In a 1967 episode of Star Trek, Captain Kirk and his crew investigate the mysterious underage murders on the planet Janus VI. It turns out the killer is a rock monster called Horta. But the Enterprise’s sensors had registered no signs of life in the creature. The Horta was a silicon-based life form. Which made it different from any on Earth where everything is carbon-based.
Still, it didn’t take long to determine that the Horta was alive. The first clue was that she moved by hopping. Spock solved the case with a mind meld. He learned that the creature was the last of its kind, protecting a multitude of eggs.
But recognizing life on different worlds probably won’t be that simple. This could prove particularly difficult if the recipe for life elsewhere does not include familiar ingredients. Some scientists even suspect that there are living organisms on Earth that have been overlooked because they don’t fit standard definitions.
Scientists who research life outside of Earth are called astrobiologists. They need ground rules – with built-in wiggle room – to know when they can confidently declare “It’s alive! “.
Christoph Adami is one of those working on the development of these rules. He is a theoretical physicist at Michigan State University at East Lansing. He saw his own silicon-based version of life develop. But this life was not real. It was a computer simulation.
“It’s easy when it’s easy,” says Adami. “If you find something walking around and beckoning you, it won’t be very difficult to understand that you have found life.” But it’s a safe bet that the first extraterrestrials that humans will encounter will not be little green men. They will probably be tiny microbes of one color or another – or perhaps no color at all.
Scientists are trying to figure out how they might recognize these alien microbes. This could be very difficult if the microbes are very strange. This has led researchers to come up with some basic criteria to distinguish living things from non-living things.
Many insist that certain characteristics must be present for all types of life, including extraterrestrials. These include an active metabolism, reproduction and evolution. Others add that life must have cells large enough to contain protein-making machines called ribosomes.
Defining “life” is not easy. A virus, such as Ebola, is not alive by most definitions (although some scientists say it is).
But these definitions may be too strict. According to Carol Cleland of the University of Colorado at Boulder, compiling a list of criteria necessary for life can give scientists a narrow view. This tunnel vision could blind them to the diversity of life in the cosmos.
Some scientists, for example, claim that viruses are not alive because they depend on their host cells to reproduce. But Adami has “no doubt” that viruses live. “They don’t carry with them everything they need to survive,” he admits. “But neither do we. What’s important, according to Adami, is that viruses pass genetic information from one generation to the next. And at its simplest, he argues, life is just information reproducing itself.
According to Cleland, evolution should also not be considered. After all, people will probably never be around long enough to know if something is changing.
Even restrictions on cell size could prevent the smallest microbes from being considered aliens. Yet that shouldn’t be the case, according to Steven Benner. He is an astrobiologist at the Foundation for Applied Molecular Evolution in Alachua, Florida. A cell too small to contain ribosomes may function in another way. Instead of proteins, it could use genetic material called RNA to carry out biochemical reactions, he speculates.
Cells were considered necessary because they separate one organism from another. But layers of clay could do it, suggests Adami. According to Cleland, life could even exist as networks of chemical reactions, which do not require separation.
It’s a fanciful thought. But that may be all it takes for scientists to recognize unusual types of life, if such extraterrestrials show up.
Up close and in person
In recent years, more than 1,000 planets have been spotted outside our solar system. With their discovery, the chances of extraterrestrial life existing are better than ever. But even the most powerful telescopes can’t image distant life, especially if it’s microscopic. The chances of finding such tiny life increase if scientists can reach out and touch it.
And that means looking into our solar system, says Robert Hazen. He is a scientist who studies minerals and works at the Carnegie Institution for Science in Washington.
“You really need a four-legged rover to analyze chemicals,” he says. Such rovers are currently sampling rocks on Mars. The Cassini space probe bathed in the geysers of Enceladus, Saturn’s icy moon.
These robot explorers could one day return signs of life. But only subtle signs of life – what scientists call “biomarkers”. And it could be very difficult to distinguish true biomarkers from a single mineral, he notes, especially from a distance.
“We really need life to be as obvious as possible,” says Victoria Meadows. By “obvious,” she partly means “Earth-like.” It also means that this signal must be one that no chemical or geological process could have left behind. Ms. Meadows is an astrobiologist with the National Aeronautics and Space Administration. She directs the Virtual Planetary Laboratory at the University of Washington in Seattle.
Some scientists argue that life is an “I’ll know it when I see it” phenomenon, says Kathie Thomas-Keprta. But life can also be in the eye of the beholder. Mrs. Thomas-Keprta knows this only too well, having studied a Martian meteorite. She is a planetary geologist. She was part of a team at NASA’s Johnson Space Center in Houston that studied a meteorite named ALH84001. (It was discovered in the Allan Hills Icefield in Antarctica in 1984).
The team was led by David McKay, Thomas-Keprta’s late colleague. In 1996, scientists claimed that the carbonate globules embedded in the meteorite looked a bit like microscopic life on Earth. The researchers found large organic (carbon-based) molecules.
This indicated that they had formed at the same time. Thomas-Keprta also identified tiny magnetite crystals covering the globules. These iron-based crystals looked a lot like those made by certain bacteria on Earth. These bacteria use the chains of crystals as a compass as they swim in search of nutrients.
In the end, the scientists concluded that they were in the presence of fossils of ancient Martians.
Other scientists disagreed. According to them, the globules and crystals could have formed by other processes, without any life being necessary.
The initial hypothesis of Martian fossils has been largely dismissed.
But it may not be necessary to leave our planet to find aliens. There is the possibility of ghost life on Earth. It could be so strange that it has not been recognized until now, says Cleland of the University of Colorado. Consider, she says, “desert polish.”
These are the dark spots on the sunny side of certain rocks in super dry climates. Some scientists believe that certain bacteria or fungi may be responsible. Weird common microbes could be sucking energy from rocks. They could use this energy to fuel their creation of this hard outer layer of minerals. These organisms could produce the glaze by cementing iron and manganese to clay and silicate particles.
Curiously, some scientists have tried to recreate the desert varnish in the laboratory. They used fungi and bacteria. And they failed.
In nature, these varnishes form over millennia. Critics have argued that it’s too slow to be something created by microbes. But how do we know, asks Cleland? “We start from the principle that life on Earth has a rhythm,” she says. Life in the shadows may develop much more slowly.
To find life, and classify it correctly, you have to look for what’s weird, suggests Hazen. He looks for messages in the minerals. Minerals are not evenly distributed across the landscape. There are 4,933 recognized minerals on the planet, Hazen says. He and his team have mapped the location of 4,831 of them. And 22% of them only exist on one site. Nearly 12% more are in just two places.
One of the reasons for this uneven distribution is that life, in the course of its evolution, has used local resources, transforming them into new minerals. Take for example hazenite. (Yes, its name comes from Hazen.) This phosphate mineral is found only in Mono Lake, California. The microbes that live there are its only source. Hazen’s group believe that other species may have given rise to pockets of equally rare minerals.
Desert varnish stains rocks burnt orange or black (top, Angel Arch, Canyonlands National Park, Utah). It can be produced by unknown living organisms. Silicates make desert glaze shiny (bottom, central Australia).
The discovery of such strange distributions of minerals on other planets or moons could indicate that life exists there or has existed there. Hazen advised NASA on how rovers could identify such mineral clues to life on Mars.
Mars was once humid. There is still occasional running water there. This shows that she may once have been able to accommodate life. This evidence and others in 2013 led Benner of the Foundation for Applied Molecular Evolution to suggest that Mars may have seeded present-day life on Earth. For this idea to hold water, it may be necessary to find Martians.
But Benner doesn’t seem worried. “I’d be surprised if we didn’t find life on Mars,” he says.
According to Dirk Schulze-Makuch, missions could easily bring astronauts to Mars to confirm an alleged discovery. He is an astrobiologist at Washington State University at Pullman. “If someone with a microscope sees a microbe and it ‘wiggles and wiggles back, it’s really hard to disprove,'” he jokes.
Go to the less obvious
But humans and even probes might have a harder time spotting life in more distant or exotic places. The preferred targets are the moons of Jupiter and Saturn. Alien hunters are drawn to Europa and Enceladus because their liquid oceans float beneath icy crusts.
Liquid water is thought to be necessary for many chemical reactions that could support life. But water is actually a terrible solvent for building complex molecules on which life could be based, notes Schulze-Makuch. Instead, he thinks the extraterrestrials may have been born in hotspots deep in the hydrocarbon lakes of Titan, Saturn’s largest moon. “We don’t know if it is possible to achieve life,” he says.
Life on Saturn’s moon Titan may exist in nitrogen-containing structures called azotosomes.
Perhaps the biggest challenge to life on Titan, according to Paulette Clancy, is the extreme cold. She is a chemical engineer at Cornell University in Ithaca, New York. This moon is so icy that its methane – a gas on Earth – is a viscous, almost frozen liquid. And the water, she says, “would be like a stone.” Under these conditions, she notes, organisms with Earth-like chemistry wouldn’t stand a chance. Indeed, the membranes that hold the innards of an Earth cell wouldn’t work on Titan.
But Clancy and his colleagues simulated experiments under Titan-like conditions. They discovered that certain short-tailed molecules could spontaneously create stable bubbles. These bubbles are similar to cell membranes.
Like the varnish of the desert, life on Titan could develop slowly. There is little sunlight or heat. Its freezing temperatures would slow down chemical reactions. So if life were to exist here, Schulze-Makuch imagines it would have a lifespan of millions of years. Organisms could reproduce – or even breathe – only once every thousand years!
With so many options, Clancy predicts that there are multiple planets or moons with life on them. Many other researchers are also optimistic about the possibility of finding life. In the future, astrobiologists may come face to face with ET. And when they do, they may even recognize it for what it is.