Comet Impacts & Water on the Moon
Due to the Moon’s orientation relative to the Sun, there are craters near the lunar poles that have not seen sunlight for more than two billion years, and are shrouded in cold, permanent shadow. Over the years, several spacecraft have detected signs of water ice within some of these craters, but we don't yet know for sure whether there are sheets of ice buried deep beneath the lunar surface, or whether the detected signatures come only from sparsely distributed ice grains, or something altogether different. Some clues to deducing how much water is present lie in asking where it could have come from. There are several answers to that question, and at CFPL, I've been investigating comet impacts as a source of lunar water.
Comets are remnant building blocks of the solar system, composed primarily of water and other ices. Over the last four billion years, a large number of these icy bodies are thought to have collided with the Moon. During impact, most of the ice in a comet vaporizes. Some of this vapor forms a temporary atmosphere that surrounds the Moon for months. The atmosphere is temporary because most of it is gradually destroyed by solar radiation, but some water could escape destruction by making its way to shadowed polar craters, where temperatures colder than 100 K (-170 C) allow ice deposits to remain stable over geological time scales. This raises a lot of interesting questions. How much water could comets have delivered to these cold traps? Also, if we can determine the location of a comet impact, then where should we look to find the water (and other volatile compounds) from this comet?
Addressing these questions increases our understanding of forces that have shaped the solar system (including our planet), and relating what numerical models suggest happened several billion years ago to what we can measure through remote sensing today is challenging and interesting work, which calls for both imagination and scientific rigor. Besides the need to interpret mission results, further interest in comets as a source of lunar ice has been motivated by recent evidence for water ice and deposits of dark, organic matter at Mercury’s North Pole, and the prospect of judicious utilization of lunar water to obtain oxygen and hydrogen to fuel interplanetary missions.
To learn more about what I've been working on, do stop by the project webpage, take a look at our recent work, or write to me at parvathy[dot]prem[at]gmail[dot]com.
To simulate volatile transport after a comet impact, I use the Direct Simulation Monte Carlo (DSMC) method, a computational technique that can be applied to a variety of engineering and scientific problems - from designing heat shields for atmospheric re-entry to modelling cometary comae. For my work, I've built on CFPL's planetary DSMC code, which other members of the group apply to other interesting planetary science problems - such as simulating the atmosphere and volcanic plumes of Jupiter's moon Io, and modeling the jets of water vapor that shoot out to space from the South Pole of Saturn's moon Enceladus.