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Monday, March 19, 2001
Earthlings, meet your parent
By ROWAN HOOPER
The four planets closest to the sun are siblings of a sort. Mercury, Venus, Earth and Mars have similar core properties and densities, suggesting that they probably formed from the same dust cloud in the early solar system, but they have very different surfaces and atmospheres. Mercury is hot, has low gravity and almost no atmosphere, Venus is hot and dense, Mars is dry. Its average surface temperature is minus 55 C. Earth is the only planet covered with oceans.
But Earth and Mars may have once had something in common that would reverberate with us more directly than their core properties: life.
The search for extraterrestrial life has always aroused great interest, but usually more among the UFO-hungry public than scientists. That's now changing, and even distinguished academic journals such as Nature are discussing the topic, with one lighthearted article last month supposing that galactic law prevents aliens from making contact with Earthlings because we are too primitive and violent.
The lighthearted tone shouldn't distract from the growing consensus that not only did Mars once harbor the conditions for life, but maybe even life itself. Life on Mars might have originated independently of that on Earth, or interplanetary "seeding" might have taken place. Evidence that Mars could support life comes from the Mars Global Surveyor's data on the Tharsis rise in the Red Planet's west quadrant.
In biblical times, Tharsis was the westernmost point in the known world, a place probably corresponding to modern-day Spain and Portugal. These days, it refers to the vast region on the western hemisphere of Mars containing the highest volcano in the solar system. The Tharsis rise covers 30 million sq. km and contains Olympus Mons, a gigantic volcano more than 550 km across and 27 km high. (In comparison, Mount Fuji is 3.8 km high and Everest is 8 km above sea level.)
Mars is smaller both in mass and volume than Earth, and being further out from the sun, it cooled more quickly as the solar system formed. Being smaller, it would also have avoided the devastating meteor impacts that sterilized the young Earth until about 3.8 billion years ago.
A meteor probably wiped out the dinosaurs 65 million years ago, but when Earth was younger the impacts were far more serious, igniting the atmosphere and boiling away the oceans. Before the new solar system settled down into its present form, a Mars-sized body collided with Earth, throwing off material that became the Moon and embedding itself in our planet to become the core. The resulting apocalypse stripped Earth of any organic materials that might have been left after the planets initially condensed.
Chris Chyba, at the University of Arizona, estimates that over the next few hundred million years, smaller comets hitting Earth supplied water and organic substances that reconstructed the scorched biosphere and primed it for life.
But Mars was already primed.
Roger Phillips and colleagues at the McDonnell Center for the Space Sciences, Washington University, published a paper in Science last week describing how the volcanic material that was ejected at Tharsis could have produced a warm climate, with liquid water present on the planet's surface.
Phillips estimates that a mind-boggling 300 million cu. km of magma erupted from the Tharsis rise, accompanied by the release of enough carbon dioxide and water to sustain a warm, wet planet, hundreds of millions of years before Earth recovered from its apocalyptic hangover. Images from the Mars Global Surveyor show a landscape riven with what appear to be dried-up river valleys. Water probably flowed on Mars over 4 billion years ago, when Earth was nothing but a parched rock, third from the sun.
Whether life evolved on Mars back then is unresolved, but the conditions were right, and the direct evidence -- fossil bacteria in Martian rocks -- is compelling.
Life present or not, conditions on Mars took a turn for the worse about 3.6 billion years ago. Volcanic activity slowed, and its atmosphere was gradually stripped away. This is just when life started on Earth: The oldest microfossils on Earth have been dated at 3.5 billion years old.
Earth had been sterilized to a depth of a kilometer by the 3,000-degree inferno that engulfed it, but now started to pick up. What was special about Earth that allowed it to keep hold of its atmosphere?
Kanako Seki and researchers at the University of Tokyo's Department of Earth and Planetary Sciences published an article March 8 in Science that suggests an answer: Earth's magnetic field.
The solar wind -- the intense flux of particles streaming outward from the sun that strips planets of their atmospheres -- is stronger nearer the sun, so the four sibling planets are the most affected. Estimates had indicated that the solar wind has cost Earth approximately 18 percent of its present-day oxygen over the last 3 billion years, but Seki and colleagues found that a significant amount of oxygen is recycled into the atmosphere.
The researchers used high-altitude spacecraft observations and found that the total loss of oxygen has been more of the order of 2 percent of the present-day total.
Earth is the only one of the four inner planets to have a strong magnetic field -- the magnetosphere. Seki's group's results strengthen the idea that the magnetosphere shields the atmosphere from the solar wind. If this proves correct, we may have the magnetosphere to thank for maintaining conditions on Earth suitable for life.
Earth has avoided -- at least until now -- a greenhouse effect like that of Venus, because most of its carbonates are locked in the sea.
"Earth's atmosphere would be roughly as dense as that of Venus, if all carbonates in Earth's seas were released as carbon dioxide to the atmosphere," said Seki.
So Earth is neither as hot as Venus nor as cold as Mars, and Seki's results show how we are protected from the solar wind: the least of Earth's problems.
And if it turns out that life on Earth originated on Mars, it would be more correct to refer to it not as our sibling, but as our parent planet. Earthlings may very well be the last Martians.
E-mail Rowan Hooper at email@example.com