Transcript Mars History

Mars Pre-Noachian and Noachian
The Early Years
THE NOACHIAN
ESP_030184_1585 Banded bedrock near Hellas basin
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Formation of the Solar System
• http://www.youtube.com/watch?v=Uhy1fucSRQI
• Three stages of planet formation
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Planetesimals
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Planetary embryos
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• Larger than 1 km diameter
• Form from aggregated dust grains
• 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
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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
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Differentiation
• Early heavy bombardment
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High impact rates
Surface solidifies when this declines
• Planets form hot and gradually cool
• Heat comes from 2 sources
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Accretionary heating from the kinetic energy of impacts
Radioactive element decay, Aluminum 26 a major player
• Differentiation, core formation
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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,
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Al,…)
Mantle is ultramafic: olivine (really garnet) (Mg,Fe)2SiO4
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Mars
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Noachian began with
the Hellas impact
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Late Noachian
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3.82 - 3.93 Ga
Middle Noachian
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3.82 - 3.93 Ga
Early Noachian
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3.93 - 4.08 Ga
Ga = Gyr = Gy =109 yrs
Nimmo and Tanaka, 2005
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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
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Where are
the largest
impact
basins on
Mars?
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Giant Impact History
Revised radiometric
dates for ALH84001 4.1
rather than 4.5 Gyr
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Internal Structure
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!
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InSight should help
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• Mars accretion was fast
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Oldest solar system solids,
CAI’s in chondrites, have
ages 4.567Ga
182Hf to 182W system times
the core formation (half life
9Myr)
Mars differentiation
sequesters all the W in the
core
Martian meteorites have 182W
levels > chondrites
Kleine et al., 2002
• i.e. this tungsten was
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produced after core formation
Implies core formation in
13±2 Myr
• Crystallization age of
ALH84001
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4.1 Ga (Lapen et al. 2010, Science
328)
Shock heating event at 3.9Ga
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Crustal Dichotomy
Northern and southern hemispheres of Mars are
very distinct:
•North
•South
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Low elevation
Few Craters – Young surface layer
Smooth terrain (km scale)
Thin Crust
No Magnetized rock
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High elevation
Heavily cratered – Old
Rough terrain (km scale)
Thick crust
Magnetized rock
Zuber et al., 2000
•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:
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Giant impact (Earth/Moon idea)
Several large basins
Degree 1 convection cell
Early plate tectonics
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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)
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Composition: northern and southern hemispheres both basaltic
•TES team reported northern plains with spectral signature of andesite
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Support from Mars pathfinder mission – elemental composition
•This is hard to understand when there’s no plate tectonics
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Reanalysis suggests that this ‘andesite’ could be chemically altered basalt
Jeff Taylor, PSRD
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New Martian Meteorite:
Northwest Africa (NWA 7034)
Courtesy Carl Agee
Meteorite NWA 7034 is a breccia, with minerals
and rock fragments set in a fine-grained glassy
matrix.
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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
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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
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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
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Back to the Dichotomy
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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
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Rules out Earth-style plate tectonics unless extremely early
Northern hemisphere is a thinly covered version of the
southern hemisphere
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Mantled by 1-2 km of material (sediments and volcanic flows)
Frey et al., 2002
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Magnetic Fields
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Mars currently has no dipole field
Areas of magnetized crust have been discovered by MGS – dipole existed once
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Vigorous core convection driven by 40K decay?
Alternating strips suggestive of seafloor spreading on Earth?
Origin of the martian magnetic stripes is an unsolved riddle!
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Seafloor Spreading on Earth
Produces magnetic stripes as lava cools through the Curie point and magnetic
poles flip
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• Lack of magnetic field over
Hellas and Argyre basins
attributed to shock
demagnetization
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Pyrrhotite (iron sulfide) likely
carrier of remnant magnetism
Demagnetized at shock > 2GPa
• Lack of remagnetization
indicates dynamo had shut
down
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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
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The Giant Tharsis Bulge
Tharsis begins forming
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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
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Valley Networks
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Valleys with dendritic patterns
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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
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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
Phillips et al., 2001 25
Noachian climate
• Primary atmosphere
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Mostly hydrogen
Xe isotopes indicate massive loss
• Secondary atmosphere
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Outgassed from interior
Delivered by impacts
• Problem to get warm Noachian
temperatures
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Faint young sun – surface temp.
190-200 K
Large greenhouse effect required
CO2 and H2O is an option
• But you need very big atmospheres (a few bars)
• May be able to reduce this with clouds
Reducing species
• NH3, CH4 etc… are very effective
• Thought to be rare because of massive H loss
• Mantle expected to be oxidized
•Carbonates?
•Big CO2 atmospheres produce lots
of liquid water…
•Quickly forms carbonic acids and
combines with Ca in rocks
•Locks up C in carbonates (CaCO3)
– 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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Summary of Mars
geologic history
(Ehlmann et al. 2011)
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Can Minerals be Used like Fossils?
Bibring et al., 2006
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18th Century geologists thought minerals could be used to date terrestrial strata
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This was disproven
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Fossils do date strata—extinction is forever
OMEGA mineralogical theory
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Clay formation ceases in Noachian
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Transition to acidic environment from sulfates
 Also requires evaporation
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Young terrains show no aqueous alteration
Problems with this theory
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Alteration can occur anytime after the rock formed, so alteration of Noachian rocks
not necessarily confined to Noachian age
There are clays of all ages in Martian meteorites
There has certainly been subsurface water since Noachian
 Hesperian and early Amazonian outflow channels, alluvial fans
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Summary
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
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