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

Meteor Crater (Arizona)Meteor Crater (Arizona)

Tycho (Moon)Tycho (Moon)

Impact Craters

The Impact Process1. 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

Meteor CraterMeteor Crater

Ariel view of Meteor Crater

Manicouigan CraterCrater Lake

Moon Near SideMoon’sGiant Basins

Moon Far SideLunarFarside

Lunar Highlands

Mare Imbrium

Lunar Mare

Crater modification

Mars Rampart Crater 3MarsRampartCrater

Venus Craters 1

Ganymede Crater Chain

Shoemaker-Levy 9

SL-9 Aftermath

HyperionHyperion

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

30km

Image of a dustdevil caught inthe act

Martian dune features

Aeolian features (elsewhere)Namib desert, Earthfew km spacing

Yardangs (elongated dunes)Mead crater, Venus

Longitudinal dunes, Earth (top),Titan (bottom), ~ 1 km spacing

Wind directionsVenus

Wind streaks, Venus

Global patterns of wind direction can be compared with general circulation models (GCM’s)

Mars (crater diameter 90m)

Fluvial features• Valley networks on Mars• Only occur on ancient

terrain (~4 Gyr old)• What does this imply

about ancient Martian atmosphere?

30 km

• Valley network on Titan• Presumably formed by

methane runoff• What does this imply about

Titan climate and surface?

100 km

• 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)?

Martian Outflow channels

50km

flowdirection

150km

Baker (2001)

Martian Gullies• A very unexpected discovery

(Malin & Edgett, Science 283, 2330-2335, 2000)

• Found predominantly at high latitudes (>30o), on pole-facing 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

Titan, 30km across

Gusev, Mars150km

Clearwater Lakes Canada~30km diameters

Titan lakes are (presumably) methane/ethaneGusev 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

Malin and Edgett, Science 2003

meander

channel

Sediments in outcrop

Opportunity (Meridiani)

Cross-bedding indicative of prolonged fluid flows