The Curious Case of the Caspian Sea’s Scars

http://www.nytimes.com/2016/04/29/science/the-curious-case-of-the-caspian-seas-scars.html

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What crafted these crisscrossing lines in the Caspian Sea?

It’s a question that had Norman Kuring scratching his head earlier this month when he first saw this satellite image of the shallow waters surrounding the Tyuleniy Archipelago.

Mr. Kuring, an oceanographer for NASA, often examines images taken by the agency’s Landsat 8 satellite, which orbits some 440 miles above Earth. But he had never before seen what looks like underwater scars stretching for miles across the green and blue seafloor.

“I had no idea what they were,” he said. “I thought maybe they were marks of trawlers which sometimes disturb the bottom.”

He and his colleagues posted the peculiar picture to Twitter and asked for help.

Others were also befuddled. Some suggested it might be marks left by a motorboat propeller. But after a few tweets they found a more likely suspect: ice.

The ice in the northern part of the Caspian Sea is thin, only about a foot and a half, but the water by the islands is also shallow, about 10 feet deep.

The theory was that large, frozen chunks had been blown across the sea until they piled on top of each other, like ice cubes in a pitcher. The stacked ice might have been deep enough to gouge the seafloor. As the winds blew, the ice was dragged across the bottom, plowing the sea grass and algae.

With his new hunch, Mr. Kuring turned back to the Landsat 8 images and rewound them to January when winter still grasped the region. The photos showed that the area was covered in ice at the exact spots where the marks were found.

“It was even clearer at that point that most of the marks were caused by ice blown around by the wind,” Mr. Kuring said. “It’s like a big rake across the bottom.”

Mr. Kuring and his colleagues’s suspicion was confirmed by Stanislav Ogorodov a researcher at Lomonosov Moscow State University who had previously observed the sea scars, known as scour marks. According to NASA, after Dr. Ogorodov looked at the images, he said the scratches were “undoubtedly” ice scour marks, melting away the mystery.