Earthlike Mars
In some ways Mars is strangely like Earth. It rotates in 24.6 hours and is tilted on its axis by 24 degrees, almost exactly the same as Earth. Its orbit wanders away from and toward the sun as much as Earth's does, giving Mars a range of climate rather similar to Earth, though much colder. Unlike the black vacuum sky of Mercury or the dull gloom of Venus, the Martian sky has clouds and colors. The ground, as seen from earthly telescopes, changes colors during the 687-day Martian year.
The geologist likewise finds Mars to be almost alive, strangely like Earth. It has volcanoes, dune fields, ice sheets and dust storms. It has traces of a magnetic field, an oxidized surface, an iron core and silicate mantle. It has large river valleys and glacial and periglacial landforms, or things that look exactly like them. Its surface, as seen through our robot landers, is covered with basalt boulders.
And yet most of these are deeply different from the earthly versions. We can be fooled. Here are a few examples of the mystery that is Mars.
Three Martian Puzzles
The Martian volcanoes are not fed by subduction, as they are on Earth. Thus we tend to think of them as hotspots—that being our best-known alternative—but until Earth's hotspots are explained this tendency should be resisted. Nevertheless, just as they do for Earth, theorists have been quick to put mantle plumes beneath the Martian volcanoes, apparently just to keep busy because firm evidence for plumes anywhere in the solar system is scant.
The Martian river valleys were probably not fed primarily by rainfall, the way ours are. They were likely carved (1) by sudden outbursts of water after volcanoes or impacts melted the permafrost we think is there, or (2) by slow melting of permafrost during climate changes related to Mars's orbital cycles. When theorists proposed that an ancient ocean once covered the northern hemisphere, the space cameras found nothing where the shorelines would have been. There was room to theorize that the liquid on Mars was not water, but carbon dioxide, or even that the flows were dry avalanches, not involving a liquid at all. The Spirit and Opportunity landers have put an end to that by documenting water-altered minerals in Mars's bedrock.
The Martian magnetic field recorded in rocks of the southern hemisphere show crude stripes in polarity, something like those mapped on the Earth's seafloor. The first easy idea was that Mars once had the kind of plate tectonics that creates the stripes on Earth. But there's no other evidence of plate tectonics and reasons to think Mars never had it: the crust is very thick, there are few or no compressive features on the planet, and volcanism does not suggest Earth-style subduction. Nor do we have a firm idea of how large Mars's core is, what composes it, and how it might have generated a magnetic field of the great strength suggested by the rock evidence.
Some clever modeling of the likely formation of Mars, loosely guided by the geochemical evidence from the landers and Martian meteorites, is giving us good working hypotheses that include all three enigmas. The planet's formation is thought to include a period when the entire rocky mantle was melted into a huge magma ocean, much like the Moon and Earth. With what we know of mineral physics and petrology, we can spin scenarios that will be useful in interpreting new data and guiding further research. That's how geology progresses.
Settling Down to Wait
The latest set of orbiters and landers will not solve these mysteries in one stroke. Remember that it took hundreds of years to learn as much about Earth as we have. Our methods today are much better, of course, but to know Mars to the same degree will still likely take decades, plus more spacecraft missions.
For a sense of where we are, visit some of these sites, plus the links in the Mars Geology list:
- Magnetic Stripes on Mars from NASA's Center for Mars Exploration (CMEX)
- Mars archive from the Planetary Science Research Discoveries site, excellent for a wide range of topics
