ICESat: Space Laser for an unprecedented view of the Earth's ice

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The NASA

The US Space Agency is about to put a laser in orbit to measure the state of the Earth's ice cover.

The satellite mission, called ICESat-2, is expected to provide more precise information on how these frozen surfaces are affected by global warming.

Antarctica, Greenland and ice floating on the Arctic Ocean have all lost volume in recent decades.

ICESat-2 will follow the changes underway with unprecedented details from its perspective located approximately 500 km above the planet.

A Delta II rocket is planned to take the satellite laser into space Saturday.

Take-off from Vandenberg Air Force Base in California is scheduled for 5:46 am local time (12:46 GMT, 1:46 pm Paris time).

As its name indicates, ICESat-2 is a complementary project. The original spaceship flew in the 2000s and was the pioneer of laser measurement of the height of polar glaciers and sea ice from space. But the mission was plagued by technical problems that limited its observations to a few months a year.

NASA has since reshaped technology, both to make it more reliable and more efficient.

"ICESat-2 will observe the cryosphere with a spatial resolution at the unseen level of space," says Pr Helen Fricker of the Scripps Institution of Oceanography.

"The beam is divided into six or three pairs – so we can further map the surface of the ice and estimate the slope of the surface, which can disrupt our interpretation of changes in height.

"The orbit reaches two degrees from the poles and the same ground tracks are sampled every three months, which gives us seasonal snapshots of the ice heights.With these data, we can identify the processes responsible for the loss. of ice in the polar regions ". told BBC News.

Why is this mission important?

Antarctica and Greenland lose billions of tons of ice every year – largely because of the hot water that can melt the land-based glaciers where they meet the ocean. This waste is slowly but surely pushing the sea level around the world.

In the Arctic, seasonal floes have also been behind. It is thought that sea ice in the Far North has lost two-thirds of its volume since the 1980s. And although it does not have a direct impact on the height of the oceans, the ice cover reduced amplifies temperature increases in the region.

Much of the change happening at the poles can be subtle, says Dr. Tom Neumann, NASA's ICESat-2 project researcher. A very precise tool is necessary to characterize it well.

"An altitude change of just one centimeter above an Antarctic ice cap represents a huge amount of water gained or lost by the icecap, a value of 140 gigatonnes."

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empics

How does ICESat-2 work?

Weighing half a ton, the new laser system is one of the largest instruments of Earth observation ever built by NASA. He uses a technique called "photon counting".

It draws about 10,000 pulsations of light every second. Each of these shots goes down to Earth and bounces on a time scale of about 3.3 milliseconds. The exact time is the height of the reflective surface.

"We shoot about one trillion photons (light particles) every time we shoot, we have about one back," says Cathy Richardson, who works in the NASA team that developed the instrument.

"We can time this photon when it comes back as exactly as when it left the instrument.And from there, we can calculate a distance of about half a centimeter on the Earth."

The laser makes a measurement every 70 cm as it advances on ice.

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The NASA

What new information will be revealed?

It is hoped that ICESat-2 can help produce the first robust maps of Antarctic sea ice thickness. At the present time, the technique of assessing pack ice really only works in the Arctic.

It is a question of comparing the height of the part of the floating ice glued on top of the water with the height of the surface of the ocean itself. As scientists know the density of seawater and ice, they can then calculate the amount of ice that needs to be submerged, and thus the total total thickness.

In Antarctica, however, this approach is problematic. In the extreme south, the sea ice can be covered with heavy snowfall. This will sometimes push the sea ice completely under the water and disrupt the calculation of the thickness.

The proposed solution is to combine the ICESat laser observations, which are reflected at the top of the snow surface, with those of the radar satellites, whose microwave beams penetrate deeper into the snow. This will reduce a lot of uncertainty.

Scientists need thickness measurements to properly access the state of the floes. Sometimes the winds will spread the ice; other times, he will stack the ice. The difference only appears when the ice is seen in three dimensions.

And no, the laser does not have the power to melt the ice itself. But on a dark night, you may see a green dot when ICESat flies over.

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