A Puzzling Moon

This week, Jasmeer Sangha talks about his work extending his simulations to include many different species of planetary volatiles bouncing around on the moon. While water was known to be present, it was LCROSS (depicted above in this artist's concept image from Northrop Grumman) that discovered a wide range of different compounds in the PSRs.

by Jasmeer Sangha

This semester I chose to extend my research past just water molecules and shift focus towards results brought back from LCROSS, the Lunar Crater Observation and Sensing Satellite. The LCROSS mission launched in 2009 and scientists found more than just water on the moon. The mission objective was to have the Centaur, a rocket stage, launch itself into the moon. It was decided that Cabeus, a large crater near the lunar south pole, would be the Centaur’s destination. Cabeus is a permanently shadowed region which allows freezing temperatures to trap particles in a layer a frost.  The debris cloud made by the impact would be analyzed by LCROSS, orbiting up above, to discern the frost's composition. From this experiment, it has been shown that carbon, nitrogen, and sulfur compounds were present in Cabeus, yet water was the dominant constituent, outnumbering all other compounds 5 to 1.

                  Though we know what is in Cabeus crater we are unsure as to how it got there. My previous work focused on the mechanism of traveling water molecules along the surface. The program randomly spawned particles on the surface of the moon and followed there lives until they were trapped in a dark hole for eternity, cooked by the Sun’s rays, or lost to space - what lovely ways to go out, right?  The results would show us how water ice is distributed on the surface. 

 

                  Thus, from observations we know where the water is located and from simulations we know how it made it there. Yet we still haven’t on touched on why there are these volatiles jumping around on the moon’s surface. My new research aims to fill this gap. It has been hypothesized that these volatiles were a result of a combination of comets and asteroid impacts alongside solar wind deposits. The solar winds consist mainly of alpha particles, protons and electrons. These are stripped down hydrogen and helium particles which could recombine with heavier elements to create water, ammonia and methane. Alternatively, comets and asteroids often carry frozen volatiles on their surface that could easily populate surrounding areas after impact.

My previous work has been modified to simulate the travelling patterns of the other molecular substances found in Cabeus crater. Each molecule type has very different properties to consider: mass, dissociation energy and residence time being the most relevant. In extreme cases, a change in mass can cause particles to be too heavy to make large enough jumps into polar regions or so light that they fly off into space with any influx of energy. Dissociation energy is linked to the robustness of these molecules: their ability to stay together while being bombarded by radiation from the sun. If a molecule is weekly bonded, it would be destroyed before it could make progress towards any PSRs. Finally, residence time tells us how long a particle will stick to a surface between hops: its easy to see how small bouncy balls would have more interesting adventures compared to Velcro. After considering these properties, the results spit out of the other end can be compared to the observations from LCROSS. This comparison will allow us to narrow down a list of candidates that could have made the deposits on our Moon.

                  It is always exciting to start new projects and this section of my research has not disappointed. Tweaking small aspects of my code and watching as it creates a cascade of different results reminds me of why I started this journey into physics. Tinkering with marble tracks as a child, exploring Python toolboxes as a student, explaining the simplicity yet complexity of planetary motion to muggles were always play outside of work. I am lucky to have this opportunity to find play in the work I am doing now, to be putting together this jigsaw or at least telling the community, “Hey, I think this piece fits here!”