The two-year mission of the Mars Science Laboratory -- better known as Curiosity -- has just begun. Curiosity packs a lot more technical punch than its precursor rovers - it's armed with sophisticated tools that can vaporize rocks in order to analyze their chemical composition and drill into the Martian surface to collect geological samples.
If it finds evidence that the Red Planet once supported life, Curiosity will have more than justified its $2.5 billion price tag. But Mars isn't the only place in our solar system that scientists think might have had "the right stuff" for life.
Red Planet, Blue Water?
Researchers have long suspected there is, or has been, life on Mars because of the planet's ice caps, and much of Curiosity's mission focuses on expanding our knowledge of any possible Martian oceans that may have existed millions of years ago.
As far as we know, the absence of liquid water is the primary deal-breaker when it comes to the development of life on other planets. Various geological features seem to resemble the shores of ancient oceans or look like canyons cut by water flowing.
This past week, a research team led by University of Texas at Austin researcher Lorena Moscardelli pointed out in a paper in the journal GSA Today that large, polygon-shaped formations on the Martian surface are eerily similar to formations on the deep-sea floor here on Earth. Moscardelli has also argued that teardrop-shaped islands on Mars were made deep beneath an ancient sea as sediment flows were warped around craters into elongated shapes.
There have been other clues pointing to Martian life. The Allan Hills 84001 meteorite found in Antarctica in 1984, thought to have fallen from Mars, vaulted into the news in 1996 when a team led by NASA scientist David McKay claimed that tiny globules on the meteorite's surface resembled fossilized bacteria-like life forms.
In 2009, NASA researchers took another look at the meteorite with more advanced instruments. While they couldn't conclusively prove that the formations were life, they were able to rule out some of the alternative explanations that various formations were actually made by heating.
"We believe that the biogenic hypothesis is stronger now than when we first proposed it 13 years ago," NASA scientist Everett Gibson said in 2009.
Life Amongst Saturn's Rings?
Another top candidate for housing extra-terrestrial life in our solar system is Saturn's moon Enceladus. At just about 314 miles in diameter, it's miniscule compared to Earth, which is nearly 8,000 miles across. Since it reflects nearly all the faint sunlight that hits it, Enceladus has a surface temperature of minus 330 degrees Fahrenheit.
These features, at first glance, may make Enceladus seem like a terrible place to look for life. But there's a good chance that there is a vast ocean trapped underneath the moon's frozen crust.
NASA's Cassini spacecraft has made several fly-bys of Enceladus since 2005. On its first pass, Cassini found an atmosphere around the moon, suggesting that gases were escaping from the surface or interior of Enceladus. When the space craft went around again, it saw four large cracks near its south pole that astronomers dubbed "tiger stripes."
Later observations showed that the tiger-striped region of Enceladus was much warmer than expected, and showed that jets within the stripes were venting plumes of gases and solids: methane, carbon dioxide and methane -- but more importantly, water vapor and ice.
Scientists weren't sure whether the plumes belched out from the moon arose primarily from the decomposition of solid ice or from liquid water. But in June 2011, a team of scientists led by German researcher Frank Postberg argued in a paper published in the journal Nature that the saltiness of the ice particles ejected by the moon and the presence of organic compounds in ice grains near the denser parts of the plumes spewed from Enceladus were most consistent with a liquid model, especially a large salt-water reservoir with a broad surface - or more simply, an ocean or sea.
Another possible hotspot of life is circling Saturn about 611,000 miles away from Enceladus' orbit: the moon Titan. Titan's the second-largest moon in the solar system, dwarfed only by Jupiter's moon Ganymede. It also has a dense, hazy atmosphere rich in nitrogen. It is the only body besides Earth that has confirmed liquid lakes on its surface - though Titan's lakes are made of methane or ethane, not water.
Thinking about life on Titan requires scientists to rejigger their vision of what life looks like. Hypothetically, to live in the lakes of Titan, an organism would inhale hydrogen instead of oxygen and exhale methane instead of carbon dioxide.
Cassini's observations of Titan show that hydrogen flowing down through Titan's atmosphere disappears at the surface; the probe also is picking up a lack of the chemical acetylene. In June 2010, researchers argued that these conditions are consistent with an exotic methane-based life form that uses acetylene as an energy source, much like most life on our planet uses glucose.
Speculating about totally new kinds of life on one of Saturn's moons is exciting, but the data we have thus far aren't conclusive proof of methane-based life.
"We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results - for example, reactions involving mineral catalysts," NASA astrobiologist Mark Allen said in a 2010 statement.
Drops Of Jupiter
Jupiter's moon Europa is slightly smaller than Earth's moon, but like Enceladus is thought to contain liquid water beneath an icy coating. Life could be hiding out in deep sea crevices and thermal vents, or floating along in the ocean.
Some scientists have argued for exploring Europa rather than Mars, as the Red Planet in most best-case scenarios will yield traces of life that have been extinct for millions of years.
"Europa, potentially, has all the ingredients for life ... and not just 4 billion years ago...but today," University of Colorado at Boulder astrophysicist Robert Pappalardo said in 2006, according to Space.com.
There's something bittersweet in the search for life on Mars, since any alien species will have in all likelihood died out eons before humanity was a twinkle in a primate's eye. But hopefully, any traces found on the Red Planet will bolster our search for any extraterrestrial neighbors that might still be around.