Pluto’s recategorization as a dwarf planet may have caused some fury in the past, but there’s never been a better time to be a Pluto fan. Ever since the New Horizons mission gave us our first real look at Pluto in 2015, researchers have been digging into the rich reality of this icy world. The last question under the magnifying glass: what did Pluto look like at birth?
Beneath its surface — which is made up of frozen water, nitrogen, carbon monoxide and more — Pluto is today thought to have an ocean of liquid water that surrounds a rocky core. But scientists are trying to figure out how long that has been the case. In the “cold start” model, Pluto’s inner ocean is believed to have frozen, but then gradually thawed due to the heat from radioactive decay in the core. But it remains possible that a “hot start” model is more accurate, one where the planet started out warm enough for some liquid water to form.
Cold start vs warm start
Although the beginnings of the dwarf planet are buried deep in the past, the two possible origins should have left traces on its surface.
The key is to think about the fact that water expands when it freezes and contracts when it melts. Just as a balloon’s skin stretches when it inflates, Pluto’s outside should contract or expand as the inside melted or froze. A cold start with an initially frozen “ocean” inside would cause shrinkage as it thaws, creating compression along the surface. But after a while, the heat provided by radioactive decay in the core would decrease enough to overcome the heat loss to space, causing the inner ocean to start freezing again. That would result in expansion, causing expansion along Pluto’s surface.
A “hot start”, on the other hand, would see a liquid inner ocean from the start. There would be a rapid phase of cooling and freezing early on, causing expansion on the surface. Then the heating of the radioactive decay from the core and the cooling of the surface would move closer to equilibrium, significantly slowing the freezing rate for the rest of Pluto’s life, but still continuing surface expansion.
So by analyzing the visible flaws on Pluto’s surface and noting how they moved, it should be possible to distinguish between compression-then-extension and extension-only versions of Pluto’s history. Carver Bierson of UC Santa Cruz and Francis Nimmo, along with Alan Stern, the leader of New Horizons, put this idea to the test using what we learned about Pluto’s geology.
Coincidentally, the researchers say there is no evidence of compression. However, there are relatively recent extension errors, which often produce a sunken valley between pairs of errors. So perhaps the “hot start” model is closer to reality than the more popular “cold start” hypothesis.
Fast assembly required
There is also a very old looking topographic feature that can represent an extension. While there is no complete and accurate estimate of the quantity of the expansion recorded by all these errors, the researchers’ rough calculation is compatible with their “hot start” model for Pluto’s history.
There are, theoretically, ways in which the available evidence can be misleading. Errors that form under compression can later change direction under extension, for example. And it’s possible that impact events (particularly the one that formed Pluto’s moon Charon) and other surface processes erased or hidden evidence of compression. Still, all observable geology seems to point to the “hot start” explanation.
How come an icy world in the outer reaches of the solar system starts out warm, you wonder? It has to do with Pluto’s assembling of the smaller pieces that swirled around during the formation of the solar system. While pieces were pulled in by gravity, energy was converted to heat on impact. So as long as Pluto accumulated fast enough to get that heat faster than it releases heat to space, its interior could reach high temperatures.
The researchers ran the numbers and found that assembly times of about 30,000 years would certainly reach the required temperatures, although if Pluto were formed from larger chunks (which collect heat more efficiently), that number could go up to a few million years.
Some models used to simulate the processes that assembled our solar system take considerably longer than that, but some newer models can quickly enough to assemble a Pluto. In any case, the requirement for quick assembly is not issue for the “hot start” idea.
However, the researchers propose another way to further test their hypothesis: A much better estimate of the amount of expansion Pluto’s surface has undergone would make it clearer which model is the winner.
Nature Geoscience, 2020. DOI: 10.1038/s41561-020-0595-0 (About DOIs).