Seismic data

Mean density and gravity data provide constraints on the interior structure of a planet, but analysis of seismic energy propagation produces the most detailed information on interior heterogeneities. The presence of volcanic and tectonic features on Mars suggests that the planet has been, and may still be, seismically active. Seismic activity likely peaked during the early period of Tharsis Bulge

Figure 3.5 Gravity anomaly map. as measured from the vertical accelerations of MGS. A strong positive gravity anomaly is seen between 240°E and 300°E longitude which corresponds to the Tharsis volcanic province, while a strong negative anomaly in the equatorial region between 300°E and 0°E correlates with Valles Marineris. However, many topographic features do not correlate with gravity anomalies. (Image PIA02054, NASA/Goddard Space Flight Center (GSFC).) See also color plate

Figure 3.5 Gravity anomaly map. as measured from the vertical accelerations of MGS. A strong positive gravity anomaly is seen between 240°E and 300°E longitude which corresponds to the Tharsis volcanic province, while a strong negative anomaly in the equatorial region between 300°E and 0°E correlates with Valles Marineris. However, many topographic features do not correlate with gravity anomalies. (Image PIA02054, NASA/Goddard Space Flight Center (GSFC).) See also color plate

Figure 3.6 MOLA revealed the large variations in topography visible across the martian surface. Topography ranges from a high at the summit of Olympus Mons to a low in the bottom of the Hellas Basin. (Image PIA02820, NASA/JPL/GSFC/MGS MOLA team.) See also color plate.

Figure 3.7 MOLA observations revealed the large variations in crustal thickness across the planet. This image shows the crustal thickness variations (gray band) across a transect from near the north pole across the Elysium volcanic region and into the southern highlands. Thickness of the gray band corresponds to the crustal thickness at each point. (Image PIA00957, NASA/GSFC.)

formation and has declined since then (Golombek et al., 1992). Nevertheless, analysis of cooling rates (Phillips, 1991) and strain rates derived from slip along surface faults (Golombek et al., 1992) suggests that Mars remains seismically active at the present time, with perhaps 14 seismic events of equivalent magnitude 4 or greater occurring each year. Meteorite impact is another source of seismic activity on Mars (Davis, 1993).

Seismic energy is released during planetary response to stress and strain. Energy is distributed both through the interior (body waves) and along the surface (surface waves) of the body. Extremely large seismic events can cause the whole planet to vibrate (free oscillations). Body waves are the most important for determining the internal structure of the planet. The fastest of the body waves, and thus the first to arrive at a seismic station after the event, are compressional waves called primary or P-waves. Secondary or S-waves are distortional, or shear, waves. The velocities of the P- and S-waves depend on the characteristics of the material through which they pass. The bulk modulus (K) is a measure of a material's incompressibility and the shear modulus (/) measures the rigidity of the material to an applied force. If p is the density of the material, the P- and S-wave velocities (VP and VS, respectively) are given by

K is always greater than zero, so VP is always greater than VS. Fluids have no rigidity (a=0), thus S-waves do not travel through liquids.

Mars is differentiated, with denser materials such as iron comprising the core and lower-density materials like silicates constituting the crust (Section 2.2.4). The increase in pressure with depth causes phase transitions among minerals and the increase in temperature towards the core can result in melting of some regions. Thus, the values of K, a, and q are constantly changing as one traverses the interior of a planet. Seismic waves also refract as they encounter layers with different material properties. By analyzing the arrival times of P- and S-waves at several surface locations, geophysicists can reconstruct the path taken by and the velocities of the seismic waves to obtain a detailed view of the interior structure of the planet.

Unfortunately, actual seismic data for Mars are non-existent. The Viking landers both carried seismometers, but the Viking 1 instrument failed to deploy and the Viking 2 seismometer was not adequately coupled to the ground, leading to large amounts of noise from lander operations and the wind (Anderson et al., 1977). No marsquakes larger than the magnitude 3 threshold of the seismometers were detected during operation of the Viking 2 seismometer. Penetrators on Mars 96 carried seismometers, but as noted in Section 1.2.2, that mission failed to leave Earth. Seismic network missions have been proposed several times by NASA and ESA, but none of these proposals has been funded to completion (see review in Lognonne, 2005). Until such a seismic network is established, our understanding of the interior structure of Mars will be limited to less detailed information obtained largely through gravity analysis.

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