Astronomy

Prospects Of Life Found On Pluto

The idea that a dwarf planet like Pluto could sustain life seemed ridiculous, till unbelievable new discoveries were made. Pluto lies in the Kuiper Belt, outside the solar system. Although it doesn’t look like this place could nurture life, evidence from NASA’s New Horizons spacecraft proves otherwise.

The evidence suggests that it could be possible for Pluto to sustain life. Pluto was even ranked next to the icy moons Europa and Titan, which are well-known nominees for habitability. Before New Horizons studied the dwarf planet intensively in 2015, scientists didn’t know much about Pluto. The results from the study were totally unexpected. It shows that Pluto is a diverse and mobile planet with a thin and hazy atmosphere. It also consists of mountains, rift valleys, and glaciers of solid nitrogen.

Is Pluto Truly Habitable?

Life's prospects on pluto
The long fracture named Virgil Fossae. Source – NASA/JHUAPL/SwRI

The habitability of an environment is generally evaluated with regards to factors such as organic molecules, liquid water, and energy available. Without a doubt, Pluto is rich in energy. Scientists were well-aware of Pluto’s density prior to the 2015 study. They were able to conclude that rocks accounted for two-third of Pluto’s mass, and the rest was due to ice.

Pluto gains most of its energy from the radioactive decay of rocks. When rocks undergo radioactive decay, they release heat over geological time. This process generates s enough energy to warm the rocks in Pluto’s interior, close to their melting point. Other heat-sources like gravitational energy may provide minor contributions to additional warming.

Organic moles too are present in Pluto, with its atmosphere comprising 0.3% methane. According to discoveries made by New Horizons, UV radiation from the sun separates these methane molecules. This produces a number of simple hydrocarbons such as acetylene, ethylene, and ethane. Methane ice is present on the dwarf planet’s surface. The reddish material on it is most likely to be hydrocarbon haze particles that are settling out from the atmosphere. Therefore it’s safe to say that the surface, at least, consists of organic molecules that can support life. Still, there’s no evidence of a mechanism to transport these organic molecules to an ocean. However, researches about comets suggest that the interior of objects outside the solar system may also contain similar components.

So now, the only issue left is liquid water. As explained above, from the amount of heat released from the radioactive decay on Pluto, all the ice on Pluto could be melted several times over. Although it was evident that a subsurface could exist under thick ice, there was no proof that an ocean actually existed. However, if a connecting shell removes heat at a significant speed, the ocean might never form. This is why spacecraft observations were vital to arrive at a proper conclusion.

Do Oceans Exist On Pluto?

Three main lines of evidence prove that a subsurface might be present on Pluto. The first proof is derived from Pluto’s surface geology. Large cracks and fissures on Pluto’s surface suggest that Pluto has experienced global expansion to a small extent. This planetwide swelling can be produced by refreezing a subsurface ocean. When the water cools down and changes its physical state back to the ice, its volume increases. Due to this, its surface is pushed outwards, pressurizing the water underneath. When this pressure exceeds a certain amount, water might erupt on the surface. This phenomenon is called ‘cryovolcanism’.

3 FROM THE SEPTEMBER 2019 ISSUE Life’s prospects on Pluto
The 90-mile-wide (150 km) Wright Mons (at lower left) shows a central depression and scalloped flanks that suggest it could be a cryovolcano. Source – NASA/JHUAPL/SwRI

The second line of evidence is with regards to a feature that’s not seen on Pluto. Some bodies like Earth’s Moon and Saturn’s satellite Iapetus appear significantly bigger around the equator. These are known as ‘equatorial bulges’. Earlier, these were formed when moons spun with a great speed. Later, they froze in place. Therefore, the equatorial bulges of the Moon and Iapetus are results of their early faster spin rate.

Due to the influence of gravity from its large moon Charon, Pluto must have spun down substantially over time. Therefore, Pluto can be a candidate for such a fossil bulge. However, a fossil bulge was not detected on Pluto by New Horizons. Out of many explanations provided by scientists to remove such a bulge, the most realistic one is to develop a subsurface ocean. Since the ice shell above is too weak to support the bulge, it will eventually collapse.

Third Line Of Evidence Is The Most Complex

The final line of evidence which begins from a bright basin known called Sputnik Planitia is the most complicated of all. The reason for the bright appearance of this region is the nitrogen ice supplied by nitrogen glaciers. Sputnik Planitia’s location is also significant. It lies directly opposite the point on Pluto that continuously overlooks Charon. If an extra mass such as a large mountain is kept on Pluto’s surface, Pluto would roll over till that mountain reaches the location of Sputnik Planitia. This phenomenon is known as true polar wander or TPW. As a result of TPW, Pluto’s surface undergoes changes when responding to movements of extra mass. When this is connected with surface expansion, fractures occur, and they match those predicted by computer models very well.

Life's prospects on pluto
Pluto’s varied geology and surface composition hint that it could support an underground ocean of liquid water. Source – NASA/JHUAPL/SwRI

Therefore, if it represents an area with extra mass, Sputnik’s location makes sense perfectly. But, the way the basin gains an extra mass is a problem, because it is a hole in the ground. Solid nitrogen being a little denser than liquid ice helps a bit. However, this is of no use if the nitrogen layer is unbelievably thick.

The thinning of ice underneath can be explained this way: When the shell is thin, denser water replaces lighter ice. This causes an excess of mass. The combination of nitrogen loading from the top and a thinned ice shell beneath easily produces an excess of mass, causing TPW.

How Can We Confirm That Oceans Exist On Pluto?

It’s possible for a spacecraft orbiting Pluto to determine whether Sputnik Planitia represents a mass excess. However, this alone isn’t enough to prove that a subsurface ocean exists on Pluto. Since the mass excess, or the ocean lies at depth and the mass deficit, or the basin lies is on the surface, their opposing contributions to gravity do not cancel each other out. Therefore, the presence of an ocean and the thickness of the ice shell can be determined by measuring how gravity distorts the orbital path of a spacecraft.

Life's prospects on pluto
Charon. Source – NASA/JHUAPL/SwRI

To What Extent Is Pluto Habitable?

Thanks to the presence of a warm interior, organic molecules, and most probably a subsurface ocean, Pluto fulfills the basic necessities for habitability. However, Pluto isn’t the best for life. Europa or Enceladus are definitely better choices, considering their oceans that are topped by thin and moving ice shells. But, when compared to large moons like Titan and Ganymede where direct contact between the ocean and the rocks below is blocked by a thick, high-pressure ice layer, Pluto is a much better choice.

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