Measuring the heat beneath the ice

Scientists studying magnetic signatures in Antarctic rocks are peering beneath the frozen continent’s ice sheets to make a detailed map of the amount of heat seeping from the Earth’s interior at various locations beneath it. 

In the process, they have not only bettered our picture of Antarctica’s geology, but improved knowledge of how the escaping heat might affect the ice sheets’ base layers, making them more or less slippery. That’s important in understanding how the ice may react to climate change.

The new map combines 50 years of magnetic measurements taken from aeroplanes, satellite observations, and data from marine research vessels, says Yasmina Martos, a geophysicist at NASA Goddard Space Flight Centre and the University of Maryland, USA, who was with the British Antarctic Survey when the research was conducted. 

The goal, she says, wasn’t to measure the Earth’s magnetic field, but to measure the intrinsic magnetism of buried rocks. 

Ferromagnetic minerals in these rocks hold magnetic signatures that produce measurable anomalies. But these minerals lose their magnetism when heated above a critical level, known as the Curie temperature, something that occurs a few tens of kilometres beneath the Earth’s surface. 

By using these magnetic anomalies to determine the thickness of the magnetised layer, the scientists were able to distinguish between regions where the Curie depth occurred close to the surface and those in which it was as far as 60 kilometres down. From that, they were able to calculate – and map – the amount of heat escaping from the Earth’s interior. They mapped base of the ice to a resolution of about 15 kilometres, substantially better than in any prior study. 

That’s important because the escaping heat melts the bottom layer of ice, in effect lubricating its flow.

“Geothermal heat is important to understanding conditions at the ice bed as it controls sliding processes,” says Martos. 

“However, it is the most unknown. This new map is giving the scientific community the base for more precise modeling on how the ice sheet is behaving and how it will behave in the future. Even a little melting at the base helps the ice sheet slide faster.”

Mapping the differences, she adds, helps scientists understand which parts of Antarctica are the most unstable, including “which parts are more probable to break, melt, or disappear.”

Conversely, pinpointing areas of low heat flux is important because these help stabilise the ice sheet. They are also important to scientists wanting to take core samples to study climate cycles from hundreds of thousands of years ago. “Such areas are important for the search for the oldest ice preserved on the planet,” Martos says.

Richard Alley, a geoscientist at Pennsylvania State University, University Park, in the US, likes the new study but cautions against misinterpreting it. 

Heat flux from beneath the ice, he says, is not a direct contributor to global warming. Not only is it not changing, barring a volcanic eruption (which would only be a local effect, anyway), but the amount of heat is small. 

The amounts of flux measured in the new study, for example, ranged from about 30 to 150 milliwatts per square metre. “Solar energy absorbed by the planet is about 240 watts per square metre,” Alley says.

But heat is important to the movement of the ice and is difficult to measure beneath several kilometres of ice. 

“People sometimes have energy audits done on homes, looking for how much heat is leaving,” Alley says. “Measuring geothermal flux under Antarctica is a lot harder.”

The new research won’t be the last word, he says, but is definitely an important advance. “I believe it will be influential and help us make better models of the ice sheet,” he says.

The study was published in Geophysical Research Letters.

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