Readers were quick to name the Caspian Sea as the location featured in our April 2016 puzzler. It took just a bit longer to puzzle out what caused the curious lines that crisscross the image. Are they gouges on the seafloor produced by trawling? Or are they are related to the movement of marine animals? Those are good guesses, but it turns out that the real culprit is ice.
Ice’s impact on the area becomes evident when you look back in time. The puzzler image (top) was acquired in springtime, on April 16, 2016; it shows open water in the vicinity of the Caspian Sea’s Tyuleniy Archipelago. On January 17, 2016, (second image) the same area is covered with fragmented ice.
Ice cover in some areas is easily deformed, rising upward and downward into hummocks. The keels of these hummocks can extend down through the shallow water to the seafloor. As wind and currents push the ice around, the keels drag along the seafloor like a rake to produce the gouges. Read more about the phenomenon in our April 23, 2016, image of the day.
Go even farther back in time and you see that the phenomenon is not new. “These scratches were found on the aerial photographs as early as the fifties of the last century,” said Stanislav Ogorodov, a scientist at Lomonosov Moscow State University. “They were published in the Russian-language scientific literature and unambiguously interpreted as ice gouges.” The image above shows ice gouges photographed from aircraft in 1954 and is described in this 2015 paper.
A number of readers suspected early on that the gouge marks had icy origins. James Varghese and Rachel were the first to comment on the blog and correctly describe the location and phenomenon. On Facebook, Jaouhar Mosbahi was the first to post a correct description. And Rodney Forster contributed insight in a twitter conversation with @NASAOceans, where the image was first released.