For those who hunt for life on other worlds, water in its liquid form is perhaps the leading indicator. Life as we know it on Earth is based on water and carbon. And if organisms can prosper here in nasty environments — in geysers, in the depths of the sea, in toxic waste, in water that is too hot, too cold, too acidic or too alkaline — why could they not prosper out there?

Scientists for years regarded liquid water as a solar system rarity, for there was no place apart from Earth that seemed to have the necessary physical attributes, except perhaps Jupiter’s ice-covered moon, Europa, which probably concealed a subterranean ocean.

The past 20 years of space exploration, however, have caused what the astrobiologist David Grinspoon calls a sea change in thinking. It now appears that gravity, geology, radioactivity and antifreeze chemicals like salt and ammonia have given many “hostile” worlds the ability to muster the pressures and temperatures that allow liquid water to exist. And research on Earth has shown that if there is water, there could be life.

On Mars and Venus, on Saturn’s moons Enceladus and Titan, and even on two outer-belt asteroids, researchers have shown that the presence of liquid water is possible and even likely. Proof of life, of course, will come only when something — or someone — puts a drop of alien water under a microscope and sees a microbe.

“Water and carbon-based life works well,” Dr. Grinspoon said. “That doesn’t mean it’s the only way, but it’s the only way we know, and it gives us something to look for.”

Finding water in space, in the form of ice, has never been a problem. Hydrogen is the most common element in the solar system, and oxygen is not far behind. When the solar system formed about 4.5 billion years ago, a spiraling disc of dust and gas spun out from the Sun to produce the planets, their moons, and an enormous cloud of comets, planetoids and other bits of cosmic flotsam. Nature endowed much of this debris with a generous helping of water ice.

Liquid water is another matter. The heat of the Sun may melt the ice, but in the vacuum of space there is little or nothing on the surface of most solar system objects to keep the heated molecules together, so they flash instantly away as water vapor. This process is called sublimation.

The physics of sublimation are unforgiving. Liquid water needs a delicate balance of temperature and pressure. Ice must be able to melt without boiling off, but the water must stay warm enough that it does not refreeze. On Earth, with a sea level atmospheric pressure of 14.7 pounds per square inch, water is liquid between 32 and 212 degrees Fahrenheit. On the unshadowed parts of our Moon, where the atmospheric pressure is zero and daytime temperatures can exceed 260 degrees Fahrenheit, the surface ice is long gone.

Ice survives at very low temperatures, however, and the chunks of debris that linger in the chill reaches of deep space beyond Neptune make up the biggest source of water in the solar system today. These dirty snowballs re-enter the planetary system periodically as comets. When they get close enough to the Sun, the ice begins to sublimate, giving the comets their characteristic tail of dust and water vapor.

Many scientists say it is likely that much of the ice in the inner solar system came from comets. On Earth, cometary impacts early in the planet’s history could have provided this raw material, and the Sun and atmospheric pressure would have done the rest. Earth is the only place in the solar system so far discovered where liquid is the default state of surface water. And Earth is where life proliferates.

But it is maybe not the only place. Dr. Grinspoon has theorized that Venus, whose spectacular volcanism boiled off all its surface water long ago, nevertheless harbored liquid moisture in the noxious clouds of sulfuric acid that cloak the planet. In 2008 the European Space Agency’s Venus Express orbiter measured water vapor in the clouds. About 30 miles above the surface in the Venusian mist, where temperatures are about 70 degrees Fahrenheit, extremophiles could find a comfort zone.

Another improbable venue for liquid water is the outer limits of the asteroid belt between Mars and Jupiter. There, using infrared telescopes, two teams of astronomers working separately in 2008 and 2009 found water ice on the surface of the asteroid 24 Themis, about 280 million miles from the Sun. Last year, the teams joined forces and found ice on a second asteroid, 65 Cybele, which, with a diameter of 180 miles, was about 1.5 times as large as 24 Themis and 45 million miles farther out.

For ice to endure on like objects with no atmosphere that close to the Sun, there must be a mechanism to replenish what is lost to sublimation. Humberto Campins, a University of Central Florida astrophysicist and leader of one of the discovery teams, suggested that the patchy ice was a thin coating of frost from a reservoir hidden below the asteroids’ topsoil regolith.

When the asteroid faced the Sun, heat penetrated the topsoil, causing subsurface ice to sublimate and migrate as water vapor to the surface, where it froze at night only to sublimate again during the day. In a variation on this theme, Dr. Campins said, meteorites could be churning up the asteroid topsoil, thus bringing ice closer to the surface. This process is called “impact gardening.”

“We suspect that something like this is happening,” Dr. Campins said, but acknowledged a third possibility: The asteroids could contain enough radioactive isotopes to melt ice deep below the surface, creating liquid water that seeps upward before vaporizing.

“You need sufficient pressure and temperature,” he said. “But conceptually it’s possible.” Pressure would come from the asteroids’ interior gravity, allowing water to exist once the isotopes melt the ice.

Radioactivity is a widespread phenomenon and a likely source of heat energy elsewhere in the solar system. Another heat source is friction, caused most commonly by tidal pressure or wobbling of an object on its axis.

The evidence that Jupiter’s moon Europa harbors an enormous liquid ocean beneath its icy shell has arisen in part from observations suggesting that tidal forces create heat by stretching and compressing the moon as it rotates around Jupiter in an eccentric orbit.

Recently scientists have been able to study tidal forces up close during fly-bys of Saturn’s moon Enceladus by NASA’s Cassini spacecraft. In 2005 Cassini found that Enceladus, with a diameter of only 300 miles, was spewing water ice grains from cracks in its south polar region. The grains were the “dust” that formed Saturn’s E-ring, and scientists soon began to suspect strongly that the particles came from a subsurface liquid water source.

“I wouldn’t say it’s virtually certain, but I’d give it 80 percent or 90 percent,” said John Spencer, a planetary scientist at the Southwest Research Institute, a member of Cassini’s composite infrared spectrometer team. “Things may be a lot stranger than we imagine, but basically, I suspect we have an ocean.”

More disputed is the theory that low-temperature “cryo-volcanoes” on Saturn’s largest moon, the hydrocarbon-rich Titan, may be belching slushy lava composed of liquid water and ammonia, or some other low-temperature mixture, that freezes on the moon’s surface.

“Titan has hydrocarbon sand dunes and methane lakes, and the cryo-volcanism could be hydrocarbon,” said Jeffrey Kargel, a University of Arizona planetary scientist. “We would have to go there to know for sure.” Still, he added, “there pretty much has to be water ice” on Titan, since there is ice everywhere else in the solar system where it is cold enough. Titan has a regular orbit, so tidal friction would be minimal. For liquid water to exist, there would have to be a radioactive heat source and antifreeze compounds.

Antifreeze is what Nilton Renno of the University of Michigan was looking for to explain the unforeseen event that befell NASA’s Phoenix Lander on the arctic plains of Mars in 2008. Hydrazine thrusters that arrested the lander’s descent had blown aside seven inches of Martian topsoil, exposing the expected layer of ice that lay below.

But four days later, something unexpected happened. Cameras examining the ice discerned a number of blisterlike globules on one of the spacecraft struts. A few days after that, the camera looked again. The globules remained.

Although Dr. Renno, the atmospheric science team leader for Phoenix, did not immediately report it, he suspected he was observing droplets of liquid water. It would have to be salty enough not to vaporize in the Martian atmosphere or freeze at surface temperatures below minus 22 degrees Fahrenheit.

For that there needed to be antifreeze. Salt was the likeliest source: “Suppose you have a swimming pool, and you fill it with saltwater,” Dr. Renno said. “When the pool cools down and starts to freeze, pure water becomes ice. The remaining water becomes more saline. It becomes harder to freeze as the salt concentration becomes stronger.”

Evidence arrived in two steps. First, the lander’s instruments found high salt concentrations in soil surrounding the spacecraft. Then, three weeks after touchdown, the Lander’s robotic arm dug a trench in the ice and encountered a soft layer that contrasted with nearby hard ice patches that the lander penetrated with a drill. The slush was a second source of water, and like the first, “probably filled with salt,” Dr. Renno said. “It was almost like ice cream.”

Meanwhile, “we kept taking pictures” of the strut, and 44 days after touchdown the largest droplet disappeared, Dr. Renno recalled. “It grew too large, and dripped off.”

Source: New York Times, Fountains of Optimism for Life Way Out There, by GUY GUGLIOTTA.