HEY, WHAT'S THE WEATHER
LIKE ON MARS?!
Current conditions on Mars are...
A Hadley
circulation transporting heated air from the
northern hemisphere to the south is
developing. At the low levels depicted in the
figure, there is strong eddy activity
(generated near the edge of the seasonal
frost cap) in the northern midlatitudes.
Say what?
Mars Today
-- Mars
Atmosphere Global Circulation Modeling Group in
the Planetary Systems Branch of the Space Science
Division at NASA Ames Research Center
Conditions
for a planet to retain an atmosphere
Astronomers are coming to believe
that comet and meteorite impacts can have a major
effect on the atmospheres of planets--both in
delivering gases like water to the atmosphere,
and in stripping away much of the pre-existing
atmosphere.
Thick
atmosphere in past despite Mars' small mass
The gravitation of Mars can hold an
atmosphere. It is doing so today, though the
atmosphere it holds is much less massive than
here on Earth (even after taking into account the
different sizes of the planets).
Liquid water could
have existed in the past if the temperatures on
the planet were warmer, say, produced by an
enhanced 'greenhouse' warming due to gases which
produce such an effect (carbon dioxide in much
greater abundance than is available today is one
possibility). Water vapor is an extremely
effective greenhouse gas, more effective than
carbon dioxide, so if temperatures were warm
enough for liquid water, the water vapor this
would provide would help the 'greenhouse' persist
and be amplified.
Additional carbon
dioxide which might have been present in Mars'
atmosphere in the past could now be present in
rocks on and below the surface. If Mars is less
tectonically active than has been Earth, Mars
would have been less efficient in recycling CO2
into rocks and then back into the atmosphere.
This would possibly have led to a cooling of the
atmosphere, which would mean less water vapor
could be contained in the atmosphere, which would
induce additional cooling, ... you can see where
I am going here.
Loss
of Mars' atmosphere
Mars certainly had more atmosphere in the past
than it does today. We see two types of evidence
for this--geological and geochemical.
The geological
evidence consists of types of features seen on
the surface that indicate that the atmosphere
must have been thicker in the past. There are
valley networks that look a lot like river
drainage systems on the Earth. For these to form,
liquid water must have been more stable at the
surface of Mars than it is today; and, the
easiest way to make water more stable is by the
presence of a thicker atmosphere. Also, the
oldest impact craters are more heavily eroded
than younger ones; again, the presence of a
thicker atmosphere is likely.
The geochemical
evidence takes the form of the ratios of
different isotopes in the atmosphere. These are
molecules that have an extra neutron in the
nucleus, so are heavier than the normal ones. The
lighter molecules escape to space more easily, so
the atmosphere that stays behind has more of the
heavier molecules. We see that the ratio of
deuterium to hydrogen is enhanced, as is the
ratio of heavy nitrogen to lighter nitrogen. In
each case, something like 90% of the gas must
have been lost to space.
Certainly,
asteroid collisions can strip off much of the
atmosphere--not from a single impact but from the
many collisions that have occurred through time.
This process may have been a major player in
removing much of the early, thicker atmosphere.
from Questions and Answers about
Atmosphere on Mars
-- Mars
Team Online, Nasa
Inverse
Greenhouse Effect
Most models for the early
atmosphere of Mars assume a much denser
atmosphere of carbon dioxide. Carbon dioxide in
the martian atmosphere gradually combined with
rocks to form carbonates, and eventually all the
carbon dioxide was removed from the air: a sort
of inverse globalwarming.
Carbonates form on
Earth, too, but we avoid planetary death because
plate tectonics recycle carbon dioxide. The
carbon dioxide in the air combines with the rocks
of the crust. It also dissolves in oceans and
lakes, where it precipitates to form carbonates
or becomes fixed by organisms that eventually die
and are buried. Plate tectonics drag these
materials back into the Earth's interior. There,
the rocks melt and release the carbon dioxide
back into the atmosphere through volcanic
eruptions.
The absence of a
recycling mechanism on Mars doomed any early
biosphere. The atmosphere disappeared, never to
return. In the process, the planet accumulated
carbonate deposits and salts like halite.
Similarly, other rocks that are rare in ancient
areas on Earth, such as evaporates, may be quite
common on Mars. These rocks were probably never
buried or disturbed by the tectonics that
constantly reshape the Earth's surface. The rocks
in these ancient southern highlands of Mars have
likely survived with little change.
from Fossil Hunting on Mars
-- J.D.Farmer
, SETI Institute, NASA Ames Research Center
Mars Ozone
"Mars
does have an ozone layer, but it's very thin in
contrast to the one that surrounds and protects
our planet Earth. Even where the martian ozone is
thickest, over the poles of Mars, it's only about
one-fiftieth as dense as Earth's ozone
layer."
-- Earthsky, 10/8/97
Hubble Monitors Weather on
Neighboring Planets
-- Space
Telescope Science Institute
Cloud Ozone Dust Imager
"...
uncertainties in Mars atmospheric cloud and dust
behaviors are potentially a major impediment to
our understanding of the current climate of Mars,
such as why the polar ice caps of the northern
and southern hemispheres of Mars are so different
in extent and composition from one another."
-- Space Science Institute, University of
Colorado at Boulder
The
Faint Sun Paradox
Climate calculations were done in
the 1970s to see how Mars' atmosphere could have
supported a greenhouse effect in the past
sufficient for liquid water. As today, such
calculations face a difficulty: according to
computer simulations of how stars evolve over
billions of years, the Sun is estimated to have
been 25-30% less luminous than today, and yet
Mars and the Earth are thought to have been
warmer (from geological and biological evidence):
a problem known as The Faint Young Sun Paradox.
Ammonia (NH3), a strong greenhouse gas, was
initially suggested as a possible component of
the early atmosphere of Mars and the Earth to
account for this problem. However, it is now
realized that NH3 could not survive long enough
due to its destruction by sunlight. Subsequently,
atmospheres of carbon dioxide (CO2) and water
vapor (H2O) were considered --- such gases are
likely to have been vented from the mantle of the
planet and produced by periodic cometary or
asteroid impacts.
Mars Today
"...produced
daily The poster depicts current conditions on
Mars and its relationship to Earth in four
panels."
-- Center for Mars Exploration, Ames Research
Center, Nasa
Theory offered to explain Martian
water flows
"There is ample evidence ... of deep
channels on the surface of Mars presumably cut by
flowing liquid water. How could Mars -- at
Pathfinder's landing site a chilly minus 100 F --
once have been warm enough to have liquid water
on its surface? The answer ... is reflective
carbon-dioxide ice clouds that retain thermal
radiation near the planet's surface. "
-- Spacer.com, 11/16/97
.. the planet's
surface is exposed to much more solar radiation
(about 44%) than our own.
from Mars Colonies
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