One of the interesting topics the lab has been pursuing in
the last couple of years is trying to gain a better understanding of the total
atmospheric pressure on Mars. This obviously has implications for the potential
stability of water on Mars surface, but also has a few important links to
volcanology. Magmas ascending to the surface degas and provide a source of
volatiles (e.g. water, carbon dioxide) to the atmosphere. Likewise the nature
of the atmosphere can drastically alter the dynamics of eruptions.
Recently we conducted a series of experiments aimed at
explaining one of the features observed by the rover Spirit at Home Plate and
use it to constrain atmospheric pressure. This feature is a package of deformed
sediments below a larger block of rock that has been interpreted as a bomb sag.
On Earth, bomb sags are generated in explosive eruptions (often produced by
magma-water interaction). During this sort of eruption base surges or
pyroclastic flows are generated that accumulated deposits surrounding the blast
region. Periodically larger chunks of rock (referred to as bombs) are ejected
and follow near ballistic trajectories. They impact the accumulating deposit,
deforming it, and leaving the impactor in place. This is then often covered by
further accumulating deposits (see attached pictures and video).
Inferred bomb sag on Mars. Courtesy: NASA/JPL.
The impact velocity of the bomb or clast is strongly
controlled by the atmospheric density. A more diffuse atmosphere will lead to
larger impact energies. By calibrating the impact energy required to produce a
certain size bomb sag we can then deduce near surface atmospheric density and pressure
if we hold other factors constant (things like gravity and size of the clast).
With a ‘pumice-gun’ we were able to propel clasts in a controlled laboratory
setting at sediment targets to do precisely this. (The pumice-gun propels the
rocks using compressed gases --- the concept is probably familiar to all who
have built a potato-gun).
Two important results came from this series of experiments.
The first is that the energies implied to make the bomb sag at the Home Plate
location requires atmospheric densities greater than 0.4 kg/m3
(roughly 20 times greater than mean present atmospheric density). Also to
produce the continuous deformation of lower layers during the impact (which is
diagnostic of a bomb sag) the sediment has to be saturated in water. This water
can come from nearby environmental sources or from the explosion source as
recondensed steam.
To learn more about these experiments see the associated
video and release, and you can also look at some of the coverage of the
Geophysical Research Letters manuscript.
http://www.gatech.edu/newsroom/release.html?nid=127981
http://www.space.com/15592-mars-water-ancient-volcano.html
http://www.livescience.com/20110-evidence-ancient-mars-wet.html
http://www.msnbc.msn.com/id/47364007/ns/technology_and_science-space/t/ancient-mars-volcano-blast-hints-wet-history/
http://www.telegraph.co.uk/science/space/9250628/Mars-was-covered-in-water-just-like-the-Earth.html
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