by Sheyna E. Gifford for Astrobiology Magazine
Moffet Field CA (NASA) Jul 29, 2014
Sometimes we get lucky: an exoplanet many light-years away passes in front of its star at the perfect angle. This transit allows us to read certain features of that planet's atmosphere. The resulting spectra - lines made by molecules like oxygen and methane - allow us a peek into that planet's chemical anatomy.
Certain features of those atmospheres make it more likely that something, or someone, inside is breathing. In the last few months, we've found even better ways of looking for these signs of life.
Airborne: Telltale Signs of Life
On Earth, and perhaps elsewhere in the Universe, methane (CH4) is both a product of life as well as one of life's basic energy-sources. For example, our ocean floor hosts an immense biodiversity that is literally swimming in methane and methane bi-products. Countless microbes living miles down feast on methane as it seeps to the surface.
"It's the methane and sulfide - not the heat - that provides the energy source for the bacteria that are at the base of the food chain," said Mandy Joye, a biogeochemist at the University of Georgia and part of a research team on methane seeps.
In addition to the activity on the bottom, it was recently discovered that SAR11, the most abundant organism in the ocean in general, produces methane when it dines upon to its second-favorite food.
Methane emanates from swamps and salt marshes. As the biomass in those places breaks down, methane combines with a substance called sulfate to produce a very distinct and awful odor.
This gaseous methane rises from wetlands to the tune of 164 Tg per year - which is about ? of all the natural methane produced annually on Earth. (1 Teragram (Tg) is equivalent to 10? kg or 2.2+ 10? pounds.)
Even exotic iron-breathing organisms living in acidic, rocky environments make methane. These environmental niches are being studied by astrobiologists as possible Mars-analogs: places where the low-oxygen circumstances may have driven life to adapt in extreme ways.
Even today, Mars may be releasing methane. This highlights a key fact: while most of the methane on Earth comes from living organisms, other sources exist.
Abiogenic methane arises when volcanically heated water reacts with rocks that contain high levels of iron and magnesium. Because of the heating, hydrogen in the water is liberated. That free hydrogen then meets with carbon from carbon dioxide dissolved in the water.
The result is life-form-free methane. These kinds of reactions occur on Earth at mid-oceanic ridges and may occur in mantle as well, where iron is subjected to intense heating, sometimes in the presence of water. Such reactions may be responsible for the recorded methane releases from Mars.
So while not all that makes methane is life, the overwhelming majority of known methane sources are alive. This makes methane a great potential biomarker for finding life on other planets: from the bottom of the sea to the thin-aired mountaintops here, there and everywhere. Knowing that methane exists in the atmosphere of a planet serves another life-related function as well: can help us understand the surface temperature of an exoplanet.
Methane is one of the most notable greenhouse gases. Like carbon dioxide (CO2), atmospheric methane acts as a sort of planetary warm blanket. It wraps around the Earth and reflects back surface radiation that would otherwise make its way into space. In fact, of the two, methane is a far more efficient warming agent. CH4 has at least 20-25 times the global warming potential of CO2, but fortunately for us, only sticks around for a few years after it's produced.
For life in general, a warm planet isn't always a bad thing. Mars, on outer edge of the habitable zone, would be warmer if it had more methane in the atmosphere. An under-abundance of greenhouse gases like hydrogen, CO2 and methane can lead to a planet too cold for life, even if the planet itself is in a habitable zone (HZ).
On the other hand Venus, on the inner edge of the HZ, is far too warm for life due to an excess of greenhouse gases. If we were observing Mars or Venus from afar, picking up greenhouse gases in the atmosphere, or failing to, would help us discern if life was likely there.
For better and worse our planet is a natural generator of methane, as are some of the life forms that dwell upon it.
While it might come as a surprise that most of our methane comes from tiny creatures in swamps and oceans, many of us are aware that closer to the top of the food chain methane slips from the digestive systems of animals that ruminante, many of which later become food-sources themselves. At the very top of the pile of methane producers - not for quality but for creativity - are humans.
Human-made methane comes from a fascinating variety of places. A 2014 study estimated that pit latrines, used as a waste-disposal system in the developing world, will be responsible for 1% of the human-produced methane on Earth this year: about 3.8 Tg.
Then there's industry, agriculture, animal husbandry, and the mining of natural gas. The amount of methane emissions generated by the natural gas industry itself is difficult to pin down. It has been cited as likely ranging from 2-4% of the total methane emissions since 2000. At the end of the day, a substantial amount of anthropogenic or life-made methane that doesn't emanate from oceans or swamps comes the way people choose to live upon this planet.
Given the goal of finding advanced life, methane raises an important question. Here on Earth, the many origins of methane include seafloor bacteria, swamp gas, the digestive emissions of animal herds and the 4-range stovetops of the human race. Looking outward to many light years away, is there any way to know if methane signatures are coming from lifeless crystal ice sculptures, single-celled organisms or gas burners?
The answer is: maybe. Earlier this year, scientists from the University of New South Wales and University College London figured out how to distinguish the spectroscopic lines of hot methane from cooler forms of the same chemical. Their paper describes more than 10 billion spectroscopic lines for methane: an improvement of 2000 times over the previously known number of methane lines.
"Current models of methane are incomplete, leading to a severe underestimation of methane levels on planets," said study co-author Jonathan Tennyson from the UCL Department of Physics and Astronomy.
"We anticipate our new model will have a big impact on the future study of planets and 'cool' stars external to our solar system, potentially helping scientists identify signs of extraterrestrial life."
Since life forms from the bacterial to the bovine to the human being pump it out as part of their normal daily activities, searching for methane lines makes sense. In the specific search for highly evolved life, however, something even better may be on the horizon - something that would tell us that not only is life flourishing, but fabricating.
Proof of (Un)Intelligent Life in the Universe
According to Loeb, this method of detecting extraterrestrial life via environmental pollution is more efficient than SETI, because, for SETI to detect a signal, the other civilization has to be broadcasting.
"You have to assume that they are producing signals that are detectable at great distances," Loeb told astrobio.net, "For over 50 years we have adopted that approach. The problem with that is that it's like searching for a needle in a haystack. You are making assumptions for what the signal might be, and then looking for it, but there is no guarantee that you will find anything."
Humans have been monitoring the skies for radio and laser signals from other species for a while. However, success in that particular mission depends to a certain extent on those races knowing that we exist - or at least wanting to make contact with another planet. Also, they would have to possess the tendency and technology to make transmissions. Searching for industrial pollution obviates these specific needs.
"If you have a civilization that tends to communicate, for example, by cable instead of radio, they will nevertheless pollute their atmospheres," said Loeb.
So rather than wait for a signal that may or may not be coming in a form we may or may not be prepared to detect, Loeb, Abad and Lin suggest that we look for the effects alien civilizations are having on their home planets' atmospheres.
There are a number of advantages to this approach. First, we are already searching exoplanet atmospheres for signs of life like methane. "It doesn't take too much extra effort," Lin told astrobio.net, "to also look for signs of intelligent life."
The specific signs Lin has in mind are chlorofluorocarbons (CFCs). As the key ingredient in many mass- produced products, CFCs are notorious on Earth for everything from holding hair in place to eating holes in our ozone layer.
In 2012, a group at the Blue Marble Space Institute of Science hypothesized that CFCs would be a great way to look for alien races. Sanjoy Som of the Blue Marble Space Institute of Science told astrobio.net that, "We are about a decade away from being able to measure detailed compositions of the atmospheres of extrasolar planets."
Advances in exoplanet spectroscopy combined with better detection of the CFCs tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F) shrunk that decade down to less than 48 months. Abad's expertise in tracing molecules like CF4 resulted in a map of those molecules in Earth's atmosphere. That map combined with Lin and Loeb's astrophysical model lead to this concrete proposal for possibly tracing civilizations by their CFC production.
In addition to needing no new instruments to find them, CF4 and CCl3F have another advantage: both are sensitive as well as specific for industrialized life. If found in an alien atmosphere, there's pretty much only one way that they got there.
"These molecules are basically only produced by human[-like] activities," Abad told astrobio.net. "Whereas methane will be everywhere regardless of whether you are polluting or not. These markers target civilizations with advanced industry."
While primitive forms of life have their own biomarkers - like oxygen and methane - these molecules are too complicated for nature to, as Loeb put it, "do this on its own." However, at least for the moment, we can't rush off and start looking for aliens by their hairspray.
"One thing that should be noted," said Loeb, "Is that the transits that we are observing right now are not suitable for finding these molecules. An Earth-like planet moving around a sun-like star would produce a very small imprint in the spectrum of the star because it covers a tiny bit of the surface area of the star."
If an advanced alien civilization is looking for us by the spectral lines of our pollution in our passage in front of the Sun, we would be very difficult to detect. The area of the Earth is 10,000 times smaller than the area of the Sun. In addition, our atmosphere itself is a very tiny amount of our area. To make the job of finding alien civilizations easier, for now, Loeb et al suggest that we focus on white dwarfs.
"White dwarfs have a size comparable to the size of the Earth, and therefore you get a much bigger effect," said Loeb.
We don't necessarily expect life to be a common phenomenon near white dwarfs. White dwarfs are 100 times smaller than the Sun, so the habitable zone is a 100 times closer. Our own Sun will become a white dwarf someday, but only after becoming a red giant and consuming the inner planets.
However, according to Loeb, a few potential candidate planets around white dwarfs have already been detected. With the James Webb space telescope (JWST) coming in 2018, if we start surveying the nearest 300 light years for white dwarfs and focus in on them, many more may be found.
"We expect to find maybe one or two Earth-like planets that transit white dwarfs, and are observable with JWST, *if* such planets at all exist around white dwarfs," said Dan Maoz from Tel-Aviv University, a collaborator of Loeb's on a paper about detecting life on white dwarfs.
It won't be easy to find an Earth-like planet around a white dwarf transiting at just the right angle, but once we do, Lin said, "We should check for crazy amounts of pollution."
With this model and a survey of white dwarfs in hand, when the JWST takes to the skies in 2018 we can begin an earnest quest for life as brilliantly self-destructive as we are. Until then, we continue to find more exoplanets whose atmospheric methane may hold the key to finding life sleeping on ocean floors or cooking over fires from millions of light years away.
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