EART160 Planetary Sciences

1. EART160 Planetary Sciences Francis Nimmo

2. 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 gas:ice:rock in ratio 100:1:0.1 (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

3. This & Next Week – Surfaces • What are solid planet surfaces made of? • What processes modify the surfaces? • Impact craters • Volcanism • Tectonics • Erosion & Sedimentation

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

5. 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?

6. SNC meteorites • Shergotty, Nakhla, Chassigny (plus others) • What are they? • Mafic rocks, often cumulates • How do we know they’re 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

7. 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)

8. APXS 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

9. Remote Sensing • Restricted to surface (mm-mm). Various kinds: • Spectral (usually infra-red) reflectance/absorption – 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

10. 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.)

11. 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?

12. 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?

13. 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 planet’s gravitational attraction • The minimum impact speed is the planet’s 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.”

14. r F F m2 m1 R M a Gravity • Newton’s inverse square law for gravitation: • Hence we can obtain the acceleration g at the surface of a planet: Here F is the force acting in a straight line joining masses m1 and m2separated by a distance r; G is a constant (6.67x10-11 m3kg-1s-2) • 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?

15. a 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 vesc=11 km/s. How big an asteroid would cause an explosion equal to that at Hiroshima?

16. Crater Basics Ejecta blanket • Typical depth:diameter ratio is ~1:5 for simple (bowl-shaped) craters Depth Mars, MOC image

17. 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

18. Timescales v 2r • Contact and compression • Time for shock-wave to pass across impactor • Typically less than 1s • 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

19. a 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

20. 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

21. Shape transitions Schenk (2002) Europa, scale bar=10km 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

22. 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 Jupiter’s tidal forces

23. 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

24. “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)

25. 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 (>75o?) impacts cause non-circular craters • Non-circular craters are rare impact Mars, D=12km Herrick, Mars crater consortium

26. 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

27. How often do they happen? (Earth) Hartmann

28. How do we date surfaces (1)? young old Saturation • 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. Slope depends on impactor population Effect of secondary craters? Increasing age • Lunar crater densities can be compared with measured surface ages from samples returned by Apollo missions

29. 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 >1km 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

30. New craters on Mars Before • Important because we can use these observations to calibrate our age-crater density curves • Existing curves look about right After Malin et al. Science 2006 Probably mis-identified

31. Evolving impactor population • One complication is that the population of impactors has changed over time • Early solar system had lots of debris => 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

32. Crater Counts saturation • 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?) 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

33. 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)

34. 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?)

35. 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)

36. 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

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

38. halo Ejecta vi vi Wind vw Wind 45o 45o d

39. A frequency B saturation size