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Is Pluto’s Ocean An Oasis For Life?
Its radioactive sub-surface ocean could be a unique home for life
Pluto is a tiny cold world billions of miles away in the outer reaches of the solar system. It’s so far out that the Sun looks like just another star in the perpetual night sky. This makes it a cold, dark world dominated by ice and frozen nitrogen. But scattered on this bleak surface are the tell-tale signs of what lies beneath, a vast sub-surface liquid water ocean warmed by the radioactive decay of the dwarf planet’s core. Could this alien ocean be a distant oasis for life in the cold desert of the outer solar system?
Before the New Horizons probe flew by Pluto in 2015, we had no idea what Pluto’s surface looked like. Even using powerful telescopes like Hubble, it appeared as a pixilated blob. That should give you a sense of scale. It is so small and far away that a telescope that can see the most distant dim galaxies in pin-sharp detail couldn’t make out Pluto.
New Horizons didn’t stick around Pluto for long and whizzed by on its way to the rest of the outer solar system, but the handful of photos it took of Pluto revealed beautiful details. Plains of frozen nitrogen, mountain ranges of ice, a thin atmosphere of methane and nitrogen gas and even organic chemistry in the form of tholins. These findings blew away the New Horizons team, they were expecting a solid lump of nitrogen with maybe a few notable features, but Pluto displayed complex geology and chemistry.
However, one feature stood out. On the west of the heart-shaped plain known as Tombaugh Regio is a region known as Sputnik Planitia, and it looked extraordinary. Sputnik is a flat plain of nitrogen ice with no impact craters, but it has cracks across the surface that cut the plain into hexagonal ‘cells’ similar to the shape of a beehive.
The team knew this plain had to have formed recently, otherwise, it would be full of impact craters like the rest of the dwarf planet, but how can such intricate shapes form on its surface? The answer came from a boiling pan of water and bees.
When you heat a body of liquid from the bottom, you get convection currents. The warm liquid rises, and the cold liquid sinks. Rather than constantly bumping into each other, the molecules line up into orderly loops circling up and down, heating and cooling. If you have many of these convection loops close together, they push up against each other, squeezing each other from the sides. In other words, it forces the convection current to maximise its cross-sectional area and reduce its contact area which it shares with its neighbouring currents.
This is the same pressure on a beehive’s cells. They need the largest possible cross-sectional area for the wax used to build the cells, this way, they can store more honey and use less wax. If you control the conditions in a pan of boiling water, you can also get the same hexagonal patterns in the rolling boil of the water.
So the team knew there was convection currents of liquid under the nitrogen ice under Sputnik Planitia, but is it liquid nitrogen? Liquid methane? Water? The team ran calculations on the size of the hexagons and concluded that there is an enormous water ocean under Sputnik Planitia, but what could be heating it?
In other distant ocean worlds like Enceladus, the heat doesn’t come from the Sun. Instead, it comes from gravitational tidal forces pulling and squeezing the core, heating it through friction, but to have that you need to be close to a gas giant planet.
After some serious head-scratching, the team concluded that Pluto must have a radioactive core that heats the sub-surface ocean as it decays away. It was the only possible heat source that could produce the hexagons on Sputnik Planitia.
So, Pluto, a world that by all rights should be cold and dead, has evidence of organic chemistry, vast amounts of warm liquid water and an energy source in the form of radiation. It seems like it has all the ingredients necessary for life. Could life thrive there?
Well, these organisms will have to be incredibly resistant to radiation, and they would have to use radiation as their sole source of energy, so they will be unlike anything we have on Earth, but do we have any evidence for such an organism here on Earth?
Yes, we do! But it probably isn’t what you think it is. I would love to say that large, complex creatures can thrive in similar conditions on Earth, but they don’t. Sadly, Godzilla and the three-eyed fish from the Simpsons still only reside in fiction. Instead, radiotrophic fungi are probably our best bet.
These species of fungi are so resistant to radiation that they have grown over the Chernobyl nuclear reactor. What’s more, they can thrive purely off the radioactive decay of the molten core. Scientists have even strapped them to the outside of the ISS and watched them thrive in the vacuum of space, absorbing the damaging, unfiltered radiation of the Sun.
So it is feasible that life could evolve to live in the radioactive oceans of Pluto, but could it start there?
Our best theory for how life started on Earth involves slow-flowing hydrothermal vents full of organic molecules that randomly combined together. Scientists then disagree on what happened next. Some say self-replicating RNA randomly formed, which could evolve to DNA, then DNA with some form of metabolism to speed up its reproduction and finally evolve a nice cell membrane to protect itself and hey presto, the first single celled life form formed! Others say this goes against the laws of entropy, and instead, metabolism randomly started first, followed by RNA and DNA to help propagate the metabolism, then cell membranes.
But either way, for life to emerge from geology on Pluto, it looks like there needs to be a period where RNA and DNA are exposed and unprotected as it takes time to evolve the ability to protect and repair these vital compounds. But RNA and DNA are easily damaged by radiation, and Pluto’s core is full of the radiation! So, it seems life couldn’t start there in the first place.
But we don’t know what Pluto’s interior looks like, all we know is that there is a massive ocean with lots of radiation. Maybe one side of the dwarf planet’s core is radioactive, and the other side isn’t but still gets heated. If this is the case, then it could mean that there are plenty of hydrothermal vents with minimal radiation under the ice, perfect for life to start and flourish. On the other hand, the core could be very radioactive all over, destroying the early building blocks of life before they had the chance to coalesce into cells.
So, could life flourish under the ice in the distant solar system? It is certainly a possibility, and NASA knows it. Pluto now joins the ranks of NASA’s ‘Ocean Worlds’ like Enceladus and Europa. These are bodies with possibly habitable vast sub-surface oceans, and they seem like our best bet for finding extra-terrestrials in our own cosmic neighbourhood. Currently, scientists are chomping at the bit to get probes to these worlds, explore the watery depths, and hopefully find life, and NASA is looking into such projects as we speak, but we have to sit tight until these missions get the green light. Until then, we can look up in awe at the possibility that our solar system is far more habitable than we ever thought possible.
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