Finding free water in the icy seas of Greenland

"Three, two, one … forget it!"

Researchers at NASA's Oceans Melting Greenland campaign heard this phrase 239 times this fall. Each time, a member of the team was forced to drop a scientific probe from a plane into the seawater along the Greenland coast. The probes are part of a five – year effort to improve our understanding of the role of the ocean in the rapid loss of ice in Greenland.

Since 2016, OMG has been collecting measurements around the Big Island three times a year. Each spring, a research aircraft measures the height of the icecap after the snow in winter. In summer, instruments embarked on boats map the seabed around Greenland. In September or October, OMG principal investigator, Josh Willis, of NASA's Jet Propulsion Laboratory in Pasadena, California, and a team of researchers, pilots and engineers travel to Greenland to dump up to 250 biodegradable probes in the ocean, thus making the tour of the coast to measure the temperature of the water touching the glaciers of Greenland.

If you have already dropped a water balloon from a height, you may imagine that the probe removal process is so simple. In reality, it is much more difficult. Each target drop site is selected in advance to provide the best scientific benefit for understanding ocean temperatures and currents. Flying at 200 mph at a height of 500 feet, the researchers must time each fall so that the probe reaches the open water – sometimes the only open water in an ice-clogged fjord. And all this is happening in Greenland, where weather conditions can make reaching the target site the biggest challenge of all.

After three years of operation, researchers learned to recognize dubious weather phenomena and to avoid difficult areas as much as possible, for example by exchanging a set of targets in the south against a set in the north. But it is impossible to know in advance the local weather conditions at a drop site. "If the fog is low and we can not see that the water is free of icebergs, people and whales, we can not give up," Willis said. In these cases, he can search for a similar site nearby to obtain comparable data or simply move to the next target.

The other major problem is sea ice. The probes weigh 14 pounds – not enough to pierce through the thick ice up to the water below. If a drop site is totally covered with ice, there is nothing to do but move on. But many potential targets are found in areas where sea ice has broken or shifted away from the shore, or where large icebergs have cut canals into the ice cover. In these cases, Willis must weigh the value of the data from this particular location with the risk that the probe will lack open water.

"There are always one or two places where it seems impossible to get a probe between icebergs and to communicate data, and then we'll do it," Willis said. "These Mission Impossible moments are kind of nice."

The campaign is also deliberately designed to avoid impacts on the environment and marine life, including using biodegradable materials and limiting noise. The probes are similar to those used by hurricane hunters to measure the temperature of the water. They look like big postal tubes. When the instruments land on the surface of the ocean, the probe is released and flows silently, measuring the temperature and salinity, while a floating part transmits data by radio waves to the plane located above. After about ten minutes, the transmission stops and the probe is out of order. The entire instrument flows to the bottom of the ocean, where it is covered with sediment and breaks down over time.

The data collected by the probes have yielded significant scientific results and are of interest to local populations. "We have had such spectacular success so far, I find it hard to believe that we can do it another two years," Willis said. "I'm like a kid in a candy store."

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