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Posted by baalke on October 19, 2006, 3:35 pm
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http://www.esa.int/SPECIALS/Mars_Express/SEMMT0O7BTE_0.html
Decoding Mars's Cryptic Region
European Space Agency
19 October 2006
Mars Express's OMEGA instrument has given planetary scientists
outstanding new clues to help solve the mystery of Mars's so-called
'cryptic region'.
In the 1970s, orbiter missions around Mars revealed that during
southern
spring, large areas near Mars's south pole became much darker than the
rest of the seasonal ice cap. How could this area be in the polar
region
and not be covered in bright ice? Intrigued, planetary scientists
called
the area the 'cryptic region' of the south seasonal cap.
The mystery deepened in the late 1990s when new observations showed
that
the temperature of the cryptic region was close to -135?? Celsius.
At
that temperature, carbon dioxide ice had to be present. So, scientists
developed the idea that a one-metre-thick slab of clear carbon dioxide
ice covered the cryptic region, allowing the dark surface underneath to
be seen.
However, the new observations from Mars Express's OMEGA instrument show
that this interpretation cannot be correct. OMEGA measures the amount
of
visible and infrared radiation bouncing off the Martian surface. In so
doing, it detects minerals and ices on the surface by charting the
specific wavelengths of radiation they absorb.
Carbon dioxide ice (dry ice) absorbs infrared light strongly at
specific
wavelengths. "We see only weak absorptions in the infrared, which would
indicate little carbon dioxide ice in the cryptic region," says Yves
Langevin, Institut d'Astrophysique Spatiale, Orsay, France, who led the
analysis of the OMEGA results.
The only way to reconcile the apparently conflicting observations is
that there is indeed a thick slab of dry ice in this area, but its
surface is so heavily covered by dust that few of the Sun?s rays make
it
to the deeper layers and back again.
How does the dust get on top of the slab? The answer could be provided
by the mysterious markings that dot the cryptic region. Known as spots,
'spiders' and 'fans' depending upon their shapes, they were discovered
in 1998?1999 by NASA's Mars Global Surveyor.
Planetary scientists believe they are caused by sunlight passing
through
the clear ice and heating the soil underneath. This causes pressure to
build up in carbon dioxide bubbles below the ice until a geyser erupts
throwing dust onto the surface, creating the spots and fans. In this
model, the spiders result from erosion of the underlying surface by
rapid gas flows below the ice. Langevin believes that this process
could
significantly contribute to the dust contamination of the icy surface,
which OMEGA observed.
"In terms of physics, this is a straightforward process and would go a
long way towards explaining our observations," says Langevin. However,
there are major questions remaining, such as why are spiders, spots and
fans only observed in a small fraction of the cryptic region? And why
are areas not covered by spots and fans already relatively dark.
To clarify these points, Langevin must wait until the next southern
spring equinox on Mars, in 2007. During the long winter, the Sun cannot
be seen from the south pole and a pristine layer of ice should build up
over the cryptic region. Langevin wants to observe the cryptic region
close to the spring equinox, before the Sun has touched it and started
the venting process. This will tell him when the dust geysers form and
whether they are the ice slab?s only source of dust contamination.
So, whilst not as cryptic as it once was, Mars's south polar region
still has a few mysteries left.
Note for editors
These findings appeared in the 17 August 2006 issue of the scientific
journal Nature, in the article titled "No signature of clear CO2 ice
from the 'cryptic' regions in Mars' south seasonal polar cap," by
Y.Langevin et al. (Nature 442, 790-792, 17 August 2006) |
doi:10.1038/nature05012).
For more information
Yves Langevin, OMEGA co-investigator, Institut d'Astrophysique Spatiale
- IAS, Orsay, France
Email: yves.langevin @ ias.u-psud.fr
Jean-Pierre Bibring, OMEGA Principal Investigator, Institut
d'Astrophysique Spatiale - IAS, Orsay, France
Email: jean-pierre.bibring @ ias.u-psud.fr
Agustin Chicarro, ESA Mars Express Project Scientist
Email: agustin.chicarro @ esa.int
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