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Concepts and Approaches for Mars Exploration (2012)
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Seismicity of Mars. J.P.Townsend and R.A. French. Northwestern University Dept. of Earth and Planetary Science. Email: [email protected].
Introduction: Little is known about the interior of
Mars. Although there have been numerous satellite
probes and lander missions to study the red planet,
many fundamental scientific questions about the structure and composition of the Martian interior remain
unanswered. Orbital surveys show that a global magnetic field once existed, but its history is difficult to
constrain [Stevenson 2001]. Both the evidence of volcanism and the hemispheric dichotomy hint at onceactive plate tectonics, yet there is conflicting evidence
of magnetic and topographic features that are the signatures of plate tectonics on Earth [Connerney et al.
1999, Breuer and Spohn 2003]. Although Mars boasts
the largest volcanoes in the solar system, it is not
known how Tharsis originated or if it remains active
[Williams et al. 2008]. Surface measurements by robotic missions combined with data from Martian meteorites give clues about crustal composition, however,
little is known about the bulk composition of Mars
[Sohl and Spohn 1997].
Seismology is the best way to address these unknowns and constrain current models of the structure
of the Martian interior. A small array of seismometers
on the Martian surface will further our understanding
of the past habitability of the surface by measuring the
crustal thickness and interior structure of the planet,
providing estimates of crust and mantle composition,
size of the core, its present state, and constraining the
evolution of the dynamo [Ruedas et al. 2009]. It has
been recognized since the 1970’s that seismometers on
Mars can answer many fundamental questions about
the red planets structure, yet no seismic mission has
been planned for Mars since Viking.
Past Seismic Studies: In 1976, Viking landed on
Mars with a primitive seismometer attached to its leg
[Anderson et al. 1977]. This seismometer collected
approximately 640 hours of data but detected no marsquakes. Because the seismometer was attached to the
landers leg, the seismometer lacked the sensitivity required to detect low magnitude or distant seismic activity [Anderson et al. 1977]. One report estimated that
a ground based seismometer could be up to 103 times
more sensitive than the leg-mounted Viking unit [Anderson et al. 1977).
Present Seismic Potential: Despite the lack of
seismic activity detected by Viking, there is reason to
expect Mars to be seismically inactive. Gravity mapping of the planet indicates that there are regions of the
Martian crust are not fully isostatically compensated,
for example the Tharsis Plateau, which imply stresses
similar to those found in the Earth and the Moon [Anderson et al. 1977]. Assuming present magmatic activity, up to 100 marsquakes per year might be detectable
[Ruedas et al. 2009]. Meteorite impacts could provide
an additional source of seismic signals and would provide an estimate for current impact rates on Mars. Additionally, background noise from wind would provide
information about weather on Mars [Anderson et al.
1977]. In the complete absence of all seismic activity
either from marsquakes or impacts, an artificial marsquake could be generated by crashing an old satellite
or other orbital debris into the Martian surface.
Science Objectives: Measuring the thickness of
the Martian crust is important for determining whether
Mars experienced plate tectonics. Gravity and topography provide the only current means to study crustal
thickness, but preliminary investigations indicate a
crustal thickness of approximately 30-150 km based on
assumed radiogenic concentrations and Airy isostasy
[Nimmo and Stevenson 2001]. Those figures suggest a
stagnant lid tectonic regime, where no material is subducted, yet they rely on many assumptions that seismology will tightly constrain [Beuer and Spohn 2003].
The existence of ancient magnetized crustal rocks
may be evidence for a magnetic field generated by a
dynamo, as on Earth. Knowing the present state of the
core (i.e. liquid or solid) would provide information
about the lifetime of the dynamo. Numerous models of
the Martian core have been proposed, yet all make
assumptions about the fundamental properties of Mars
[Okal and Anderson 1978, Nimmo and Stevenson
2001, Gudkova and Zharkov 2004]. Okal and Anderson assume a liquid core to estimate composition of the
mantle, however, the core of the core is not presently
known. Both Nimmo and Steven, and Gudkova and
Zharkov make assumptions about the variety and
abundances of light elements in the core.
Answering these two questions alone would do
much to further our understanding of the interior properties of Mars, as well as past conditions for life on its
surface. The past habitability of Mars is one of the
biggest unanswered questions in planetary science because Mars shows evidence of ancient liquid surface
waters, an ancient magnetic field, and topography that
suggests once active plate tectonics, all important ingredients for habitability of the Earth. Mars seems to
have evidence for some of the most important geological processes for life, yet little is known about the past
or present state of its interior. We therefore recommend that the Lunar and Planetary Institute and NASA
Concepts and Approaches for Mars Exploration (2012)
make seismic investigations of the Martian interior a
science priority for future lander missions.
Proposed Configuration: To gather adequate information about the deep interior of Mars, an appropriately spaced array of seismometers is required. The
advantage of an array of seismometers includes triangulation of marsquake locations, which strongly affects
the inferred ray path, and waveform stacking, which
reveals small amplitude arrivals from potential deep
structures [Weber et al. 2011]. Previously, an array of
at least nine seismometers was suggested because it
would maximize the possibility of inferring both internal structure and core state [Solomon et al. 1991]. This
configuration was composed of three groups of three
seismometers positioned 100 km apart, with each triad
spaced 3500 km apart. Another report indicated that
regions with elevations from -2km to 6km would be
acceptable for seismic stations, corresponding to green
and red/brown regions in Figure 1 [Solomon et al.
1990]. We suggest a similar configuration with preliminary proposed locations in the vicinity of (1) the
Tharsis region, (2) the southern highlands, and (3) the
northern lowlands (Fig.1).
Figure 1: Topographical map of Mars. Three proposed
seismic station locations are indicated. Map from
NASA/GSFC.
Acknowledgements: The authors wish to thank Dr.
Seth Stein for insightful conversation and inspiration.
References:
Anderson DL, Miller WF, Latham GV, Nakamura Y,
Toksoz MN, Dainty AM, Duennebier FK, et
al. (1977). Seismology on Mars. J. Geophys.
Res. 82: 4524–4546.
Breur D, Spohn T (2003). Early plate tectonics versus
single-plate tectonics on Mars: evidence from
magnetic field history and crust evolution. J.
Geophys. Res. 108-8: 1-12.
Connerney JEP, Acuna MH, Wasilewski PJ, Kletet
schka G (2001). The global magnetic field of
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Mars and implications for crustal evolution.
Geophysical Research Letters 28: 4015-4018.
Gudkova TV, Zharkov VN (2003). Mars: interior
structure and excitation of free oscillations.
Physics of Earth and Planetary Interiors 142:
1-22.
Nimmo F, Stevenson DJ (2001). Estimates of Martian
crustal thickness from viscous relaxation of
topography. J. Geophys. Res. 106: 50855098.
Okal EA, Anderson DL (1978). Theoretical models
for Mars and their seismic properties. Icarus
33: 514–528.
Ruedas T, Schmerr N, Gómez Pérez N, and 10
endorsers (2009): Seismological investigations of Mars' deep interior. White Paper for
the US National Research Council Planetary
Science Decadal Survey 2013-2022.
Sohl F, Spohn T (1997). The interior structure of Mars:
Implications from SNC meteorites, J.
Geophys. Res. 102(E1): 1613–1635.
Solomon SC, et al. (1991). Scientific rationale and
requirements for a global seismic network on
Mars. LPI Tech. Rpt. 91-02, Lunar and Planetary Institute, Houston. 51 pp.
Stevenson DJ (2001) Mar’s core and magnetism. Na
ture 412: 214-219.
Weber RC et al. (2011). Seismic detection of the lunar
core. Science 331: 309-312.
Williams J-P, Nimmo F, Moore WB, Paige DA (2008).
The formation of Tharsis on Mars: What the
line-of-sight gravity is telling us. J. Geophys.
Res. 113.