Analysis of ancient volcanic eruptions offers more evidence early Mars was saturated with water and that its atmosphere was considerably thicker—at least 20 times more dense—than it is today. The atmosphere of Mars today is less than 1 percent the density of Earth’s—one of the reasons liquid water covers much of our planet but can’t exist on the Red Planet. As more research points toward the possibility of water on early Mars, scientists have increased their studies on the density of its atmosphere billions of years ago, but it’s not an easy task. In fact, it’s difficult to even determine Earth’s atmospheric pressure from the same time frame. “Atmospheric pressure has likely played a role in developing almost all Mars’ surface features,” says Josef Dufek, assistant professor in the School of Earth and Planetary Science at Georgia Institute of Technology (Georgia Tech). “The planet’s climate, the physical state of water on its surface and the potential for life are all influenced by atmospheric conditions.” As reported in the journal Geophysical Research Letters, 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. His 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. 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. “Our study is consistent with growing research that early Mars was at least a transiently watery world with a much denser atmosphere than we see today,” says Dufek. “We were only able to study one bomb sag at one location on the Red Planet. We hope to do future tests on other samples based on observations by the next rover, Curiosity.” Curiosity is scheduled to land on Mars on August 5. The research was supported by the National Aeronautics and Space Administration.