[meteorite-list] New Theory: Water On Mars Came After Asteroid Impacts

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:33 2004
Message-ID: <200212051748.JAA04580_at_zagami.jpl.nasa.gov>

http://www.saljournal.com/stories/120502/tec_mars.html

New theory: Water on Mars came after asteroid impacts
By PAUL RECER
Associated Press
December 5, 2002

WASHINGTON (AP) -- Mars never had oceans as some researchers have claimed,
but instead is a cold, dry planet that was pounded by water-bearing
asteroids and showered with scalding rain that carved vast gullies and
valleys. This suggests the Red Planet was a less than favorable place for
life as we know it, a new study claims.

The study, appearing this week in the journal Science, sheds new light on a
continuing debate by Mars researchers about how much water there was on
Mars, where did it go and how did it form the planet's intricate pattern of
canyons, river beds and deltas.

Using Mars photos and computer simulations, researchers at the University of
Colorado at Boulder concluded that immense asteroids pounded Mars some 3.6
billion years ago, bringing vast quantities of water to the young planet and
releasing powerful shock waves of heat that melted existing underground
deposits of ice.

Owen B. Toon, senior author of the study, said at least 25 craters on Mars
were gouged out by asteroids 60 to 150 miles in diameter. The impact of such
large space rocks would have propelled into the atmosphere millions of tons
of superheated rock vapor and melted ice. It also would have unleashed a
blast wave heated to more than 4,000 degrees and blanketed the entire planet
with heated rock several hundred feet thick.

"The atmosphere would be hotter than a self-cleaning oven," said Toon. "When
the water was released from the atmosphere, it would fall as scalding rain."

The blanket of hot rocks "would be a global thing, causing rivers to form
anywhere. The ice would be melting all over the planet," he said.

Based on the erosion features on Mars, the researchers estimated that after
a major impact, more than 150 feet of water would flow in some areas,
carving the riverine features. By some calculations, they said, there could
be many decades with rainfall of 6 feet per year.

Between impacts, Toon said, Mars would eventually cool, turning again into a
dry, chilled planet with water present only as subsurface ice.

"We believe these events caused short periods of a warm and wet climate, but
overall, we think Mars has been cold and dry for the majority of its
history," said co-author Teresa Segura.

Toon said that since the moist and warm periods were short, the conditions
were not favorable for life to evolve on the planet's surface.

Peter H. Smith, a University of Arizona planetary scientist, said that if
warmth and liquid water were available on Mars only episodically, "then you
have a pretty gloomy picture for life."

But he said there were other forces on the planet, particularly volcanic
action, that may have created subsurface pools of water where microscopic
life could have lived.

"In my opinion, they haven't closed the book on the prospects for the
evolution of life on Mars," said Smith.

He applauded the study, saying, "Assuming their calculations are correct,
this must have happened on Mars."

Ronald Greeley, a planetary researcher at Arizona State University, said the
study by Toon, Segura and others "has the potential to tie together several
loose ends regarding Mars surface history." Water ejected into the
atmosphere by asteroid impacts, he said, "could account for many of the
apparently water-eroded features."

However, Greeley said the study "doesn't put a nail in the coffin" for
evolution of life on Mars. Like Smith, he said hydrothermal systems powered
by volcanic action and subsurface brine pools could still exist and would be
favorable for the evolution of life.
Received on Thu 05 Dec 2002 12:48:36 PM PST