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Observing the behavior of ants helped NASA solve the problem of measuring ice thickness accurately. Let’s see how the scientists did it
Lidars for space
The NASA ICESat-2 satellite is equipped with a lidar. It is the main component of unmanned vehicles. It is an optical sensor that measures the distance or speed of objects using an infrared laser. As a result, the computer system receives a three-dimensional image of the surrounding world and determines the exact distance to obstacles and other vehicles, as well as information about whether the object is moving. In fact, lidar is a radar where light waves are used instead of radio waves (which is reflected in the names of the devices).
A satellite sends trillions of photons to Earth via lidar and then analyzes what bounces back. Since scientists know the speed of light, they can use lidar to determine altitude: a photon bouncing off a mountain top will take slightly less time to reach ICESat-2 than one returning from a valley floor.
Photons in the snow
Similarly, scientists have tried to use lidars to measure the depth of snowpacks. Hu Yongqian, a researcher at NASA Langley Research Center, explains: “We can measure the distance that each individual photon travels inside the snow. Some of them are capable of diving to depths of tens of meters before resurfacing and returning to the satellite.” Accordingly, this delay reveals the depth of the snow. However, measurements often turned out to be inaccurate due to the behavior of photons.
Data confusion
The path of photons through the snow is not always easy and often random. Some reach the very ground and bounce off it before returning to the surface. Some bounce back halfway, hitting snow grains. According to Yongqian, “Most of them move a few centimeters and come back, but some go very deep, for very long distances, getting stuck in the snow and bouncing back and forth.” All this creates inaccuracies in the final data.
Ants to the rescue
Hu Yongqian learned that biologists have experienced something similar: While the path of an individual ant cannot be precisely determined, scientists can predict the average time it will take for an ant to travel underground, carrying a load, before reaching the surface. The behavior of photons also has its own regularity. Although the particles move along a random path, scientists can mathematically represent the average distance that each of them travels. The team calculated that, on average, a photon travels twice as far as the depth of snow it travels through. The data were compared with the results of measurements of the depth of snow cover in the same areas, made using aircraft radar. Everything came together.
Benefits of the new method
Ben Smith, a glaciologist (glacier researcher) at the University of Washington, who was not involved in the study, is surprised: “This is really a bold application of theory to real measurements. My initial reaction was: “It can’t be possible for this to work.” But they seem to have succeeded.” Researchers will be able to use the new method to analyze sea ice thickness to better understand how the Arctic is changing: due to climate change, it is warming four times faster than the rest of the planet. Analysis through space lidar is much more environmentally friendly and cheaper than using aircraft. The satellite also helps to penetrate into the most inaccessible places on Earth.