EART160 Planetary Sciences

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EART160 Planetary Sciences
Francis Nimmo
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Last Week Solar System Formation

• Solar system formation involved collapse of a
  large gas cloud, triggered by a supernova (which
  also generated many of the elements)
• Solar system originally consisted of gasicerock
  in ratio 10010.1 (solar photosphere primitive
  meteorites)
• Initial nebula was dense and hot near the sun,
  thinner, colder further out
• Inner planets are mainly rock outer planets
  (beyond the snow line) also include ice and (if
  massive enough) gas
• Planets grow by collisions Mars-sized bodies
  formed within 1 Myr of solar system formation
• Late-stage accretion is slow and involved large
  impacts

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This Next Week Surfaces

• What are solid planet surfaces made of?
• What processes modify the surfaces?
• Impact craters
• Volcanism
• Tectonics
• Erosion Sedimentation

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Surface Compositions

• How can we tell?
• Samples (Earth, Moon, Mars, Vesta?)
• In situ measurements by spacecraft (Venus, Mars,
  Moon, Titan)
• Remote sensing (elsewhere)

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Samples

• Very useful, because we can analyze them in the
  lab and we (usually) know where they came from
• Generally restricted to near-surface
• For the Earth, we have samples of both crust and
  (uniquely) the mantle (peridotite xenoliths)
• We have 382 kg of lunar rocks (29,000 per pound)
  from 6 sites (7 counting 0.13 kg returned by
  Soviet missions)
• Eucrite meteorites are thought to come from
  asteroid 4 Vesta (they have similar spectral
  reflectances)
• We also have meteorites which came from Mars
  how do we know this?

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SNC meteorites

• Shergotty, Nakhla, Chassigny (plus others)
• What are they?
• Mafic rocks, often cumulates
• How do we know theyre from Mars?
• Timing most are 1.3 Gyr old
• Trapped gases are identical in composition to
  atmosphere measured by Viking. QED.

2.3mm
McSween, Meteoritics, 1994
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In Situ Measurements

• In situ measurements give us information without
  needing samples returned (difficult)
• Problem is that only limited data can be returned
• Still useful e.g. we know that the surface of
  Venus is basaltic, and that the surface of Titan
  has the texture of crème brulee
• The Viking spacecraft even carried life detection
  experiments, but the results were negative or
  ambiguous

Venusian surface (Venera 14)
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In Situ Measurements (Mars)

• Pathfinder (1997) measured rock and soil
  compositions using an Alpha Proton X-Ray
  Spectrometer (APXS)
• This works by irradiating a sample with Alpha
  particles and detecting the particles/radiation
  given off
• One problem was the desert varnish coating the
  rocks

• The Mars Exploration Rovers (2004- ) carried a
  rock abrasion tool to scrape off the varnish
  before carrying out their measurements
• The results suggested ancient water had
  percolated through the sediments and produced
  concretions nicknamed blueberries

blueberries
RAT
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Remote Sensing

• Restricted to surface (mm-mm). Various kinds
• Spectral (usually infra-red) reflectance/absorptio
  n gives constraints on likely mineralogies e.g.
  Mercury, Europa
• Neutron good for sensing subsurface ice (Mars,
  Moon)

• Most useful is gamma-ray gives elemental
  abundances (especially of naturally radioactive
  elements K,U,Th)
• Energies of individual gamma-rays are
  characteristic of particular elements

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Physical Properties

• In the absence of other processes, ancient crusts
  will have been broken up by impacts at all scales
• Lunar surface consists of fine-grained dust
  (produced by impacts) overlying brecciated,
  unconsolidated material (regolith)

• Whether a surface is dusty or consists of solid
  rock can be inferred from its thermal inertia
  (rocks have a higher T.I.)

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Summary Planetary Crusts

• Surfaces are expected to be broken up by impacts
  (regolith)
• Remote sensing (IR, gamma-ray) allows inference
  of surface (crustal) mineralogies compositions
• Earth basaltic (oceans) / andesitic (continents)
• Moon basaltic (lowlands) / anorthositic
  (highlands)
• Mars basaltic (plus andesitic?)
• Venus basaltic
• In all cases, these crusts are distinct from
  likely bulk mantle compositions indicative of
  melting
• The basaltic compositions are all very similar,
  suggesting planetary mantles have similar
  compositions
• The crusts are also very poor in iron relative to
  bulk nebular composition where has all the iron
  gone?

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Impact Cratering

• Important topic, for several reasons
• Ubiquitous impacts occur everywhere
• Dating degree of cratering provides information
  on how old a surface is
• Style of impact crater provides clues to the
  nature of the subsurface and atmosphere
• Impacts produce planetary regolith
• Impacts can have catastrophic effects on planets
  (not to mention their inhabitants)
• What we will cover
• What are the physical effects of impacts?
• What can we infer about a planet from its
  cratering record?

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Why do impacts happen?

• Debris is left over from solar system formation
  (asteroids, comets, Kuiper Belt objects etc.)
• Object perturbed by something (e.g. Jupiter) into
  an orbit which crosses a planetary body
• As it gets closer, the object is accelerated
  towards the planet because of the planets
  gravitational attraction
• The minimum impact speed is the planets escape
  velocity, typically many km/s

The next big event for astronomers will be
Friday April 13th 2029. Scientists predict that
the asteroid Apophis (400m diameter) will be
coming only 32,000 kilometres from the Earth,
which is close enough to hit a weather satellite
and even be visible without a telescope.
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Gravity

• Newtons inverse square law for gravitation

Here F is the force acting in a straight line
joining masses m1 and m2 separated by a distance
r G is a constant (6.67x10-11 m3kg-1s-2)

• Hence we can obtain the acceleration g at the
  surface of a planet

• We can also obtain the gravitational potential U
  at the surface (i.e. the work done to get a unit
  mass from infinity to that point)

What does the negative sign mean?
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Escape velocity and impact energy
M

• Gravitational potential

r
R

• How much kinetic energy do we have to add to an
  object to move it from the surface of the planet
  to infinity?
• The velocity required is the escape velocity

• Equally, an object starting from rest at infinity
  will impact the planet at this escape velocity
• Earth vesc11 km/s. How big an asteroid would
  cause an explosion equal to that at Hiroshima?

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Crater Basics
Ejecta blanket
Depth

• Typical depthdiameter ratio is 15 for simple
  (bowl-shaped) craters

Mars, MOC image
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Crater Formation
1. Contact/compression

• Impactor is (mostly) destroyed on impact
• Initial impact velocity is (usually) greater than
  sound speed, creating shock waves
• Shock waves propagate outwards and downwards
• Heating and melting occur
• Shock waves lead to excavation of material
• Transient crater is spherical
• Crater later relaxes

2. Excavation
3. Modification
Note overturned strata at surface
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Timescales
v

• Contact and compression
• Time for shock-wave to pass across impactor
• Typically less than 1s

2r

• Excavation
• Free-fall time for ejected material
• Up to a few minutes

d

• Modification
• Initial faulting and slumping probably happens
  over a few hours
• Long-term shallowing and relaxation can take
  place over millions of years

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Crater Sizes

• A good rule of thumb is that an impactor will
  create a crater roughly 10 times the size
  (depends on velocity)
• We can come up with a rough argument based on
  energy for how big the transient crater should be

Does this make sense?
v
2r
2R

• E.g. on Earth an impactor of 0.1 (1) km radius
  and velocity of 10 km/s will make a crater of
  radius 2 (12) km
• For really small craters, the strength of the
  material which is being impacted becomes
  important

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Craters of different shapes

• Crater shapes change as size increases
• Small simple craters (bowl-shaped)
• Medium complex craters (central peak)
• Large impact basins
• Transition size varies with surface gravity and
  material properties

BASIN Hellas, Mars
SIMPLE Moltke, Moon, 7km
COMPLEX Euler, 28km, 2.5km deep
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Shape transitions
Schenk (2002)

• Europa, scale bar10km
• Note change in morphology as size increase

Lunar curve

• Depth/diameter ratio decreases as craters get
  larger
• Gravity on icy satellites similar to that on the
  Moon
• Transition occurs at smaller diameters than for
  Moon due to weaker target material? (ice vs.
  rock)

Ganymede
complex
simple
basins
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Unusual craters

• 1) Crater chains (catenae)
• 2) Splotches
• 3) Rampart Craters (Mars)
• 4) Oblique impacts

• Crater chains occur when a weak impactor (comet?)
  gets pulled apart by tides

Crater chain, Callisto, 340km long
Comet Shoemaker-Levy, ripped apart by Jupiters
tidal forces
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Rampart Craters (Mars)

• Probably caused by melting of subsurface ice
  leading to slurry ejecta
• Useful for mapping subsurface ice

Tooting crater (28 km diameter)
Tooting crater, 28km diameter
Stewart et al., Shock Compression Condens. Matt.
2004
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Airbursts

• Venus dark splotches

• Tunguska, Siberia 1908

• Result of (weak) impactor disintegrating in
  atmosphere

300km across, radar image

• Thick atmosphere of Venus means a lack of craters
  smaller than about 3 km (they break up in
  atmosphere)

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Oblique Impacts

• Impacts are most like explosions spherical
  shock wave leads to circular craters
• Not understood prior to the space age argument
  against impact craters on the Moon
• Only very oblique (gt75o?) impacts cause
  non-circular craters
• Non-circular craters are rare

impact
Mars, D12km Herrick, Mars crater consortium
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Atmospheric Effects

• Small impactors burn up in the atmosphere
• Venus, Earth, Titan lack small impact craters
• Venus thick atmosphere may produce other effects
  (e.g. outflows)

After McKinnon et al. 1997
Radar image of impact-related outflow feature
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How often do they happen? (Earth)
Hartmann
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How do we date surfaces (1)?
young
old
Saturation
Slope depends on impactor population
Effect of secondary craters?

• Crater densities a more heavily cratered
  surface is older
• The size-distribution of craters can tell us
  about the processes removing them
• Densities reach a maximum when each new crater
  destroys one old crater (saturation). Phobos
  surface is close to saturated.

Increasing age

• Lunar crater densities can be compared with
  measured surface ages from samples returned by
  Apollo missions

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How do we date surfaces (2)?

• It is easy to determine the relative ages of
  different surfaces (young vs. old)
• Determing the absolute ages means we need to know
  the cratering rate (impacts per year)

Number of craters gt1km diameter per km2

• We know the cratering rates on the Earth and
  the Moon, but we have to put in a correction
  (fudge factor) to convert it to other places
• So the uncertainties tend to be large,
  especially for intermediate-age surfaces

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New craters on Mars

• Important because we can use these observations
  to calibrate our age-crater density curves
• Existing curves look about right

Before
After
Malin et al. Science 2006
Probably mis-identified
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Evolving impactor population

• One complication is that the population of
  impactors has changed over time
• Early solar system had lots of debris gt high
  rate of impacts
• More recent impact flux has been lower, and size
  distribution of impactors may also have been
  different
• Did the impact flux decrease steadily, or was
  there an impact spike at 4 Gyr (Late Heavy
  Bombardment)?

Hartmann W are numerical simulation results,
boxes are data from Moon/Earth
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Crater Counts

• Crater size-frequency plots can be used to infer
  geological history of surfaces
• Example on left shows that intermediate-size
  craters show lower density than large craters
  (why?)

saturation
frequency
size

• Smallest craters are virtually absent (why?)
• Most geological processes (e.g. erosion,
  sedimentation) will remove smaller craters more
  rapidly than larger craters
• So surfaces tend to look younger at small scales
  rather than at large scales

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Complications

• Rate of impacts was certainly not constant, maybe
  not even monotonic (Late Heavy Bombardment?)
• Secondary craters can seriously complicate the
  cratering record
• Some surfaces may be buried and then exhumed,
  giving misleading dates (Mars)
• Subsurface impact basins (Mars)
• Very large uncertainties in absolute ages,
  especially in outer solar system

Pwyll crater, Europa (25 km diameter)
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Cratering record on different bodies

• Earth few craters (why?)
• Titan only 2 craters identified so far (why?)
• Mercury, Phobos, Callisto heavily cratered
  everywhere (close to saturation)
• Moon saturated highlands, heavily cratered
  maria
• Mars heavily cratered highlands, lightly
  cratered lowlands (plus buried basins) and
  volcanoes
• Venus uniform crater distribution, 0.5 Gyr
  surface age, no small craters (why?)
• Ganymede saturated dark terrain, cratered light
  terrain
• Europa lightly cratered (0.05 Gyr)
• Io no craters at all (why?)

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Where do impactors come from?

• In inner solar system, mostly asteroids, roughly
  10 comets (higher velocity, 50 km/s vs. 15
  km/s)
• Comets may have been important for delivering
  volatiles atmosphere to inner solar system
• In outer solar system, impactors exclusively
  comets
• Different reservoirs have different freq.
  distributions
• Comet reservoirs are Oort Cloud and Kuiper Belt
• Orbits are perturbed by interaction with planets
  (usually Jupiter)
• There may have been an impact spike in the
  inner solar system when the giant planets
  rearranged themselves (not quite as unlikely as
  it sounds)

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Summary

• Planetary crustal compositions may be determined
  by in situ measurements or remote sensing
  (spectroscopy)
• Most planetary crusts are basaltic
• Impact velocity will be (at least) escape
  velocity
• Impacts are energetic and make craters
• Crater size depends on impactor size, impact
  velocity, surface gravity
• Crater morphology changes with increasing size
• Crater size-frequency distribution can be used to
  date planetary surfaces
• Atmospheres and geological processes can affect
  size-frequency distributions

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Key concepts

• Spectroscopy (IR, gamma-ray)
• Regolith
• SNC meteorite
• Gravitational potential
• Escape velocity
• Simple vs. complex crater vs. impact basin
• Depthdiameter ratio
• Saturation
• Size-frequency distribution

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