Green Photons Track Melting Ice

Using the most advanced Earth-observing laser instrument NASA has ever flown in space, a team of scientists led by the University of Washington has made precise measurements of how the Greenland and Antarctic ice sheets have changed over sixteen years.

Researchers found the net loss of ice from Antarctica, along with Greenland’s shrinking ice sheet, has been responsible for 0.55 inches (14 millimeters) of sea level rise to the global ocean since 2003. In Antarctica, sea level rise is being driven by the loss of the floating ice shelves melting in a warming ocean. These ice shelves help hold back the flow of land-based ice.

This shows the amount of ice gained or lost by Antarctica between 2003 and 2019. Dark reds and purples show large average rates of ice loss near the Antarctic coast, while blues show smaller rates of ice gain in the interior. The ice lost near the coasts, especially West Antarctica and the Antarctic Peninsula, vastly outweigh gains in the interior. Thwaites and Crosson ice shelves (seen just below the peninsula) have thinned the most. The two ice shelves have lost 5 m (16 ft) and 3 m (10 ft) of ice per year, respectively. The circle in the middle is over the South Pole where the instrument does not collect data. (Source: Smith et al. / AAAS)

The findings come from the Ice, Cloud and land Elevation Satellite 2 (ICESat-2), which was launched into orbit in fall 2018 and began taking detailed global elevation measurements, including over Earth’s frozen regions. By comparing the new data with measurements taken by the original ICESat from 2003 to 2009, researchers have generated a comprehensive portrait of the complexities of ice sheet change – and insights into the future of Greenland and Antarctica.

Previous studies of ice loss or gain often analyze data from multiple satellites and airborne missions. The new study takes a single type of measurement – height as measured by an instrument that bounces laser pulses off the ice surface – providing the most detailed and accurate picture of ice sheet change to date.

To do that, ICESat-2 carries a single instrument, the Advanced Topographic Laser Altimeter System, or ATLAS, built by NASA Goddard Space Flight Center. It generates a single laser beam, which is split into six by a diffractive optical element as it exits the instrument, arranged in three pairs to better gauge the slope of Earth’s surface.

About twenty trillion photons leave ATLAS through its box structure with each pulse; only about a dozen return to the satellite. To catch these photons, the satellite is equipped with a beryllium telescope, 2.6 feet in diameter. To ensure that the telescope is aligned with the returning photons, the engineers have designed and built the Laser Reference System. This device picks up a fraction of the laser light before it leaves the satellite, and compares the laser positioning to the pointing of the telescope. A steering mechanism then moves the laser beam, if needed.

The photons that return to the telescope are focused on six fiber-optic cables in the focal plane, corresponding with where the six laser beams will return. From those fibers, the photons pass through a series of filters, which only let through light that is at precisely 532 nanometers. The filters are key to preventing sunlight that naturally reflects off Earth from swamping the detectors.

ICESat-2 in orbit (illustration; source: NASA)

The time-tagged piece of data about each returned photon is communicated to the electronics and communication system on ICESat-2’s spacecraft and sent to the ground. Computer programs can take that travel time and satellite position, do a calculation involving the speed of light, and tell researchers the distance the photon traveled.

The laser sends 10,000 pulses of green light per second. With this 10 kHz pulse rate, ATLAS can take measurements every 2.3 feet along the satellite’s ground path. With six beams arranged in pairs, it can also estimate the slope of the surface it flies over. The mission will gather enough data to estimate the annual elevation change in the Greenland and Antarctic ice sheets even if it’s as slight as 4 mm – the width of a pencil.

The lasers, delivered to NASA by Fibertek in 2015, are expected to operate continuously for the three-year mission life, firing over one trillion laser pulses. The laser performance requirements and long lifetime represents a significant increase in complexity and reliability compared to previous space-based laser systems. (Sources: U. Washington / NASA / ESA / Fibertek)

Further reading: Lasers Will Fly on NASA’s ICESat-2, photonicsviews.com, 30th August 2017

Reference: B. Smith et al.: Pervasive ice sheet mass loss reflects competing ocean and atmosphere processes, Science, 30 Apr 2020; DOI: 10.1126/science.aaz5845

Links: Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA, USAPrograms, Fibertek Inc., Herndon, VA, USAICESat-2, NASA Goddard Space Flight Center, Greenbelt, MD, USA

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