Scientists may have discovered the largest flood channels in the solar
system on Mars, currently a cold desert planet.

A system of gigantic ancient valleys — some as much as 200 kilometers wide
— lies partly buried under a veneer of volcanic lava flows, ash fall and
wind-blown dust in Mars’ western hemisphere. New observations made with Mars
Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft reveal
northwestern slope valleys (NSVs) northwest of the huge martian volcano,
Arsia Mons, and south of Amazonis Planitia, site of a postulated ocean.

The northwestern slope valley system is ten times larger than Kasei Valles,
the largest previously known outflow channel system on Mars, said James M.
Dohm of the University of Arizona. The best explanation is that they were
formed by catastrophic floods that at their peak potentially discharged as
much as 50,000 times the flow of the Amazon River, Earth’s largest river,
Dohm said. Smaller outflows flooded the valleys later in martian history.

Dohm and others from the University of Arizona Department of Hydrology and
Water Resources, UA Lunar and Planetary Laboratory, NASA Jet Propulsion
Laboratory, U.S. Geological Survey-Flagstaff and Smithsonian Institution
reported the discovery in the June 2001 issue of the Journal of Geophysical

“The implications of uncovering such a significant flood record of the
ancient martian past is of great significance in that such activity supports
northern ocean (s) and (or) large paleolakes in the northern plains,” the
researchers wrote.

At sustained peak discharge rates, floods through the valleys would have
filled a large ocean (96 million cubic kilometers) hypothesized for northern
Mars in about 8 weeks, and a smaller ocean (14 million cubic kilometers) in
the same region in about 8 days, according to the scientists’ calculations.

The large ocean is equivalent to about a third the volume of the Indian
Ocean, or more than three times the volume of the Mediterranean Sea,
Caribbean Sea, South China Sea and Arctic Ocean combined. The smaller ocean
is equal in volume to the Arctic Ocean.

“The Tharsis region has had pulses of major magmatic activity that triggered
catastrophic floods that sculpted the surface, ponded, perhaps forming
oceans and lakes that in turn perturbed the climate, ” Dohm said. “After
picking the complex geologic picture apart like a jigsaw puzzle, we think
there must have been several episodes of magmatic heating creating
catastrophic floods. Dave Scott and others have documented that there was
not just one flooding episode at Kasei Valles, but several.”

Dohm, the late David Scott of the U.S. Geological Survey at Flagstaff, and
other collaborators have been mapping Mars’ terrain as shown by Viking data
for more than 10 years. They mapped basins that appear to be dried lake
beds, outflow channels and other water-related features, and determined the
relative ages of the features. They determined that almost 25 percent of the
martian valleys represent relatively recent hydrologic activity, including
flooding events.

(Until future sample return missions give geologists absolute dates for
surfaces on Mars, they can determine only relative ages for geological

New Mars Global Surveyor results are consistent with the idea that Mars is
an episodically wetter, warmer planet.

“To me, the NSVs document magmatic-driven, catastrophic floods of enormous
magnitude that mark a previously unrecognized northwest watershed,” Dohm
said. “It has taken years of work to piece this story together. Individual
pieces of evidence by themselves represent a weak argument, but collectively
they give greater creedence to a working hypothesis. I am excited because
the results of our work collectively fit a consistent and coherent picture.

Individual pieces of evidence collectively give greater creedence to a
working hypothesis that Mars is an episodically warmer, wetter planet.

“When the Mars Global Surveyor data started coming in, from a mapper’s point
of view, I could understand why there seemed to be anomalies. Part of the
story appears to be the result of a highly productive aquifer in the Tharsis
region, and the northwestern slope valleys may be a feature of that.”

The northwestern slope valleys coincide in time with the early development
of outflow channels that flooded Chryse Planitia, plains in the northeastern
part of the Tharsis region. The channels formed before hot magma formed the
northeast-trending chain of gigantic shield volcanoes, Tharsis Montes, that
later severed the two watersheds, the researchers said.

If Chryse flooding occurred simultaneously with Amazonis flooding, the
postulated large and smaller oceans would have filled very quickly, they
added. At sustained peak discharge rates, floods from the northwestern slope
valleys and Chryse outflow channels would have filled the larger ocean in
under 8 days and the smaller ocean in under 6 days.

Dohm, who joined the UA in July 1999, collaborates with UA Regents’
Professor Victor R. Baker on planetary- and terrestrial-related research.
>From 1987-98, Dohm was with the U.S. Geological Survey in Flagstaff, where
he was assistant coordinator of NASA-funded Mars and Venus mapping programs,
now known as the Planetary Mapping Program.

Baker, head of the Department of Hydrology and Water Resources, and Lunar
and Planetary Laboratory Professor Robert G. Strom are among the authors of
the JGR article. They and other UA scientists proposed in 1991 that an
“Oceanus Borealis” repeatedly formed over the northern plains of Mars. Baker
and colleagues have further developed the theory, naming it the “MEGAOUTFLO”

Basically, the hypothesis states that over the long term, water and
volatiles remain frozen as ground ice in the subsurface because Mars is so
extremely cold, due to its distance from the sun and atmospheric conditions.
The perennially frozen permafrost acts like a cap on a soda bottle. Just as
gas and water in a capped soda bottle explode when heated, sporadic bursts
of internal planetary heat likewise trigger the dramatic release of gas and
water locked under the permafrost.

They theorize that so much water is released in such episodes that a
temporary ocean forms repeatedly over the northern hemisphere. Massive
martian volcanism near the Thasis Bulge has – and may again – trigger a
northern plains ocean or lake, they said.

Carbon dioxide released to the atmosphere promotes the warming greenhouse
effect so that liquid water is stable near the martian surface. If Mars
lacks Earth-like bio-rich soils, water from precipitation, outburst
flooding, or both may filter underground more rapidly than it does on Earth.
Local valleys and other observed martian features form when near-surface
water springs from below. But when it snows or rains, water removes carbon
dioxide from the atmosphere, so that Mars chills to the point that
permafrost reforms, plunging the planet into another dry, frigid
long-lasting epoch.

Contact Information

James M. Dohm