A warmer, wetter Mars might have been a habitable world, but it's not clear exactly how the conditions on Mars changed and became so different. Our neighboring planet had the best chances for harboring life as we know it, even if it doesn't anymore. There's really no proof that Mars supported life (yet?), but it's not impossible to think that things once lived there. Here are just a few clues that could lead to finding alien lifeforms.

• NASA's Curiosity rover measured a spike of methane gas that could indicate biological activity. Of course, this methane plume might have non-biological origins, but it's very interesting to see fluctuations of organic molecules. [url]
• Sediment found on Mars might have been home to microbial colonies, analogous to life on Earth, forming microbially-induced sedimentary structures (MISS). Similar-looking rock structures don't necessarily mean microbes lived on Mars, but it would be an extraordinary coincidence. [url]
• An ancient freshwater lake likely existed on Mars for at least 100,000 years. The presence of fine-grained sedimentary rocks and their chemical composition point to a sizable body of water that might have been common on the red planet in its distant past. [url]

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Astrobiology is a field that doesn't have a whole lot of experimental evidence, but it's interesting to entertain the possibilities of life that might exist elsewhere in the universe. So far, we've been looking for ET biology that's similar to our own, but there's nothing that says biology must be based on DNA/RNA/proteins/etc. Here are just a few more links on possibly finding life on other worlds.

• Stars in our galaxy generally travel only a few hundred miles per second relative to their peers, but some hypervelocity stars traveling at hundreds of thousands of miles per second might escape our galaxy -- and spread life across the universe. It could happen theoretically, but no one has yet observed this phenomenon directly. [url]
• Data from the Kepler mission suggests possibly-habitable planets exist near a significant fraction of all stars -- providing better estimates for values in the Drake equation. However, we're still probably very far away from our nearest neighbors (if they even exist). [url]
• How tough is DNA? Tough enough to survive on the outside of a rocket going to space and also re-entry from orbit. Presumably, this also means that it's not impossible for ET DNA to arrive on the tons of meteorites that hit our planet every day.[url]

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Panspermia isn't a crazy idea, especially when we seem to keep finding extremophiles that can survive in very harsh environments. The components of life may be traveling between planets or solar systems at a non-zero rate, seeding the universe with living materials. That said, life is still relatively fragile, but there may be some optimism in finding living specimens elsewhere than Earth. (First, though, we have to make sure we're not the ones contaminating our own solar system.) Here are just a few links on various organisms that might survive a trip in space -- without a space ship.

• The International Space Station (ISS) may have microorganisms living on the outside of the station, and evidence of sea plankton has been found on the exterior surface of the ISS. No one knows exactly how sea plankton could have gotten on the ISS, but contamination on interplanetary objects (eg. comets, asteroids, etc) could spread life farther than previously suspected. [url]
• Astrobiology researchers are just starting to figure out which known organisms can survive the harsh conditions of space. Spore-forming bacteria (eg. Bacillus pumilus SAFR-032) have been seen to survive in simulated space environments, and similar organisms could support lithopanspermia theories suggesting an extra-terrestrial origin of life as we know it. [url]
• Biologists can make very tiny space suits for insects if they really wanted to. In order to study live insects in a scanning electron microscopes (SEM), biologists can coat bugs with a polymer film that protects them from drying out in a vacuum chamber (and lets them live while they're being bombarded with charged particles). [url]

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Water is an essential substance for life (as we know it), so it's not too surprising that as we look for evidence of extraterrestrial life, we often start by looking for signs of water. (We also look for water on places like the moon because locations near water/ice would be nice if we ever want to build a lunar base.) Here are just a few links on finding water in our solar system and beyond.

• NASA has released its tentative plans for the next 30 years, and its goals are to answer 3 questions: Are we alone? How did we get here? And how does the universe work? Upcoming telescopes will be looking for exoplanets and may be able to detect atmospheres on distant worlds. [url]
• Stardust is mostly made up of silicates floating in space, and when these dust particles encounter solar wind (charged hydrogen ions), water can form from these interplanetary dust particles. Assuming similar stardust exists in other star systems, water and the basic ingredients for life as we know it might exist throughout the universe. [url]
• The Herschel space observatory has detected water spewing from the dwarf planet Ceres (located in the asteroid belt between the orbits of Mars and Jupiter). The European Space Agency (ESA) reports that Ceres has a very thin water vapor atmosphere, and this is evidence of an icy surface. NASA's Dawn mission is scheduled to visit Ceres in 2015, so we might get a closer look in the near future. [url]

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Studies have shown that spore-forming bacteria can survive in space under certain conditions, so it doesn't seem so implausible that life -- as we know it -- has accomplished interplanetary travel successfully. Extremophiles living in unexpected places also seems to suggest mounting evidence that life exists on other worlds, even though we have no direct proof of extraterrestrial beings (yet). Here are just a few links related to the growing field of astrobiology.

• NASA microbiologists have classified a new genus (not just a species) of bacteria that can survive in the clean rooms designed to prevent spacecraft from contaminating the rest of the solar system with terrestrial organisms. The bacterium Tersicoccus phoenicis was first found in 2007 in a clean room made for the Mars lander Phoenix, and it's been seen again in another clean room 4,000 km away that housed the ESA's Herschel space telescope. [url]
• When the universe was just 15 million years old, the 'Goldilocks' conditions for supporting life might have existed nearly everywhere -- 10 billion years before life on Earth likely began. No one is actually certain if planets even existed when the universe was that young, but if there were, liquid water and nice ambient temperatures were probably not that hard to find. [url]
• The asteroid that killed off all the dinosaurs about 65 million years ago hit with such force that rocks from Earth could have been ejected out of our atmosphere -- to travel as far as Saturn. Rocks from our planet could have carried life to Mars, Venus, Jupiter and Saturn (as well as the moons of those planets), so if we ever find life elsewhere in our solar system, it might have come from Earth. [url]
• Biomolecules may have formed on asteroids when our sun was much younger and warmer than it is now. The 'Goldilocks' zone for our sun may have included the asteroid belt between Mars and Jupiter, but our sun has cooled with age and the asteroid belt isn't a very likely place to find liquid water anymore. [url]

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Discovering an alien life form would be quite an achievement, but we've been burned before by over-eager press releases that claimed to find evidence of life from beyond our planet. NASA might be more careful about making any announcements about life based on peculiar and potentially extra-terrestrial-based life, but NASA seemed to have forgotten about the extraordinary claims over ALH84001. Overall, though, it's probably good that NASA hasn't given up on searching for aliens, so here are a few links on looking for life from outer space.

• Recently, some UK scientists claimed to find evidence of alien life in the Earth's upper atmosphere, suggesting that alien life is just raining down from space all the time. The concept of panspermia is interesting, but the evidence for it isn't quite convincing yet. [url]
• There are a bunch of ways to look for aliens, such as a search for extraterrestrial technology (SETT) to find non-natural shapes in space. Looking for alien crop circles on exoplanets ain't easy, though. [url]
• Some astronomers are looking for evidence of Dyson Spheres -- a massive array solar panels that advanced alien civilizations might use for sustaining enormous energy needs. This search is actually being funded by a grant from the Templeton Foundation. [url]
• NASA's Curiosity rover hasn't detected much methane in the Martian atmosphere, so the odds of finding familiar living organisms seems a bit more distant. Mars was most likely suitable for Earth-like life at some point, but it's looking pretty dead right now. [url]

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We've discovered thousands of exoplanets beyond our solar system, and some of them are even in the "Goldilocks zone" where liquid water could possibly exist. Some astronomers think life could be abundant in the universe, but there's not that much hard evidence (yet!). Here are just a few astronomical discoveries that might encourage researchers to look for signs of life a bit more carefully.

• Exoplanets have been discovered circling some extremely old, metal-poor stars, creating interesting curiosities that might expand the theories of how planets form. These exoplanets might not harbor any kind of life... or we may want to get a bit more creative about how we envision life on other worlds. [url]
• A free-floating planet named CFBDSIR2149 is not orbiting a star, and it's only one of about two dozen or so known examples of a starless planet. This rogue planet is relatively young compared to the Earth, and some spectroscopic measurements suggest this planet's temperature is about 430° Celsius -- a bit too warm for our tastes (but maybe not for aliens?). [url]
• Within our own solar system, NASA's Cassini spacecraft has detected spurts of water from Saturn's moon Enceladus. This evidence leads to some speculation of a habitable zone on some icy moons where microbes might be able to survive. [url]

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The origin of life is a pretty enormous mystery. There are several theories for how life might have come about, but it's difficult to design experiments to narrow down these options. In the meantime, researchers continue to look for clues and evidence for life that didn't originate on our planet. Here are just a few examples that could one day lead us in the right direction.

• Meteorite fragments found in Sri Lanka *might* contain extra-terrestrial fossils -- if you kinda squint and tilt your head, the algae-like fossils support a panspermia theory for the origin of life. This isn't the first time a peer-reviewed journal has published a story like this, and it won't be the last. The real question is when will these papers actually be convincing to skeptics.... [url]
• In 1996, a rock named ALH84001 made a lot more headlines when scientists claimed that it might contain evidence of life from Mars in the form of microfossils. Since then, the ALH84001 debate seems to have settled on the view that those small fossils weren't necessarily created by ET life, and the media hoopla re-affirmed that "extraordinary claims require extraordinary evidence." [url]
• Maybe we'll see evidence of life outside our solar system by looking for evidence of asteroid mining operations around other stars -- if we see unusual amounts of dusty debris, it could mean intelligence life has been exploring for interplanetary resources. It might be hard to determine the difference between natural and unnatural debris, though. [url]
• NASA's Curiosity rover has discovered that Mars once had an environment suitable for life. This is an important find, and it raises questions of why Mars doesn't seem to have ubiquitous life now. [url]

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There are about 700 confirmed exoplanets that astronomers have indirectly observed. The Kepler Space Telescope has gathered a vast amount of data, and it suggests that there are probably planets orbiting nearly every star. Now that we know about all these planets, it's fun to speculate about how much life could exist on them. So here are just a few links for those of you who want to know where our alien neighbors might be.

• In just our galaxy, there could be 10 billion other planets that are similar to our own Earth. Gas giant exoplanets the size of Jupiter are easier to find, but they might not be as common as small rocky planets like ours. [url]
• The Habitable Exoplanets Catalog has a list of 7 planets that might support life, and this list has only just started to be compiled. A lot of these exoplanets are "super-Earth" sized, and it's tough to know much about their atmospheres with current telescopes. [url]
• Tau Ceti is a star system just 12 light years away that seems to have similar characteristics to our solar system. Astronomers are guessing that there may be 5 rocky planets in orbit around Tau Ceti, and that star is about twice as old as our Sun, so there would have been plenty of time for life to arise (but also to perish). [url]
• ATLAST is a space-based telescope specifically designed to look for signs of life on other planets, but it's only in the concept stage -- with vague plans to launch sometime before 2032. Directly observing exoplanets and their atmospheres could tell us more about how unique our Earth is and how lonely we might be in the universe. [url]

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Looking for extraterrestrial life has been a largely fruitless task for many decades. There have been a few times when people thought they might have found evidence of life that wasn't from Earth, but upon further analysis, those discoveries weren't so clear cut. Still, the search for ETs is on-going, and here are just a few links on some ways to find alien friends.

• Spectroscopic biosignatures could be detected in light reflections off of alien planets. Reflected sunlight is polarized when it bounces off the Earth, and plant life on Earth absorb specific bands of light -- so alien worlds may exhibit similar phenomena (if only we could detect it). [url]
• In the 1970s, there was some speculation that cellulose could be detected in interstellar space. However, it's not so easy to figure out how to detect biopolymers in the vast vacuum of space. [url]
• The Drake Equation estimates how many alien civilizations might exist in our galaxy. It's just an estimate, and there are a lot of unknown variables... [url]
• Moqui marbles are unusual spherical rocks that have a rusty iron oxide exterior and a sandy interior -- and were formed with the help of microorganisms. So if we find similar rocks on other worlds, we might be able to infer that extra-terrestrial microbial life also exists. [url]

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