Mars: Dead or Alive?
|Olympus Mons rises 24 kilometers high and measures 550 km across. By comparison, Earth’s largest volcano, Mauna Loa in Hawaii, rises 9 km high and measures 120 km across.
"Mars ain’t the kind of place to raise the kids. In fact it’s cold as hell." So goes the 1970s song by Elton John.
The words expressed the public’s notions at the time about the Red Planet: dry, cold, dusty, barren and pockmarked by craters – the very images sent back by the Mariner series of spacecraft in the 1960s. Those same missions brought us pictures of the vast bulk of Olympus Mons, the largest volcano in the solar system. But while it loomed huge, it was apparently extinct. After examining these images, most scientists concluded that geologic activity on Mars had ceased long ago.
More recent research on the planet, however, has convinced some scientists that Mars may still be geologically active. The area that is raising a few eyebrows is Tharsis, an elevated region that occupies approximately twenty-five percent of the surface area of the planet. Tharsis contains the biggest shield volcanoes in the solar system – Olympus Mons, Tharsis Montes and Ascraeus Mons. It also contains some intriguing tectonic, or crustal, features.
At the Lunar and Planetary Science Conference in March, 2001, Bob Anderson, Investigating Scientist for the Mars Exploration Rovers (MER) project at the Jet Propulsion Laboratory (JPL) in Pasadena, asked the question: "Is Mars tectonically active today?" He and his colleagues used data obtained by NASA’s Viking orbiters in the mid-1970s to map more than 24,000 tectonic features in Tharsis: tension cracks; simple grabens (long, narrow, flat-floored troughs bounded by inward-dipping faults); complex grabens; troughs and channels; and wrinkle ridges (linear or arch-shaped asymmetrical highs on otherwise smooth plains).
All these features can be caused by crustal movements associated with volcanic activity, which, as we know from Earth, creates cracks, troughs and wrinkles as the crust expands and contracts. But very little Martian seismic data is available. (The Viking landers contained seismometers, but they didn’t return conclusive information.)
If we can’t seismically detect quakes on Mars, how can we tell whether the crust is moving? And without a dated sample from the rocks that have been affected by these crustal movements, how do scientists know how old the activity was?
The answer comes from the heavens, for throughout its life, Mars, like all the terrestrial planets, has been bombarded by objects from space. Each collision produces a crater on the surface the object strikes. On average, the longer the time the surface has been exposed, the more hits it will receive, and if the surface is not destroyed by some geologic process-erosion or burial, for example-it will preserve a record of the punishment it has suffered.
|Counting craters on the Moon and comparing these numbers to Mars impacts has helped Hartmann estimate the age of lava flows on Mars.
In the 1960s, Bill Hartmann of the Planetary Science Institute in Tucson, AZ, counted craters on the large flat lava plains on the Moon, and by comparing the counts with the number of known craters in Canada, predicted that the age of the lunar maria, relatively uncratered regions of the Moon’s surface, would be around 3.4 billion years. Rocks brought to Earth by Apollo astronauts proved him correct.
"We can reconstruct the rate of cratering with time on the Moon," says Hartmann. "My work has been to take that and use it to convert to Mars, using various people’s estimates of the ratio of Mars-to-Moon impacts by asteroids and comets." He estimates that some lava flows on Mars may be fewer than 100 million years old, perhaps as young as 10 million years old.
The problem is a difficult one because blowing dust on Mars can obscure small craters, but Mars Global Surveyor (MGS) images, obtained in the past few years for some areas by the Mars Orbital Camera (MOC), show neither significant mantles of dust nor many craters, which leads Hartmann to believe that the flows are very young. He argues that this work is essentially confirmed by dating of meteorites from Mars. They include igneous rocks, such as those that form volcanic lava flows, only 170 million years old. Other scientists are more cautious.
Tim Parker, Research Scientist at JPL notes that "there are huge ranges in ages depending upon which surface you’re looking at. On Tharsis, most of that’s on the order of a billion years old, but some of it may be as young as one hundred million years. There is talk of flows that are as young as ten million years, but the uncertainty in that number can be potentially quite large."
Where there are no surface volcanic flows, dating geological events can be more difficult. Some of the structural tectonic features identified by Anderson and his colleagues are long and narrow, and might not receive a statistically sufficient number of hits from space debris to make crater counting a reliable means for determining their ages. Because fewer hits mean younger ages, these narrow structural features might be older than calculations show, and the volcanic activity that caused them might also be more ancient.
Sean Solomon, a planetary geologist/geophysicist at the Carnegie Institution of Washington and a member of the NASA Astrobiology Institute, believes that there is a good prospect of recent tectonic and volcanic activity, but adds that "the smoking gun would be an eruption or a change in a surface." But that gun may not have been fired in the short period we have been looking closely at Mars – only thirty years.
|The NetLander mission will collect atmospheric, seismic and geodetic data to help determine if Mars is geologically active.
Bob Anderson puts it succinctly: "Until we put seismometers on the surface of Mars, we are really going to have a hard time telling whether Mars is still active." That is the goal of the European Space Agency’s NetLander Mission to Mars, scheduled for launch in 2007. Four landers will deploy from an orbiter to collect atmospheric, seismic and geodetic (planetary surface motion) data for at least one Earth year.
"We intend to put three of them in the Tharsis region, the region where we expect most of the activity to be," says Bruce Banerdt, a planetary geophysicist at JPL and the representative of U.S. science for the mission. He and his colleagues estimate that fifteen globally-detectable marsquakes may occur every Earth year, and if one of them is recorded during the NetLander mission, then we will know for sure that Mars, while red, is not dead.