GEOLOGY / MARS : Scientists Use Photos to Spot Seismic Activity on Red Planet
Earth is not the only planet that regularly rumbles and rolls, scientists with the Jet Propulsion Laboratory and the U.S. Geological Survey have concluded. The first observational evidence of so-called marsquakes comes from a review published today in the journal Science of thousands of faults visible in Viking orbiter photographs from the 1970s.
Marsquakes are probably seldom larger than a magnitude 5.8, but shocks that powerful may occur every 4 1/2 years; a magnitude 4.9 shakes the Martian surface about every seven months, and smaller temblors are even more frequent, said Bruce Banerdt, a geophysicist at JPL in Pasadena and one of four authors of the report.
Mars is apparently thousands of times less seismically active than Earth, but thousands of times more so than the Earth’s moon, Banerdt said.
Seismometers on the two Viking landers, which reached the surface in 1976, failed to clearly detect any movement, said Raymond E. Arvidson, chairman of the Earth and planetary sciences department at Washington University in St. Louis.
One of the instruments, however, did not function. The other was poorly positioned atop the probe, where it was blown around by the wind. “Whenever it operated, we didn’t know if it was a marsquake or the wind,” Arvidson said.
The moon’s surface, by contrast, was known to be occasionally rocked by quakes because Apollo mission seismographs registered unmistakable activity.
JPL is proposing to send seismometers to Mars as part of a new lander mission late in the decade, but this time the equipment would be stationed directly on the red Martian soil--perhaps carefully placed there by a tiny roving robot.
The team that examined the photographs believes that a magnitude 4.0 marsquake could be detected from anywhere on the planet. “We don’t have to be right next to an active fault,” said David M. Tralli of JPL, who also worked on the study. Based on the frequency of quakes the group found, its members support sending seismometers to Mars.
Scientists say that if marsquakes actually are measured, much could be learned about the interior of the Red Planet. Researchers hope to compare information about the composition of Mars to Earth, gleaning insights into the original materials of the solar system and how gravity worked to shape them into planets.
Tralli said he and his colleagues are beginning a similar study of Venus, which is crisscrossed with faults, to determine whether that planet is also active, and if so, whether quakes are frequent enough to justify sending seismometers. Such devices might last only days or weeks on the Venusian surface, where temperatures are hot enough to melt lead.
Those high temperatures also may allow more sliding along Venusian faults, which could relieve pressure and lead to fewer, if any, quakes.
The most powerful marsquake would probably measure about magnitude 6.7, Banerdt said--much smaller than the Landers earthquake in June or the San Francisco quake of 1906. The largest quakes on Earth occur where tectonic plates are sliding past each other, separating or converging. But, Banerdt said, the researchers doubt that Mars has such plates.
Instead, he said, all seismic activity on Mars appears to result from the cooling of the planet. The movements tend to be vertical rather than horizontal.
Still, he said, “there’s a lot of uncertainty here” about the calculations.
It was easier to see details of the Martian faults, however, than it would have been on Earth, where there is much more erosion, Banerdt said. “On Mars,” he said, “we have things that are preserved as they were a billion years ago.”
The group measured the faults’ length and depth and estimated the total amount of energy released over billions of years to form ridges, valleys and other geologic features. They calculated how many quakes, at what magnitude, would have been generated by the released energy.
The other authors are Matthew P. Golombek of JPL and Kenneth L. Tanaka of the geological survey in Flagstaff, Ariz.
