A flattened 1,800-mile-wide section of the moon was likely caused by a collision with an asteroid the size of Austria, Japanese researchers say.

Scientists at the National Institute of Advanced Industrial Science and Technology said the moon's Procellarum basin, the dark part as seen from Earth many say resembles a "man in the moon," is what remains of the impact crater.

The researchers studied the distribution of minerals in mapping data obtained by the Japanese moon exploration orbiters, The Asahi Shimbun reported.

One of the minerals present is low-calcium pyroxene, likely generated when rock melted under the heat of an impact and concentrated in a near-circle around the Procellarum basin, the researchers said.

They said they believe an asteroid more than 180 miles in diameter hit the moon some 3.9 billion years ago, leaving a crater and melting much of the moon's rock under the energy of the impact.

The study was published in the British journal Nature Geoscience.