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Giant Impacts on early Mars and the cessation of the Martian Dynamo

Giant Impacts on early Mars and the cessation of the Martian Dynamo. James Roberts 1 , Rob Lillis 2 , Michael Manga 2 1 Johns Hopkins University Applied Physics Laboratory 2 UC Berkeley CIDER 19 May 2009. From CIDER 2008. 20 exposed and buried basins > 1000 km diameter Frey, 2008, GRL

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Giant Impacts on early Mars and the cessation of the Martian Dynamo

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  1. Giant Impacts on early Mars and the cessation of the Martian Dynamo James Roberts1, Rob Lillis2, Michael Manga2 1Johns Hopkins University Applied Physics Laboratory 2UC Berkeley CIDER 19 May 2009

  2. From CIDER 2008

  3. 20 exposed and buried basins > 1000 km diameter Frey, 2008, GRL Crater retention age from counting smaller basins on rim and interior (both QCDs and non-QCD CTAs > 300 km) Spike in Basins (LHB?) Highland Basins Lowland Basins Tharsis Basins Ages of impact basins Visible basins are the youngest

  4. Basin Magnetization Lillis et al., 2008, GRL

  5. Impact generates shock pressure [Melosh, 1989] Isobaric core centered on impact hypocenter, scales with impactor size [Watters et al., JGR, 2008] Pressure decays with distance outside Isobaric core Temperature increased based on shock pressure. Impact Heating Core Mantle

  6. Temperature Profile through Utopia

  7. Spikes in surface heat flow associated with impacts Only the larger impacts seen at CMB. Largest impacts (e.g. Utopia) can result in 10% drop in heat flow Mantle heating effect is greater than increased convective vigor If dynamo is subcritical, a single negative spike in CMB heat flow can kill it A supercritical dynamo may recover

  8. Heat Flow Anomaly

  9. Conclusions • Impact heating mostly at shallow depths • Large impacts (D> 2500 km) can heat the core • Decrease in CMB heat flow (~10%) seen in our models may be sufficient to end the dynamo • Dynamo must be subcritical at time of impact or magnetic field will recover in ~20 My • One giant impact may be sufficient to cut off CMB cooling. • Multiple impacts may build up heat flow anomaly • May be more older, undiscovered giant basins • Shift in climate from wet to dry, acidic conditions. • Mineralogy observed with OMEGA consistent with the Phyllosian-Theiikian transition [Bibring et al., 2006]. • Heat flow drop depends strongly on crater scaling • Need more experiments!

  10. CIDER and Planets • How much do we want to expand into the solar system? • Cooperative Institute for Deep EarthResearch and Planetary REsearch in the Solar System • Comparative Planetology • Processes are universal (impacts, tectonics, volcanism) • Outcome is not universal • No other examples of plate tectonics • Don’t always get a dynamo • Origin of the Moon • Moon is a proxy for the early Earth • Earth and Moon are chemically distinct, but isotopically identical • Mixing? • Single source? • Challenges • No seismology (apart from the Moon) • No samples we can place in context • Hard to do fieldwork

  11. Your Ad Here

  12. If the movie doesn’t work

  13. Collaborators at UC Berkeley Rob Lillis Michael Manga

  14. Giant Impacts on Mars MOLA Topography, Image: NASA/GSFC

  15. Highland Basins Lowland Basins Tharsis Basins Ages of Impact Basins From counting smaller basins on rim and interior (both QCDs and non-QCD CTAs > 300 km)

  16. Magnetism on Mars • Mars currently has no global magnetic field • Strong evidence for such a field in the past • Widespread crustal magnetism [e.g. Acuña et al., 2001]

  17. The Martian Dynamo • Magnetic field generated by convection in the conducting fluid core. • Heat flow at the CMB controls core convection • Need > ~0.5 TW to sustain convection [Nimmo & Stevenson, 2000] • Unfavorable heat flow  Dynamo activity stops • In subcritical dynamo simulations, 1% drop in heat flow can kill global magnetic field [Kuang et al., 2008, GRL] • Need ~25% increase in heat flow to recover dynamo. • Geophysically difficult • How do giant impacts affect core-mantle heat flow?

  18. Coincidence or Causality? • Dynamo is thought to have shut down towards the end of a series of large impacts. • [Lillis et al., 2008, GRL] • Younger basins are demagnetized, formed after the dynamo stopped • Utopia is the largest basin and is the oldest that is clearly demagnetized. • Could the giant impacts be responsible for the death of the Martian dynamo?

  19. Impacts dump a lot of heat into the interior Mantle temperature rises Temperature gradient at CMB drops Core HF is reduced Dynamo shuts down Impacts heat regions of the interior Lateral temperature variations  Buoyancy More vigorous convection Core HF is enhanced Dynamo survives Two Hypotheses Two competing effects: Mantle heating vs. Convective vigor Which one is more important?

  20. Mantle Convection Modeling • CitcomS • 3D spherical FE mantle convection code • [Zhong et al., JGR, 2000] • Temperature- and pressure-dependent viscosity • 1.7 million elements in numerical grid • Isothermal and Free-slip Boundary Conditions • Random initial perturbation to temperature field • Internally heated by radioactive decay [Wanke and Drebius, 1994] and potentially by impacts • Simulated effects of 20 giant impacts in convection models. • Also ran control cases with no impact heating

  21. Utopia Acidalia Amazonis Isidis Ares Daedalia Basins not really to scale! Hellas Argyre Hellas is not all that impressive!

  22. Impact Ages • Timing of the impacts may be important • If they all happen at once, all the heat is dumped in at once, may overheat the mantle • More gradual time spacing of impacts gives the heat a chance to dissipate • N(300) Crater Age  Absolute Model Age

  23. Subcritical Dynamo • Heat flow too low to initiate a dynamo. • An existing magnetic field generates a Lorenz force. • Maintains dynamo below the critical point • <2% drop in heat flow at this stage can kill magnetic field • Drop in Rth from 2460 to 2420 • Need ~25% increase to recover it. Kuang et al., GRL, 2008

  24. Temperature profile after Utopia impact • Temperature scaling is nonlinear • Distance from heated region to CMB is more important than size of heated region. Holsapple (1993) Dtr Df1.08 McKinnon & Schenk (1985) Dtr Df1.13 No Scaling Dtr= Df

  25. If the transient and final basins are the same, the largest impacts can actually reverse core heat flow! No impacts Holsapple McK & Sch No scaling

  26. Crater Scaling Laws • Heating is based on impactor size • We observe impact basin diameters • Transient crater size scales with impactor • Final basin size scales with transient crater • Quite some uncertainty in scaling

  27. Climate Change • Shift in climate from wet to dry, acidic conditions. • Mineralogy observed with OMEGA consistent with the Phyllosian-Theiikian transition [Bibring et al., 2006]. • Climate shift (> 3.92 Gya) probably occurred AFTER dynamo stopped (4.12 Gya) • Loss of magnetic field exposes atmosphere to erosion by solar wind • Early solar wind much stronger than today

  28. NP Ac Ut Az IA Cr Th Is Sc Am SE Ar Ze So Da SW He Ag Si

  29. Isosurface (T = 2000 K) Ra = 4.1×107 E = 157 kJ mol-1H = 8.6×10-8 W m-3 V = 8 cm3 mol-1

  30. Sequence of impacts Based on absolute model ages for 20 giant impact basins [Frey, 2008, GRL] • Either exposed or buried • Identified from QCDs and CTAs “Borealis” basin not considered here

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