Ice mission delights scientists

By Molly Bentley
BBC Science, in San Francisco

Pulses of light are flashed to Earth 40 times a second and a telescope collects the reflections

A US space agency (Nasa) satellite launched early this year and designed to measure the world's ice sheets is showing novel maps of Antarctica and Greenland in remarkable detail - down to the last ice crevasse.

Despite the early loss of one of its lasers; the Ice, Cloud and Land Elevation Satellite, (IceSat) is beaming home high-resolution measurements of polar ice and sea ice elevation as well as the stratification of clouds and aerosols.

The instrument sends out pulses in two "colours" - green for the atmospheric properties and infrared for the ice altimetry.

The eventual result is a 3D map of the Earth's surface.

Up to now climate scientists have had only spotty measurements of the height of the remote ice sheets.

"It's the difference between looking down at the tops of buildings and seeing them flat, and then discovering how high the buildings are," said Jay Zwally, IceSat project scientist from Nasa Goddard at the fall meeting of the American Geophysical Union in San Francisco.

Into a third dimension

The satellite data will help scientists determine just how the Earth's ice sheets are changing and what this may mean for global sea level.

The Greenland and Antarctic ice sheets cover 10% of Earth's land area.

Even a small variation in their average thickness affects sea level, but glaciologists do not even know whether these massive blocks of ice are expanding or shrinking.

Their vast size makes accurate ground measurements impossible.

When you saw [the first measurements] you knew right away you were seeing something that would revolutionise the way we study ice sheets Bob Schutz

Knowing their height - the Antarctic ice sheet is more than 4,000 metres high at its tallest - allows scientists to calculate total ice mass and how it changes with time.

"We know a lot about the Earth in two-dimensions," said Nasa programme scientist Waleed Abdalati. "But the major challenge is the vertical dimension."

This comes from the craft's single scientific instrument, the Geoscience Laser Altimeter System (Glas) which sweeps over the Earth in a near pole-to-pole orbit, measuring the brief period it takes a light pulse to travel to Earth and bounce back.

Nasa's 'shooting star'

As the Earth rotates beneath the spacecraft, the resulting crisscross pattern is a bit like winding a ball of string, with the greatest coverage at the high latitudes.

Stargazers may catch the green pulse of the laser - the infrared is invisible - as the spacecraft arcs through the sky. The laser is safe on the eyes, but you have to be in just the right place and know just when to look to catch it.

"People think it's a shooting star," said Dr Abdalati.

Ice cover plays a crucial role in the Earth's climate system

While the space shuttle also used laser altimetry to measure topography it was lower in resolution, not as accurate and did not cover the poles.

Glas is the first satellite laser altimeter to be used to measure ice mass balance and its 15-centimetre accuracy is unprecedented.

This will help scientists determine exactly how much melting has occurred in areas where previous surveys reveal significant ice loss.

Aircraft measurements show increased melting at the edges of the Greenland ice sheet and satellite radar has revealed a thinning in the West Antarctic Ice Sheet that scientists believe is contributing to sea level rise.

Part-time mission

The first few months of IceSat operation have provided measurements and surface details that were never before visible from space.

"When you saw them you knew right away you were seeing something that would revolutionise the way we study ice sheets," said Bob Schutz, the leader of the Glas Science team.

This is good news from a mission that suffered a disappointment early this year.

We're seeing megadunes on the surface of Antarctica that ripple Waleed Abdalati

A little more than a month after the spacecraft's launch from Vandenberg Air Force Base in California in January, one its three lasers failed.

Scientists found a fault in the photo-diode, which pumps the light into the lasing rod.

While the two remaining lasers are working fine, scientists are running them in intervals rather than continuously to maximize their lifetime.

This will reduce the mission somewhat, said Dr Zwally, but still provide scientists with unprecedented ice measurements.

Which include a close-up of polar ice dynamics. Who knew the ice sheet was so busy...

"We're seeing megadunes on the surface of Antarctica that ripple... ice falls where the ice dips sharply down... and the shear margins of ice streams where fast streams and slow-moving ice lie adjacent to each other," said Dr Abdalati.

He said the 3D images would help scientists understand the mechanics of ice flow and how flow patterns change.

Climate uncertainty

IceSat is also providing a new look into activity above the ice with 3D maps of cloud and aerosol layering.

"We can see the atmosphere in a way we haven't before," said Jim Spinhirne, senior scientist at Nasa Goddard, who showed the first 3D images of the "brown cloud" of pollution over India and dust over Iran.

Unprecedented detail on the elevation of ice sheets

He said that the effect of aerosols on radiation and clouds is the largest current uncertainly in global warming.

Clouds are important in climate change because they trap sunlight. Aerosols are highly variable - carbon soot absorbs heat and warms the Earth while the natural aerosols such as sulphates reflect heat and cool it.

It is not clear what their overall contribution is to climate change.

The IceSat mission is scheduled to continue for another 2-4 years. Scientists hope new laser-altimeter missions will follow.

Europe will launch its own version of IceSat next year called CryoSat.

CryoSat is a radar altimetry mission. Its aim is to study possible climate variability and trends by determining the variations in thickness of the Earth's continental ice sheets and marine sea ice cover.