The ASU Center for Meteorite Studies is pleased to announce that Brandon Johnson, a graduate student at Purdue University, is the recipient of the 2012 Nininger Meteorite Award!
Brandon's paper, “Impact spherules as a record of an ancient heavy bombardment of Earth”, demonstrates a method to estimate the size and velocity of objects impacting Earth that create a spherule layer, based on the layer thickness and the average spherule size within the layer.
Asteroids hitting the Earth typically vaporize a mass of target rock comparable to the projectile's mass. As this vapor expands in a large plume or fireball, it cools and condenses into molten droplets. For asteroids larger than ~10 kilometers in diameter, these spherules are deposited in a global layer. Spherule layers preserved in the geologic record accordingly provide information about an impact even when a source crater cannot be found.
On Earth, impact craters can be quickly obscured or destroyed by surface weathering and tectonic processes. Thus, Earth's impact history is inferred from estimates of the current day impactor flux or the incomplete impact chronology of the moon. However, using geologic observations of spherule layers, and their method to determine the sizes of the projectiles that created the layers, Brandon and co-author Jay Melosh make the first direct estimates of the Earth's bombardment history. These estimates imply that the impactor flux was significantly higher 2.5 to 3.5 billion years ago than it is today. This conclusion is consistent with a steady decline in the post Late Heavy Bombardment impactor flux and can help constrain dynamical models of the solar system.
Brandon’s research was conducted under the advisement of Dr. H. Jay Melosh.
Each submission was reviewed by a panel of experts from a broad array of fields in meteoritical science.