Concepts and Approaches for Mars Exploration (2012) 4349€¦ · · 2012-05-10many fundamental...
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Seismicity of Mars. J.P.Townsend and R.A. French. Northwestern University Dept. of Earth and Planetary Sci- ence. 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 struc- ture and composition of the Martian interior remain unanswered. Orbital surveys show that a global mag- netic field once existed, but its history is difficult to constrain [Stevenson 2001]. Both the evidence of vol- canism and the hemispheric dichotomy hint at once- active plate tectonics, yet there is conflicting evidence of magnetic and topographic features that are the sig- natures 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 ro- botic missions combined with data from Martian mete- orites 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 un- knowns 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 mar- squakes. Because the seismometer was attached to the landers leg, the seismometer lacked the sensitivity re- quired to detect low magnitude or distant seismic ac- tivity [Anderson et al. 1977]. One report estimated that a ground based seismometer could be up to 10 3 times more sensitive than the leg-mounted Viking unit [An- derson 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 map- ping 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 [An- derson et al. 1977]. Assuming present magmatic activ- ity, 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 pro- vide an estimate for current impact rates on Mars. Ad- ditionally, 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 mar- squake 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 topogra- phy 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 sub- ducted, yet they rely on many assumptions that seis- mology 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 Ander- son 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 prop- erties 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 be- cause Mars shows evidence of ancient liquid surface waters, an ancient magnetic field, and topography that suggests once active plate tectonics, all important in- gredients for habitability of the Earth. Mars seems to have evidence for some of the most important geologi- cal processes for life, yet little is known about the past or present state of its interior. We therefore recom- mend that the Lunar and Planetary Institute and NASA 4349.pdf Concepts and Approaches for Mars Exploration (2012)
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Transcript of Concepts and Approaches for Mars Exploration (2012) 4349€¦ · · 2012-05-10many fundamental...
Seismicity of Mars. J.P.Townsend and R.A. French. Northwestern University Dept. of Earth and Planetary Sci-ence. 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 struc-ture and composition of the Martian interior remain unanswered. Orbital surveys show that a global mag-netic field once existed, but its history is difficult to constrain [Stevenson 2001]. Both the evidence of vol-canism and the hemispheric dichotomy hint at once-active plate tectonics, yet there is conflicting evidence of magnetic and topographic features that are the sig-natures 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 ro-botic missions combined with data from Martian mete-orites 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 un-knowns 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 mar-squakes. Because the seismometer was attached to the landers leg, the seismometer lacked the sensitivity re-quired to detect low magnitude or distant seismic ac-tivity [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 [An-derson 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 map-ping 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 [An-derson et al. 1977]. Assuming present magmatic activ-ity, 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 pro-vide an estimate for current impact rates on Mars. Ad-ditionally, 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 mar-squake 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 topogra-phy 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 sub-ducted, yet they rely on many assumptions that seis-mology 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 Ander-son 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 prop-erties 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 be-cause Mars shows evidence of ancient liquid surface waters, an ancient magnetic field, and topography that suggests once active plate tectonics, all important in-gredients for habitability of the Earth. Mars seems to have evidence for some of the most important geologi-cal processes for life, yet little is known about the past or present state of its interior. We therefore recom-mend that the Lunar and Planetary Institute and NASA
4349.pdfConcepts 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 in-formation about the deep interior of Mars, an appropri-ately spaced array of seismometers is required. The advantage of an array of seismometers includes trian-gulation 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 inter-nal 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 pre-liminary 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
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: 5085-5098.
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 investiga-tions 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 Plane-tary 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.
4349.pdfConcepts and Approaches for Mars Exploration (2012)
