The coldest place in the world is a desert. An icy, white mountain top called Dome A, or Dome Argus, which also happens to be the highest point in Antarctica and possibly the best place on Earth to look back in time at how the universe formed.
If you were to stand on Dome A, the 1.8 miles of ice between the soles of your frozen shoes and the peaks of the buried mountains below would seem to contradict the idea that the place is a desert. But most of that ice is over a million years old. Above you would be a broad blue disc of sky, usually unbroken by clouds. Dome A receives less than an inch of precipitation in a typical year. That is less precipitation than Death Valley.
The fact that Dome A is so dry and high also makes the peak one of the most interesting places in the world to astronomers. In a letter published recently in the online journal Nature Astronomy, Qizhou Zhang, an astrophysicist with the Harvard-Smithsonian Center for Astrophysics, writes that Dome A would be the perfect place to put a new radio telescope—one that operates in the little-studied terahertz frequency range.
Terahertz radiation is a band on the electromagnetic spectrum squeezed in between microwaves and the infrared. Although it is plentiful in the universe, the opacity of our atmosphere makes it difficult to study from Earth. Water vapor in the atmosphere typically hides and distorts reception of light and radio waves reaching the Earth from distant parts of the universe, including the terahertz frequency range.
“The water vapor [in most of Earth's atmosphere] is a major inconvenience for those wavelengths,” Zhang says. “The atmosphere is quite opaque. So we need to find a place with minimal water vapor in the air.”
“Dome A is very high,” he says. “It's probably the driest place on the ground that we know of.”
The location is so arid that if all the water vapor in a narrow column stretching straight up from the ground to the edge of space were condensed, according to the researchers, it would form a film less than about 1/250th of an inch, or twice the width of a human hair, and about 10 times less than the water vapor at Hawaii's Mauna Kea Observatories, one of the world's best astronomical observing sites.
At the top of Dome A, is a smattering of antenna masts, small shipping containers, scientific equipment and a lot of footprints that take years for the snow and meager wind to cover up. No people. Dome A is an unmanned station. It is visited only a few times each year by maintenance crews that travel 750 miles from the coast in specially outfitted tractors. The trip takes about three weeks. Crew members sleep in the cab.
Other observatories and weather stations on Dome A are operated by the Australian and Chinese governments. But a terahertz observatory, utilizing a five-meter telescope, would be different from other equipment on Dome A because it would allow scientists to look at the details of how important parts of the universe formed.
“At very early stages those clouds [of gas in space] are very dense and cold,” he says. “When you probe a very cold universe the black body radiation peaks close to the frequency range where this telescope operates. It is good for origin science: stars, planets and galaxies.”
The terahertz radiation reaching the Earth from distant galaxies took millions and even billions of years to reach us. By observing it, astronomers would be looking back in time at events that took place before Dome A existed and before Antarctica was a frozen, polar wasteland almost as desolate as the other planets in our solar system.
But in order to properly design and build a terahertz observatory to be installed at Dome A, the research team needed to find out exactly how much water vapor lies between the surface of Dome A and the edge of space. And it turned out that the existing science doing spectral analysis of water in the atmosphere was somewhat lacking.
“The problem with that is that the spectroscopy of water is not well calibrated into the far infrared,” says Scott Paine, an astrophysicist also at the Harvard-Smithsonian Center for Astrophysics and an author of the letter. “So we needed an instrument that could measure through the pure rotation band of water vapor. . . . Where this got interesting in a multidisciplinary way, the same things we have to see to find out if its a good observatory site also help you to improve our knowledge of the spectral absorption of water vapor.”
Zhang, Paine and their Chinese, Japanese, American and Australian collaborators, led by principal investigator Sheng-Cai Shi, found that their data was also useful as a piece of climate science that can help climatologists understand the dynamics of global warming.
“The column of air you are looking at gives you a natural laboratory for understanding what is happening in the upper troposphere all over the planet,” Paine says. “The real key point is that astronomers are on the ground trying to look out and the view is blocked mostly by water vapor. And the greenhouse effect is caused mostly by water vapor.”
Getting 19 months of continuous spectral observations from Dome A was not an easy task. Diesel generators are notoriously cranky things to keep working long-term in the best of circumstances. To keep the station operating without humans to perform regular maintenance, the generators automatically traded off duty to one another and allowed periodic rest. If one generator broke down, the others had enough capacity to make up for it. Solar panels can provide power for much of the year but aren't much help during the months when the polar region doesn't receive sunlight.
The next step will be to build and install the terahertz observatory. The Chinese government paid most of the expenses for the research to date but additional funding has not been finalized. “They don't have the final green light to go-ahead for construction yet,” Paine says.