14 December 2009

Mars: A Study of a Dead Planet

Posted by mjvinas

Mars, taken by the Hubble TelescopeWhen I saw the title of Monday afternoon Whipple lecture (P14C), “Mars Exploration: Bridging Our Past and Future,” I was cautiously intrigued.  As a student of solid Earth sciences, planetary science seemed too esoteric—lots of hypotheses about places that you couldn’t readily ground truth.

In particular, the words “Mars exploration” set off warning bells—I remember my college’s pushy members of the Mars Society, and I recalled with fascinated horror the message given by the Apollo astronauts at NASA’s 40th anniversary celebrations of humans landing on the Moon: that it was time to send humans not back to the Moon but on a one-way ticket to Mars.

The invited speaker for the Whipple lecture, Jean-Pierre Birbing of Institut d’Astrophysique Spatiale (Orsay, France), carried with him none of that baggage.  Instead he presented a fascinating talk about how through understanding the evolution of phenomena on Mars, we can understand more about Earth.

Fundamentally, what drives planetary evolution “Is Mars a dead planet? How does planet die?” Bibring asked.  The answer lies in spectral analysis of Mars’ surface, observed through ESA’s Mars Express satellite and the NASA’s Mars Reconnaissance Orbiter.  These data show that at the poles, CO2 ice covers only a few meters while water ice is kilometers thick, begging the question when liquid water persisted on Mars.

Liquid water would have left traces, seen by the presence of hydrologically altered minerals. But in general, the crust has preserved its igneous composition, particularly in the southern hemisphere, Bibring said.  What of the north?  “We know it is covered with red dust, and we know it is rust. But spectral analysis shows that the dust is anhydrous, altered by peroxide in the atmosphere.  Thus, liquid water is not responsible for Mars being red!”

The idea that red rust implies water and water implies life vanished before my eyes.  But are there other hydrologically altered minerals dotted across Mars, Bibring said. These are phyllosilicates—sheet minerals typically found in clays can only form in long-standing bodies of water. “The phyllosilicates record an era of potential habilitability,” Bibring said.

So when were these minerals deposited?  And what happened to the water?  Mars has little atmosphere, making it impossible to sustain a stable ocean. The trigger for the vanishing atmosphere probably is that Mars’s rotating core—its dynamo—stopped. Without a dynamo creating an active magnetic shield, the young Sun would have stripped away Mars atmosphere.

“Why did this happen on Mars and not on Earth?” Bibring asked.  “The seismicity of Mars was not enough to sustain convection in the mantle, and without that core convection couldn’t be sustained.  Core convection drops, and the dynamo dies.”

So was Mars ever habitable? Bibring explained that from Mars and the Earth, heavy bombardment from meteors was always considered prebiotic.  However, on Earth, oxygen isotope compositions from ancient zircon crystals show that water likely existed on Earth during the latter part of the period of heavy meteoric bombardment—before the onset of the Archean 3.8 billion years ago. And if water existed on Earth during this time, why not on Mars? And if water was available at both locations, why not assume that life could have been present at both locations during these ancient times?

Bibring said that Mars may have the answer to all these questions.  “There is no record at all on Earth of time between early and late heavy bombardment.  But the record is on Mars.”

–Mohi Kumar, AGU Science Writer