Painted Glaciers: Mapping the Dust, Soot and Rock on Earth's Ice from Space

For a short while after fresh snow, a glacier is a blank white canvas. It does not stay that way. Wind brings smoke and dust in black, grey, metallic, red and yellow tones; volcanoes lay down ash and tephra from millimeters to meters thick; landslides and lahars spread soil, crushed rock and mud across the ice. A NASA glaciologist, who once weighed a career in art against one in science, became one of the first to use satellites to tell these coatings apart, from fine aerosols to volcanic ash to boulders. Working at NASA's Goddard Space Flight Center and the University of Oslo, she produced satellite maps of debris on key glaciers around the world that say not only where the ice is dirty but what the dirt is and where it came from.

A world of shrinking glaciers

Glaciers are rivers of ice fed by snow in places cold enough to keep it all year, at high latitudes or high altitudes. About 10 percent of Earth's land is covered by ice, in roughly 200,000 glaciers. As part of the European Space Agency's GlobGlacier project, she spent three years improving ways to count and monitor them from space, which is harder than it sounds: clouds get in the way, debris hides the edges of the ice, and shrinking glaciers break into pieces too small for satellites to see.

Glaciers lie at high altitudes or high latitudes. Map by NASA Earth Observatory, from Randolph Glacier Inventory and Natural Earth data.

Retreat is the rule. The World Glacier Monitoring Service found that 87 percent of the world's 2,100 best-studied glacier fronts retreated between 2000 and 2005, and retreating glaciers outnumber advancing ones by about 8 to 1. Yet neighbouring glaciers under the same weather can behave very differently, some basins lose ice far faster than expected, and glaciers in the Karakoram of central Asia have been gaining mass. Some glaciologists suspect the coatings of dust and debris explain part of it, but most glacier inventories record nothing about them.

Exit Glacier, Alaska, in July 1986. NASA Earth Observatory, Landsat 5 data.

Exit Glacier in August 2009; debris along its edge makes the retreat hard to measure. NASA Earth Observatory, Landsat 5 data.

Why the dirt matters

Colour. Like dark clothes on a sunny day, dark particles, soot from pollution and wildfires or basalt ash from volcanoes, absorb sunlight and warm the ice, so it melts faster than under pale salt or silica-rich dust. A surface's reflectivity is its albedo: pure snow reflects more than 95 percent of visible light, desert sand about 40 percent, pure soot less than ten percent. Small amounts go a long way. In 1980 a scientist then at the National Center for Atmospheric Research showed that a few parts per billion of soot could cut the albedo of snow by up to 15 percent, and leading climatologists have argued that soot and pollution from South Asia are a main reason Tibetan glaciers retreated so fast.

Composition. Salts dissolve into meltwater. Mixtures of soot, dust, pollen and crushed rock melt round pits called cryoconites, which fill with water and host colonies of cyanobacteria, fungi and other microbes that can themselves darken the ice. Over time the pits grow into suncups and then into moulins, pipes that carry water down to the bedrock and can change how fast the glacier slides.

Thickness. A thin dark layer speeds melting, but a thick cover of rock, ash or soil does the opposite, insulating the ice like a blanket.

Into the field

To check what the satellites see, she climbed to the glaciers themselves, as a geochemist trained to read particles under the microscope: in the Rockies, the Himalayas, the European Alps, New Zealand's Southern Alps, Peru, Norway, Tanzania and Svalbard.

• Nepal. A two-month expedition took her to 5,500 meters (18,000 feet) on the Khumbu Glacier, fed by snow from Mount Everest and the world's highest glacier, and Ngozumpa, on Cho Oyu, the sixth highest mountain. Avalanches and rockfall bury both in thick debris.
• New Zealand. Mount Ruapehu, an active stratovolcano and the largest mountain on North Island, regularly showers ash and sends lahars across its eight small glaciers. Near its warm crater lake, sulfur gas gave the ice a yellow sheen.
• Switzerland. Near the Matterhorn she sampled the clean Findelen Glacier and the debris-covered Zmutt.
• Svalbard. On Spitsbergen, 1,000 kilometers (600 miles) from the North Pole, two small glaciers, Grønfjordbreen and Aldegondabreen, collect salt from the sea and coal dust from nearby mines.

Reading the spectrum

Colleagues warned her that mapping glacier debris from orbit was a fool's errand: too much noise, too little resolution. But each material, ash, dust or soot, leaves its own spectral fingerprint in wavelengths the eye cannot see. The ASTER instrument on NASA's Terra satellite, built by Japan's Ministry of Economy, Trade and Industry, has six shortwave-infrared bands (1.0 to 2.5 microns) and five thermal-infrared bands (3 to 12 microns), and the instruments on Landsats 7 and 8 have similar bands. With ASTER she told dark, iron-rich debris from paler, iron-poor debris on the Zmutt, and dark schist from lighter granite on the Khumbu.

Near-, shortwave- and thermal-infrared views of the Ngozumpa and Khumbu glaciers. NASA Earth Observatory, ASTER data.

For a few glaciers she used Hyperion, on NASA's Earth Observing-1 satellite, which records more than 200 separate wavelengths where most instruments record a few dozen, a flood of detail, but over a narrow strip and hard to interpret. It gave detailed readings of Lhotse Shar and Imja glaciers in Nepal, Whakapapa and Mangatoetoenui in New Zealand, and the Hofsjökull Ice Cap in Iceland, which her field samples confirmed.

What comes next

The next steps are teaching computers to do the mapping that she did image by image, and studying the particles in the air just above glaciers. The goal is global maps that let scientists ask where coatings are changing, why, and what that means for the ice as dust storms, wildfire smoke and salt spray shift with a warming climate. The tools are a worry: ASTER was 13 years old and some of its most useful bands stopped working in 2008, with no NASA replacement planned; the European Space Agency planned Sentinel 2, with similar bands, and NASA was developing a hyperspectral imager like Hyperion.

Sources

• NASA Earth Observatory, "Painted Glaciers: Examining Debris on top of Earth's Icy White Canvases"; rewritten in hubnx's own words. Images: NASA Earth Observatory. Figures adapted from a United Nations report and a journal paper, and photographs without a stated credit, are not reproduced here.