New observations of Mars reveal that the planet's flat northern lowlands
were an early zone of high heat flow that later may have been the site of
rapid water accumulation, according to a view of the Martian interior
generated using data from Mars Global Surveyor, managed by NASA's Jet
Propulsion Laboratory, Pasadena, Calif. Elevation and gravity
measurements, which have been used to probe beneath the surface of Mars,
indicate a period of rapid cooling early in Martian history, and evidence
for large, buried channels that could have formed from the flow of
enormous volumes of water.
By combining highly accurate topographic
maps with new plots of the planet's
gravitational field, researchers have developed
a working draft of what the planet's interior
looks like several kilometers below the surface.
This global view of the Martian interior was generated from gravity
measurements with the radio science experiment and elevation measurements
from the Mars Orbiter Laser Altimeter (MOLA) instruments. Gravity and
topography measurements were combined to reveal the structure of the crust
on Mars, which preserves the record of melting of the interior and the heat
loss from the planet over time.
"The crustal thickness map shows that, as for Earth, Mars has two distinct
crustal provinces," explained Dr. Maria Zuber of the Massachusetts
Institute of Technology, Cambridge, Mass., and lead author of a study to be
published in the March 10 issue of Science. Beneath the rough southern
highlands and Tharsis volcanic province the crust, estimated at 80
kilometers (50 miles) thick, thins progressively from the South pole toward
the North. In contrast, the northern lowlands and Arabia Terra region of
the southern highlands have a crust of uniform thickness, about 35
kilometers (22 miles) deep.
Below is a global slice of the crustal structure of Mars along 0° E
longitude as derived from MGS gravity and topography data. In the figure,
the south pole is at the far right and the north pole is at the far left.
For illustrative purposes the crustal structure is vertically exaggerated
and is about 40 km thick under the northern plains and 70 km thick at high
southern latitudes. The sloping region under part of the southern
highlands (yellow/orange) and the uniform thickness region under the
northern lowlands (blue) and Arabia Terra region (green) represent the two
distinct crustal provinces. The global dichotomy boundary occurs at the
lowlands/Arabia Terra (blue/green) transition. This boundary does not
correlate with the crustal structure, which indicates that the geological
manifestation of the boundary is primarily due to surfical rather than
The crustal structure accounts for the elevation of the Martian northern
lowlands, which controlled the northward flow of water early in Martian
history, producing a network of valleys and outflow channels. The new
gravity and topography data suggest that the transport of water continued
far into the northern plains. The gravity shows features that have been
interpreted as either buried channels or relics of an earlier topographic
boundary that has been massively eroded. Undoubtedly the Valles Marineris
and Kasei Valles outflow regions played a role in forming the present-day
gravity lows, but the timing and extent of these channels is the subject of
||Evidence suggests that rapid heat flow in the Northern Hemisphere
produced a wide lowland area, encouraging the formation of channels, which
could have sluiced water resources into a large basin, even an early
ocean. Shown in this image is one such putative channel,
draining from the giant Valles Marineris into the wide, flat area of the north.
The features are about 200 kilometers (125 miles) wide and over 1,600
kilometers (1,000 miles) long, with characteristics that can be explained
by water flow on the surface or in a submarine environment, later buried by
sediments. The large size of these channels implies that any bodies of
water in the northern lowlands could have accumulated rapidly. The
now-buried channels may represent the means for filling an early ocean.
The gravity and topography also provide information on the cooling of Mars
over time, which bears on the early climate and history of water. "The
observations suggest that the northern lowlands was a location of high heat
loss from the interior early in Martian history, probably due to a period
of vigorous convection and possibly plate recycling inside of Mars," said
Dr. Sean Solomon, Director of the Department of Terrestrial Magnetism of
the Carnegie Institution in Washington, D.C., and a co-author of the study.
The high heat-loss zone corresponds to the part of Mars proposed to have
been the site of an ancient ocean. The rapid transport of heat to the
surface in this region would have released onto the surface and into the
atmosphere gases and water or ice trapped in the interior. The time of
rapid interior heat loss may correspond to the period when Mars had a
warmer climate, liquid water flowed on the surface, and the planet's
surface was shielded from the solar wind by a global magnetic field.
Links & Resources
This text was taken from the
For more information and figures, see:
Internal Structure and Early Thermal Evolution of Mars from Mars Global
Surveyor Topography and Gravity,
Science, Vol 287, March 10, 2000.