Correlation of subsurface ice content and gully formations on Mars: testing the shallow aquifer theory of gully formation

This Master of Science diploma project in Space Engineering was performed at NASA Ames Research Center and is based on images from the Mars Orbiter Camera. Pictures taken by this camera onboard the Mars Global Surveyor show gully features resembling water-carved gullies on Earth. One theory of gully formation on Mars contends that the source of the water feeding the gullies is a shallow liquid water aquifer. Interestingly, the gullies tend to form in locations of relatively low ice content. The shallow aquifer theory was quantitatively tested by calculating the temperature and pressure of the Martian subsurface at the measured alcove base depths in order to determine if liquid water can exist at these locations.

Since the density and thermal conductivity of the soil depend on the amount of ice in the soil, the ice-to-soil ratio is an important factor. The thermal conductivity and density of the soil were calculated for the location of the gullies, assuming that the ice content in the soil would be the same at all depths down to the alcove base depth.

Around 59% of the gullies were found to fall outside of the temperature and pressure regime of liquid water at the alcove base depth when assuming an overburden consistent with the observed GRS ice content. However, it may be unrealistic to assume that the measured GRS ice content extends down to the depth of the gully alcoves. Therefore the thickness of a dry layer that must exist within the overburden column for the water to be liquid at the alcove base depth is estimated. These calculations build on the assumption that the soil has a fraction of overburden with dry and icy components where the icy layer has the same concentration of ice as measured by GRS. According to these calculations, liquid water could exist in approximately 81% of the gully locations.

Some 19% of the gully locations could not have liquid water at the depth of the alcove base because the required thickness of the dry layer exceeds the alcove base depth. For the gullies where liquid water cannot exist under the surface, no outstanding characteristics were found regarding the albedo, elevation, channel length and thermal inertia. However, all of these gullies had very shallow alcove bases.

It is possible that the gullies that could not have liquid water at the alcove base depth have been formed in a different way than the other gullies, such as melting ground ice, snowmelt or a deep aquifer source.

Furthermore, the thesis shows that PCA is a very effective method to characterize the changes in the control action.

It can also be concluded that the control for basis weight is the most important variable if multiple CD-controls are analysed.

Author: Edlund, Jeanette

Source: Lulea University of Technology

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