The atmosphere of Mars is less than 1 percent the density¹ of Earth's. It's one of the reasons liquid water covers much of our planet but cannot 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. It's not an easy task. In fact, it's very difficult to even determine Earth's atmospheric² pressure from the same time frame. Georgia Tech Assistant Professor Josef Dufek is attempting to learn more about the past atmospheric conditions by analyzing³ two unlikely sources: ancient volcanic⁴ eruptions⁶ and surface observations by the Mars rover Spirit. His new findings, published by the journal Geophysical Research Letters, provide more evidence that early Mars was saturated⁷（饱和的） with water and that its atmosphere was considerably⁸ thicker, at least 20 times more dense⁹, than it is today.

"Atmospheric pressure has likely played a role in developing almost all Mars' surface features," said Dufek, an instructor¹⁰ in the School Earth and Atmospheric Sciences. "The planet's climate, the physical state of water on its surface and the potential for life are all influenced by atmospheric conditions."

Dufek's first research tool was a rock fragment propelled into the Martian atmosphere during a volcanic eruption⁵ roughly 3.5 billion years ago. The deposit landed in the volcanic sediment¹¹, created a divot（草皮） (or bomb sag¹²), eventually solidified¹³ and remains¹⁴ 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.

Dufek and his team then went to the lab to create bomb sags¹⁵ 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 craters¹⁶ similar in shape to the Martian bomb sag.

By varying the propulsion speeds, Dufek's team also determined¹⁷ that the lab particles must hit the sand at a speed of less than 40 meters per second to create similar penetration¹⁸ depths. In order for something to move through Mars' atmosphere at that peak velocity¹⁹（速率） , 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. Click here for a video demonstration²⁰.

"Our study is consistent with growing research that early Mars was at least a transiently watery²¹ world with a much denser²² atmosphere than we see today," said 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 material is based upon work supported by NASA under award No NNX09AL20G.