Titan Could Have An Alien Biosphere – But It Might Be Dog-Sized

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7 hours ago

Titan, Saturn's largest moon, could harbor life in its hidden underground ocean – yet Titan's entire aquatic biosphere may weigh no more than a miniature poodle, according to new research.
In disappointing news for alien hunters, the cycle of energy and nutrients in Titan's hypothetical biosphere may barely be enough to satisfy a single fermenting cell per liter of the moon's deep ocean, says evolutionary biologist Antonin Affholder from the University of Arizona.
Tantalizing Titan is unique in the Solar System. Compared to other icy moons, some of which may also harbor life in subsurface oceans, Titan has plentiful organic content, Affholder says.
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Hydrocarbons on its surface liquify at a frigid -179 degrees Celsius (-290 degrees Fahrenheit), collecting in rivers and pools as large as our own Great Lakes.
Titan also hides a subsurface water ocean beneath its icy crust. In cross-section, the 5,150-kilometer (3,200-mile) wide Titan may resemble a planetary-scale jawbreaker candy, with five distinct layers, according to Cassini-Huygens data.
These layers hypothetically begin with a rocky core, then a layer of otherworldly 'ice-VI' – an alien ice that only exists at extremely high pressures. Above this sits the salty water ocean. The ocean is sealed by an outer layer of water-ice, which may be about 100 kilometers thick.
This outer water-ice layer forms Titan's bedrock, which is continually dusted by organic molecules that fall as raindrops from methane clouds, or settle as solid particles out of the hazy, yellowish atmosphere.
In the upper reaches of the atmosphere, molecules of nitrogen and methane are split apart by UV radiation from the Sun, and then recombine into a variety of heavy organic molecules rich in carbon and oxygen. As they gather on the surface they form tall, dark dunes that resemble mounds of coffee grounds as they fall back to the surface.
This bounty of organic molecules may seep into Titan's underground ocean through melt pools formed by meteorite impacts that crack and melt the ice crust. Organic molecules may also drift up into the watery ocean from Titan's rocky core.
In the new study, researchers used bioenergetic modeling to ascertain whether these organic molecules could offer the energy to sustain a community of microbes in Titan's ocean. These microbes could have evolved to produce energy by breaking down glycine, not unlike the class of bacterium Clostridia does here on Earth.
Here on Earth, a diverse array of life forms have made use of oxygen as way to conveniently rearrange energetic compounds for growth and energy.
Deprived of this powerful element, Titan's microbes could utilize a process of anaerobic respiration similar to one here on Earth known as fermentation.
Accordingly, the researchers say they chose this "simplest and most remarkable of all biological metabolic processes" because it does not require any speculations on wildly unknown alien metabolisms.
Fermentation on other ocean worlds could be plausible because it's a proven strategy on Earth; a ubiquitous and ancient process that now provides Earthlings with culinary favorites like sourdough, yogurt, and beer – yet also food spoilage if left unchecked.
Additionally, glycine and its precursors are very common throughout the Universe. These molecules are embedded in asteroids, comets, and the clouds of gas and dust that condense into stars and planets.
However, even though organic molecules like glycine have been enriching Titan's ocean on geologic timescales, only a piddling portion of this organic inventory may be suitable for microbial consumption.
This may mean that across Titan's vast ocean, the overall weight of life could measure "only a few kilograms at most – equivalent to the mass of a small dog," Affholder explains.
In other metrics, researchers add that the biosphere would average much "less than 1 cell per kg [2.205 lb] of water over the entire ocean." Or, the carbon content of a single, approximately 110-pound human.
With such a Lilliputian population scattered throughout an expansive environment, discovering a living cell would be akin to finding a needle in a haystack – a haystack approximately 800 million miles away.
This study was published in The Planetary Science Journal.
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