About the Sun
Pole an ideal spot for astronomers
Posted January 21, 2007
Searching for a minute change in microwave radiation that has traveled across space for some 14 billion years requires not only state-of-the-art equipment, such as the South Pole Telescope, but it needs to occur in special locations.
Antarctica is, among other things, Earth’s highest, driest and coldest continent. The South Pole epitomizes those qualities, and it is also home to support facilities, making it an ideal vantage point for peering deep into our past.
When the South Pole Telescope (SPT) aims its huge 10-meter dish into the six-month night sky this winter, its operators know that their cold, isolated setting will enhance their opportunity to crack open secrets of the origin of the universe.
What makes the South Pole such a perfect spot for observing the universe is the minimal amount of moisture in the air. Water vapor absorbs microwaves, which is why a microwave oven works.
“We want to be in a place where the atmosphere is not going to absorb much,” said John Carlstrom, principal investigator for the SPT. “There are two reasons for that. One is that a good absorber is a good emitter.” Emitted radiation would interfere with readings obtained from space.
One way of limiting the amount of moisture in the atmosphere is to rise above it. The fact that the South Pole sits atop more than three kilometers of ice helps in that regard. And then there is the legendary cold.
“It’s so cold that, even if the atmosphere is a hundred percent saturated, a hundred percent [relative] humidity, there’s very little water vapor,” Carlstrom said. “If you took all the water at the South Pole and compressed it down to a sheet, just got it out of the atmosphere, a typical average thickness of that sheet would be 250 microns,” about the thickness of heavy-duty photo paper.
The second good reason for a dry atmosphere is uniformity. Since astronomers are seeking incredibly small fluctuations in temperature, it’s nice when the atmosphere does little to interfere.
“We can deal with the atmosphere changing,” he said. “There’s no way the atmosphere can be perfectly stable, but the more stable it is, the better it is.”
One important ingredient in that stability is the fact that the sun does not rise all winter. In lower latitudes, the daily rising and setting of the sun heats the lower atmosphere and creates unwanted turbulence.
Sitting on Earth’s axis of rotation has distinct advantages, too. The same sky is visible all winter; it doesn’t sweep from east to west. That enables observers to make prolonged exposures.
“All you’ve got to do is account for the fact that it’s spinning underneath you,” Carlstrom said, “but the spot is always there.”
Six months of darkness has another advantage, he said, in that it offers an opportunity to get an instrument working well and keep it going through the winter.
The last important element, he said, is the infrastructure. At Amundsen-Scott South Pole Station, support is in place for housing, food, telephone and Internet accesss, and electricity.
Carlstrom said the entire station has been involved in seeing the project through to completion.
And those providing the support are aware of their roles, said Jerry Marty, National Science Foundation representative at the South Pole.
“The community is excited about the projects and they’re proud to be part of them. All the planning and all the work to bring the magnitude of materials and the support to bring the projects to where we are now is something that everyone feels pride in being a part of, and everyone’s excited about the possibility of returns on the scientific research.”