Sophie Gruber, physics student at the University of Innsbruck and OeWF member is about to finish her Bachelor thesis. The Austrian Space Forum supported her research topic. In this blog post, she explains her thesis:

Where to look for life on Mars?

That question is for sure one of the most fundamental ones to answer, prior to crewed missions to Mars. But what does life as we understand it need to develop? Water! As it is one of the key ingredients to life. Therefore, when asking about extra-terrestrial life, one could ask as well where water (or water ice) can be preserved over a long period of time. That is exactly where my Bachelor thesis comes in. I spent the last 5 months with answering the question:

“What is the influence of dust layers on the lifespan of ice deposits in Martian caves?”

But why caves? And why are dust layers interesting? I will try to explain that in the following sections to give you an insight about my Bachelor thesis on “The sublimation and lifespan of water ice in caves on Mars”, which I wrote at the Austrian Space Forum (OeWF) to conclude my Physics studies at the University of Innsbruck.

As the title of my thesis already indicates, are caves on Mars somehow interesting locations. Especially on Mars, protection from radiation and dust storms is very important, when looking for habitable locations. Due to the fact that Mars no longer has a global magnetic field to protect the surface from solar radiation, it is more likely to find traces of life in environments, which are sheltered from that radiation. Additionally, provide caves generally more stable physicochemical conditions (e.g. temperature, humidity) than the Martian surface, hence favouring the deposition of ice and minerals.

Based on the model from Williams et al. (2010)¹, I studied the sublimation of water ice deposits in a cave environment as seen in figure 2. The major features which influenced the sublimation rate of the ice deposit are: the “chimney”, which allows for an air and humidity exchange with the outside air, the temperatures of the ice surface and the cave air, and dust layers, which are covering the ice deposit.

Figure1: The blue markings label the areas where water ice deposits are suggested to be stable in Martian caves

Sublimation is the phase transition from solids to vapor and thus can lead to mass loss of ice deposits. It occurs more naturally on Mars than on Earth, because the Martian atmospheric pressure is just about 1% of the Earth´s. The sublimation process is generally governed by the saturation water vapor pressure (ρsat) and the partial water vapor pressure (ρ). This means, the more humidity there is in the cave air, the less sublimation will take place, because the concentration difference between the ice surface and the cave air gets less and without a concentration difference there is no mechanism driving the sublimation process.

Figure 2: Diagram of the cave environment I studied for my thesis.

If we now also introduce dust layers on top of the ice deposits, things get slightly more complex. The diffusion process through a porous medium, as for instance Martian regolith, is regulated by its geometric properties. Such being: the porosity, quantifying the reduction in the cross-sectional space through which diffusion can take place, the tortuosity, which measures how convoluted the medium is and the pore size. Those parameters are only known for regolith near the surface at the Viking landing sites, but for the most part of the Martian regolith, they are unknown. Therefore, experiment data obtained from materials, which are analogous to Martian regolith, are essential for the theoretical assessment of locations on Mars, which are potentially harbouring water ice.

Taking those parameters into account yields the sublimation rates (and with that the mass loss rates) of ice deposits in specific caves on Mars. From that, a comparison of the lifespans of pure water ice deposits and ice deposits, covered by regolith layers, can be obtained. By this means can a 20 cm regolith layer prolong the lifespan of water ice deposits by factors up to 6. The resulting lifespans ranged from 350 years for pure ice deposits to 2100 years for covered ice deposits, depending on the different regolith types respectively.

Simulating Mars caves mission at the Dachstein Ice Cave in 2012. (c) OeWF (Katja Zanella-Kux)

Summarizing the findings from my thesis leads to the following criteria, which favour a prolonged lifespan of water ice deposits in caves on Mars:

• Regolith layers on top of the ice deposits with small pore space, high tortuosity and low porosity can significantly slow down the sublimation process.
• In cavities without air exchange with the surface, the air can be saturated well within the calculated lifespans, which would then prevent further sublimation.
• Cavities with a thick ceiling, would not be influenced by insolation (i.e. the cave air temperature would generally be colder).

When taken under consideration in the planning phase for crewed missions to Mars, those criteria can even help to enhance the chance of finding life on Mars.

If you have any further questions about my thesis, I would be happy to answer them!

Author: Sophie Gruber

References:

• Williams, K. E., McKay, C. P., Toon, O. B. and Head, J. W. (2010), “Do ice caves exist on Mars?”, Icarus 209, 358–368.

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