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ASTR 330: The Solar System
Announcements
• Homework assignment #5 on-line.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 21:
Large Satellites I: Callisto,
Ganymede and Europa
Picture credit: NASA/JPL - Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
From planets to moons
• We have now completed out tour of the nine ‘official’ planets of the
solar system.
• We now turn our attention to the large moons of the outer planets by
which we mean:
• The four ‘Galilean’ moons of Jupiter: Io, Europa, Ganymede and Callisto.
• Saturn’s giant moon Titan, the only moon with a substantial atmosphere.
• Neptune’s large moon, Triton.
• Each one of these large moons would have a significant claim to being
a planet in its own right, had it been orbiting the Sun rather than a planet.
• Note that only Uranus, of the four ‘giant’ planets, lacks a 3000-5000 km
sized moon. The two largest, Titania and Oberon are around 1500 km.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
By the numbers
• Facts and figures for the six large moons, and also Mercury and Pluto
are shown in the table below.
• Which one is the:
Satellite
Period
Diameter
Mass
(days)
(Moon=1)
(Moon=1)
(g/cm3)
Density
• largest?
Callisto
16.69
1.38
1.47
1.9
• most massive?
Ganymede
1.16
1.51
2.02
1.9
Europa
3.55
0.91
0.65
3.0
Io
1.77
1.04
1.22
3.6
Titan
15.95
1.48
2.00
1.9
Triton
5.88
0.84
0.30
2.1
Mercury
1.40
4.49
5.4
Pluto
0.66
0.17
2.1
• densest?
• Which moons are
bigger than:
• Mercury?
• Pluto?
Table: Morrison and Owen
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cratering On Icy Moons
• Most of the large moons (those with densities around 2 g/cm3) are
roughly equal mixtures of rock and ices.
• These moons have differentiated internally to varying amounts, but in
general the surfaces are quite icy.
• What then is the implication for the cratering record? On the terrestrial
planets there was no doubt that cratering of rocky surfaces was capable
of retaining crater imprints over billions of years, if left in peace.
• But can icy surfaces retain crater records?
• The answer depends on temperature. On the Earth, ice is clearly quite
plastic and flows down mountains in the form of glaciers.
• At Saturn, water ice is frozen as hard as rock. However, at the distance
of Jupiter, the ice may not only deform and lose shape, but may also
sublime and evaporate.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Impacts in the Outer Solar System
• For the Earth and Moon, 75-90% of craters are thought to have been
caused by asteroid impacts.
• However, few asteroids reach Jupiter, and virtually none can reach
Saturn, so we expect asteroid craters rates to be pretty insignificant.
• Jupiter’s gravity partly shields the inner solar system from comets, so
we expect more cometary impacts in the outer solar system.
• At Jupiter, most impacts come from ‘Jupiter family’ comets: objects
diverted from the Kuiper Belt.
• At Saturn, most impacts come from long-period comets, from the Oort
Cloud.
• The impact rates at Jupiter are about half that of the Earth, at Saturn
the rate is one quarter the Earth rate.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Influence of Gravity on Cratering
• There is an important correction we must make when comparing
cratering on different satellites.
• The gravity of the parent planet tends to pull comets inwards: the
more massive the planet, the stronger the gravitational effect.
• We can imagine the planets as situated at the bottom of a slope: the
closer the moon is to the planet:
1. the more impacts will occur, and
2. the higher the impacts speeds will be.
• For example: Mimas, the innermost large moon of Saturn would
expect 20 times the impact rate of Iapetus, far from the planet.
• Inner moons of Jupiter and Saturn therefore have higher cratering
rates than the Earth and Moon, whereas distant moons would have a
lesser rate.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cratering Rates Over Time
• We have just seen that the gravitational focusing of the
parent planet can compensate for the lower number of
potential impactors, so that the cratering rates are not too
different for bodies in the outer solar system, compared to
the inner solar system.
• In fact, we do see crater densities not much different from
the Moon, which we have noted are in contradiction to the
number of available impactors at the present day.
• Hence, cratering rates were higher in the past than now,
and we are led to the conclusion that the ‘Late Heavy
Bombardment’ period we surmised in the inner solar system
also applied to the outer solar system.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Galilean Satellites Of Jupiter
• The composite image above shows the four large Galilean satellites of
Jupiter to scale. From left to right they are: Ganymede, Callisto, Io and
Europa. While Ganymede is larger than Mercury, Europa is slighter
smaller than the Moon.
• The moons are all named after ‘conquests’ of Jupiter/Zeus in mythology.
Picture credit: NASA/JPL - Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Callisto
• The image (right) was
taken by Voyager 2.
• What can you tell from
this picture about the:
• atmosphere?
• surface
composition?
• geology or history
of the surface?
Picture credit: NASA/JPL Voyager 2
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Callisto Interior
• Callisto is about the same size as Mercury, but only 1/3 as massive.
Hence, it is about half water ice and half rock, rather than rock and iron.
• Callisto, like all the large satellites, should have been able to fully
differentiate, due to heating from radioactive elements in the rocky
component. This would imply a rock-mud core and an icy mantle and
crust.
• However, gravitational field measurements from Galileo showed that the
planet does not have a big change in density towards the core, and has
therefore not fully differentiated.
• Additionally, a weak magnetic field was detected by Galileo, which may
be explained by a salty fluid below the surface: a possible ocean layer.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Model Callisto Interior
• A model
Callisto interior is
shown right.
• The outer layer
is a 200 km thick
ice crust with a
10-km ocean
overlying the
interior mantle.
• The mantle is a
mostly
homogeneous
mixture of rock
and ice.
Picture credit: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Callisto Surface
• The temperature of Callisto varies from about 100 K at noon to 150 K at
night, low enough for ice to evaporate extremely slowly: a few meters
over the last 4 billion years.
• Ice is observed on the surface of Callisto (by spectroscopy), but the
reflectivity of just 0.18 shows the ice is rather dirty, perhaps mixed with
meteoric dust.
• As expected for a world with almost no internal activity, the surface
retains an ancient cratering record, with a density almost as great as the
lunar highlands: 250 10-km craters per million square km.
• Hence, we can safely say that the surface of Callisto appears at least
as old as the lunar maria, and probably formed during a similar period of
Heavy Bombardment.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cratering
• On close examination, the craters are different shapes to those of the
Moon. Instead of being bowl-shaped, they are flattened.
• This is because at Callisto’s temperature, the ice is not perfectly solid,
and flows very slowly over time, gradually smoothing the initially sharp
crater forms over hundreds of millions of years.
• The image (right) from
Voyager 2 shows the Gipul
Catena, a crater chain 620
km long.
• Features such as this
mystified scientists until the
S-L 9 break-up was
observed.
Picture credit: NASA/JPL Voyager 2
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Giant Impacts
• Callisto apparently has suffered
similar giant impacts to those which
produced basins such as Imbrium
on the Moon and Caloris on
Mercury.
• However, on Callisto we see only
the concentric rings, not the basin
itself. Presumably, ice is too plastic
to retain the sunken basin shape.
• The Vahalla impact (right) is 3000
km across, and a similar bulls-eye
pattern occurs at the Asgard
impact, which is 1700 km across.
Picture credit: Bill Arnett/Nine Planets
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Small-scale topogrpahy
• At the smallest scales of just 10s of meters, other distinctive features
become apparent.
• The Galileo image (right) shows a
region just 13 km across, with the
smallest visible craters here 130
meters in size.
• A process of evaporation and
condensation has taken place here.
Ice has evaporated from some
regions, leaving behind dark surface
material.
• Some of the ice has re-condensed
on cooler slopes which face away
from sunlight.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Ganymede
• Ganymede is the
largest moon in the
solar system, and
the same density as
Callisto.
• The large dark
region, called the
Galileo Regio, is
about 3200 km in
diameter.
• Parts of this terrain
may be covered in a
bright frost.
Picture credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Ganymede Interior
• Ganymede, unlike
Callisto is definitely
differentiated, as
tracking of the
Galileo spacecraft
proved.
• Ganymede has a
large, dense core,
presumably of
silicates,
surrounded by an
icy mantle and
crust.
Picture credit: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Ganymede Surface
•
Ganymede’s surface may be divided into two categories:
1. About half the surface resembles the ancient, dark terrain of
Callisto, with crater ghosts and concentric ridges from long-ago
impacts. This terrain has a similar level of cratering, and hence
age, as Callisto. The albedo is about 25%, suggesting a relatively
low admixture of ice with the ‘dirt’.
2. The rest of Ganymede is a lighter colored, less-cratered type of
terrain. The densities here are 100-200 10-km craters per 1
million km2, indicating an age of 1-2 billion years. The albedo is
40%, indicating a higher proportion of ice.
• Note that the age/brightness difference is the opposite to the Moon,
where the darker areas (the maria) are younger than the lighter
highlands.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Indications of Internal Activity
• Many of the younger areas of
Ganymede show parallel ridges
and valleys, separated by 10-15
km. These are similar to some
areas of the Appalachians.
• Although these are tectonic
features, they are not the same as
the fold mountains we see on the
Earth.
• Rather than being produced by a
horizontal compression, these
features are believed to be parallel
cracks produced by uplift and
subsidence: vertical motions.
Picture credit: NASA/Galileo/Nine Planets Page
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Craters
• Ganymede has a small number of very bright craters, suggesting that
nearly pure water ice has been ‘splashed’ across the surface by an
impact.
• Callisto does not show similarly bright crater ejecta, suggesting that the
sub-surface ice on Ganymede is much cleaner: further evidence for
greater differentiation.
• This Galileo image
shows yet another
crater chain (the
Enki Catena): notice
that the impact has
occurred right
across the boundary
between light and
dark terrains.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
History of Ganymede
• The evidence we have accumulated so far seems to indicate that
Ganymede suffered a series of internal upheavals sometime in the first 2
billion years of its existence.
• One possible cause could be a change of density and structure of
internal ices, as the moon gradually cooled since formation.
• This may have caused a series of expansions and contractions which
could crack the surface and flood some of the valleys with water ‘lava’,
explaining the flattened floors of some valleys.
• Magnetic field measurements indicate a weak magnetic field, but there
is a possibility of a liquid ‘slush’ layer deep in the mantle which is
electrically conducting (similar to Callisto).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Europa
• Below: natural (left) and false color (right) images of Europa’s trailing
hemisphere, from the Galileo spacecraft.
Picture credit: NASA/JPL - Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Europa: Overview
• Europa is the smallest of the ‘big four’ Galilean moons of Jupiter.
• Europa’s density of 3 g/cm3 indicates a much smaller amount of ice in
the compositional mixture (10%) than either Ganymede or Callisto.
• However, the surface is the brightest of the four (reflectivity 70%) and
appears to be nearly pure water ice. In this respect, Europa resembles
what the Earth would be like, if all the oceans were frozen.
• The surface ice appears to comparatively pure at the present day,
unlike the ‘dirty ice’ appearance of Ganymede and Callisto.
• Europa is the smoothest planetary object in the solar system, with
almost no impact craters visible at all, despite the ‘focusing’ effect of its
closeness to Jupiter. Any record of historical bombardment has been
erased. How?
• The surface age is thought to be just 10 Myr: younger than the Earth’s
surface!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Linear Markings
• Europa’s most remarkable features are the many linear markings: similar
to the fanciful drawings of Martian canals by Lowell .
• The markings are in fact tectonic cracks produced by stresses in the
crust, which can stretch for 100s or 1000s or km.
• The implication is that the crust is floating on a layer of liquid water or
slush.
• The tectonic cracks are often a ‘double ridge’ type, a few km in width, and
several hundred meters in height.
• Scientists suspect that these may have formed when slushy material was
injected into a crack from below, or due to pressure at the closing of the
crack.
• We see similar pressure ridges in the Earth’s Arctic ice pack, but on a
much smaller scale.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
‘Double Ridge’ Cracks
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Scalloped Ridges
• Even more remarkable are the ‘scalloped’ ridges, which show multiple arcs
along their length. These are believed to have been caused by successive
daily tides.
• Each Europan day, the crack would form in an arc, due to the changing
direction of tidal forces. The next day (3.5 Earth days), another arc is
created, adding to the first.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Europa: Chaos Terrain
• A few percent of Europa’s surface is classified as ‘chaos’ terrain
(remember the chaos terrains of Mars?).
• These areas show pieces
of older crust, crisscrossed with many ridges,
which have apparently
rotated and drifted into new
positions, even tilted, and
then been re-frozen back
into the solid crust again
(like a mixed up jigsaw
puzzle).
• Periodically, the crust
must melt, allowing liquid
water to reach the surface.
Picture credit: NASA/JPL Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Europa Interior
• The idea of a global ocean on Europa is the most certain of the three
candidate moons of Jupiter (Callisto, Ganymede, Europa).
• Aside from the chaos
terrain, magnetic field
measurements indicate
a conducting fluid (e.g.
salty water) is present
sub-surface.
• The crust is probably
10-20 km thick, and the
water ocean is the
largest in the solar
system, Earth included!
Picture credit: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
1. Should large moons have differentiated at all? What factors sould
affect the amount of differentiation in a given moon?
2. Name the largest moon of (i) Jupiter (ii) Saturn (iii) Neptune.
3. Why might the cratering record be preserved less well on the surfaces
of the outer planet moons, than say the Moon or Mercury?
4. Would we expect the same populations of objects to cause craters on
the outer planet moons, as on the inner planets? Why?
5. What effect does the parent planet have on the amount of impactors
reaching (I) a close-in moon such as Mimas (ii) a far-out moon such as
Iapetus?
6. Was there a period of heavy bombardment in the outer solar system?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
7. Callisto and Ganymede are similar sizes and densities. Are they the
same inside?
8. What differences are there between craters on Ganymede and Callisto,
and those on Mercury and the Moon?
9. What two types of surface terrain do we see on Ganymede? Which is
also seen on Callisto? Which is older?
10. Which is more like a billiard ball: Europa or Callisto?
11. What causes the linear double-ridges on Callisto?
12. Europa’s scalloped ridges are caused by tides; what length of time does
each scallop shape correspond to?
13. Which Galilean moon is most likely to have a liquid ocean? How much,
e.g. relative to the Earth’s oceans?
Dr Conor Nixon Fall 2006