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Published September 30, 1977 | Supplemental Material + Published
Journal Article Open

Seismology on Mars


A three-axis short-period seismometer has been operating on the surface of Mars in the Utopia Planitia region since September 4, 1976. During the first 5 months of operation, approximately 640 hours of high quality data, uncontaminated by lander or wind noise, have been obtained. The detection threshold is estimated to be magnitude 3 to about 200 km and about 6.5 for the planet as a whole. No large events have been seen during this period, a result indicating that Mars is less seismically active than earth. Wind is the major source of noise during the day, although the noise level was at or below the sensitivity threshold of the seismometer for most of the night during the early part of the mission. Winds and therefore the seismic background started to intrude into the nighttime hours starting on sol 119 (a sol is a Martian day). The seismic background correlates well with wind velocity and is proportional to the square of the wind velocity, as is appropriate for turbulent flow. The seismic envelope power spectral density is proportional to frequency to the -0.66 to -0.90 power during windy periods. A possible local seismic event was detected on sol 80. No wind data were obtained at the time, so a wind disturbance cannot be ruled out. However, this event has some unusual characteristics and is similar to local events recorded on earth through a Viking seismometer system. If it is interpreted as a natural seismic event, it has a magnitude of 3 and a distance of 110 km. Preliminary interpretation of later arrivals in the signal suggest a crustal thickness of 15 km at the Utopia Planitia site which is within the range of crustal models derived from the gravity field. More events must be recorded before a firm interpretation can be made of seismicity or crustal structure. One firm conclusion is that the natural background noise on Mars is low and that the wind is the prime noise source. It will be possible to reduce this noise by a factor of 10^3 on future missions by removing the seismometer from the lander, operation of an extremely sensitive seismometer thus being possible on the surface.

Additional Information

Copyright © 1977 American Geophysical Union. Received March 30, 1977. Revised May 9, 1977. Accepted May 9, 1977. Paper number 7S0408. The Viking seismometer was designed and tested by F. L. Lehner and W. Miller at the California Institute of Technology. Flight hardware was designed and fabricated by J. Lewko, D. Gibson, M. Van Dyke, T. Gaffield, and D. LaFeniere of Bendix Aerospace. The large-scale integration circuitry was fabricated by American Microsystems, Inc. Wyatt Underwood has been in charge of downlink operations. George Sutton and Ken Anderson were involved in various aspects of the experiment. We thank W. Kaula, R. Phillips, B. Bills, A. Ferrari, and E. Okal for useful discussions and for some of the calculations given in this paper. The cooperation of the Meteorology Team headed by S. Hess is gratefully acknowledged. This work was supported by NASA contract NAS1- 9703. Contribution 2910 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91225.

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Supplemental Material - jgr82msapp.pdf


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