hot stuff

New evidence suggests something strange and surprising about Pluto

The findings will make scientists rethink the habitability of Kuiper Belt objects.


On July 14, 2015, a small, shimmering spacecraft was the first to explore Pluto up-close, and probe at the icy objects of the Kuiper Belt.

Data from that rare encounter during NASA's New Horizons mission may have just rewritten the origin story of the most popular dwarf planet, and changed our understanding of Pluto's early history.

Astronomers have long believed that Pluto started off as an icy, cold rock in the Kuiper Belt. However, the new findings suggest that Pluto may have actually started off hot, and had a subsurface ocean when it first formed.

The study, published Monday in the journal Nature Geoscience, not only increases the chances of Pluto having been habitable at some point during its early history, but also increases the potential for habitability for distant, icy objects in the Kuiper Belt.

Pluto already has a notorious past. In August, 2006, the cold world was officially demoted from a budding planet of the Solar System to a dwarf planet located in the doughnut-shaped ring of icy objects in the outer Solar System known as the Kuiper Belt.

The decision remains controversial more than 10 years later, even NASA Administrator Jim Bridenstine was recently quoted as saying that Pluto is still a planet in his books, much to the delight of Pluto fans.

Whether or not it's a full-grown planet, astronomers did agree that Pluto likely formed as an icy sphere, and that a liquid ocean that likely flows beneath the planet's icy shell may have formed later on in its history due to the ice warming from radioactive decay.

NASA's New Horizons captured close-up images of Pluto.


However, images captured by New Horizons of Pluto's surface revealed its geology in detail and suggests that the beloved dwarf planet actually started off hot with a liquid ocean.

If Pluto had a cold start, the ice would have melted internally and water contracts when it melts. Therefore, Pluto would have contracted and evidence of that compression process would have been showing on its sufrace. On the other hand, if Pluto had a hot start, then the water would have frozen over time, expanding in the process, and expansion features would be seen on the planet's surface.

"We see lots of evidence of expansion, but we don't see any evidence of compression, so the observations are more consistent with Pluto starting with a liquid ocean," Carver Bierson, graduate student at University of California, Santa Cruz, and lead author of the new study, said in a statement.

Evidence of expansion revealed on Pluto's surface.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

Additionally, the study suggests that Pluto may have formed over approximately 30,00 years and weathered several impacts that warmed its surface, making it hot enough for an ocean to form.

Today, Pluto is a freezing cold world with a surface temperature of about 45 Kelvin, or -380 degrees Fahrenheit.

However, the research suggests that early on during its ancient history, Pluto had higher chances of being habitable.

Therefore, the team of researchers behind the new study suggest that other, similarly icy objects in the Kuiper Belt may have also started off warm with possible oceans flowing on their surface despite previously excluding them from having an inviting, habitable past.

"Even in this cold environment so far from the Sun, all these worlds might have formed fast and hot, with liquid oceans," Bierson said.

Abstract: Pluto is thought to possess a present-day ocean beneath a thick ice shell. It has generally been assumed that Pluto accreted from cold material and then later developed its ocean due to warming from radioactive decay; in this ‘cold start’ scenario, the ice shell would have experienced early compression and more recent extension. Here we compare thermal model simulations with geological observations from the New Horizons mission to suggest that Pluto was instead relatively hot when it formed, with an early subsurface ocean. Such a ‘hot start’ Pluto produces an early, rapid phase of extension, followed by a more prolonged extensional phase, which totals ~0.5% linear strain over the last 3.5 Gyr. The amount of second-phase extension is consistent with that inferred from extensional faults on Pluto; we suggest that an enigmatic ridge–trough system recently identified on Pluto is indicative of early extensional tectonics. A hot initial start can be achieved with the gravitational energy released during accretion if the final stage of Pluto’s accretion is rapid (<30 kyr). A fast final stage of growth is in agreement with models of the formation of Kuiper belt objects via gravitational collapse followed by pebble accretion, and implies that early oceans may have been common in the interiors of large Kuiper belt objects.
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