Monday, August 20, 2012

Eruptive Paleobarometers on Mars

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).

Martian Bomb Sag Image
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.

No comments:

Post a Comment