Download Notes

Survey of terrestrial planets
• All have similar bulk chemical composition, dominated by
O, Mg, Si, Fe
• Fraction of total mass in metallic iron decreases with
increasing distance from the Sun (Mercury → Mars)
• Table below which roughly ranks the importance of
various surface influences on each terrestrial body; 3 is
most important and 0 no significance.
Planet
Moon
Mercury
Mars
Venus
Earth
Impacts Volcanism Tectonics Erosion
3
3
2
1
1
1
1
2
3
3
0-1
0-1
2
3
3
0
0
1-2
0-1
3
• Bodies are listed in order of increasing mass;
¾ impacts become less important as mass increases,
¾ both volcanism and tectonics become more important
as mass increases,
¾ erosion (atmosphere and surface water) effects are
also more important as mass increases.
• Planetary mass influences planetary evolution.
¾ The amount of internal heating and the rate at
which a planet cools are key.
¾ All of the heating mechanisms depend on mass
(proportional volume or R3) while cooling depends on
surface area (proportional to R2). Consequently the
larger body will cool more slowly/retain internal heat
longer.
•
•
•
•
•
•
•
The Smallest Moons
Small moons are characterized by:
¾ Heavy or saturated cratering
¾ Irregular shapes
Deimos: smooth and blanketed by regolith
Phobos:
¾ Dark, coated with carbonaceous material
¾ Low density (porous)
¾ 100m regolith. But more rugged, grooved terrain
than Deimos. More recent impact?
Impacts knock debris off these moons, form “dust
belts” which continue to sandblast the surface and add
to the regolith
Amalthea is a small moon of Jupiter that is unusually
red
¾ Coated with sulfurous material blown off Io?
Hyperion: largest irregular satellite
¾ Saturated with craters; oldest surface in the
Saturn system
¾ Reddish colour, similar to the dark material on
Iapetus
¾ Shallow craters filled with unknown dark material:
may help to melt surrounding ice
Phoebe: roughly spherical satellite of Saturn
¾ Very dark surface (albedo 0.06).
•
•
•
•
•
Intermediate-size moons
Mimas, Tethys, Dione, Rhea:
¾ All have cratered surfaces of clean, bright water ice
¾ Currently inactive
¾ Tethys has signs of resurfacing
¾ Dione shows bright wisps on a darker background,
shown to be ice cliffs: tectonic fractures.
Cratering rate is especially high in Saturn system
¾ The inner moons (esp. Mimas) may have been
fragmented and reassembled several times
Iapetus:
¾ Has well-defined “dark side”, made up of
carbonaceous material probably blasted off
neighbouring moon Phoebe
¾ Unusual equatorial ridge. Tectonic activity?
Preserved from a period of more rapid rotation?
Enceladus:
¾ Relatively young surface (<1 Gyr)
¾ Tidal heating produced eruptions of water ice
A fine spray of small, icy particles emanating from the
warm, geologically unique province surrounding the
south pole of Enceladus was observed by Cassini
¾ water vapor escaping from warm ice that is exposed
to the surface?
¾ Another possibility is that at some depth beneath
the surface, the temperatures are hot enough for
water to become liquid, which then, under pressure,
escapes to the surface
•
•
•
•
Triton and Pluto
similar densities
both have tenuous atmospheres of N2, CH4 and CO,
with comparable surface pressure.
¾ When Pluto moves away from the Sun, most of its
atmosphere freezes and falls to the ground.
Both probably large members of Kuiper belt
Triton:
¾ Atmospheric methane gas and surface methane ice
¾ Surface ices of nitrogen mixed with methane and CO
¾ Young, nearly uncratered, icy surface
¾ At least two active vents, probably N2 jetting from
beneath the surface.
¾ Heating may be due to tidal forces resulting from
capture by Neptune. Or perhaps seasonal heating
from the Sun.
•
•
•
•
•
•
•
•
Earth’s Moon
Moon’s surface is dominated by two major regions:
¾ old, brighter (A~11-18%), heavily cratered highlands
¾ younger, darker (A~7-10%), smooth, much less
cratered maria (<20% of lunar surface).
¾ The smoothness of the maria suggests they are
basins filled with hot lava flows.
Highland rocks are mainly basalt; samples returned by
Apollo missions are lacking in both metals and volatiles.
Volcanism is evident in the lava flows of the maria and
features such as sinuous rilles which are probably
exposed lava tubes.
Tectonic activity indicated by fault features, but there
is no evidence of plate tectonics either past or present.
Moonquakes originate mainly from a depth of ~1000km,
suggesting that beneath this level the
temperature/pressure combination is high enough to melt
the rock.
The Moon’s low density means that it cannot contain a
very high % of metals and other heavier elements so its
core, if iron-rich, must be very small and probably no
more than about 10% of the Moon’s mass.
The remnant magnetic field found in lunar samples would
seem to indicate that the Moon once had a molten
metallic core.
The lunar crust is thinner under the maria (~20km thick)
than the highlands (50-100km), with the result that the
centre of mass of the Moon is displaced slightly closer to
Earth than the “centre of figure”.
•
•
•
•
•
•
Large Moons of Jupiter
Surface melting, or eruptions, resurface these moons
with water ice.
¾ Probably due to tidal heating
Some of Ganymede’s bright, sparsely cratered regions
are actually older ice that has been intensely fractured
Callisto nearly saturated with craters, smooth between.
¾ Thin atmosphere of CO2, constantly replenished
from sublimating ice.
Darkest parts of Ganymede also nearly saturated with
craters
¾ lighter regions are still old, but have grooves and
ridges; clearly tectonic
Bright surface of Europa is pure water ice.
¾ One of the smoothest SS objects, with very few
craters
¾ Streaks may be equivalent of sea-floor spreading.
¾ Cycloidal ridges are unique – compressional ridges?
Io
¾ Most vocanically active world in the solar system
¾ Volcanoes eject material at 0.5 to 1.0 km/s
¾ Ejecta temperatures probably ~300 K, up to ~900 K
at the vent itself. Ejecta is probably sulfur, which
is black when molten and reddens as it cools. It is
yellow-orange at 400 K
¾ White deposits may be SO2 frost.
¾ However, the hottest spots have temperatures up to
2000 K, too hot for molten sulfur. Silicate rock
likely to be a major component of the lava.