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Researchers have discovered the largest fresh meteor-impact crater ever spotted on Mars.

The before-and-after images were captured by NASA's Mars Reconnaissance Orbiter.

The huge crater spans half the length of a football field and first appeared in March 2012.

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Before and after: This pair of images taken one day apart by the Mars Color Imager (MARCI) weather camera on NASA's Mars Reconnaissance Orbiter reveals when an asteroid impact made the scar seen in the right-hand image. The left image was taken during Martian afternoon on March 27, 2012; the right one on the afternoon of March 28, 2012.

The largest crater associated with a March 2012 impact on Mars has many smaller craters around it, revealed in this image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.

The impact that created it likely was preceded by an explosion in the Martian sky caused by intense friction between an incoming asteroid and the planet's atmosphere.

'This series of events can be likened to the meteor blast that shattered windows in Chelyabinsk, Russia, last year,' the researchers say.

'The air burst and ground impact darkened an area of the Martian surface about 5 miles (8 kilometers) across.'

The darkened spot appears in images taken by the orbiter's weather-monitoring camera, the Mars Color Imager (MARCI).

Since the orbiter began its systematic observation of Mars in 2006, scientist Bruce Cantor has examined MARCI's daily global coverage, looking for evidence of dust storms and other observable weather events in the images.

About two months ago, Cantor noticed an inconspicuous dark dot near the equator in one of the images.

'It wasn't what I was looking for,' Cantor said.

'I was doing my usual weather monitoring and something caught my eye.

These images from the Context Camera (CTX) on NASA's Mars Reconnaissance Orbiter were taken before (left) and after (right) an apparent impact scar appeared in the area in March 2012 and was detected in images from the orbiter's Mars Color Imager (MARCI). Comparing the Jan. 16, 2012, image on the left with the April 6, 2014, image on the right confirms that two adjacent fresh craters appeared during that interval.

'It looked usual, with rays emanating from a central spot.'

He began examining earlier images, skipping back a month or more at a time.

The images revealed that the dark spot was present a year ago, but not five years ago.

He homed in further, checking images from about 40 different dates, and pinned down the date the impact event occurred; the spot was not there up through March 27, 2012, and then appeared before the daily imaging on March 28, 2012.

Once the dark spot was verified as new, it was targeted last month by CTX and the orbiter's sharpest-sighted camera, the High Resolution Imaging Science Experiment (HiRISE).  

Six-and-a-half month journey across Mars cut into 4min film

This April 6, 2014, image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter shows numerous landslides in the vicinity of where an impact crater was excavated in March 2012.

Of the approximately 400 fresh crater-causing impacts on Mars that have been documented with before-and-after images, this is the only one discovered using a MARCI image, rather than an image from a higher-resolution camera.

'The biggest crater is unusual, quite shallow compared to other fresh craters we have observed,' said HiRISE Principal Investigator Alfred McEwen of the University of Arizona, Tucson.

The largest crater is slightly elongated and spans 159 by 143 feet (48.5 by 43.5 meters).

McEwen estimates the impact object measured about 10 to 18 feet (3 to 5 meters) long, which is less than a third the estimated length of the asteroid that hit Earth's atmosphere near Chelyabinsk.

Because Mars has much less atmosphere than Earth, space rocks of comparable size are more likely to penetrate to the surface of Mars and cause larger craters.