In the shadow of lunar waters

A map from McGovern, J. A., et al. (2012), Mapping and characterization of non-polar permanent shadows on the lunar surface, Icarus, 223, 566 – 581, doi: 10.1016/j.icarus.2012.10.018 showing the permanently shadowed regions of the lunar south (in red) from a polar perspective. Jasmeer has been expanding on PVL's work in this area, adding in additional cold traps for our exospheric model and collaborating with researchers in Hawaii. His preliminary results are being presented as a poster here in Houston this week.

By Jasmeer Sangha

As you may have guessed from the posts preceding this one, I along with most of PVL am attending LPSC. This seems like as good a time as any to introduce my project which will be there in poster form. My project has grown and evolved since I last mentioned it on this blog, titled ‘The Waiting Game’. My current project is aimed to understand why the lunar poles ice abundances look as they do today. Observations have shown that water ice signatures are found near the lunar poles. However, unlike Earth, the local maximas of these ice signatures do not occur at the rotational poles. In order to obtain a full understanding of the processes on the lunar surface, my results and interpretations of those results will be built off of the groundwork done by three different people.

                  Firstly, to simulate particles on the moon I will be altering and adding to a code created by Prof. N. Schorghofer from the University of Hawaii. This code can be broken up into three main aspects: surface conditioning, Monte-Carlo based particle simulation and, particle trapping. The code initially creates a thick longitude and latitude grid representing the Moon’s surface. My simulation uses surface temperatures to imbue particles with the energy to hop to a new random location thus, to ensure the correct temperatures are used, the code uses the 1D heat equation to accurately reproduce the lunar surface. As the heat equation is time dependent, this process necessarily includes a spin up time. The lunar surface grid is exposed to sunlight for the equivalent of 12 synodic months to develop the appropriate equilibrium in temperature.

Particles are placed randomly anywhere on the surface, while keeping in mind that such particles are less likely to land further from the equator due to less surface area between lines of longitude. As aforementioned, the water particles rely on surface temperature to govern its flight time. The thermal energy transfer causes the ice particles to vaporize and hop to a new location on the planet. Once landing, particles have a chance to sublimate and freeze for a time based on surface temperature. As you can see, this process if fraught with randomness: where the particle starts, how much energy it has to move, where it lands and how long its going to stay there. To remedy this, the simulation starts with over a million particles and continues to slowly produce more as time goes by. But where do the particles end up?

The water particles will complete their journey in one of three ways. Least likely of the three, the particles can gain enough thermal energy to go fast enough and escape the Moon’s gravitational pull but these high temperatures are rare. Most likely, these particles will eventually disassociate, that is they are essentially cooked by the radiation coming in from the Sun. The most interesting ending though, is when these particles fall into traps. Due to the Moon’s low rotational obliquity, there exist areas on the surface where sunlight never reaches. These permanently shadowed regions (PSRs) are most common near the poles as crater walls block the Sun’s rays but can also be found at lower latitudes as well. PSRs are home to some of the coldest temperatures found in our solar system. It can be shown that when water particles land in these regions, they will never be able to gain enough thermal energy to jump back out. My project looks at where these particles find their final resting spots and why they may seem more abundant in certain PSRs.   

This phenomenon has been tested for mid latitude regions and the South pole but never for the North and never with this level of freedom (in terms of particle initial positions). Bringing me to my second pillar of research. My advisor, John, has applied an exospheric model to the lunar South pole and found that concentrations of water molecules were governed by diffusive properties. That is to say that concentrations were lower at higher latitude PSRs because the water would be less likely to jump over the low latitude PSRs in favour of their equatorward neighbors. This is similar to rain shadows on Earth, some areas will not get precipitation due to a potential energy barrier (mountain, or in the Moon’s case a crater).

The final pillar is Prof. M. A. Siegler. Prof. Siegler proposed that lunar water migration in the last couple of billion years is a non-factor when it comes to the distribution of water ice near the poles. He proposes that due to large mass deficits, particularly associated with the Procellarum KREEP Terrane (PKT), the Moon has changed its rotation axis over time. According to Siegler, the local maxima of icy material near the North and South poles are nearly antipodal: there is approximately 180º between their positions on the surface. This suggests that the Moon may have been rotating around these poles at one time. Due to a large impact, the Moon’s rotation axis would have changed by a few degrees giving the new rotation axis today. This process would explain why the maxima are off by a few degrees but relies on the fact that no ‘new water’ would accumulate at the present rotational poles.

After a plethora of simulation time, I am able to contrast and compare these two hypotheses to the results given by a robust exospheric modeling method. Find out about my conclusions by visiting my poster and I on Thursday (or take a look at this link of the e-poster: http://www.hou.usra.edu/meetings/lpsc2017/eposter/2144.pdf).