EAPS Graduate Student Studying Evolution of Impact Basins and the Moon’s Thermal History
Writer(s): Logan Judy
Alex Trowbridge is no stranger to the fascinating field of geology. Growing up surrounded by the mountainous terrain of Colorado, Trowbridge majored in physics with an emphasis on geology at Colorado College. That started a path that would ultimately lead to applications in planetary science.
“I always knew I wanted to do something with geology and physics,” he said. “After receiving my undergraduate degree, I worked at Lawrence Livermore National Labs, which captured my interest in high-pressure, high-temperature experiments.”
Now at Purdue EAPS as a PhD student in planetary science, Trowbridge’s research focuses on modeling impacts into the moon, using the principles of his national lab experience on a larger level. Based on research by his co-advisors, EAPS professors Dr. Jay Melosh and Dr. Andy Freed, Trowbridge’s work uses a state-of-the-art model for crater evolution. He applies the model to explain large concentrations of mass (called “mascons”) hidden beneath the lunar surface that change the moon’s gravitational field. These mascons caused problems for early lunar missions, but EAPS research has led to a greater understanding of them.
Previous studies led by Dr. Freed and coauthored by Dr. Melosh found that mascons were caused by massive asteroid impacts billions of year ago. Impacts excavate crust and deposit it at distance, which leads to a thinned inner basin and thickened crustal collar. Over millions of years, both the inner basin and the crustal collar rise a few kilometers. If a strong enough crust forms in time, the uplift of the inner basin is boosted by the crustal collar forming a mascon. This forms the basis for Trowbridge’s research.
“I’m expanding upon the work that they did to look at the distribution of mascons, because they only exist for a certain crater size range,” Trowbridge said. “Beyond just learning about crater formation and development, this will also help us obtain information about the thermal structure and composition of the moon during the time of impact, which are things that are really advantageous and kind of hard to figure out through other means.”
This research has led to Trowbridge receiving a NASA Earth and Space Science Fellowship, which is a competitive program. Approximately 10 percent of this year’s proposals were selected for funding. It is an important area of research, made possible because of the connection between the moon’s thermal structure and the development of basins. A hotter geothermal gradient, for example, will allow more crust to flow back into the basin center during crater formation. This allows Trowbridge to deduce facts about the moon’s thermal history by looking at the mascons.
“By modeling these basins with various geothermal gradients, we get different crustal structures for the same basin. We then compare to observed crustal structure for that crater and constrain the geothermal gradient at the time of impact.”
Trowbridge anticipates graduating with his PhD in 2019.