Ants, like group, they are creatures of habit. While an individual’s path is unsafe, biologists who have spent a lot of time observing the behavior of entire colonies can predict the average time an ant can walk underground before resurrecting. This made NASA physicist Yongxiang Hu wonder if the same predictability could be true with photons (light particles) traveling through the snow packet. If so, this would allow scientists to use a laser powered from an orbiting satellite to estimate the depth of the snow, potentially a powerful new way to monitor water supply and sea ice health in the world. Arctic.
NASA’s ICESat-2 satellite is equipped with lidar, the same variety of laser systems used by autonomous cars to build 3D maps of their surroundings. This extremely sensitive instrument shoots billions and billions of photons into the Earth, and then analyzes what bounces off the satellite. Because scientists know the speed of light, they can use the lid to determine altitude: a photon bouncing at the top of a mountain will take a little less time to reach ICESat-2 than a photon bouncing at the bottom of the mountain. valley.
The same goes for shooting a snow bank. “We can measure this distance from each individual photon that travels through the snow,” says Hu, a researcher at NASA’s Langley Research Center. Some photons can enter tens or even a hundred feet deep into the layer of snow before coming to the surface and returning to the satellite. (Photons penetrate the snow like a beam, instead of spraying them sideways. Imagine how a laser shot through a cloud of smoke looks like a single line.) This delay exposes the depth of the snow, just as than a photon bouncing in a valley. a little longer to return to the lidar instrument than one bouncing on top of a mountain.
The path of a photon is not always easy. Just as an ant roams its underground colony, a photon fired from a space laser makes a random route through the snow. A few will travel to the underlying ground and reflect on it before returning to land. Some bounce halfway after hitting snow particles. “Most of them go inches into the snow and come back,” Hu says. “But then there are many who go very deep and very long distances trapped in the snow, bouncing back and forth, back and forth.” All the bouncing around makes the data noisy.
But inside it, there is a pattern, just as there is in the way groups of ants, as a whole, move through a colony. Although each photon follows an erratic path, scientists can mathematically represent the average distance each person travels. The team calculated that, on average, a photon travels twice the depth of the snow through which it moves.
Once they had this formula, the team was able to estimate the depth of snow across the planet using ICESat-2 global lidar data. They then compared these estimates with snow depth measurements of the same areas taken by the aircraft by radar. (A third option is to insert special sticks into the snow.) “They compare very well,” Hu says of the methods. “We are very happy that the theory works.”