The Red Planet has intrigued humanity for more than 3,000 years. The ancient Babylonians and Romans named it for their gods. A 19th-century astronomer caused an international uproar when he said he found "canals" on its surface.
But now, with probes and satellites, scientists are gaining unprecedented insight into Mars' present and mysterious past.
In one of the most significant finds of the past decade, researchers announced today that they have found evidence that the geologic forces that have molded the Earth's surface may once have been at play on Mars.
Such a finding could help astronomers piece together the evolution of the fourth planet from the sun. But also, it would indicate that at some time deep in Mars' past, it might have had water and a strong magnetic field. On Earth, the magnetic field prevents the withering solar wind from stripping away our atmosphere.
To be sure, scientists are still being very cautious in their pronouncements. Yet some say the findings suggest that life on Mars, albeit more than 4 billion years ago, is no longer as remote a possibility.
"It would make the environment even more conducive for exobiology," says geologist Ronald Greeley of Arizona State University, referring to the possibility of life outside Earth.
The discovery, reported in today's issue of the journal Science, was made by the Mars Global Surveyor, a satellite orbiting closer to Mars than any previous satellite. It found two regions of enormous magnetic stripes on the planet's southern hemisphere. Scientists say the stripes - some of which extend for as long as 1,240 miles along the surface - might indicate that the Red Planet once had a system of drifting continental plates similar to Earth's.
"With just a few passes over the surface these huge signals jumped out in the data and blew away anything else we were measuring," says Jack Connerney of NASA's Goddard Space Flight Center in Maryland.
Magnetic stripes on the earth's surface characterize regions where magma bubbles up to form new crust as tectonic plates spread apart. The iron-bearing minerals in the newly formed crust align themselves to point north or south, depending on which which magnetic field is dominant at the time - creating these bar-code-like stripes. Earth's magnetic field reverses about every 10,000 years.
Reminiscent of Earth
The discovery of a similar set of stripes on the Atlantic sea floor cemented the scientific acceptance of the theory of continental drift and plate tectonics. It holds that Earth's crust is made up of thin, solid interlocking plates that float on a mantle of partially molten lava. Heat escaping from the superheated liquid core of the planet causes the mantle to shift and move. As a result of this motion, the plates on the surface constantly collide, split, and slide by, under and over each other in an intricate dance of massive continents.
Plate tectonics explains the reasons for earthquakes, volcanic eruptions, the formation of mountain ranges, and other geological phenomena that were once assumed to be totally unrelated. The theory, which emerged in the 1960s, radically altered man's understanding of the geological dynamics of the earth.
Now continental drift may revolutionize the scientific view of Mars, as well. "It had been thought that plate tectonics only worked on the Earth. While other planets had mantle convection, only the Earth had plate tectonics," says Dr. Connerney. "If we were to find plate tectonics had been operating on Mars, even briefly, it will have widespread implications for the evolution of Mars."
In addition to the indications that Mars may have had a thicker atmosphere, proof of plate tectonics on Mars might help answer the question of whether it ever had water. Water is believed to be a crucial lubricant that keeps the Earth's plate tectonics mechanism working smoothly.
This most recent discovery is just one of many new insights into Mars' past that are coming from the Mars Global Surveyor. Sending back images that are 20 times sharper than those taken during the Viking mission in the 1970s, it has given researchers better looks at deep canyons suggesting extensive volcanic activity and empty fissures that hint at water erosion.
Contrast these findings with the past knowledge of the fourth planet from the sun as a cold and inhospitable place, and it's clear that science is dealing with a new Mars. "Mars has been viewed as more moonlike than Earthlike," says Dr. Greeley. "If these results are correct, it would indicate an internal history that is much more complicated than we had envisioned previously."
Still, some scientists are wary of taking the comparisons with Earth too far. "I would stop short of making the connection with plate tectonics," says geologist Bruce Jakosky of the University of Colorado, who can envision at least one other geological scenario that could be responsible for the stripes. "On the [Earth's] sea floor, the stripes clearly indicate both an age and a spreading relationship.
"Here on Mars we see an age relationship but not necessarily a spreading relationship," he says. "You don't see a clean signature of the symmetric stripes."
But even critics admit that the discovery of stripes and the confirmation that a magnetic field once acted strongly on Mars widens the possibilities.
"I don't think there is a strong connection between whether Mars did or did not have plate tectonics and whether it could or could not support life," says Dr. Jakosky. "But this is a significant discovery. This tells us about the interior structure and thermal history of Mars and helps us to understand Mars as a planet. And, by extension, it helps us to understand how planets work."