When Mars Was the Most Earthlike Planet

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When Mars Was the Most Earthlike Planet

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Title: When Mars Was the Most Earthlike Planet

1
When Mars Was the Most Earth-like Planet
Sean C. Solomon Department of Terrestrial
Magnetism Carnegie Institution of Washington
A.O.C. Nier Memorial Lecture University of
Minnesota 2 October 2003
2
Outline

Exploration of Mars
Geological Time on Mars
Modern (Amazonian) Mars
Ancient (Noachian) Mars
Global Differentiation
Early Crust
Magnetic Field
Water and Climate
Volcanism Tharsis
Some Interconnections
Prospects for Further Discovery

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Mars Exploration

Flybys
Mariner 4 (1965)
Mariner 6-7 (1969)
Orbiters
Mariner 9 (1971 - 72)
Viking 1-2 (1976 - 80)
Mars Global Surveyor (1997 - )
Mars Odyssey (2001 - )
Landers
Viking 1-2 (1976 - 82)
Pathfinder (1997)

Mariner 6-7
Viking Lander
Mars Global Surveyor
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Martian Meteorites

Some two dozen known.
Evidence for Martian origin Trapped gas matches
Martian atmosphere.
All are igneous rocks.
All but one are relatively young, 0.2 - 1.3 Gy
(AL84001 is 4.5 Gy old).
Isotopes preserve a record of earlier Martian
events.

Wiens and Pepin 1988
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Geological Time on Mars

Crater density gives relative age (more densely
cratered units are older).
Lunar cratering flux can, with assumptions, be
used to infer absolute ages (with uncertainty).
Martian geological time is divided into three
epochs.

Hartmann and Neukum 2001
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Modern (Amazonian) Mars

Thin CO2 atmosphere.
No global magnetic field.
Cold, dry climate.
Abundant evidence for ice in polar deposits and
regolith.

Northern polar deposits (Viking mosaic over MOLA
topography)
Epithermal neutron flux from Mars Odyssey (blue
colors show hydrogen enrichment, inferred to be
ice-rich soil)
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7
Modern (Amazonian) Mars

Very limited recent volcanism.
Limited recent water-surface interaction (e.g.,
gullies in crater walls).
Evidence for climate change driven by variations
in planets spin-axis orientation.

MOC image of Olympus Mons
MOC image of gullies in the wall of an impact
crater
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8
Ancient Mars Accretion

Formation of the Sun begins by collapse of a
giant molecular cloud of gas and dust to a
nebular disk.
In the inner solar system, planetesimals accrete
to kilometer size in 104 years.
Runaway growth of planetary embryos up to Mars
size accrete by accumulation of planetesimals in
105-106 years.
Final terrestrial planets form by gravitational
interaction of embryos in 107 years.

Solar System Origin W. K. Hartmann
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Core Differentiation

All Martian meteorites contain radiogenic 182W
(from 182Hf, 9-My half-life).
Core-mantle differentiation occurred within 10-15
My after solar system formation.
Superheating of the core and widespread melt
production in the mantle are possible outcomes.

From Lee and Halliday 1997
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Crust/Mantle Differentiation
Lee and Halliday 1987
Brandon et al. 2000

Excess 182W correlates with isotopic tracers of
crust-mantle differentiation (146Sm - 142Nd,
103-My half-life, 187Re-187Os, 42-Gy half-life).
Early melting and differentiation of the mantle,
probably in a magma ocean environment, is implied.

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Crustal Structure

Mars Global Surveyor has determined the global
topography and the global gravity field of Mars.
From the two fields, a crustal thickness map can
be derived, subject to an assumed crust-mantle
density contrast and an uncertain mean thickness.

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Crustal Structure
There is a crustal thickness dichotomy on Mars.

In the southern crustal province, crustal
thickness tends to thin progressively northward
(a consequence of S-to-N topographic slope).
In the northern crustal province, crustal
thickness is approximately uniform ( 40 km).

Updated from Zuber et al. 2000
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Crustal Heterogeneity

Possible origin of crustal thickness dichotomy
Heterogeneous magma ocean evolution
Early mantle dynamics and melt generation
Impact excavation and transport
Distinguishing among possibilities is possible,
if challenging (e.g., N-S differences in crustal
chemistry or early heat flux).

S
N
Zuber 2001
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Early Crust Crater Density
From Frey et al. 2002

Topographic identification of partially buried
impact basins indicates that much of the present
crust had formed by early Noachian.
Large-scale crustal recycling after early
Noachian can therefore be ruled out.

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Early Crust Models

Thermal history models are strongly constrained
by limits to additions to at least the upper
crust after early Noachian.
This constraint favors wet mantle rheologies,
near-chondritic heat production, and some
fractionation of U, Th, and K into the crust.

Hauck and Phillips 2002
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Core Dynamo
Two scenarios for dynamo timing

(1) Acuña et al. 1999
Dynamo was active early (Noachian) and ceased
prior to end of heavy bombardment.
(2) Schubert et al. 2000
Dynamo onset postdated youngest impact basins
(Hesperian or later).

From Acuña et al. 1999
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Core Dynamo
Arguments favoring a Noachian dynamo

Concentration of regions of high
magnetization in the ancient southern
uplands.
Lack of correlation of magnetic anomalies
with late Noachian or younger volcanic units or
impact structures.
Magnetization of carbonates at least 3.9 Gy
old in ALH84001.

From Weiss et al. 2002
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Dynamo Shut-Down

A magnetohydrodynamic dynamo can shut down for
one of several reasons
Insufficient core heat flux
Thinning of fluid outer core.
Dynamo simulations suggest that maintaining a
dynamo is difficult at an inner core radius
Ri / Rc gt 0.5.
Timing of inner core growth is sensitive to
initial conditions, mantle heat transport, and
core composition.

S. Hauck 2002
Aurnou, Al-Shamali Heimpel 2002
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Water and Climate

Isotopic evidence for a larger early water budget
and a more massive early atmosphere.
Noachian Valley networks indicate drainage of
surface or very-near-surface water.
Topographic data indicate widespread erosion of
Noachian uplands (e.g., Arabia Terra).

MOLA data indicate valley networks better
developed than formerly appreciated Hynek and
Phillips, 2003.
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Water and Climate

The episodic flooding events that carved the
outflow channels may have led to short-lived
lakes or oceans.
Large impact events may have evaporated surface
and subsurface ice, raising atmospheric
temperature and precipitable water for brief
periods.
Such events decreased in volume and frequency
after the Noachian.

Possible lake and ocean shorelines in the
northern plains Fairén et al., 2003.
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History of Tharsis
From Anderson et al. 2001
Tharsis was a site of voluminous magmatism and
concentrated deformation by the middle Noachian.
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History of Tharsis
From Phillips et al. 2001
Late Noachian valley networks formed after the
N-to-S slope was established and after much of
Tharsis magmatism occurred.
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Noachian Tharsis

A major volcanic center at Syria Planum and
formation of the Thaumasia highlands may have
been linked by gravity-sliding of the upper crust
of the Thaumasia plateau.
Excess mantle heat flux and melt delivery (by a
plume?) may have weakened the lower crust in this
region relative to surrounding areas.
Plume activity, fed by core heat loss, would
likely have accompanied core dynamo activity.

Tectonic sketch map of the Thaumasia and Syria
Planum regions Webb and Head, 2002.
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Why So Few Northern Magnetic Anomalies?

Northern crustal province postdates dynamo
(unlikely).
Magnetic anomalies have wavelengths lt 200 km
(testable).
Reheating by volcanism and intrusion (small
effect).
Burial by sediments and post-dynamo lavas (small
effect).
Hydrothermal alteration.

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Possible Hints from Oceanic Crust

Central magnetic anomaly generally greater in
amplitude than older anomalies.
Magnetization decreases steadily off axis to ages
of 20 to 30 Myr.
Attributed to off-axis hydrothermal alteration of
titanomagnetite to titanomaghemite (lower in
specific magnetization).

Raymond and LaBrecque 1987
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Possible Hints from Oceanic Crust

Hydrothermal circulation is enabled by the
penetration of seawater along fissures and
faults.
Time scales for changes in magnetization range
from tens of thousands to millions of years.
Axial vent areas can be sites of very low
magnetization.
Depth of hydrothermal circulation at least 10
km (300 MPa) on the basis of depth of brittle
faulting and oxygen isotopes in ophiolites.

Tivey et al. 2002
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Hydrothermal Alteration Hypothesis for Mars

Hydrothermal circulation along deep faults led to
oxidation of magnetic carriers.
Preferred sites for deep, long-term circulation
were the interiors of major drainage basins.
Effects of hydrothermal alteration included
lessened specific magnetization and changes to
the spatial scales of magnetization coherence.
Dominant scale of alteration, if comparable to a
depth of circulation similar to oceanic
lithosphere, would be 25 km.

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Testable Consequences of Hypothesis

There should be a strong correlation between the
central regions of major drainage basins and an
absence or paucity of strong magnetic anomalies
observable from orbit.
Such a correlation is observed.

Banerdt and Vidal 2001
Purucker et al. 2000
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Testable Consequences of Hypothesis

Magnetic anomalies should tend to be suppressed
or unresolvable from orbit within topographically
well-preserved basins (e.g., Hellas, Argyre) even
if the impact event predated dynamo shut-off.
Older basins may preserve significant volumes of
crust remagnetized during loss of impact heat and
any later magmatism, particularly if their
initial topographic relief had been relaxed by
crustal and mantle flow.

U Utopia, D Daedalia,
Frey et al. 2003
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Testable Consequences of Hypothesis

There should be shorter-wavelength magnetic
anomalies throughout the northern lowlands and
perhaps the youngest impact basins than can be
resolved from orbit.
Such anomalies should be detectable from the
surface (landers, rovers) or low elevations
(balloons, aircraft).

ARES Scout concept (http//marsairplane.larc.nasa
.gov)
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Water and Crustal Cooling

Deep hydrothermal circulation would have
accelerated crustal cooling of the Martian crust
compared with conductive heat transport alone.
Timescale for cooling must be less than
timescales for relaxation of crustal thickness
differences by lower crustal flow.
Longest timescale for crustal relaxation is for l
1 (hemispheric variation), consistent with
geometry seen today.

Relaxation of relief by flow of a low-viscosity
lower crust.
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Tharsis and Volatiles
Phillips et al. 2001

Tharsis added 3 x 108 km3 of igneous material
to the crust, much of it in the Noachian.
From recent upward revisions to the probable
water content of Martian (shergottitic) magmas,
water equivalent to a 100-m global layer would
have been released.
This water, and released magmatic CO2, had the
potential to influence Martian climate.

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Summary of Some Interconnections

Water played multiple roles when Mars was the
most Earth-like planet.
Dominated erosion and deposition.
Release to atmosphere during large impacts and
volcanic events may have modified climate.
May have dominated cooling of the crust.
May have controlled pattern of magnetic
anomalies.
Crustal hydrothermal systems provided habitats
for organic synthesis and possibly life.

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Near-Term Exploration Mars Express