New type of impact crater discovered on Mars

Lessons from underground nuclear tests and explosive volcanoes may hold the answer to how a category of unusual impact craters formed on Mars. The craters feature a thin-layered outer deposit that extends well beyond the typical range of ejecta, said Nadine Barlow, professor of physics and astronomy at Northern Arizona University. She has given them a name -- Low-Aspect-Ratio Layered Ejecta Craters -- and presented the findings this week at the American Astronomical Society Division for Planetary Sciences in Denver. Barlow found the LARLE craters while poring over high-resolution images to update her highly popular catalog of Martian craters. "I had to ask, 'What is going on here?' " Barlow said. Delving into "explosion literature," Barlow said she and her collaborators learned more about a phenomenon known as base surge. After a large explosion, fine-grain material forms a cloud and moves out along the surface. The cloud erodes the surface and picks up more material, creating an extensive outer deposit. By adjusting equations from volcano research for Martian conditions, Barlow said, the researchers, including Joe Boyce, an NAU alum from the University of Hawaii, could accurately explain the "thin, sinuous, almost flame-like deposits." "So we think we're on to something," Barlow said. The craters are found primarily at higher latitudes, a location that correlates with thick, fine-grained sedimentary deposits rich with subsurface ice. "The combination helps vaporize the materials and create a base flow surge," Barlow said. The low aspect ratio refers to how thin the deposits are relative to the area they cover. Barlow, Boyce and Lionel Wilson, of Lancaster University, relied on the stream of data that continues to flow from ongoing surveillance of Mars. Older data from the Mars Odyssey Orbiter was used for a global survey, but more detailed studies referred to high-resolution images from the Mars Reconnaissance Orbiter -- about six meters per pixel. "We're looking in more detail at these deposits to find out what their characteristics are," Barlow said. "We can see dune-like structures and the hollows that occur in the outer deposit." Barlow said she hopes to complete the revision of her catalog within a year, and welcomes surprises such as the LARLE finding along the way. "That's part of the fun of science, to see something and say, 'Whoa, what's that?' " she said. "Projects like this end up leading to proposals."