Antarctica: High and Dry (and Cold and Calm)
What makes a good observing site? Nearly every kind of astronomical telescope benefits from a site that provides clear, dry, and dark conditions. While the stormy weather on the Antarctic coast is legendary, the high plateau in the center of the continent is quite the opposite. It lies at the eye of the storm; it is a calm, clear polar desert that offers space-like observing conditions while comfortably standing on terra firma. Thus, siting a telescope at the summit of the Antarctic plateau has significant advantages over essentially anywhere else on Earth:
|Characteristic||Ridge A advantage|
|Clear||No clouds >80% of the time|
|High||>13,000 ft elevation for 100 miles|
|Dry||Super-dry polar desert (winter PWV is 0.1 mm)|
|Cold||Winter temperature -100F, mid-summer a balmy -30F.|
|Clean||Cleanest air on Earth, few aerosols, little scattering|
|Dark||Almost 6 months of continuous darkness|
|Good atmospheric seeing||~0.2 arcsec above a ~15m ground layer|
|Stable climate||No prevailing weather, seemingly constant|
|Minimal lightning||What’s lightning?|
|Low wind throughout atmosphere||No jet stream, calm surface winds (4 knots avg)|
|Low seismic activity||Seismically quiet|
|Accessible||Land a plane right next to your telescope!|
|Continuous observing possible||Most southern astronomical objects are circumpolar|
In particular, one’s ability to do infrared, submillimeter-wave and terahertz astronomy hinges on the amount of water vapor in the atmosphere, since water molecules absorb this light very effectively before it reaches the ground.
Really! In the summer, there is enough moisture in the air at most midlatitude locations on Earth, at the frequency of the ionized carbon line (1900 GHz), light can only travel 10-50 meters before it is mostly absorbed by the intervening water vapor.
This effectively forces such astronomical observatories to the highest and driest sites where this atmospheric absorption is minimized. The bitterly cold air holds no water vapor, and what little remains freezes out into tiny ice crystals. This makes the summit of the Antarctic plateau the driest place on Earth.
We selected the Ridge A site from satellite data to be the best location for an astronomical observatory on the Antarctic plateau, and indeed, anywhere on Earth. It is located on the summit ridge of the ice plateau at 81:40:25 South latitude and 72:42:58 East longitude at a physical elevation of 13,260′ (4040 m) with a typical pressure altitude of 15,200′ (4650 m). It constitutes the origin of the continent’s famous katabatic winds and is perhaps the calmest place on Earth, with typical winds of 4 knots (2 m/s). Even more important for infrared and terahertz telescopes is the extreme cold. Winter temperatures routinely drop below -100F (-70C), providing for a very dry, stable, clear atmosphere. The extremely low amount of water vapor that results allows observations to be routinely performed here that cannot be done reliably anywhere else on Earth. While it is perhaps the most remote site on the planet, it is nevertheless still accessible by aircraft or ground traverse.