[meteorite-list] Scientists Say Ancient Asteroids, Comets May Have Caused Mars Rain

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:33 2004
Message-ID: <200212052131.NAA11095_at_zagami.jpl.nasa.gov>

http://amesnews.arc.nasa.gov/releases/2002/02_126AR.html

Dec. 5, 2002
Kathleen Burton
NASA Ames Research Center, Moffett Field, Calif.
Phone: 650/604-1731 or 604-9000
E-mail: kburton_at_mail.arc.nasa.gov

RELEASE: 02-126AR

NOTE TO EDITORS AND NEWS DIRECTORS:

You are invited to a special
session at the American Geophysical Union (AGU) meeting titled
"Mysteries of the Martian Rivers," to be held on Dec. 6 from 2 p.m.
to 4 p.m. PST in the Moscone Convention Center (MCC) Theatre, Exhibit
Hall C, San Francisco. The session is an oral debate and discussion
about the formation of rivers and valleys on Mars. After the session,
researchers will be available for follow-up questions from the media
in the MCC Press Room between 4 p.m. and 5 p.m. PST. In addition,
there will be a poster session on the topic beginning at 8:30 a.m.
PST on Dec. 6 in MCC Hall D (P51B).

SCIENTISTS SAY ANCIENT ASTEROIDS, COMETS MAY HAVE CAUSED MARS RAIN

Scientists from NASA and the University of Colorado suggest the
bombardment of comets and asteroids on early Mars caused cycles of
rain that led to global flooding and the formation of Mars' river
valleys and other water-sculpted landscapes.

The researchers emphasize that the period when large comets and
asteroids struck Mars appears to correlate with the formation of
ancient rivers when water once flowed on Mars, and that both 'events'
seem to have ended about the same time, between 3.5 billion and 3.8
billion years ago. The research will be published on Dec. 6 in
Science magazine in an article entitled "Environmental Effects of
Large Impacts on Mars."

"The river valleys and large craters on Mars may both be about the
same age geologically," said Teresa Segura, the paper's lead author.
"We think that the two must be related, and our paper describes one
possible connection." Segura, a graduate student in atmospheric and
oceanic sciences at the University of Colorado, is based at NASA Ames
Research Center, in California's Silicon Valley.

The researchers modeled the impacts of asteroids and comets between
60 miles and 150 miles in diameter that bombarded Mars billions of
years ago. Such impact events packed a huge energy wallop, equal to
about 10,000 million megatons of TNT depending on collision
velocities, which were lower back then.

The impacts released water on Mars in four ways, the scientists say -
from the vaporized asteroid or comet itself, from Mars' icy polar
caps, from the ground where the crater formed and from the heat from
hot 'ejecta' (a mixture of soil, rocks and water) that gradually
baked water out of the martian soil.

When Mars eventually cooled down after an impact episode, scientists
theorize, water that had evaporated into the atmosphere condensed
into rain. During Mars' rainy periods, precipitation rates probably
averaged between 1 meter and 2 meters a year, similar to Earth's
average annual rainfall today.

"This happened dozens of times, maybe more, but after it rained, Mars
would go dry," said Dr. Kevin Zahnle, a co-author from NASA Ames. "In
the times between impacts, the water sank back into the soil, where
it lay dormant until the next time an impact occurred."

Scientists think the martian rains lasted for episodes ranging from
months to decades and that, between bombardments, Mars returned to
its typical cold, dry state. Besides bringing moisture, the impacts
also caused Mars to warm up, they say. During bombardment episodes,
hot 'ejecta' from impacts kept Mars' surface warm for hundreds of
years at a time.

The martian cratering record shows that there are at least 30 craters
carved by impactors that are 100 kilometers or more in diameter.
These were created during the planet's period of heavy bombardment by
comets and asteroids more than 3.5 billion years ago.

Scientists do not know why a late heavy bombardment stopped about 3.5
billion years ago, according to Segura. "Our research provides some
insight into what early Mars might have been like, but we've fit only
a couple of pieces into the puzzle of Mars' past," Segura said.

Team members include Dr. Owen B. Toon, University of Colorado, and
Dr. Anthony Colaprete, NASA Ames.

The project is funded by the University of Colorado Center for
Astrobiology in Boulder, and the NASA Astrobiology Institute (NAI)
through NASA Ames Research Center. The NAI is an international
research consortium with its central offices located at NASA Ames.

                    -end-
Received on Thu 05 Dec 2002 04:31:48 PM PST