This is the largest fresh impact crater anywhere ever clearly confirmed from before-and-after images. This HiRISE image is from May 9, 2014 / NASA/JPL-Caltech/Univ. of Arizona

A series of before-and-after images captured by NASA's Mars Reconnaissance Orbiter reveal the largest fresh impact crater ever clearly documented anywhere, not just on Mars. It spans 48.5 by 43.5 meters (that’s half the length of a football field) and first appeared in the spring of 2012.

Researchers think the meteor impact that created this new crater was preceded by an explosion in the sky when an asteroid hit the Red Planet’s atmosphere. When the air burst, the ground impact darkened the surface of an area 8 kilometers across. It was this darkened spot that showed up in images taken by the orbiter's weather-monitoring camera, the Mars Color Imager (MARCI). Of the 400 or so fresh impacts on Mars that have been documented with before-and-after shots, this is the only one discovered using a weathercam image -- rather than one from a higher-resolution camera.

By looking through earlier images, he figured out that the dot existed a year ago -- but not five. When he refined his search, he pinned down when the impact event must have occurred: between the afternoons of March 27 and 28 in 2012. Here's the before-and-after images from MARCI taken one day apart.

Last month, researchers targeted the spot using the orbiter's telescopic Context Camera (CTX) and its sharpest-sighted camera, the High Resolution Imaging Science Experiment (HiRISE). Two craters appear in an April 2014 CTX image that weren’t in an earlier one, confirming the dark spot is related to a new impact crater. HiRISE revealed more than a dozen smaller craters near the two larger ones seen in the CTX image. These were probably created by chunks of the exploding asteroid or secondary impacts of material ejected from the main craters during impact. Here is the before-and-after taken by CTX comparing January 2012 and April 2014 images.

The impact object probably measured about 3 to 5 meters long, according to Alfred McEwen of the University of Arizona in Tucson. Since Mars has much thinner atmosphere than Earth, space rocks of the same size are more likely to penetrate to the surface of Mars and cause giant craters.

HiRISE also revealed many landslides that could have darkened the slopes in the 8-kilometer surrounding area. Landslides could result from shock waves generated by the explosion of an asteroid through the thin Martian atmosphere, or from impacts of those fragments striking the ground.

Images: NASA/JPL-Caltech/Univ. of Arizona (top, fourth) & NASA/JPL-Caltech/MSSS (middle two)