A bomb sag provides more evidence of water on early Mars

Georgia Tech Assistant Professor Josef Dufek is attempting to learn more about the past atmospheric conditions of Mars. Dufek’s first research tool was a rock fragment propelled into the Martian atmosphere during a volcanic eruption roughly 3.5 billion years ago. The deposit landed in the volcanic sediment, created a divot (or bomb sag), eventually solidified and remains in the same location today. Dufek’s next tool was the Mars rover. In 2007, Spirit landed at that site, known as Home Plate, and took a closer look at the imbedded fragment. Dufek and his collaborators at the University of California-Berkeley received enough data to determine the size, depth and shape of the bomb sag.

This bomb sag on the surface of Mars was created during a volcanic blast approximately 3.5 billion years ago. Assistant Professor Josef Dufek has replicated the sag in his lab while studying the possibility of water and atmospheric conditions of early Mars. Credit: NASA/JPL

Dufek and his team then went to the lab to create bomb sags of their own. They created beds of sand using grains the same size as those observed by Spirit. The team propelled particles of varying materials (glass, rock and steel) at different speeds into dry, damp and saturated sand beds before comparing the divots with the bomb sag on Mars. No matter the type of particle, the saturated beds consistently produced impact craters similar in shape to the Martian bomb sag.

By varying the propulsion speeds, Dufek’s team also determined that the lab particles must hit the sand at a speed of less than 40 meters per second to create similar penetration depths. In order for something to move through Mars’ atmosphere at that peak velocity, the pressure would have to be a minimum of 20 times more dense than current conditions, which suggests that early Mars must have had a thicker atmosphere.

Sources:

• Ancient Volcanic Blast Provides More Evidence of Water on Early Mars, Georgia Tech, May 3, 2012
• Mars Impact Experiments, YouTube video
• Manga, M., A. Patel, J. Dufek, and E. S. Kite (2012), Wet surface and dense atmosphere on early Mars suggested by the bomb sag at Home Plate, Mars, Geophys. Res. Lett., 39, L01202