Looking at Earth from space, we have learned a lot more about our extraordinary planet. But sometimes we are left absolutely clueless on the origin of the features we see.
The latest such feature is the peculiar scour marks spotted by the Operational Land Imager on the Landsat 8 satellite, which is part of NASA’s Earth Observatory. The image is a natural-color view of the shallow waters surrounding the Tyuleniy Archipelago in the Caspian Sea. It shows clearly the islands, the dark green vegetation on the sea floor, and some mysterious "scars".
An image of the Archipelago from January 17, 2016, shows the markings present underneath the ice. NASA
These markings were first observed in January, and researchers thought they might have been an ephemeral feature, but they soon came to realize they were actual marks on the seabed. “You can tell this by the fact that marks laid down in January have not moved by April,” Norman Kuring said in a statement. “If those were water features, they would not persist through one tidal cycle.”
NASA decided to put the question to Twitter, giving the public the chance to solve the puzzle. Several explanations were put forward, with human action being a likely suspect. Both propellers and fisheries employing bottom trawling can rip the vegetation from the sea floor.
But the answer is actually a natural phenomenon: the scour marks were created by ice.
The latest image of the Tyuleniy Archipelago with the markings clearly visible. NASA
“Undoubtedly, most of these tracks are the result of ice gouging,” said Stanislav Ogorodov, a scientist at Lomonosov Moscow State University, who has previously published research on the phenomenon.
This area of the Caspian Sea is very shallow, no more than 3 meters (10 feet) deep. When it freezes over the ice doesn’t reach the bottom, but when the sea starts thawing ice fragments are pushed against each other, forming "hummocks" – which is a bit like a hill coming out of the sea. Some pieces rise above the surface, and others are pushed down to the bottom. Wind and water currents move these hummocks around, and their bottoms drag across the sea floor, producing the marks seen by NASA.