Impacts and Erosion
14 September 2015
Four Basic Geological Processes
• Impact cratering
– Impacts by asteroids or
comets
• Volcanism
– Eruption of molten rock
onto surface
• Tectonics
– Disruption of a planet’s
surface by internal stresses
• Erosion
– Surface changes made by
wind, water, or ice
Possible Subduction and Crustal Recycling on Europa
Impact Cratering
High kinetic energy of
impacting object
 Excavation
 Heat
Shape round for all impact
directions
Crater Morphology:
• Impactor in the inner
solar system can
have 10s of km/sec
relative velocity
• May vaporize or
liquefy a significant
amount of the target
for a large impactor
• “Simple” craters are
bowl shaped
• “Complex” craters
are flatter, and have
a central uplift or
peak
Impact
Craters
Tycho (Moon)
Meteor Crater (Arizona)
The Impact Process
1. Detonation - shockwave propagates through
target, projectile vaporized
2. Excavation - target is heated, vaporized,
liquified, solid material is ejected (possibly above
escape velocity!)
3. Rebound - in larger craters, the target may
“bounce back” viscously, forming a central uplift
4. Relaxation - crater walls subside, melt pools in
the crater
Few Hundred Confirmed Craters on Earth
eteor Crater
Meteor Crater
Ariel view of Meteor Crater
Manicouigan Crater
Crater Lake
Moon’s
Giant Basins
Moon Near Side
Lunar
Farside
Moon Far Side
Lunar Highlands
Mare Imbrium
Lunar Mare
Crater modification
Mars Rampart Crater 3
Mars
Rampart
Crater
Venus Craters 1
Ganymede Crater Chain
Shoemaker-Levy 9
SL-9 Aftermath
Hyperion
Hyperion
Erosion and Deposition
• Erosion and deposition require the
presence of a fluid (gas or liquid) to pick
up, transport and deposit surface material
• Liquid transport more efficient
• These processes tend to be rapid
compared to other geological processes
• So surface appearance is often controlled
by these processes
• Earth, Mars, Titan, Venus have erosional
or sedimentary features
Aeolian Features (Mars)
• Wind is an important process on Mars at the present day
(e.g. Viking seismometers . . .)
• Dust re-deposited over a very wide area (so the surface of
Mars appears to have a very homogenous composition)
• Occasionally get global dust-storms (hazardous for
spacecraft)
• Rates of deposition/erosion almost unknown
Martian dune features
Image of a dust
devil caught in
the act
30km
Aeolian features (elsewhere)
Namib desert, Earth
few km spacing
Longitudinal dunes, Earth (to
Titan (bottom), ~ 1 km spacin
Yardangs (elongated dunes)
Mead crater, Venus
Wind directions
Venus
Mars (crater diameter 90m)
Wind streaks, Venus
Global patterns of wind
direction can be compared
with general circulation
models (GCM’s)
Fluvial features
• Valley networks on Mars
• Only occur on ancient
terrain (~4 Gyr old)
• What does this imply
about ancient Martian
atmosphere?
100 km
30 km
• Valley network on Titan
• Presumably formed by
methane runoff
• What does this imply about
Titan climate and surface?
Martian Outflow channels
• Large-scale fluvial features,
indicating massive (liquid)
flows, comparable to ocean
currents on Earth
• Morphology similar to giant
post-glacial floods on Earth
• Spread throughout Martian
history, but concentrated in the
first 1-2 Gyr of Martian history
• Source of water unknown –
possibly ice melted by volcanic
eruptions (jokulhaups)?
flow
direction
50km
150km
Baker (2001)
Martian Gullies
• A very unexpected discovery
(Malin & Edgett, Science 283,
2330-2335, 2000)
• Found predominantly at high
latitudes (>30o), on polefacing slopes, and shallow
(~100m below surface)
• Inferred to be young – cover
young features like dunes
and polygons
• How do we explain them?
Liquid water is not stable at
the surface!
• Maybe even active at
present day?
Lakes
Clearwater Lakes Canada
~30km diameters
Gusev, Mars
150km
Titan, 30km across
Titan lakes are (presumably)
methane/ethane
Gusev crater shows little evidence
for water, based on Mars Rover data
Erosion
• Erosion will remove small, near-surface craters
• But it may also expose (exhume) craters that were
previously buried
• Erosion has recently been recognized as a major
process on Mars, but the details are still extremely poorly
understood
• The images below show examples of fluvial features
which have been exhumed
channel
meander
Malin and Edgett, Science 2003
Sediments in outcrop
Opportunity (Meridiani)
Cross-bedding indicative of prolonged fluid flows