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 180ยบ 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).
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