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Infrared astronomy peers into origins of life

Eye on Technology - astronomy

From oemagazine April 2001
31 April 2001, SPIE Newsroom. DOI: 10.1117/2.5200104.0003

A Small Explorer Project (SMEX) used to investigate interstellar gas clouds, SWAS studies water, molecular oxygen, and carbon in star-forming regions. (NASA)

Infrared astronomy is extending scientists' view of the universe into new wavelength regions, giving them new information about the workings of the cosmos and providing tools that may ultimately help reveal life on other planets.

"Everybody would like to know whether there's life elsewhere in the universe," says Martin Harwit, professor emeritus of astronomy at Cornell University (Ithaca, NY). At the American Association for the Advancement of Science meeting (15–20 February; San Francisco, CA), Harwit led a session titled "Infrared Astronomy: In Search of the Molecules of Life."

Infrared radiation is of interest to astronomers because it travels easily through the galaxy, while visible light undergoes scattering by interstellar dust particles. More importantly, many of the molecules believed to be required for life radiate at infrared wavelengths. However, the Earth's atmosphere prevents ground- based telescopes from seeing much of the infrared spectral region, especially at longer wavelengths. Most meaningful infrared astronomy takes place from space- based instruments.

Spacecraft launched in the 1990s, such as the Infrared Space Observatory (ISO) and the Submillimeter Wave Astronomy Satellite (SWAS; see figure), already have provided insights into the development of planets and the formation of complex molecules. Missions planned for the coming decade, such as the Herschel satellite of the European Space Agency (ESA) and NASA's Terrestrial Planet Finder, will offer views never seen before.

"To get maximum sensitivity and maximum wavelength coverage, what you want to do is put a cooled telescope in space," says ESA's Martin Kessler, who reviewed the SWAS mission at the meeting.

distant stars, distant wavelengths

According to Kessler, SWAS is the first satellite to observe star formations and interstellar chemistry at submillimeter wavelengths. Launched in late December 1998 and scheduled to operate through at least January 2002, SWAS looks for the spectral lines of water at 538.66 µm, isotopic water at 546.77 µm, isotopic carbon monoxide at 544.54 µm, neutral carbon at 609.56 µm, and molecular oxygen at 615.70 µm. The satellite incorporates a 55 X 71-cm elliptical off-axis Cassegrain telescope and a pair of passively cooled subharmonic Schottky photodiode receivers.

Gary Melnick, a senior astronomer at the Harvard-Smithsonian Center for Astrophysics (Cambridge, MA), says the satellite showed that the concentration of water in the gas clouds that collapse to form solar systems was different than what scientists had predicted. There was more water in areas where temperatures were relatively high, but there was much less than predicted in cool parts of the clouds. Melnick says the discrepancy is probably because the water forms ice crystals on grains of dust within the clouds. As a cloud collapses under gravity, it heats up, freeing the water from the dust particles.

As gas heats, it also expands, counteracting the collapsing forces of gravity. In order for a cloud to fully condense into stars and planets, the system has to lose energy. Molecules collide with one another and are excited into emitting photons, which carry off energy. Using spectroscopy to view the chemistry of these clouds helps astronomers understand the processes involved.

technology of the future

According to Melnick, the detector technology on SWAS dates back to the late 1980s. The Herschel satellite, set to launch in 2007, will carry a 3-m telescope and will use superconductor- insulator-superconductor technology that requires cryogenic cooling. It will view wavelengths from 80 to 670 µm.

To detect life on other planets, astronomers will want to look at absorption spectra to detect water vapor, oxygen, ozone, and other gases in a wavelength band covering most of the visible spectral region out to about 10 µm. At the same time, instrumentation will have to discriminate between radiation from the surface of a planet and the glare of its star. The Terrestrial Planet Finder will be a space-based interferometer consisting of a range of telescopes separated by about 1 km. Over that distance, the individual scopes will have to be aligned to a tolerance of about one wavelength, which will be difficult to achieve, according to Harwit. "It'll probably take 15 or 20 years for the technology to come online for these planets' searches and look for the molecules of life," he says. Once we reach that point, though, we may find that we're not alone.