CRYO2ICE – ESA and NASA satellites jointly measure Antarctic sea ice | Polarjournal
CryoSat-2 measures the ice thickness of the polar ice caps and sea ice by radar from an altitude of 720 kilometers. Photo: ESA

There was promising news yesterday for climate scientists, oceanographers and other Earth system scientists: in the future, thanks to a joint venture between ESA and NASA, they will be able to make better statements about processes in the atmosphere and the ocean, at least according to expectations. The two “ice satellites” CryoSat-2 and ICESat-2 will meet at short intervals over the poles and provide coupled data from sea ice.

On Tuesday, managers of the European Space Agency (ESA) approved the joint CRYO2ICE campaign with NASA. For this purpose, the orbit of the European spacecraft CryoSat-2 is to be increased by almost one kilometer. This small change will greatly increase the number of simultaneous observations that CryoSat-2 can take with the American ICESat-2 mission. One result of this new strategy will be the very first reliable maps of sea ice thickness in Antarctica.

At present, it is hardly possible to reliably measure the thickness of Antarctic sea ice floes. Heavy snow can pile up on the floating ice and obscure the actual vertical dimension. Powerful snow conditions can even push the sea ice under the surface of the water.

The European CryoSat-2 satellite radars the thickness of the ice protruding from the water (freeboard) and uses the known ratio of 9:1, with 90 of the ice being underwater and 10 above the water surface, the total thickness of the sea ice. ICESat-2 measures the ice thickness in the same way, but cannot penetrate the snow cover with its laser. Graphic: ESA

But the researchers believe that the collaboration of the various instruments on the two satellites can help resolve this complexity. NASA’s ICESat-2, which orbits Earth at an altitude of about 500 km, uses a laser to measure the distance to the Earth’s surface and thus the height of objects. This beam of light is reflected directly from the top of the snow.
ESA’s CryoSat-2, on the other hand, uses a radar system at an altitude of about 720 km as an altitude-measuring instrument that penetrates much deeper into the snowpack before being thrown back.

So far, scientists are working with relatively old climate models to estimate the likely depth of snow cover when observing sea ice in both the Arctic and Antarctic. And while these models work quite well in the north, they are virtually worthless in the south.

“When ICESat and CryoSat work together, it’s like a closed measurement system where we no longer have to rely on outdated data sets. In the future, we will be able to estimate the snow cover and thus the sea ice thickness more precisely. In the Arctic, this will reduce our mistakes. In Antarctica, I don’t think we really know how cool that could be.”

Dr. Rachel Tilling, NASA’s radar and laser altimetry scientist

Sea ice in Antarctica is very variable in space and time. In winter, it can extend to an area of up to 18 million square kilometers – more than the area of the continent with 14 million square kilometers. In summer, however, the drift ice melts to only two to three million square kilometers. But without an accurate assessment of the third dimension – the floe thickness and thus the volume – scientists lack some key insights into the effects of this large fluctuation in ice extent.

The data from the two satellites working with different systems will be brought together for the first time to enable detailed measurements of the ice thickness and snow cover at the poles. Video: ESA/NASA

“If you look at the thicker sea ice in the western Weddell Sea, for example – this is finally exported to the southern Atlantic. And what happens is essentially the export of fresh water (sea ice contains little salt when it freezes), which affects the distribution of ocean salinity and ocean circulation when the ice eventually melts,” says Dr. Mark Drinkwater, Head of Earth & Mission Science at ESA. “And if we can say something about the snow load on the sea ice, then we can also say something about the rainfall in Antarctica. This is something we are completely clueless about. So we would get both atmospheric and oceanic insights.”

CryoSat-2 will ignite its engines on July 16 to rise a few hundred meters higher into the sky. The maneuver, which will take several weeks, is not intended to affect the longevity of the mission, as the spacecraft has plenty of fuel on board.

ESA’s CryoSat mission leader Dr Tommaso Parrinello told BBC News: “ICESat is quite far below us, so we can’t go down to meet them, but by going up we find this incredibly resonant orbit where we’ll meet for 19 earth orbits each for us and 20 earth orbits for them – within a certain time delay. Basically, we meet every 1.5 days within a few hours over the poles, and that means that we can observe almost the same ice at almost the same time. […] This could be revolutionary. Perhaps we will never again have the opportunity to do this with two instruments that are so different.”

Perhaps the closest thing to this dual observation mode is a future radar spacecraft codenamed CRISTAL. This would map sea ice in two frequency bands to capture some of the differences in the backscattering of satellite signals through snow and ice, but the effect will not be as pronounced as between CryoSat and ICESat.

Last week, ESA’s Industrial Policy Committee gave Airbus in Germany green lights to start developing CRISTAL. The mission could start by the end of the decade if sufficient funding can be raised by the Agency and its project partner, the European Commission.

Source: Jonathan Amos, BBC

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