satellites?
4. What probably happened to Dione, and how do we know?
5. Which moon has a huge trench, bigger than the Valles Marineris as a
proportion of the satellite size? How did it probably form?
6. Why is Mimas the most beat-up of Saturn’s regular satellites?
7. Why is Enceladus so remarkable?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
8. What similarities are there between Enceladus and (I) Io (ii) Europa?
9. Which moon looked most like a panda bear? What could have
happened to give it this appearance?
10. Are the densities of the Uranian satellites higher, lower or the same as
the Saturnian ones?
11. Many of the Uranian moons exhibit graben, or dropped-down fault
blocks which have created extensive valley systems. What couldd have
caused this rifting?
12. Why is Miranda the most astonishing of the Uranian moons?
13. Explain the strange orbits of Janus and Epimetheus.
14. How could Jupiter have captured families of irregular, retrograde
moons?
Dr Conor Nixon Fall 2006