When astronauts finally land on Mars, a safe bet is that they’ll head
for northern climes if they intend to spend much time there. That’s
because nearly all the available water is frozen as ice at the north
pole. Planetary scientists have been aware of this for some time, but
they now have a new clue why it is so.

In the March 21 issue of the journal Nature, California Institute of
Technology researcher Mark Richardson and his colleague John Wilson
of the National Oceanic and Atmospheric Administration reveal that
the higher average elevation of the Red Planet’s southern hemisphere
ultimately tends to drive water northward.

Their evidence is based on a computer model the two have worked on
for years (Wilson since 1992, Richardson since 1996), coupled with
data returned by NASA’s Mars Global Surveyor.

"We’ve found a mechanism in the Martian climate that introduces
annual average hemispheric asymmetry," explains Richardson, an
assistant professor of planetary science at Caltech. "The circulation
systems of Mars and Earth are similar in certain ways, but Mars is
different in that water is not available everywhere."

The key to understanding the phenomenon is a complicated computer
modeling of the Hadley circulation, which extends about 40 degrees
of latitude each side of the Martian equator. A topographical bias
in circulation pretty much means there will be a bias in the net
pole-to-pole transport of water, Richardson explains.

A plausible explanation is that water ice is found at the north pole
and carbon dioxide ice is found at the south for reasons having to
do with the way the sun heats the atmosphere. As the Martian orbit
changes on time scales of 50,000 years and more, these effects tend
to cancel, with no pole claiming the water ice cap over geological
time. It has been suggested that topography determines where carbon
dioxide forms, and hence, where water ice can form, but the
processes controlling carbon dioxide ice caps are poorly understood.

However, the mechanism Richardson and Wilson describe is independent
of this occasional realignment of the pole’s precession and the
planet’s eccentric orbit. The mechanism means that, while there is
never a time in the past when water ice can be discounted at the
south pole, one is more likely to find it more frequently at the
north pole.

The importance of the study is its furthering of our understanding
of the Martian climate and Martian water cycle. A better
understanding of how water is transported will be particularly
important to determining whether life once existed on Mars, and what
happened to it if it ever did.

The Web address for the journal Nature is

Related Links

* The Division of Geological and Planetary Sciences at Caltech

  • 21 March 2002: Martian climate: A tale of two hemispheres, Nature

    “The martian climate displays large seasonal and hemispheric asymmetries, generally ascribed to variations in solar heating caused by orbital eccentricity. But a numerical study of the effect of the high southern plateau on martian atmospheric circulation suggests there may be more to it than that. The fact that the southern hemisphere is on average about 5 km higher than the north
    is an important characteristic of the general circulation, and may even be responsible for the asymmetry in the ice caps.”