[meteorite-list] First Mars Express Gravity Results Plot Volcanic History

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 26 Apr 2012 11:41:21 -0700 (PDT)
Message-ID: <201204261841.q3QIfLD4002800_at_zagami.jpl.nasa.gov>

http://www.esa.int/esaSC/SEM6HJNW91H_index_0.html

First Mars Express gravity results plot volcanic history
European Space Agency
26 April 2012

Five years of Mars Express gravity mapping data are providing unique
insights into what lies beneath the Red Planet's largest volcanoes. The
results show that the lava grew denser over time and that the thickness
of the planet's rigid outer layers varies across the Tharsis region.
 
The measurements were made while Mars Express was at altitudes of
between 275-330 km above the Tharsis volcanic "bulge" during the closest
points of its eccentric orbit, and were combined with data from NASA's
Mars Reconnaissance Orbiter.

The Tharsis bulge includes Olympus Mons - the tallest volcano in the
Solar System, at 21 km - and the three smaller Tharsis Montes that are
evenly spaced in a row.

The region is thought to have been volcanically active until 100-250
million years ago, relatively recent on a geological timescale.

The large mass of the volcanoes caused tiny "wobbles" in the trajectory
of Mars Express as it flew overhead; these were measured from Earth via
radio tracking and translated into measurements of density variations
below the surface.

Overall, the high density of the volcanoes corresponds to a basaltic
composition that is in agreement with the many martian meteorites that
have fallen to Earth.

The new data also reveal how the lava density changed during the
construction of the three Tharsis Montes volcanoes. They started with a
lighter andesitic lava that can form in the presence of water, and were
then overlaid with heavier basaltic lava that makes up the visible
surface of the martian crust.

"Combined with the varying height of the volcanoes, we can say that
Arsia Mons is the oldest, then Pavonis Mons formed and finally Ascraeus
Mons," says Mikael Beuthe of the Royal Observatory of Belgium and lead
author of the paper published in the Journal of Geophysical Research.

At Ascraeus Mons, however, the density of the lava decreased at a later
stage, so that the top of the volcano is of lower density."

The transition could reflect changes in heating beneath the surface in
the form of a single mantle plume - an upwelling of abnormally hot rock
from deeper within the viscous mantle, created in a process that can be
likened to a lava lamp but on a gigantic scale - that slowly moved
sideways to create each of the three Tharsis Montes in turn. This is the
exact opposite of Earth where "plates" of crust move above a stationary
plume to form chains of volcanoes, such as the Hawaiian islands.

The data also describe the thickness of the lithosphere - the outermost
shell of the planet, including the upper portion of the mantle - and
find surprising lateral variations between Olympus Mons and the Tharsis
Montes, with the three smaller volcanoes having a much higher density
underground "root" than Olympus Mons.

These roots could be dense pockets of solidified lava or an ancient
network of underground magma chambers.

"The lack of a high-density root below Olympus Mons indicates it was
built on a lithosphere of high rigidity, while the other volcanoes
partially sank into a less rigid lithosphere," says co-author Veronique
Dehant, also of the Royal Observatory of Belgium. "This tells us that
there were large spatial variations in the heat flux from the mantle at
the time of their formation."
 
Since the three Tharsis Montes sit on top of the Tharsis bulge, whereas
Olympus Mons stands on the edge, the greater crustal thickness at the
centre may have acted as an insulating lid to increase the temperature,
creating a less rigid lithosphere. Here rising magma interacted with the
pre-existing bulge, whereas the magma forming Olympus Mons ascended
through the older crust that is supporting the Tharsis bulge, perhaps
creating the observed density differences between the volcanoes.

"These results show that data on the Mars interior are key to
understanding the evolution of the Red Planet," says Olivier Witasse,
ESA Mars Express Project Scientist. "One option for a future mission to
Mars would be a network of small landers, simultaneously measuring
seismic activity in order to probe the interior."
 
 
Contact for further information
<http://www.esa.int/SPECIALS/Mars_Express/SEM75LNW91H_0.html>
 
Received on Thu 26 Apr 2012 02:41:21 PM PDT