Life in the Universe, Search for

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Life in the Universe, Search for

It is an old question, and a persistent one: Is there life elsewhere in the cosmos? Is the universe more than just an enormous collection of dead rock and glowing gas, with only one inhabited world?

While speculation about life in space is an old pastime, a serious, scientific search for it is very new. Despite the impression one may get from movies and television, scientists still have not found any conclusive evidence of biology beyond Earth—not even evidence of the simplest microbes. But many scientists expect that this situation will soon change.

Part of their optimism is due to an astounding fact revealed by centuries of studying the heavens: The physics and chemistry of the farthest galaxies are the same as the physics and chemistry found on Earth. Astronomers have proven this by analyzing the light of distant objects with spectrographs. When they use these instruments to break up starlight into its constituent colors, they see the telltale "fingerprints" of atoms that are found on Earth: the ninety-two elements listed on the familiar periodical table of elements.

The light elements such as hydrogen, carbon, nitrogen, and oxygen are especially plentiful in space. These are the building blocks of all life on our planet. If the stuff of life is so commonplace, might not life itself also be widespread?

How to Find Extraterrestrial Life

There are several obvious—and a few not-so-obvious—methods used in the hunt for extraterrestrial biology.

We could simply send rockets to other worlds and look for it. Since the mid-1960s, this has been done on a limited basis. Spacecraft have landed on the Moon, Venus, and Mars (although only the Moon has been visited by humans), and camera-toting probes have investigated all the other solar system planets except Pluto. Of these familiar locales, only the Moon and Mars have been examined in much detail. The Moon is sterile, which given its lack of atmosphere and liquid water, is hardly surprising. Mars is less obviously dead.^*

Another approach is to use spacecraft to gather rocks from other worlds so they can be scrutinized in the laboratory. The Moon rocks lugged back by Apollo astronauts are an example of this, and the National Aeronautics and Space Administration (NASA) hopes to eventually use robot craft to bring back small pieces of Mars.

Still another way of getting extraterrestrial evidence is to find it on Earth. When meteorites hit nearby worlds, they kick up bits of rock, some of which might have enough speed to escape from their planet entirely. These rocky runaways then wander around the inner solar system. Some, by chance, will hit Earth. If they are large enough to avoid being completely incinerated as they plunge through our atmosphere, they could end up in a laboratory collection. A dozen meteorites from Mars have been found to date, brought here by nature rather than NASA.

For investigating distant worlds that orbit other stars, there is no hope of sending rockets or collecting meteorite samples. Instead, astronomers can use incoming electromagnetic radiation (more commonly known as light and radio) to search for certain "signatures" of life. Making a spectrographic analysis of the light reflected by the atmosphere of a far-off planet would permit scientists to check for the presence of oxygen or methane. Either one might be a clue to the presence of bacteria or possibly more advanced biological forms. The oxygen in Earth's atmosphere is the result of billions of years' worth of exhaust gases from bacteria and plants. Much of the methane is due to the digestive activities of cows and pigs. Finding large amounts of either of these gases in the atmosphere of a distant, Earth-sized planet would suggest an inhabited world.

A final technique is to look for radio or light signals that have been deliberately sent by sophisticated beings on other planets. Hunting for artificially produced signals is known as SETI, the Search for Extraterrestrial Intelligence. Since 1960, SETI scientists have used large radio antennas (and more recently, specially outfitted conventional telescopes) to scan for signals from intelligent aliens.

Where Do We Expect to Find Life?

All life on Earth is based on carbon chemistry and uses DNA as its blueprint for reproduction. Alien life might not sport DNA, but the odds are good that it would still be carbon-based. This is a sure bet because carbon has an exceptional ability to link up with other atoms into long chains, or polymers. To encourage this sort of chemical complexity, a solvent in which the atoms and molecules can easily move and meet is essential. Liquid water is the best such solvent, and therefore most researchers assume that the first step in tracking down extraterrestrial life is to find cosmic niches where liquid water is likely to both exist and persist.

Until recently, astronomers felt that liquid water would be abundant only on Earth-like worlds that were situated at the right distance from their suns—neither too close, where water would boil, nor too far, where it would freeze. In our own solar system, orbital radii greater than that of Venus and less than that of Mars seem right, a region referred to as the Habitable Zone (HZ). For stars dimmer than the Sun, the HZ would be closer in and smaller; for brighter stars, it would be larger and farther out.

This straightforward idea has lately been modified. For one thing, an atmosphere can make a big difference in keeping a planet's surface warm. Mars is cold and dry today, but in the past, when it had a thicker atmosphere of carbon dioxide—an efficient greenhouse gas—there was liquid water gurgling across its landscape. So the extent of the HZ depends on a planet's atmosphere.

In addition, life has been discovered on Earth thriving in decidedly unfriendly environments. Tube worms and bacteria coexist in the inky darkness of ocean deeps. No type of photosynthesis will work in this environment, so the inhabitants of this strange ecosystem take advantage of the chemical nutrients that come churning out of hot water vents (some above 100°C [212°F]) in the ocean crust. Bacteria have also been found in another unexpected environment: kilometers under the ground, where they can live off of chemical nutrients naturally present in rock. The conditions in this environment are brutal: temperatures are high (again, often above 100°C), and the elbowroom, consisting of pores in the rock, is low. A little bit of liquid water in these pressure cooker environments allows these extremophiles , as they are called, to survive.

If life can exist in such difficult conditions on Earth, why not in space? These discoveries have challenged scientists with new thoughts about exactly what kinds of worlds are "habitable." The conventional concept of an HZ has been stretched to include icy moons and underground retreats, and this has encouraged scientists to look for life in what were once considered all the wrong places.

What Have We Found?

What is the scorecard on the search for life? Broadly speaking, the quest for extraterrestrial biology has been a two-pronged affair: a search for nearby, simple biology (e.g., microbes on Mars) and a hunt for distant, intelligent beings (SETI).

Mars.

Of the possible nearby sites for life, Mars has traditionally been everyone's favorite. In the late nineteenth century, some astronomers astounded the world (and their colleagues) by claiming that thin, straight lines could be seen crisscrossing the surface of Mars. These "canals" bespoke the existence of an advanced society on the Red Planet. Unfortunately, the canals turned out to be optical illusions. Nevertheless, of all the worlds in our solar system, Mars is most like Earth. It beckons us with the prospect of nearby, alien life.

In 1976 NASA placed two robot spacecraft on the rusty surface of Mars: the Viking Landers. They were essentially mobile biological laboratories and spent days analyzing the Martian soil for the presence of microbes. They did not look for alien bacteria directly (for example, with a microscope), but searched for organic molecules in the soil, or soaked it with nutrient solutions and watched for exhaust gases that would betray microbial metabolism. The conclusion of the Viking science team was that the Martian surface was sterile, although it is worth noting that two team members disagreed. This indicates how difficult it may be to design unambiguous experiments to look for extraterrestrial life.

Despite the failure of this sophisticated effort to find Martians directly, there is growing evidence that Mars may once have been a more hospitable environment for life. High-resolution photos from the orbiting Mars Global Surveyor reveal what look like sedimentary rock layers, strongly suggesting that more than 3 billion years ago Mars had lakes—environments that might have spawned life. This same spacecraft had an onboard altimeter and discovered an enormous flat region in Mars's northern hemisphere. This may once have been an ocean.

In 1996, NASA scientists examined one of the known Martian meteorites (ALH 84001) and claimed to find several lines of evidence for fossilized microbes within. This evidence included the presence of various chemicals associated with biology, as well as small bits of iron (magnetite) that is commonly found in earthly bacteria. The scientists also made microscope photos of the meteorite's interior, which showed tiny rod-and wormlike structures that look very much like single-celled creatures. Unfortunately, there is great disagreement in the science community about whether this evidence is really due to long-dead Martians or to some inorganic phenomenon.

NASA is planning to send additional orbiters and rovers to Mars in the early years of the twenty-first century. The major goal of these expeditions is to learn more about the history of liquid water on the planet, as this is the key to an improved search for life. Ancient fossils may yet turn up, and some researchers speculate that the descendents of these ancient microbes (if there were any) might still be eking out a dark existence deep under the Martian surface where it is still relatively warm and wet.

Other Solar System Sites.

Mars may not be the only solar system site for life other than Earth. Ever since the late 1970s, when the Voyager spacecraft made the first close-up photos of Jupiter's large moons, astronomers have considered whether life might exist even in these cold, dim environs. Europa is the most promising of the moons for biology. Its surface is bright white ice, cracked and glazed like a billiard ball with a bad paint job. The temperature on Europa is -160°C (-256°F), and one might naively assume that no liquid water could exist. But Europa is in a gravitational tug-of-war with its sister moons, and this keeps it in an egg-shaped orbit. The consequence is that Jupiter's changing gravitational pull squeezes and squishes Europa, heating it up the way pastry dough gets warm when kneaded. There is increasing evidence that beneath Europa's granite-hard, icy skin is a 100-kilometer-thick (62-mile-thick) liquid ocean, one that has been there for billions of years. At the bottom of this ocean, vents may spew hot water and chemicals, much as they do on Earth. Needless to say, if this picture of Europa is correct, some simple forms of life may be swimming in these dark, unseen waters.

In 1995, NASA's Galileo spacecraft began taking photos of Europa and other Jovian moons. That mission will be followed by an improved orbiter, probably to be launched in 2009, that will carry radar equipment to examine the Europan ice. The plan is to find out if the unseen ocean really exists, and if so, whether there any thin spots in the ice where future landers might be able to drill holes and drop equipment down into Europa's briny deep.

Even Saturn's large moon Titan (which is bigger than Mercury) might conceivably host a bit of biology. Titan sports a substantial atmosphere, one that is denser than Earth's and that seems to be perpetually shrouded in smog. The air on Titan is mostly nitrogen and neon, but hydrocarbons and complex polymers make up the smog, together with a haze of methane (natural gas) crystals and ethane clouds. Some researchers suspect that lakes of liquid ethane, or even a moon-girdling ocean of ethane, methane, and propane, may exist on Titan.

All this hydrocarbon chemistry is discouragingly cold, -180°C (-292°F). Nevertheless, despite resembling an arctic oil refinery gone wild, it is possible that over the course of billions of years, Titan's hydrocarbons have spawned exotic life-forms. In 2004 a probe from the Cassini spacecraft will be dropped into Titan's chilly clouds for the first close-up glimpse of this oddball moon.

SETI.

While NASA and other space organizations search for relatively simple living neighbors, SETI scientists turn their large antennas in the directions of nearby stars, hoping to find broadcasts from intelligent beings. The type of signals they look for are called narrowband, which means they are at one spot on the radio dial. Such transmissions could pack a lot of radio energy into a small frequency range, making detection even light-years away much easier. The most sensitive of these searches is Project Phoenix, which uses the 305-meter (1,000-foot) diameter radio antenna at Arecibo, Puerto Rico, to scrutinize about 1,000 Sunlike stars less than 150 light-years distant. Another SETI experiment is called SERENDIP, a project that is less sensitive but searches large tracts of the sky.

While SETI scientists still have not come up with a confirmed, extraterrestrial signal, they are greatly improving their equipment. In the next decades, they will scrutinize as many as a million star systems or more. In addition, new experiments using conventional optical telescopes have been started up. These look for very short (a billionth of a second), very bright laser pulses that an alien civilization might be sending earthward to catch our attention.

The discovery in recent years that many Sun-like stars have planets has greatly encouraged this type of search. It has also prompted space agencies around the world to consider building mammoth space telescopes that could uncover Earth-like planets around other stars. If this is done, then a spectrographic analysis of the atmospheres of these planets might turn up the traces of life—even simple life.

What Finding Extraterrestrial Life Would Mean

As noted earlier, we still have no convincing proof that there are any life-forms other than those found on Earth. Life is complex, and we still do not understand how it got started on our own planet. But to find living creatures—even microbes—on other worlds would tell us that biology is not some miraculous, extraordinary phenomenon. If SETI succeeds, and we find other intelligence, we might learn much about the universe and long-term survival. In either case, we would know that Earth and its carpet of living things is not the only game in town, but that we share the universe with a vast array of other life.

see also Extrasolar Planets (volume 2); First Contact (volume 4); SETI (volume 2); Robotic Exploration of Space (volume 2).

Seth Shostak