Tidal heating could sustain dwarf planets’ subsurface oceans
Icy dwarf planets in the Kuiper Belt could maintain subsurface liquid oceans if they have moons that generate tidal heating, thereby preventing the oceans from freezing, according to a new study published in the journal Icarus.
The possibility of microbial life existing in these underground oceans significantly expands the number and types of locations scientists consider habitable for extra-terrestrial life.
While dwarf planets beyond Pluto are far too cold to host liquid water on their surfaces, with temperatures below minus 350 degrees Fahrenheit (minus 200 degrees Celsius), there is growing evidence that many have subsurface layers of liquid water.
Heat within these objects is also generated by the radioactive decay of internal elements left over from the time of their initial formation.
This internal heat could generate and sustain subsurface oceans for millions of years. However, eventually, internal radioactive elements would decay into more stable ones and then heat generation would stop, causing the dwarf planets’ interiors to cool and the underground oceans to eventually freeze.
However, if a dwarf planet has a moon, especially one generated in a large-scale collision such as the one that formed Pluto’s moon Charon, the gravitational interaction between the two objects could produce heat that prevents interior cooling, preserving the subsurface liquid oceans.
Moons formed through collisions with their parent dwarf planets initially have unstable orbits around those planets. But over time, these orbits gradually stabilize to the point that gravitational interaction between the two worlds causes their interiors to alternately stretch and relax.
Known as tidal heating, this process generates friction, which produces heat.
“These objects need to be considered as potential reservoirs of water and life. If our study is correct, we may now have more places in our Solar System that possess some of the critical elements for extra-terrestrial life,” said Prabal Saxena of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who led the study.
Significantly, measurements of the bulk densities of several Kuiper Belt dwarf planets revealed them to be similar to those of worlds known to harbor underground oceans.
Analysis of light reflected from these dwarf planets showed signatures of both water ice and crystalline ammonia hydrates.
Because of both their low surface temperatures and their interaction with space radiation, these substances cannot survive for any length of time on these worlds’ surfaces.
This indicates they may originate in interior liquid oceans and erupt on the surfaces through cryovolcanism.
Saxena’s research team used tidal heating equations to calculate heat levels produced by several systems of dwarf planets with moons, including Eris and its small moon Dysnomia.
“We found that tidal heating can be a tipping point that may have preserved oceans of liquid water beneath the surface of large TNOs (Trans-Neptunian Objects) like Pluto and Eris to the present day,” said research team member Wade Henning of NASA Goddard and of the University of Maryland at College Park.
Tidal heating could also move deeply buried subsurface oceans closer to a world’s surface by melting the layers of ice directly above them. This would result in the oceans being easier for scientists to observe, research team member Joe Renaud of George Mason University in Fairfax, Virginia, explained.
Both radioactive decay and tidal heating could produce the hydrothermal vents through which energy-rich chemicals needed by microbes for food are transported to the subsurface oceans.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.