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The Mars mystique

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"It went into the oven, and it came out differently as a function of how far [planets] were from the sun. The last 10 years has challenged that assumption and shown us that there are specific and fundamental differences among the terrestrial planets, even though [they are] really close together."

The geological processes that shape these planets "are fundamentally the same," she says. "But the way they play out in detail, which we can see with the exploration program we've had over the last 10 years, is very different."

It turns out, for instance, that Mars' crust has much higher levels of sulfur than Earth's crust. While the distinction might seem esoteric, it suggests a far different environment early in the planet's history than Earth's – one that would have a direct influence on any life that might have emerged then, Dr. Dyar says.

Efforts to follow the water have revealed a remarkable planet whose prospects for serving as a habitat – even if it ultimately didn't host life of any sort – seem to increase with each new mission.

From high above the planet, two orbiters – Mars Global Surveyor and Mars Odyssey (the first mission in a revamped NASA Mars exploration program following the 1999 debacles) – returned images of fans and flows of sediment from the earliest period of Martian history. The teams interpreting the images, published in 2003, concluded that the features represented evidence of long-lived flows of water moving across the surface.

At the same time, images from Mars Odyssey showed channels cut into the sides of craters and along other slopes, suggesting brief catastrophic releases of water in the geologically recent past.

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