Download Earth, Venus and Planetary Diversity

Earth, Venus and Planetary Diversity Dave Stevenson Caltech Compara2ve Tectonics & Geodynamics May 4 2015 Lissauer et al, 2014 •  There are many rocky exoplanets –  Typically somewhat more massive than Earth (“superEarths”) •  Many (perhaps most) of these have atmospheres, almost certainly hydrogen rich –  Even those that do not may have started out with such an atmosphere –  Testament to rapid forma2on (Earth & Venus formed slowly by comparison) •  These planets are mostly not Earthlike! The atmosphere creates a basal T that can readily sustain a magma ocean •  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 system
•  Use the same periodic table –  But not necessarily the same radioac2vity! •  Use the same physics (& obey the same rules of chemistry) •  Have a broadly common genesis story? –  Gravita2onal collapse; disk because of angular momentum –  Accre2on of solids followed (some2mes ) by accre2on of gas Guidelines for the Diversity of Planets •  STRONG CONSTRAINT: Dynamically stable (most of the mass sa2sfies hydrosta2c equilibrium); stable to “evapora2on” for ~109 yrs. •  MODERATE CONSTRAINT: Consistent with cosmic abundances. •  WEAK CONSTRAINT: Conceivable forma2on scenario. (In prac2ce, theorists are very good at explaining any observa2on, even when it’s wrong!) Why do Planets of Similar Mass & Composi;on have different Behaviors? •  Distance from star plays a role (but not necessarily dominant) •  Environment of forma2on plays a role –  Water –  Carbon dioxide, or… •  Planets (unlike stars) are made of thermodynamically complex materials –  Complex mel2ng behavior (volcanism) –  Effect of alloying (e.g., sulfur in iron) –  Effect of water on rock rheology •  Role of chance in forma2on –  Major aspect of Earth-­‐Venus difference? •  Life Main Proposi;ons •  The planets in our solar system exhibit great diversity, in part because of the role of minor cons2tuents (e.g. water, sulfur) •  The planets in our solar system are a small, unrepresenta2ve sample of all planets. •  The extrasolar planets found thus far are different but s2ll unrepresenta2ve! •  Most no2ons of the habitable zone are anthropocentric. The habitable zone may be almost everywhere in the Universe if we include all places with energy sources & liquid water. (False?) Concept of a Habitable Zone Goldilocks scenario •  Loca2on, loca2on loca2on! •  Size mafers •  Also depends on availability of H2O. Very abundant in the Universe… very under-­‐abundant on Earth •  Adopts a narrow perspec2ve •  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 •  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 interes2ng ideas about the opera2ng regime(s) for plate tectonics –  But s5ll, we know li>le about Venus Q. Why do we know so lifle 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 forma2on, evolu2on and nature of terrestrial planets. Earth Venus 6400 km radius 6050 km radius 5.53 g/cc mean density 5.24 g/cc Basal2c volcanism Large Fe-­‐rich Core Surface consistent with basal2c crust; volcanic constructs Large Fe Core inferred ~100 bars equivalent of CO2 100 bars of CO2 Earth Venus Thin atmosphere& water ocean Thick atmosphere and much less water(?) Plate tectonics Stagnant lid (but previously mobile?) 24 hr day 243 day rota2on period (retrograde) Large Moon No moon (requires explana2on?) Dynamo No intrinsic magne2c field How to think about a Planet? •  Could discuss provenance-­‐ the proper2es 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 stochas2c nature of that accre2on… do Earth & Venus differ in part because of chance? •  Opera2ng Condi2ons –  Planets are heat engines whose evolu2on depends on the materials, the ini2al condi2ons, temperature ,etc. In current terrestrial accretion
models, the material that goes
into making Earth or Venus
comes from many different
regions…
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 EarthMoon similarity
(Mastrobuono-Basttisti et al,
2015) might suggest greater
Earth-Venus similarity (but
not the same sequence of
events)
Results from Chambers, 2003
(Similar results from Morbidelli)
What do Planet Forma2on Stories tell us about Venus-­‐Earth differences? •  It is not difficult to make models where Earth ~Venus in mass & composi2on •  It is difficult to make models where Earth is different from Venus in bulk composi2on •  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 diagnos2c –  Oxygen isotopic difference, if any, would be of interest Where has All the Water Gone? Long 0me 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) –  Magne2c field may mafer? (Did early Venus have a dynamo?) Origin of the Atmosphere •  Presumably (as with Earth) part of primary accre2on –  “late veneer” is a minor component for Earth; of interest to establish its presence for Venus –  Comets also minor? •  Ingassing of solar nebula of interest (e.g., Ne isotopes) •  History of outgassing related to volcanism ; expressed in noble gases Stagnant lid vs. Plate Tectonics •  We do not know why Earth has plate tectonics –  Water, mel2ng, con2nents may all play a role •  Stagnant lid is the “natural” form for convec2on when you have a very high viscosity near surface region –  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 hofer and have greater capability for magma2sm –  But pressure release mel2ng dominates on Earth Did Venus undergo a Change in Convec2ve Style over Geologic Time? •  Catastrophic resurfacing at 500Ma to 1Ga ago? Compa2ble with but not required by the surface age deduced from cratering record (e.g. McKinnon, Zahnle). •  Convec2on models are permissive (i.e., can be tuned); some2mes show episodic behavior (e.g. ,Armann & Tackley, 2012) •  May connect to evidence for a current thick lithosphere (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
With plates
t
“With plates” means
any convective style
that involves recycling
of the surface layer
Mantle T does not have to decrease
monotonically. This can also affect
core cooling and the presence of
plumes or extent of igneous
activity
Plate Tectonics & the Role of Water (Earth Perspec2ve) •  Water lubricates the asthenosphere •  Water defines the plates •  Maintenance of water in the mantle depends on subduc2on; this may not have persisted except on Earth •  Plate tectonics helps the dynamo by promo2ng core cooling Water
Plate tectonics
These all influence…
LIFE
Magnetic
field
Neith -­‐ Venus’ Moon •  “Discovered” by Cassini in 1672 •  “Confirmed” by Lagrange in 1761 •  Named ater Egyp2an goddess •  Consistent with philosophy of the 2me wherein other planets should have Earthly afributes The Moon that Wasn’t Helge Kragh (2008) What would happen if you placed Earth-­‐Moon at Venus orbit (4.5Ga ago)? •  Moon stays well within Hill sphere of Venus –escape does not seem likely in any unperturbed story •  Moon+Sun do not despin Venus to orbit period=spin period in less than 4.5Ga (unless 2dal Q is even smaller than for Earth, which is very unlikely) –  It must be conceded , however, that our ignorance about 2dal Q’s is very large! (In general, not just in this case!) •  But if you increase moon mass by factor of 2 (or decrease ini2al angular momentum budget by factor of 2) then moon would end up absorbed by Venus.. Incompa2ble 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 ques2onable •  Had one and lost it early on –  Escaped & subsequently crashed –  Spiraled in & crashed •  WORKING HYPOTHESIS: –  Giant impacts that make moons 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 Origin of the moon 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 ini2al condi2on for Earth evolu2on A Story to Consider •  Giant impact led to Neith. This impact occurred earlier than the Earth-­‐Moon event (perhaps only ~10 Ma ater solar system forma2on) •  Moon evolves out quickly to 10 or 20 Venus radii. •  This reduces the spin of Venus •  Large impactors (~Mars mass) s2ll 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. Rota2on State of Venus •  Slow rota2on more significant than retrograde rota2on •  Balance of solar solid body and thermal 2des •  Plausible but not firmly established; would in any event imply small changes with 2me that we should seek to measure •  Length of Day (LOD) varia2ons can be diagnos2c of atmospheric-­‐interior coupling •  True Polar Wander (TPW) should be fast… geode2cally detectable? State of the Core: Why no Dynamo? •  Liquid because the minimal freezing point is below plausible mantle opera2ng temperatures •  Slow rota2on is NOT slow for core fluid mo2ons of interest (i.e. Ro =v/ ΩL <<1). Might even favor the dynamo! •  Core convec2on must be absent –  Core unable to cool efficiently? –  Core is stably stra2fied? –  No inner core? Things We Don’t know (I wish we did!) •  Seismic ac;vity & structure Essen2al for understanding interior . •  Isotopes (oxygen, Hf/W) Crucial or placing Venus in context. Not just for chronology! •  Rota;on & Geodesy Essen2al dynamical constraints. Can be done partly from earth. •  Atmospheric origin and evolu;on •  True Water content Mantle water not known. •  Magne;sm Even small effects (paleo or present) would be of great interest. –  Electromagne2c induc2on •  Heat flow Difficult. (But indirect constraints exist). •  Igneous Ac;vity,Volcanism •  Gravity & Topography beyond Magellan Earth-­‐Venus Summary •  A lot may be due to chance –  Both in our solar system and for exoplanet analogs •  Absence of a dynamo & absence of a moon may both arise because because of a different sequence & 2ming of giant impacts •  In other planetary systems ,you could have a Venus-­‐like planet outboard of an Earthlike planet –  Runaway greenhouse is not the only possible determining factor