1. 1 THE NOACHIAN The Early Years Mars Pre-Noachian and Noachian ESP_030184_1585 Banded bedrock near Hellas basin

2. 2 Formation of the Solar System http://www.youtube.com/watch?v=Uhy1fucSRQI Three stages of planet formation Planetesimals Larger than 1 km diameter Form from aggregated dust grains Planetary embryos Gas drag causes circular orbits and a disk shape Embryos become large enough to have appreciable gravitation Begin accreting more planetesimals Mars didn’t grow a large as Earth and Venus due to gravitational effect of Jupiter Late-stage impacts Rapidly increase size and mass Or erode the young planet like the Earth-Moon system Depends on impact parameters Composition inside solar nebula Temperature and Pressure control the volatiles (like water) Refractory materials (resist vaporization) near the Sun Water ice condenses near Jupiter’s orbit (5 AU) Hydrated minerals are stable near Earth-Mars orbits

3. 3 Differentiation Early heavy bombardment High impact rates Surface solidifies when this declines Planets form hot and gradually cool Heat comes from 2 sources Accretionary heating from the kinetic energy of impacts Radioactive element decay, Aluminum 26 a major player Differentiation, core formation Semi-fluid state of planet Density variations cause Fe to sink to form a core, and lighter elements to form the crust Chemical affinities attract other elements Siderophile elements move to core (Ni, Co, S, Pt,…) Lithophile elements follow oxygen and come to surface (K, Na, Ca, Mg, Al,…) Mantle is ultramafic: olivine (really garnet) (Mg,Fe) 2 SiO 4

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5. 5 Noachian began with the Hellas impact Late Noachian 3.82 - 3.93 Ga Middle Noachian 3.82 - 3.93 Ga Early Noachian 3.93 - 4.08 Ga Ga = Gyr = Gy =10 9 yrs Nimmo and Tanaka, 2005 Mars

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7. 7 Scaling from the Moon to Mars In terms of the number of impactors: Moon >1700 craters w/ ≥20 km diameters during the Nectarian (Wilhelms, 1984, 1987) Nectarian (moon) = Hadean (Earth) = Noachian + EH (Mars) Mars Using the scaling ratio of Ivanov (2000), >6,500 similarly-sized craters are implied for the same period

8. 8 Where are the largest impact basins on Mars?

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10. 10 Giant Impact History Revised radiometric dates for ALH84001 4.1 rather than 4.5 Gyr

11. 11 Martian core Previous core-dynamo driven by core solidification Interaction with solar tides shows current core is not entirely solid – radius 1500-1800km Additional modeling of Fe-Ni-S materials at high pressure indicate core may still be completely liquid In other words—we don’t know much! InSight should help Internal Structure

12. 12 Mars accretion was fast Oldest solar system solids, CAI’s in chondrites, have ages 4.567Ga 182 Hf to 182 W system times the core formation (half life 9Myr) Mars differentiation sequesters all the W in the core Martian meteorites have 182 W levels > chondrites i.e. this tungsten was produced after core formation Implies core formation in 13±2 Myr Crystallization age of ALH84001 4.1 Ga (Lapen et al. 2010, Science 328) Shock heating event at 3.9Ga Kleine et al., 2002

13. 13 Northern and southern hemispheres of Mars are very distinct: North Low elevation Few Craters – Young surface layer Smooth terrain (km scale) Thin Crust No Magnetized rock South High elevation Heavily cratered – Old Rough terrain (km scale) Thick crust Magnetized rock Dichotomy boundary mostly follows a great circle, but is interrupted by Tharsis No gravity signal associated with the dichotomy boundary - compensated Theories on how to form a dichotomy: Giant impact (Earth/Moon idea) Several large basins Degree 1 convection cell Early plate tectonics Crustal Dichotomy Zuber et al., 2000

14. 14 Recent attempt to explain the crustal dichotomy Andrews-Hanna et al. (2008) a) Topography and original boundary by Wilhelms and Squyres (1984) b) Crustal thickness of Mars c) Removal of Tharsis using a model and a new boundary showing an elliptical boundary Large moderately oblique impacts should produce elliptical basins (for smaller craters only highy oblique impacts make elliptical craters)

15. 15 Composition: northern and southern hemispheres both basaltic TES team reported northern plains with spectral signature of andesite Support from Mars pathfinder mission – elemental composition This is hard to understand when there’s no plate tectonics Reanalysis suggests that this ‘andesite’ could be chemically altered basalt Jeff Taylor, PSRD

16. 16 New Martian Meteorite: Northwest Africa (NWA 7034) Meteorite NWA 7034 is a breccia, with minerals and rock fragments set in a fine-grained glassy matrix.

17. 17 NWA 7034 Older Than Most Martian Meteorites Martian meteorite ages ALH 84001: 4100 Ma NWA 7034: 2089 Ma Nakhlites & Chassigny: 1300 Ma Shergottites: 170-575 Ma Martian meteorite ages ALH 84001: 4100 Ma NWA 7034: 2089 Ma Nakhlites & Chassigny: 1300 Ma Shergottites: 170-575 Ma

18. 18 New Martian Meteorite is Similar to Typical Martian Crust TAS diagram for classification of igneous rocks. NWA 7034 falls with rocks and soils from the surface (red dots, from Spirit rover) and mean surface measured from orbit (GRS) Martian meteorites (SNC) are depleted in alkalis. McSween, H. Y., Jr., Taylor, G. J., and Wyatt, M. B. (2009) Elemental Composition of the Martian Crust, Science, v. 324, p. 736-749, doi: 10.1126/science.1165871

19. 19 Mars Crust: Made of Basalt Compositions from Mars meteorites, rovers, and orbiters reveal that Mars is dominated by basaltic rock TES data indicate that most of the surface has been weathered

20. 20 The N-S age difference is only skin deep Buried impact basins in the northern hemisphere have been mapped Before this burial the northern and southern hemispheres were indistinguishable in age Rules out Earth-style plate tectonics unless extremely early Northern hemisphere is a thinly covered version of the southern hemisphere Mantled by 1-2 km of material (sediments and volcanic flows) Frey et al., 2002 Back to the Dichotomy

21. 21 Mars currently has no dipole field Areas of magnetized crust have been discovered by MGS – dipole existed once Vigorous core convection driven by 40 K decay? Alternating strips suggestive of seafloor spreading on Earth? Origin of the martian magnetic stripes is an unsolved riddle! Magnetic Fields

22. 22 Seafloor Spreading on Earth Produces magnetic stripes as lava cools through the Curie point and magnetic poles flip

23. 23 Lack of magnetic field over Hellas and Argyre basins attributed to shock demagnetization Pyrrhotite (iron sulfide) likely carrier of remnant magnetism Demagnetized at shock > 2GPa Lack of remagnetization indicates dynamo had shut down Hellas = beginning of the Noachian Key Result: Mars dynamo shut down very early Hypothesis: loss of shielding from solar wind led to atmospheric loss and climate change Hood et al., 2003

24. 24 Tharsis begins forming Initial mantle formation led to unstable density structure Remains active throughout Martian history Long-lived mantle plumes hard to understand Volcanism may have outgassed a substantial early atmosphere Location on dichotomy boundary is a puzzle Flows as recent as a few 10 Myr Volcanic rock sequences 10km thick can be seen in the walls of Valles Marineris Pole-to-pole slope and Tharsis bulge control the planet’s shape Mass of Tharsis likely caused some polar wander The Giant Tharsis Bulge

25. 25 Valleys with dendritic patterns Low-order tributaries Alcove heads Indicates erosion by groundwater sapping not precipitation Or not… some cases look like surface runnoff Valley networks exist on the oldest terrain of Mars Erosion rates in the Noachian time were enormous (or Earth-like) compared to modern Mars Valley orientations indicate they formed after the bulk of Tharsis was in place. Formation of Tharsis rise therefore occurred very early Valley Networks Phillips et al., 2001

26. 26 Primary atmosphere Mostly hydrogen Xe isotopes indicate massive loss Secondary atmosphere Outgassed from interior Delivered by impacts Problem to get warm Noachian temperatures Faint young sun – surface temp. 190-200 K Large greenhouse effect required CO 2 and H 2 O is an option But you need very big atmospheres (a few bars) May be able to reduce this with clouds Reducing species NH 3, CH 4 etc… are very effective Thought to be rare because of massive H loss Mantle expected to be oxidized Noachian climate Carbonates? Big CO 2 atmospheres produce lots of liquid water… Quickly forms carbonic acids and combines with Ca in rocks Locks up C in carbonates (CaCO 3 ) – no plate recycling on Mars People have spent a long long time looking for these carbonates Alternate Model: Formation of Valley Networks by large impacts (Segura et al. papers) The mass deposited (and volatiles released) by impacts is large, and comparable with the mass from the Tharsis volcanic construct. Steam atmospheres formed after large impacts can produce more than 600 m of rainfall, followed by rainfall from water-vapor greenhouse atmospheres, and snowmelt. Mars never had a “stable” warm wet climate

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28. 28 Summary of Mars geologic history (Ehlmann et al. 2011)

29. 29 Can Minerals be Used like Fossils? l 18 th Century geologists thought minerals could be used to date terrestrial strata l This was disproven l Fossils do date strata—extinction is forever l OMEGA mineralogical theory n Clay formation ceases in Noachian n Transition to acidic environment from sulfates wAlso requires evaporation n Young terrains show no aqueous alteration n Problems with this theory n Alteration can occur anytime after the rock formed, so alteration of Noachian rocks not necessarily confined to Noachian age n There are clays of all ages in Martian meteorites n There has certainly been subsurface water since Noachian n Hesperian and early Amazonian outflow channels, alluvial fans Bibring et al., 2006

30. 30 Mars forms Accretion and core formation in about 13 Ma Crust forms from magma ocean ALH84001 crystallizes at ~4.1 Ga (not ~4.5 Ga as originally believed) Crust develops asymmetry Perhaps due to degree-1 mantle convection or large elliptical basin Core-Dynamo switches off Magnetic remnants frozen in to crustal rocks Atmospheric loss Major impact basins form Both hemispheres are heavily cratered Remnant magnetism erased over large basins Tharsis rise is constructed Vigorous volcanism outgasses significant atmosphere Polar wander Valley networks form Orientation controlled by pole-to-pole slope and Tharsis bulge Erosion rates orders of magnitude higher than Hesperian or Amazonian epochs Strong greenhouse needed to offset faint young sun Lack of carbonates from long-lived greenhouse atmosphere Summary