Mojave Crater on Mars is source of Mars rocks found on Earth

Mojave crater showing its well-preserved morphology and ray pattern. 

The direction of the red arrow indicates distant examples of secondary crater clusters shown in Figure 1B in the manuscript. 

Credit: Science/AAAS

A trio of researchers, two from France and one from Norway, has published a paper in the journal Science claiming to have found sufficient evidence to identify a specific crater on Mars as the origin of Mars rocks found on the Earth's surface.

In their paper Stephanie Werner (CEED Norway), Anouck Ody (France) and François Poulet describe their extensive research and how they came to their conclusions.

Scientists have known for many years that some of the meteorites that strike the Earth came from Mars. Such rocks are ejected from the surface of Mars when struck by meteorites themselves.

Mars rocks have been classified into three main categories depending on their chemical makeup: chassignites, nakhlites and shergottites.

The latter make up roughly three quarters of all such rocks found and have been studied extensively over the years.

In this new effort, the researchers contend that all shergottites come from a single source on Mars: Mojave Crater.

To establish Mojave Crater as the source, the researchers combined several types of data, each of which suggest the crater as a likely suspect, though there is one, the estimated age of the rocks, that is not quite as clear-cut as the others.

MRO-CTX image mosaic of Mojave’s crater interior. 

Credit: Science/AAAS

The researchers began by analyzing imagery of Mars surface taken by various spacecraft over the years—they were looking for reasonably recent formation, a large size and rays leading away from the crater indicating a blast capable of sending rocky debris into space.

Mojave Crater stood out as one of the best candidates for further study.

The team next examined data gathered by spacecraft that have orbited the Red Planet over the years, specifically those that had taken mineral scans (measures of wavelengths of light bounced back off of them) of the Mojave Crater and the area around it.

Analysis showed that the mineral composition of rocky material on the lip of the crater matched closely with the Mars rocks found on Earth.

The team also sought to determine the age of the crater by studying the surface area around it—the number of craters around it and their size allows for a model to be made based on techniques developed with the Apollo moon project.

Their study showed that the crater was likely formed approximately three million years ago. Finally, prior research has shown that the Martian rocks likely were in transit on average less than five million years, based on cosmic ray exposure.

THEMIS daytime image mosaic overlain by MOLA color-coded topography. 

Craters were counted for the plateau units (brown), channel units (blue-grey), and the continuous ejecta unit of Mojave Crater (red line). 

Credit: Science/AAAS

Prior research has shown that the plateau in which Mojave Crater exists is approximately 4.3 billion years, which suggests that rocks blasted from its surface should be near the same age. Unfortunately, that's where things don't match as well.

Most studies of Mars rocks have found them to be only 150 to 600 million years old.

The researchers suggest this anomaly can be explained by events (shock waves, etc.) that transpired on Mars that set their age clock back.

More information: The Source Crater of Martian Shergottite Meteorites, Science DOI: 10.1126/science.1247282

NASA MARS Rovers: Mojave Crater

Mars Rover Picture of Mojave Crater

A digital terrain model generated from a stereo pair of images provides this synthesized, oblique view of a portion of the wall terraces of Mojave Crater in the Xanthe Terra region of Mars.

This view, in which the vertical dimension is exaggerated three-fold compared with horizontal dimensions, shows the ponding of material backed up behind massive wall-terrace blocks of bedrock.

Hundreds of impact craters on Mars have similarly ponded features with pitted surfaces. These "pitted ponds" are thought to result when material melted by the crater-causing impacts is captured behind the wall terraces.

Mojave Crater is approximately 60 kilometers (37 miles) in diameter, centered at 327.0 degrees east longitude, 7.5 degrees north latitude. The portion of its northwestern edge shown here spans about 3.5 kilometers (about 2.5 miles) in width halfway between the bottom and top of the image. The view is toward the north.

Mojave is one of the freshest large craters on Mars. A survey of its features indicates very few overprinting craters on them, and an analysis of that infrequency suggests the crater may be as young as about 10 million years, very young for a crater of this size. The depth of the crater -- about 2.6 kilometers (1.6 miles) -- also demonstrates that Mojave has experienced little infilling or erosion.

Mojave gives us a glimpse of what a very large complex crater looks like on Mars. In a sense, it is a "Rosetta Stone" of craters, given that it's so fresh and most others -- especially this size -- have been affected by erosion, sedimentary infilling and overprinting by other geologic processes. Such fresh craters give insight into the impact process: ejecta, melt-generation, deposition, etc.