As far as we know, life in our Solar System is confined to one lonely planet: Earth. Yet over the past decade or so, several other candidate worlds have emerged that scientists believe have—or, at least, had in the past — the potential to support some kind of life. These include Jupiter and Saturn’s moons Europa and Enceladus, both icy worlds with the potential for liquid oceans lurking beneath their ice, as well as planets such as Mars, which may have been much hotter and wetter in the past.
For all of their potential, however, Europa, Enceladus, and Mars are not scientists’ sole candidates for life-harboring worlds. While these three are perhaps the most likely, several more outlandish theories propose the existence of life on even the most strange and seemingly inhospitable bodies in our Solar system, from microbes piggybacking on asteroids and comets to single-celled organisms living within Venus’ clouds.
Of all of these more “out-there” theories, the two most interesting surround Saturn’s moon Titan and the far-off dwarf planet Pluto—both cold, icy worlds where life as we know it could never exist, a reality that hasn’t dissuaded many scientists who believe fervently in the capacity for life to adapt and become something other than what we know on Earth. Below are some of the theories that speculate on life’s potential on these two hellish worlds, theories that, if ever proven true, could radically challenge our assumptions on where life can adapt and thrive in the universe.
Titan: Saturn’s Most Interesting Moon
First is Titan, the largest moon of Saturn and unique for being the only moon that possesses and atmosphere. Much like Earth, Titan’s atmosphere is mostly nitrogen, and like Earth it possesses stable bodies of liquid on its surface—the only other world in the Solar System that is known to do so. At first glance, then, Titan appears perfectly suited for life—that is, until one takes a closer look.
Whereas Earth sits at a comfortable average temperature of 15 degrees Celsius, Titan is far colder, averaging -180 degrees Celsius on its surface. This is far too cold for liquid water to exist as its does on Earth. Instead, Titan’s liquid lakes and oceans are made of methane, its mountains water ice so cold that it acts like granite. Yet for all of this, there are some small signs that point to something other than geological processes going on.
For one, Titan’s atmosphere contains methane and several other organic compounds, much as Earth’s does. Yet sunlight constantly breaks down the compounds upon interacting with a world’s atmosphere. On Earth, biological processes by and large replenish the atmosphere’s methane supply, but short of constant volcanic activity, few other explanations can be given for the methane in Titan’s atmosphere.
If life does exist on Titan, however, it’s unlike anything we’ve ever seen before. Water is too frozen to act as a catalyst for life as on Earth, so what life that could exist would have to adapt to live off methane instead of water. Yet this may, in fact, be possible: according to a team of chemical engineers and astronomers at Cornell University, cells that would have developed on Titan could have replaced their liquid bilayers- a component of cells on Earth that separates intracellular water-based fluids from extracellular ones- with a methane-based “shell” composed of nitrogen, carbon, and hydrogen molecules, all of which can be found in Titan’s seas. This “nitrogen body” would serve much like lipid bilayers on Earth, but would have a lower freezing point that would allow life to survive in the freezing temperatures of Titan’s methane sees.
These “oxygen-free” lifeforms, if they truly exist, would live off its own carbon cycle, breathing hydrogen instead of oxygen and eating acetylene. And there’s more promising news: in the 1970s, acclaimed astronomer Carl Sagan and chemist Bishun Khare showed that the reactions in Titan’s atmosphere could produce amino acids- a crucial building block for any kind of life.
If life can be found on Titan, than it would represent something extraordinary: a “second genesis” of life in our own solar system that must have develop independently from life on Earth, and without relying on water. This would mean that life is far more adaptable than previously thought, and would expand the possibilities for where life could life tremendously.
What about Pluto?
Pluto, meanwhile, is colder than even Titan: some 50 degrees colder, in fact. Until the New Horizons probe reach Pluto last year, most of the scientific community assumed the world to be a dead, lifeless body, much like our own Moon, pocket marked by impacts from Kupier belt objects with little else to show. After the New Horizons fly-by, however, assumptions have begun to change, with massive mountains over 11,000 ft high standing besides great chasms rivaling our own Grand Canyon pointing to a world much more active and interesting than we ever had previously thought.
This new information has caused some scientists to wonder if Pluto itself could even harbor life. Even more so than Titan, Pluto’s surface is far too cold for life to exist; yet beneath its layers and layers of ice, a liquid ocean- much like that which most likely lies beneath Europa and Enceladus- may in fact exist.
For the presence of a liquid ocean beneath Pluto, something must warm the ice enough for it to melt and not re-freeze. Luckily, Pluto’s core composes of 40 percent of its volume, and should it possesses something close to 75 parts per million of radioactive isotope potassium-40, the energy released from its decay could be sufficient to create liquid seas. This isn’t that far of a stretch, either: Earth itself possesses ten times that amount of potassium-40 in its core, and it is actually theorized to have less of the element present than bodies that formed closer to the outer edges of the solar system. In addition, the ice of Pluto itself can act as an “insulator”, preventing the heat caused by the radioactive decay of its core from escaping into space.
As exciting as this is, the likelihood of life on Pluto is still astronomically low. As stated by the physicist Bryan Cox, “liquid water is necessary but not sufficient for life”. Things like amino acids and hydrocarbon compounds are needed for life to get the kick-start it needs to begin, which all but prevents life from beginning on Pluto.
Yet this doesn’t stop many scientists from wondering “what if”. What if, for example, an asteroid or comet impact carrying amino acids broke through Pluto’s ice and into its sub-surface seas, in doing so creating enough heat to jump-start the processes that would allow for life to exist?
Perhaps we’ll never know for sure, but that life could – no matter how remote the chance- exist on worlds like Pluto and Titan, than perhaps it could exist anywhere, and has been lying right under our noses in our own solar system just waiting to be found.
For More Information:
- Life on Titan? – European Space Agency
- Here’s what methane-based life could look like on Titan- Popular Science
- Saturn’s largest moon would host really weird life- National Geographic
- Pluto could have astrobiological potential- Daily Galaxy
- There could be aliens beneath Pluto’s crust- Daily Mail
About the Author:
Devin is a junior at Northeastern University in Boston MA where he majors in international relations. Devin is responsible for background work on many of the articles on the site, as well as some science writing.