Astronomy on Ice Jonathan Shanklin British Antarctic Survey, Cambridge, England Published in the journal of the British Astronomical Association August 28, 1996 Two meetings on the subject of Antarctic astronomy were held in Cambridge in early August in association with the XXIV meeting of the Scientific Committee on Antarctic Research. This is a biennial international meeting, which attracts scientists and administrators of National Antarctic Programs and is designed to ensure co-operation amongst all groups working on the continent. Associated with the meeting are specialist working groups covering all the scientific disciplines with interests in the Antarctic and this year two workshops were devoted to Astronomy in the Antarctic. It turns out that the high plateau of Antarctica is perhaps the best place in the world to carry out some branches of astronomy. The highest parts of the plateau rise in a series shallow peaks known as domes, to over 4000 metres, which puts them above nearly half the atmosphere. In addition, during the polar night, temperatures can fall lower than -80C at the surface due to intense radiative cooling (snow is nearly black in the infrared) setting up an inversion where the temperature initially rises sharply with altitude. This creates a stable boundary layer in the lowest 200 metres, but unfortunately this also contains most of the bad-seeing, amounting to around 1". Above this however there are no major sources of turbulence because the plateau is inside the polar vortex, which limits the high altitude jet-streams (and contains the Antarctic ozone hole). If one could get above the stable layer, the mean visual seeing would be better than 0.4". Most of the turbulence in the boundary layer is caused by gravity waves trapped in layers of air of slightly different density which flows down from the highest points on the plateau. The gentle slope of the plateau means that these katabatic winds are relatively light, rarely exceeding 10 knots. The astronomers think that getting to the highest domes will minimise the turbulence as there is no katabatic flow, however the meteorologists are doubtful because gravity waves can travel very long distances (and where they don't they are breaking causing turbulence). However a solution may lie with adaptive optics: because most of the bad seeing is generated close to the telescope, the aplanatic disc moves through large angles. This makes it relatively easy to correct with an artificial guide star or even a nearby bright star. With an isoplanatic angle of 5-10 arcmin one will nearly always be found in any part of the sky. A further advantage of the cold Antarctic atmosphere is that it contains very little water vapour and it carries less than 0.1 mm of precipitable water. The interior of the Antarctic is in fact classed as a desert due to the low amount of precipitation, despite the kilometres of ice that lie below the surface. This dryness makes it ideal for millimetre, sub-millimetre and very far infra red observations which are otherwise frustrated by water vapour in the air. Not only is the amount low, but it is consistently low and the South Pole is always better in this respect than sites such as Paranal and Hawaii. In the window of 2.3 to 2.5 microns there are 90% of usable days compared with 19% at Mauna Kea. The sites are also very dark in the infra-red as there is minimal air-glow, with the sky brightness being nearly comparable to visual wavelengths. In the K band (2.4 microns) the sky brightness is just 17 mags/arcsec^2 compared to 13 mags/arcsec^2 at Siding Spring and the L band (3.5 microns) is about 50 times darker and photometrically more stable. Further into the infra-red, observations at 50 microns will be possible and sometimes even at 200 microns. Opening these new windows will produce new views of the universe and with them, new discoveries. Although Antarctica is the most pristine continent, pollution still rears its head. In the vicinity of bases more and more outside lights are being put up. Traditionally radio is used for communication and no-one has worried too much about the frequencies being used. However this is a problem to the new radio-astronomers who find their observations being interrupted by meteorologists passing the latest weather observations or mechanics heating up food in the micro-wave after a late shift. Another potential problem is the exhaust from aircraft which can still be detectable 24 hours after a plane has departed; a proposal for no fly zones is on the table. Despite these threats the air of Antarctica is still exceptionally clear, which could be a benefit for observations in the ultra-violet. Most measurements have so far been made at the South Pole, however meteorologically it is not a special place and in order to measure the seeing at other sites an Australian group is developing an Automated Astrophysical Site Testing Observatory. This will be able to make measurements from the ultra-violet to infra-red and will work in temperatures as low as -85C. It will measure the sky brightness and seeing and also meteorological parameters. The plan is to run it for two years at the South Pole under test, then move it to Dome C, followed by Dome A. Preliminary meteorological measurements suggests that skies are clear 90% of the time in summer at Dome C, but are cloudier in winter. The geographical position of the South Pole makes it possible for continuous observations to be made for several days (the record is about a week). This can benefit the study of oscillations in our sun and the study of certain types of variable stars. Observations are best carried out when no more than 2 air masses are being looked through and this places a limit of around -15S on observations. Working at the South Pole requires different working practices to normal observatories and it is not possible for an astronomer to travel there for a fortnight's observing run (especially not in winter when the continent is totally isolated). The telescopes will be much more automated, though technicians will be on hand to repair some of the inevitable breakages. One disadvantage of the South Pole is the light from the aurora australis, though fortunately this is only emitted at well defined wavelengths. Another is that it is out of view of geostationary satellites, limiting connections to the internet. Several experiments are being run at the South Pole under the auspices of the Centre for Astrophysical Research in Antarctica or CARA, which coordinates the work of a number of American universities. Perhaps the best known project is SPIREX which provided very good coverage of the SL9 impacts on Jupiter. The South Pole Infra Red EXplorer uses a 0.60-m telescope, optimised for the near infra red (2.3 microns in the K band) and a 128 x 128 detector array, which can reach 23.5 mags/arcsec^2. Observation programs include looking at nearby galaxy halos to try and find dark matter, star formation in antenna galaxies, RR Lyra stars (the period/luminosity relation may be independent of metallicity in the near infra-red), face on spirals (observing carbon monoxide to see the contrast in spiral arms) and micro lensing (trying to detect planetary systems through nano-lensing effects when looking towards the galactic centre). They observed 16 of 20 of the SL9 events and have determined a consistent sequence of precursor, bolide trail, explosion and re-impact cloud. AST/RO: Between the infrared and the microwave is a region astronomers call the sub-millimetre, where absorption from water vapour makes life difficult for astronomers. A telescope built to exploit the dry conditions at the South Pole is the Antarctic Sub-millimetre Telescope/Remote Observatory. The 1.7-m AST/RO operates at 100 micron to 2 mm wavelengths and has an off axis secondary which sends radiation to a coude room. In its first winter of operation it surveyed the southern galactic plane in an emission line of atomic carbon at 492 GHz. Carbon is abundant in molecular clouds and this line promises to be an even better tracer of such clouds than the traditional observations of carbon monoxide. COBRA: COsmic Background Radiation Anisotropy experiment. Using the 0.75-m Python telescope, COBRA sees greater anisotropy than COBE did, seeing fluctuations of around 30 micro-Kelvin which are repeatable from year to year, over degree size-scales, at both 45 and 90 GHz. Next year the team hopes to use the very low background noise of the Antarctic environment to produce real maps of the background radiation, rather than the difference maps seen so far. There is no evidence for a non gaussian distribution of variation on scales from between 1 and 5 degrees, which implies that no cosmic strings are present. A successor to Python known as Viper is under construction and will resolve scales down to 0.1 degrees. AMANDA, the Antarctic Muon And Neutrino Detector Array uses the earth as a shield to observe high energy neutrinos from supernova events, active galactic nuclei and dark matter particles coming upwards. It cannot detect solar neutrinos as they are below its energy threshold, but it can detect muons from air-showers coming downwards and is the largest muon detector in existence. The Antarctic ice provides a good working platform, making it (relatively) easy to lower strings of 20 cm diameter photomultiplier tubes down holes drilled through the ice to depths of up to 2000 meters. Although there are some air bubbles trapped in the ice, it is one of the most transparent substances on earth in the ultraviolet, with an absorption length of around 300 metres. It is also non-radioactive, reducing the background count considerably compared to sea-water. SPASE: The South Pole Air Shower Experiment. The South Pole is a good site to observe high energy cosmic rays, and a team from Leeds has carried out studies there over the past ten years which puts limits on the flux of particles with energy greater than 10E15 eV. The site is good because sources are at a constant elevation; it is near the magnetic pole which enables lower energy air-showers to penetrate to the surface; it is high which maximises the intensity of showers from 100 TeV cosmic rays; and southern hemisphere objects are on view. The team can calculate the arrival direction of air showers to better than 1 degree using AMANDA and surface located Cerenkov detectors which see the flashes of radiation caused by particles having to slow down to avoid traveling faster than the local speed of light. They also hope to be able to determine the mass composition of high energy particles by measuring the number of muons generated compared to the energy of the primary. The GASP (Gamma Astronomy at the South Pole) telescope also uses Cerenkov detectors to look for gamma-ray interactions in the upper atmosphere. Future plans for telescopes include VCA (Very Compact Array) which is an interferometer for studying the cosmic microwave background at scales from 0.25 to 1.4 degrees. A ten metre telescope, observing at 100 microns to 2 mm and capable of resolving the CMB at scales between 0.02 and 0.2 degrees, as well as being used as a sub-mm telescope, is planned for deployment in 2002/3 at a cost of $10.5M. Finally there is a plan for a 2.5-m telescope, optimised for the near infrared and using adaptive optics. A final topical astronomical interest in Antarctica is the search for meteorites, such as the stone which possibly holds evidence for life on Mars. Although not a topic of the meetings, Antarctica is one of the most prolific sources of meteorites. Certain areas of the continent are known as blue-ice zones, and in these zones ablation of the snow surface exceeds precipitation. In a few of these areas, ice is transported in by slow glacial flow, sometimes from great distances and the upwelling ice brings meteorites to the surface. These will have fallen many kilometres from where they are found and many thousands of years in the past. No doubt the discovery of possible life on Mars will bring more life to the Antarctic continent: if so I hope they remember their flame throwers and blood-banks - some Thing is out there waiting for them!