Issue 6, December 2002
Evidence for an Ancient Ocean on Mars
Planetary Sciences, Hampshire College
southern hemisphere of Mars is a jagged, pummeled, and scarred terrain.
Deep chasms gorge the crust, craters of all sizes pockmark the surface,
towers of rock split the horizon where volcanoes lie dormant, and
enormous channels cut across hundreds of miles of rocky terrain.
Mars Odyssey, an orbiting spacecraft
designed to determine the composition of the planet's surface,
detect water and shallow buried ice, and study the radiation
environment. Image courtesy of NASA
billions of years of meteor impacts, volcanic eruptions, and flash
floods, the surface of Mars's southern hemisphere is a testament
to its violent history, and looks exactly as a planet with that
history would be expected to look.
contrast, the northern hemisphere of Mars is smooth, flat, and nearly
untouched by the natural disasters that have marred the southern
hemisphere. No great chasms have cut through the crust, few
Ringing the northern third of the planet
is a precarious line of cliffs, which may represent the edge
of an ancient sea.
disrupt the landscape, and no channels cut its surface.
solar system is a violent place for a planet. Meteors, comets, tidal
interactions, and volcanism all gradually tear up a planet's surface
until it is as battered as the moon. Earth is somewhat protected
from these events by its thick atmosphere, and oceans and winds
quickly remove evidence of impacts; however, other planets hold
onto their impact craters for billions of years. To find, then,
that more than one-third of a planet is smooth and without craters,
is puzzling. The origin of one-third of Mars is still unexplained.
Many models and theories have attempted to explain the odd form
of the Martian northern lowlands. In the late 1980s, geologist Timothy
Parker proposed one of the most controversial theories. Looking
at the abrupt cliffs separating the cratered highlands from the
low, flat northern plains, Parker thought he saw the shorelines
of an ancient ocean. Ringing the northern third of the planet is
a precarious line of cliffs, which may represent the edge of an
ancient sea. Farther north, covering a much smaller area, is another
such ring, where a small sea may have existed in more recent times.
of Mars. Data from Mars Orbiter Laser
Altimeter (MOLA). MOLA is an instrument currently in orbit around
Mars on the Mars Global Surveyor (MGS) spacecraft. Image
courtesy of Jet Propulsion Laboratory, www.marsglobalsurveyor.com
the Martian ocean, if it ever existed, would have dried up more
than two billion years ago - almost half of the planet's four-and-a-half-billion-year
existence - deciphering the clues it would have left is difficult.
Nevertheless, several lines of evidence point to the possibility
of an ancient sea. In addition to the smooth, low plains and steep
shorelines, numerous tall, narrow mesas are sprinkled about the
plains-rather like islands on Earth-and the vast river networks
crisscrossing the southern highlands end abruptly at the margin
of the northern plains-like rivers flowing to the sea on Earth.
The lack of large craters on the plains reveal their relatively
young age - less than about two billion years old-and the severely
degraded remains of ancient craters suggest that a prolonged eroding
force was acting upon the plains. In addition, several studies have
found sediments and sands deposited by water covering basin floors
in the plains.
Global Surveyor. The spacecraft's
prime mission is to map Mars. The surveyor has studied the entire
Martian surface, atmosphere, and interior, and has returned
more data about the red planet than all other Mars missions
Image courtesy of NASA
shorelines, river networks, island-like mesas, eroded craters, and
water-deposited sediments all support the ocean hypothesis; however,
they in no way prove it. Recently, contradictory evidence has emerged
from Mars Global Surveyor data. The MGS, which began mapping Mars
with the Mars Orbiter Camera and the Mars Orbiter Laser Altimeter
(MOLA) in 1998, was intended to last for about two years. But the
mission has been extended, and MGS will continue mapping Mars until
its maneuvering propellant runs out. MOLA has been mapping elevations
on the planet, and has found that the outer shoreline proposed by
Parker varies in elevation by as much as five miles in places.
a level sea would have left a level shoreline, the MOLA data casts
doubt on the idea that the outer contact is actually an ancient
shoreline. However, it is possible that the shoreline was affected
by later uplift via tectonic activity. MOLA data actually supports
the hypothesis that the inner contact line - which may have represented
a younger lake or smaller sea - is a shoreline, since the elevation
of the line differs very little.
of the first problems that would need to be solved is to determine
where all the water went.
presence of ancient oceans on Mars would have far-reaching implications
in the hunt for evidence of life on the planet as well as for our
understanding of planet formation and evolution. One of the first
problems that would need to be solved is to determine where all
the water went. Most likely, as Mars's core cooled, the surface
of the planet did as well, and as the temperature dropped, the ocean
froze over. The resulting sheet of ice would have eventually been
covered by dust, dirt, and lava, hiding it from view. The thick
layers of permafrost discovered just beneath Mars' surface support
this idea, and it has been speculated that liquid water may still
exist in underground rivers.
The Mars ocean debate is still unfolding. As new images and data
pour in from Mars Global Surveyor and Mars Odyssey, new evidence
emerges. If Mars did indeed have oceans, the possibility of finding
evidence for ancient life on Mars increases. The discovery of shorelines
and ancient lakebeds, then, may be an exciting step toward unearthing
the origin of life and the existence of life elsewhere in the solar
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Parker, et al. "Mars: Implications for modification of the lowland/upland
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Abstract # 1894, Lunar and Planet. Inst., Houston (CD-ROM), 1999.
Journal of Young
Investigators. 2002. Volume Six.
Copyright © 2002 by Selby Cull and JYI. All rights reserved.