STEMJazz Talket with Alex Evan
Abstract
Alex Evans in the Department of Earth, Environmental & Planetary Sciences regaled us with tales of Luna and how she’s aging. Or more generally, how one can guess the age of surface features on a solar system orb (and by extension, elsewhere in the galaxy if you could see the surface). The problem was immediately captivating since it’s about stochastic estimation tied to fundamental physics, and Alex is a computational planetary scientist.
The question is whether you can look at an orb (a solid one - not a gas giant) and date its surface features. On earth that’s tough because even aside from life, it’s a dynamic surface – atmosphere, ocean, tectonics, volcanism. The moon, however, is moderately “dead” – no appreciable atmosphere, not a bunch of volcanism and no tectonics. So, how do you date the surface?
Well, you first have to think about solar system formation. It was violent and there were lots of rocks flying around – somewhat chaotically. And stuff smashed into each other even after things had cooled down quite a bit. So on an airless body, you’ll get impacts and those impacts will cause craters. And on the moon, those craters persist because there’s nothing to erase them – except more impacts of course.
But not so fast! If you look at the nearside and farside of the moon, you’d think that the nearside is so very much younger cuz it’s so smooth. Well, there’s not a LOT of volcanism, but there WAS volcanism in its past, so smooth looking patches could be very very old – volcanos erased the crater evidence. Then there’s the complicating factor that when there’s an impact (or an eruption), the material can go a REALLY long distance (~1/6 earth gravity and no air) and create NEW craters.
What Alex does, through a combination of physics, observation and powerful computation, is suss out the conditions in the early and not so early (but prebiotic) solar system and through that determine how bodies probably formed and how old various parts of those bodies are. His clock (aside from geological events) is crater spectra (number and size).
The question Alex left us with was a puzzling gap in the observed ages of lunar features. The moon got a lot of craters early on (lots of data for that). And this happened again about 4-3.9Ga (Giga years) ago, but not quite as intensely. In between and going forward there are unexplained gaps. It feels like the five order of magnitude size gap in matter atoms (1A) and protons (1/10000 A). And the gaps imply something fundamental about the dynamics/physics.
In any case, thanks Alex, for a highly accessible talk.
