Severe glacial cycles on Mars (Forwarded)

Observatoire de Paris
Paris, France

Contacts:
Benjamin Levrard
Observatoire de Paris, IMCCE; Now at Laboratoire de Planétologie et de Télédétection
UCBL1/ENS Lyon, 69622 Villeurbanne


27 October 2004

Severe glacial cycles on Mars

Since the arrival of Mars Global Surveyor and more recently Mars Odyssey
spacecrafts, a range of facts has revealed the existence of frozen water ice in
the top meters of high latitudes near-surface (~60 deg - 90 deg) of both martian
hemispheres. However, its origin was still unexplained. Climatic simulations
directed by astronomers from Paris Observatory and researchers from IPSL
Planetology Departement (Paris VI) and published in the journal "Nature", show
that this ice may come from an ancient reservoir of equatorial ice created
during high obliquity episodes on Mars but which became unstable during the more
recent episodes of low obliquity. This study has permitted to illustrate the
existence of glacial cycles on Mars even more severe than on Earth.

Even if the presence of ice caps has been observed on the Mars poles for more
than three centuries, the arrival of Mars Global Surveyor and Mars Odyssey
spacecrafts in 1996 and 2001 has permitted to show that important quantities of
ice (more than 70% of volume) had undoubtedly also been present in the top two
meters of martian high latitudes (Figure 1).

It seemed difficult to explain the existence of such a quantity of ice so nearby
the surface: more than half a milimeter of water frost is currently laying down
during autumn and winter at high latitudes. Nevertheless, this ice cap
sublimates completely at the end of spring. This ice was proposed to be resulted
from a slow diffusion of water between the Martian regolith and the atmosphere
but the in situ measurements of porosity from Viking spacecrafts have shown that
the regolith can not contain any ice with such a concentration. The study
directed by the researchers from Paris Observatory and IPSL suggests that the
solution may come from astronomical forcing of Martian climates.

For almost thirty years, sedimentary and ice cores have confirmed that the
variations of the insolation received on the Earth's surface resulting from slow
changes of the orbit and the Earth's obliquity had given rise to
glacial/interglacial periods. However, the martian obliquity variations are
chaotic and much more significant than on Earth. The Martian obliquity has
indeed varied between 25 deg and 45 deg during the 5-10 Ma time intervall and
between ~15 deg and 35 deg during the last 5 Ma, with a "periodicity" close to
120 000 years.

A climatic 3-dimensional model of General Martian Circulation developed by the
team of François Forget (IPSL, Paris VI) and simulating faithfully the current
seasonal cycle of water has been used to determine the path of Martian ice
through these large variations. These simulations have brought the intense
latitudinal redistribution of Martian ice to light. When the obliquity
overpasses 35 deg (compared to the current average value which is of ~25.19
deg), the summer insolation becomes too strong to maintain the stability of the
current Northern cap which provokes a quick atmospheric transfer of ice towards
the equatorial high topography region of Tharsis (Arsia, Pavonis, Ascraeus et
Olympus Montes). Remarkably, these summits sides present morphological traces
which may be the result of the recent presence of glaciers. When the obliquity
is below the current value, the equatorial ice becomes unstable and is carried
not only to the polar zones but also to the high latitudes of the both
hemispheres. The latitudinal distribution of stable ice obtained is then very
close to the Mars Odyssey observations, illustrating a severe martian ice age.

How this ice can be preserved? As it is currently observed on Mars, ice is
expected to be co-deposited with dust. When ice begins to sublimates, a dust lag
is forming and prevents some ice from complete sublimation at every cycle so as
to permit a "regular" forming of sedimentary meters-thick and ice-rich layers.
These deposits are visible at high latitudes and more spectacularly in the polar
caps. The ice observed by Mars Odyssey would also be the mark of an ancient
Martian glacial age (probably inferior to 5 Ma), covered nowadays with a thin
cover of dry layer. If this is true, there must be some ice not only on the top
meters but on hundreds of meters depth. The radars MARSIS and SHARAD
respectively aboard Mars Express (in progress) and Mars Reconnaissance Orbiter
which is forseen to be launched in 2005 will probably brought additional
constraints on these underground reservoirs.

References

Recent ice-rich deposits formed at high latitude on Mars by sublimation of
unstable equatorial ice during low obliquity
Levrard, B., Forget, F., Montmessin, F. and Laskar, J., Nature, 28 octobre 2004

Long term evolution and chaotic diffusion of the insolation quantities of Mars.
Laskar, J., Correia, A., Gastineau, M., Joutel, F., Levrard, B., Robutel, P.: 2004,
Icarus, 170, 343-364

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/nov04/mars04-f1.png (46KB)]
The cartography of frozen water concentration in the two top meters of the
Martian subsurface provided by the Gamma spectrometer of the Odyssey spacecraft.
It is the concentration of the element hydrogen which is in fact determined by
the gamma spectrum analysis of the radiations sent out by the Martian ground
overwhelmed with cosmic rays. The blue and violet zones (high latitudes)
indicate a strong concentration of ice, higher than 50% of volume.

[Figure 2:
http://www.obspm.fr/actual/nouvelle/nov04/mars04-f2.png (37KB)]
The ice evolution on Mars over a caracteristic obliquity cycle: the angle
between the white arrows and the dotted line denotes the Martian obliquity. At
high obliquity, the northern cap becomes unstable and looses a few centimeters
of ice each year. This ice is then deposited in equatorial zones. When the
obliquity decreases, ice comes back at high latitudes. When the equatorial
reservoir disappears, high-latitude ice deposits become unstable too. A fraction
sublimates and lays out again towards the poles which contributes to the
creation of Martian polar caps, while an other fraction is buried under
a protecting dust lag (ASD/IMCCE-CNRS, adapted from Jim Head/Brown University
and NASA/JPL).