Third Conference on Early Mars (2012)
7079.pdf
A
GEOLOGICAL
THEORY
FOR
THE
ROLE
OF
WATER
IN
THE
EVOLUTION
OF
EARLY
MARS.
V.R. Baker 1,2,
J.M. Dohm 1, S. Maruyama 3, 1Department of Hydrology and Water Resources, The University of Arizona, Tucson,
AZ, 85721, [email protected], 2Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ,
85721, 3Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro 152-8551, Japan
Introduction: A dozen or so years ago we developed a working hypothesis [1-6] with the goal of understanding the geological evolution of early Mars. As
a working hypothesis, this conceptual model was not
meant to provide a final, definitive picture. Rather it
was to serve in pointing, in a productive way, to the
qualities of an eventual “true” representation of causative factors in Mars’ geological history. The working
hypothesis was partly driven by the physical insight
that terrestrial planets, depending on their first-order
geophysical parameters, should evolve through various
modes of mantle convection, including magma-ocean,
plate tectonic, and stagnant lid convective processes
[7]. For example, the surface environment of Earth has
been largely controlled by plate tectonics and superplume activity over the 3.9 Gy of its history that is
recorded in preserved rocks and structures [4,8].
The other motivation for this incipient theory,
termed GEOMARS (Geological Evolution Of Mars
and Related Syntheses), was to attempt to provide a
unified explanation for the many otherwise, puzzling
anomalous discoveries made during the 1990s, especially those related to the role of water over the geological history of Mars, including the following: (A) a
Martian crust extensively layered to the depth of several kilometers [9,10], probably indicating an early
history of extensive fluvial denudation [11] and sedimentation, contemporaneous with and immediately
subsequent to the late heavy bombardment cratering
episode; (B) many geological indicators pointed to the
role of an episodically present, ancient ocean on the
northern plains that played a critical role in Mars’
long-term hydrological cycle [12], (C) areas of hematite mineralization indicated an ancient phase of aqueous activity [13], (D) a globally bimodal Mars hypsometry with discrete highlands in the south, associated with much thicker crust (~60 km), and low-lying
plains in the north, underlain by relatively thin crust
(~30 km), thereby comprising a dichotomy analogous
to that of Earth, but explained differently by the conventional theory [14]; (E) striking linear crustal magnetization anomalies of remarkable intensity that occur
in the southern highlands [15]; (F) persistence of water-related activity on Mars to the present day, with
extensive evidence of very recent hillslope gullies [16],
glaciers [17], outlflow channels [18, 19], and related
phenomena; (F) episodic volcanism and related tectonic processes persisting through later Martian history
to the present time despite physical theory predicting
greatly reduced heat flow through time for the planet,
(G) progressive concentration of volcanism and tec-
tonic activity at Tharsis and Elysium, episodically operating through most of Martian history, extending
from the later Noachian to the present [20].
GEOMARS: The GEOMARS theory has been
described in several full papers [21, 22, 23], and therefore the following is but an extremely brief sketch of
this working hypothesis. It holds that Mars, like Earth,
initially accreted from water-rich planetesimals. Following differentiation to a metallic core and rocky
mantle, Mars subsequently developed a steam atmosphere, rich in CO2 and H2O. Above a primordial
magma ocean, this steam atmosphere facilitated the
development of a solid crust, above which a global
ocean of water could condense. The resulting situation
would have then been optimal to initiate subduction in
a proto-lithosphere, thereby leading to a very ancient
phase of plate tectonics, which would have been associated with a remarkably intense core dynamo and
consequent magnetic field. This, in turn, led to the
formation of intensely magnetized oceanic plateaus
that accreted to protocontinental terrains now in evidence as the linear anomalies of remnant magnetism in
the Martian southern highlands.
The early phase of plate tectonics, would , via subduction, convey water, carbon dioxide, and sulfates to
the core-mantle boundary zone, thereby depleting the
reservoir of these materials from the surface and transferring them to the deep mantle. The whole-mantle
subduction processes also cooled the evolving core,
thereby terminating the dynamo by the time of the late
heavy bombardment. Thus, this very
early
phase
of
plate
tectonics,
occuring
prior
to
the
late
heavy
bombardment,
could
have
generated
the
Martian
highland
crust
by
continental
accretion.
Moreover,
by concentrating
volatiles
in
a
local
region
of
the
Martian
mantle,
the
early
plate‐tectonic
phase of
Mars
would
have
led
to
the
superplume
at Tharsis.
The
immense
concentration of
volcanism
at
Tharsis
in
later
Mars
history
would
have
a great
influence
on
climate
change
and
the
generation
of
megafloods,
in
analogous
fashion
to
Earth
super‐
plume
centers.
The
persistence
of
this
volcanism
episodically
through
later
Martian
history
would
provide
a
mechanism
for
the
episodic,
short
dura‐
tion
aqueous
phases
that
generated
transient
“oceans”
and
fluvial
landforms.
Internal
planetary
heat
provided
the
trigger
for
the
massive
outflows
that
transformed
Martian
climate
during
the
geologically
short
epochs
of
ocean
formation.
Superimposed
on
the
long‐term
monotonic
decline
in
mantle
heat
flux
for
Mars
were
short‐duration
Third Conference on Early Mars (2012)
episodes
of
higher
heat
flow
to
the
surface.
Such
episodes
of
higher
heat
flow
seem
consistent
with
both
the
magmatic
and
tectonic
history
of
Mars.
Recent
Discoveries: . Our GEOMARS working
hypothesis for the geological evolution of Mars continues to provide unified explanations for recent discoveries concerning the role of water in the early history of that planet, including (A) the importance of
phyllosilicates, including clay minerals in the early
history of the planet [24], (B) the later development of
extensive sufate mineralization [24], and (C) the role
of an ancient ocean in providing a base level for sedimentation and a critical component in the ancient hydrological cycle of the planet [25].
Conclusion: Our working hypothesis for the geological evolution of Mars (GEOMARS --Geological
Evolution Of Mars and Related Syntheses) continues
to provide a consistent, coherent, and consilient explanations for recent discoveries concerning the role of
water in the early history of that planet.
References: [1] Dohm, J.M. et al. (2000) LPSC XXXI.
Lunar and Planetary Institute, Abstract # 1632. [2]
Baker, V.R. et al. (2001) Eos 82 (47), F700. [3] Dohm
et al. (2001) Eos 82 (47), F713. [4] Maruyama S. et al.
(2001) GSA, 33(7), A310. [5] Baker V.R. et al. (2001)
GSA, 33(7), A432. [6] Dohm J. M. et al. (2001) GSA,
33(7), A309. [7] Sleep N. H. (2000) JGR, 105,
17,563-17,578. [8] Condie K. C. (2001) Mantle plumes
and their record in Earth history (Cambridge Univ.
Press, N.Y.). [9] McEwen A.S. et al (1999) Nature,
397, 584-586. [10] Malin M.C. and Edgett K.S. (2001)
JGR, 106, 23,429-23,570. [11] Craddock, R.A. and
Howard A.D. (2002) JGR, 107, doi:
10.1029/2001JE001505. [12] Baker
V.R.
et
al.
(1991)
Nature,
352,
589‐594. [13] Christensen, P.R. et al.
(2001) JGR, 106, 23,873-23,875. [14] Zuber, M.T.
(2001) Nature, 412, 220-227. [15] Connerney J. E. P.
et al. (1999) Science 284, 794-798. [16] Malin, M.C.
and Edgett, K.S. (2001) Science, 288, 2330-2335. [17]
Baker, V.R. (2001) Nature, 412, 228-236. [18] Berman, D.C. and Hartmann, W.K. (2002) Icarus, 159, 117. [19] Burr, D.M. et al. (2002) Icarus, 159, 53-73.
[20] Dohm, J.M. et al. (2001) JGR, 106, 32,94332,958. [21] [21] Baker, V.R. (2006) Bolletino Della
Societa Geologica Italiana 125 (3), 357-369.
[22]Dohm, J.M., et al. (2007) In Yuen, D.A., Maruyama, S., Karato, S.-I., and Windley, B.F., eds., Superplumes: Beyond plate tectonics: Springer, p. 523-537.
[23] Baker, V.R., et al. (2007) In Yuen, D.A., Maruyama, S., Karato, S.-I., and Windley, B.F., eds., Superplumes: Beyond plate tectonics: Springer, p. 507–523.
[24] Bibring et al. (2006) Science 312, 400. [25] DiAchille, G. and Hynek, B. (2010) 23, 459-463.
7079.pdf