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Earth, and Planetary Diversity

Dave Stevenson Caltech Comparave Tectonics & May 4 2015 Lissauer et al, 2014

• There are many rocky – Typically somewhat more massive than Earth (“superEarths”) • Many (perhaps most) of these have , almost certainly rich – Even those that do not may have started out with such an – Testament to rapid formaon (Earth & Venus formed slowly by comparison) • These are mostly not Earthlike! The atmosphere creates a basal T that can readily sustain a • But perhaps a few of them have Earthlike or Venus-like atmospheres.

What do all (most*) Planets have in Common *there will always be oddballs, for example pulsar planets, which were the first planets found outside our solar • Use the same periodic – But not necessarily the same radioacvity! • Use the same (& obey the same rules of chemistry) • Have a broadly common story? – Gravitaonal collapse; disk because of angular momentum – Accreon of solids followed (somemes ) by accreon of gas Guidelines for the Diversity of Planets

• STRONG CONSTRAINT: Dynamically stable (most of the sasfies hydrostac equilibrium); stable to “evaporaon” for ~109 yrs. • MODERATE CONSTRAINT: Consistent with cosmic abundances. • WEAK CONSTRAINT: Conceivable formaon scenario. (In pracce, theorists are very good at explaining any observaon, even when it’s wrong!) Why do Planets of Similar Mass & Composion have different Behaviors? • Distance from plays a role (but not necessarily dominant) • Environment of formaon plays a role – – dioxide, or… • Planets (unlike ) are made of thermodynamically complex materials – Complex melng behavior () – Effect of alloying (e.g., in ) – Effect of water on rock • Role of chance in formaon – Major aspect of Earth-Venus difference? • Main Proposions

• The planets in our exhibit great diversity, in part because of the role of minor constuents (e.g. water, sulfur) • The planets in our solar system are a small, unrepresentave sample of all planets. • The extrasolar planets found thus far are different but sll unrepresentave! • Most noons of the habitable zone are anthropocentric. The habitable zone may be almost everywhere in the if we include all places with sources & liquid water. (False?) Concept of a Habitable Zone Goldilocks scenario

• Locaon, locaon locaon! • maers

• Also depends on availability of H2O. Very abundant in the Universe… very under-abundant on Earth • Adopts a narrow perspecve • Magellan was launched 26 ago today. – Magellan died 21 years ago. – It was the last Venus mission designed to tell us something about the of Venus • Magellan was launched 26 years ago today. – Magellan died 21 years ago. – It was the last Venus mission designed to tell us something about the geophysics of Venus – But we know more about the atmosphere – And we have learned much more about Earth, including interesng ideas about the operang regime(s) for – But sll, we know lile about Venus Q. Why do we know so lile about Venus? A. Because it is a hard place to get to know.

Q. Why do we want to know about Venus? A. Because it is as important as Earth in understanding the formaon, evoluon and of terrestrial planets. Earth Venus

6400 km radius 6050 km radius

5.53 g/cc mean 5.24 g/cc

Basalc volcanism consistent with basalc ; volcanic constructs Large Fe-rich Core Large Fe Core inferred

~100 bars equivalent of CO2 100 bars of CO2 Earth Venus

Thin atmosphere& water Thick atmosphere and ocean much less water(?) Plate tectonics Stagnant lid (but previously mobile?) 24 hr 243 day rotaon (retrograde) Large No moon (requires explanaon?) Dynamo No intrinsic magnec field How to think about a ?

• Could discuss provenance- the properes of an apple depend on the environment in which the tree grows • Or could discuss it as a machine (cf. Hero[n], 1st century AD) • Need to do both How Should we Compare Earth & Venus? • Provenance – The materials that were accreted – The stochasc nature of that accreon… do Earth & Venus differ in part because of chance? • Operang Condions – Planets are engines whose evoluon depends on the materials, the inial condions, ,etc. In current terrestrial models, the material that goes into making Earth or Venus comes from many different … It is very unlikely that the Venus is substantially different from Earth but it is very likely that Venus started out somewhat different from Earth by chance Recent work on the Earth- Moon similarity (Mastrobuono-Basttisti et al, 2015) might suggest greater Results from Chambers, 2003 Earth-Venus similarity (but (Similar results from Morbidelli) not the same sequence of events) What do Planet Formaon Stories tell us about Venus-Earth differences? • It is not difficult to make models where Earth ~Venus in mass & composion • It is difficult to make models where Earth is different from Venus in bulk composion • It is difficult to make models where Venus receives much less water than Earth – But what do we make of the 36Ar puzzle? • Isotopic differences are diagnosc – isotopic difference, if any, would be of interest Where has All the Water Gone? Long me passing… • Delivery seems likely • Surface & crust are dry • “Runaway “ greenhouse likely but not assured – More correct to say that Earth “healed” and Venus did not? – 2D models needed? LeConte et al , 2013. • What about the interior? • Loss from the atmosphere – Water photolysis and hydrogen loss affected by presence of CO2 (e.g., Wordsworth &PierreHumbert, 2013) – Magnec field may maer? (Did early Venus have a dynamo?) Origin of the Atmosphere

• Presumably (as with Earth) part of primary accreon – “late veneer” is a minor component for Earth; of interest to establish its presence for Venus – also minor? • Ingassing of solar of interest (e.g., Ne ) • History of related to volcanism ; expressed in noble gases Stagnant lid vs. Plate Tectonics

• We do not know why Earth has plate tectonics – Water, melng, connents may all play a role • Stagnant lid is the “natural” form for convecon when you have a very high near surface – Venus has this despite the high surface temperature – Maybe this actually favored stagnant lid (Bercovici & Ricard, 2014) • Other things being equal, stagnant lid planets run hoer and have greater capability for magmasm – But pressure release melng dominates on Earth Did Venus undergo a Change in Convecve Style over Geologic ? • Catastrophic resurfacing at 500Ma to 1Ga ago? Compable with but not required by the surface age deduced from cratering record (e.g. McKinnon, Zahnle). • Convecon models are permissive (i.e., can be tuned); somemes show episodic behavior (e.g. ,Armann & Tackley, 2012) • May connect to evidence for a current thick (possible low heat flow) compared to earth – Most of Earth’s heat flow is secular cooling, not radiogenic at present (Urey number problem) Changes in Convective Style (existence or extent of plate tectonics) can have big effects on Thermal History T

No plates T does not have to decrease monotonically. This can also affect core cooling and the presence of plumes or extent of igneous With plates activity

t “With plates” means any convective style that involves recycling of the surface layer Plate Tectonics & the Role of Water (Earth Perspecve) • Water lubricates the • Water defines the plates • Maintenance of water in the mantle depends on subducon; this may not have persisted except on Earth • Plate tectonics helps the dynamo by promong core cooling Water Plate tectonics Magnetic field These all influence…

LIFE Neith - Venus’ Moon

• “Discovered” by Cassini in 1672 • “Confirmed” by Lagrange in 1761 • Named aer Egypan goddess • Consistent with philosophy of the me wherein other planets should have Earthly aributes The Moon that Wasn’t Helge Kragh (2008) What would happen if you placed Earth-Moon at Venus (4.5Ga ago)? • Moon stays well within Hill of Venus –escape does not seem likely in any unperturbed story • Moon+ do not despin Venus to orbit period=spin period in less than 4.5Ga (unless dal Q is even smaller than for Earth, which is very unlikely) – It must be conceded , however, that our ignorance about dal Q’s is very large! (In general, not just in this case!) • But if you increase moon mass by factor of 2 (or decrease inial angular momentum budget by factor of 2) then moon would end up absorbed by Venus.. Incompable with observed low spin; might also leave a signature on surface Why Venus has no Moon?

• Never had one (certainly a possibility) – BUT why then does Venus have such low spin? – Standard story for this is quesonable • Had one and lost it early on – Escaped & subsequently crashed – Spiraled in & crashed • WORKING HYPOTHESIS: – Giant impacts that make are common – Earth-Moon differs from Venus-Neith in that it was the last major event in the terrestrial zone. This is suggested by isotopic evidence is the most striking consequence of giant impacts, but it is singular only in being the last event of its kind. (Earlier moons could have existed)

Sets the inial condion for Earth evoluon A Story to Consider

• Giant impact led to Neith. This impact occurred earlier than the Earth-Moon event (perhaps only ~10 Ma aer solar system formaon) • Moon evolves out quickly to 10 or 20 Venus radii. • This reduces the spin of Venus • Large impactors (~ mass) sll present; one will eventually impact Venus (without making another Moon?) • But close encounters are much more common than impacts… what are the consequences? Can further reduce the spin or eject the moon. Rotaon of Venus

• Slow rotaon more significant than retrograde rotaon • Balance of solar solid body and thermal des • Plausible but not firmly established; would in any event imply small changes with me that we should seek to measure • Length of Day (LOD) variaons can be diagnosc of atmospheric-interior coupling • (TPW) should be fast… geodecally detectable? State of the Core: Why no Dynamo?

• Liquid because the minimal freezing point is below plausible mantle operang • Slow rotaon is NOT slow for core fluid moons of interest (i.e. Ro =v/ ΩL <<1). Might even favor the dynamo! • Core convecon must be absent – Core unable to cool efficiently? – Core is stably strafied? – No inner core? Things We Don’t know (I wish we did!)

• Seismic acvity & structure Essenal for understanding interior . • Isotopes (oxygen, Hf/W) Crucial or placing Venus in context. Not just for chronology! • Rotaon & Essenal dynamical constraints. Can be done partly from earth. • Atmospheric origin and evoluon • True Water content Mantle water not known. • Magnesm Even small effects (paleo or present) would be of great interest. – Electromagnec inducon • Heat flow Difficult. (But indirect constraints exist). • Igneous Acvity,Volcanism • & beyond Magellan Earth-Venus Summary

• A lot may be due to chance – Both in our solar system and for analogs • Absence of a dynamo & absence of a moon may both arise because because of a different sequence & ming of giant impacts • In other planetary ,you could have a Venus-like planet outboard of an Earthlike planet – Runaway greenhouse is not the only possible determining factor