Along with much information about the differences and distinctions of these various destinations, the spacecraft also have found something remarkably consistent: the basic elements of life are plentiful and common, at least in this galactic neck of the woods.

The molecules that comprise all known living things require the same four elements: hydrogen, nitrogen, carbon and oxygen. Together, these substances, along with smidgens and smatterings of a dozen or so other constituents, such as potassium, calcium, iron and magnesium, will pr oduce life - if given the right conditions and enough time.

Given the latest contributions to the extraterrestrial compendium, there is little doubt the ingredients are there.

Take the current missions to Mars. The twin robotic rovers, Spirit and Opportunity. By Monday, both craft will have been operating on the surface of the red planet for a year or more. Both have discovered strong evidence that liquid water once flowed there.

At the same time, high above the Martian surface, the European Space Agency's Mars Express and NASA's Mars Odyssey and Global Surveyor orbiters have photographed many locations where water appears to have cut deep channels into the planet's soil.

Even more intriguing, Mars Express has detected methane in the atmosphere. Methane, along with ammonia, is a strong signal that some organic process is at work.

The ESA's Huygens probe, which landed on Saturn's gian t moon Titan last Friday, set down in a virtual organic soup of liquid methane. The craft took images during its descent that looked exactly like the shoreline of an ocean - although this ocean is hundreds of degrees Fahrenheit cold.

An ocean of liquid water could exist beneath the icy surface of Jupiter's large moon, Europa. The Jovian system experiences strong tidal influences from the biggest planet in the solar system, enough to keep the ice thawed inside Europa and perhaps generate enough heat to support a biosystem.

Jupiter itself could play host to life in its vast atmosphere, as could Saturn.

All these areas - including, obviously, Earth - possess the basic building blocks from which living systems can be constructed. Organic molecules have even been found inside meteorites.

Though the answer to the overall question of how life started remains unknown, science has established certain components.

Billions of years ago, a giant star exploded in a supernova that forged the heavy elements necessary for life. In the nebular remnant of that titanic explosion, the elements combined to form simple molecules such as carbon monoxide, water, ammonia, methane.

Whether within that cloud or afterward, those molecules spontaneously combined further to form amino and nucleic acids.

From there the record quickly becomes fuzzy, because the simple molecules contain only a couple of dozen atoms or so, while DNA, the biggest molecule of all, contains billions.

Still, the similarities abound in terms of ingredients. The quest is turning to establishing the conditions required to create the complex organic molecules and resulting life.

Even here, the gaps are filling in. Somehow, scientists think, lightning played a role in forging the longer molecular chains. Lightning is common on Jupiter and Saturn and, acco rding to Huygens, Titan.

Ultraviolet light from the sun is another player. It breaks down some molecules and creates others. Earth's famous ozone band protects living things from much of the harmful effects of solar UV light. A counterpart has not yet been found on any other planet or moon, but it is possible ozone becomes less important as the distance from the sun increases.

Disturbance is a surprisingly critical constituent in the life process. Living systems require a balance of quietude and disruption, such as from comet and asteroid impact, or from volcanism. In the resulting turmoil, chemical changes sometimes produce or redirect evolution.

Mars, lacking volcanism, is relatively quiet. Titan and the large inner moons of Jupiter - Europa, Ganymede, Callisto and Io - are another story. All are subject to the gravitational tugs of very large parents and numerous lunar neighbors. Those tugs generate heat, perhaps enough to turn water ice into liquid.

Water remains the most critical ingredient for life, but now the existence of water - albeit in the past - has been essentially confirmed on Mars. In 2015, if all goes as scheduled, NASA will launch its Prometheus mission to Jupiter's moons. Its mission is to zero in on Europa, sending a probe plunging through the ice to attempt to determine if there is, as expected, water below and whether anything is swimming in it.

It is possible, of course, to jump to incorrect conclusions about the mounting evidence, but it is just as likely to become overly and unnecessarily pessimistic based on preliminary observations.

Toby Owens, an astronomer at the University of Hawaii, recently discussed this possibility with Astrobiology Magazine.

"In the case of Mars exploration, we went there three times with three different spacecraft, looking at three different parts of the planet, " he said in the interview, referring to the twin Viking spacecraft, that landed in 1976, and the Pathfinder, with its Sojourner rover, which landed in 1997.

"We never saw the volcanoes, the riverbeds, the big canyon. All we saw were craters. Some scientists were so disappointed with that, they said, 'Let's cancel the program. Mars is just like the moon. There's no point in spending this money. But fortunately, we had an orbiter in the works and it discovered that we had visited the three most boring places on Mars."

That lesson has not been lost on the scientific community. There are, no doubt, future wonders to be discovered, some of which might require a more sophisticated mission to bring home a soil sample.

Still, it is a good bet that meeting a neighboring ecosystem is a prospect that lies within this century - perhaps even within one or two decades.

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