Cracks in Pluto's moon could indicate it once had an underground ocean

NASA | JUNE 18, 2014

If the icy surface of Pluto's giant moon Charon is cracked, analysis of the fractures could reveal if its interior was warm, perhaps warm enough to have maintained a subterranean ocean of liquid water, according to a new study.

This artist concept shows Pluto and some of its moons, as viewed from the surface of one of the moons. Pluto is the large disk at center. Charon is the smaller disk to the right. Image Credit: NASA, ESA and G. Bacon (STScI)

Pluto is an extremely distant world, orbiting the sun more than 29 times farther than Earth. With a surface temperature estimated to be about 380 degrees below zero Fahrenheit (around minus 229 degrees Celsius), the environment at Pluto is far too cold to allow liquid water on its surface. Pluto's moons are in the same frigid environment.

Pluto's remoteness and small size make it difficult to observe, but in July of 2015, NASA's New Horizons spacecraft will be the first to visit Pluto and Charon, and will provide the most detailed observations to date.

Some moons around the gas giant planets in the outer solar system have cracked surfaces with evidence for ocean interiors – Jupiter's moon Europa and Saturn's moon Enceladus are two examples.

As Europa and Enceladus move in their orbits, a gravitational tug-of-war between their respective parent planets and neighboring moons keeps their orbits from becoming circular. Instead, these moons have eccentric (slightly oval-shaped) orbits, which raise daily tides that flex the interior and stress the surface. It is thought that tidal heating has extended the lifetimes of subsurface oceans on Europa and Enceladus by keeping their interiors warm.

In Charon's case, this study finds that a past high eccentricity could have generated large tides, causing friction and surface fractures. The moon is unusually massive compared to its planet, about one-eighth of Pluto's mass, a solar system record. It is thought to have formed much closer to Pluto, after a giant impact ejected material off the planet's surface. The material went into orbit around Pluto and coalesced under its own gravity to form Charon and several smaller moons.

Initially, there would have been strong tides on both worlds as gravity between Pluto and Charon caused their surfaces to bulge toward each other, generating friction in their interiors. This friction would have also caused the tides to slightly lag behind their orbital positions. The lag would act like a brake on Pluto, causing its rotation to slow while transferring that rotational energy to Charon, making it speed up and move farther away from Pluto.

Based on observations from telescopes, Charon's orbit is now in a stable end state: a circular orbit with the rotation of both Pluto and Charon slowed to the point where they always show the same side to each other. Its current orbit is not expected to generate significant tides, so any ancient underground ocean may be frozen by now.

YOU MAY ALSO LIKE

New Clues to TRAPPIST-1 Planet Compositions, Atmospheres

In the year since NASA announced the seven Earth-sized planets of the TRAPPIST-1 system, scientists have been working hard to better understand these enticing worlds just 40 light-years away.

ESO Discovers Earth-Size Planet in Habitable Zone of Nearest Star

A newly discovered, roughly Earth-sized planet orbiting our nearest neighboring star might be habitable, according to a team of astronomers using the European Southern Observatory's 3.6-meter telescope at La Silla, Chile.

Cassini Reveals Surprises with Titan's Lakes

On its final flyby of Saturn's largest moon, NASA's Cassini spacecraft gathered radar data revealing that the small liquid lakes in Titan's northern hemisphere are surprisingly deep, perched atop hills and filled with methane.

New Horizons Collects First Science on a Post-Pluto Object

Warming up for a possible extended mission as it speeds through deep space, NASA’s New Horizons spacecraft has now twice observed 1994 JR1, a 90-mile-wide (145-kilometer-wide) Kuiper Belt object (KBO) orbiting more than 3 billion miles (5 billion kilometers) from the sun.

Lava or Not, Exoplanet 55 Cancri e Likely to have Atmosphere

Twice as big as Earth, the super-Earth 55 Cancri e was thought to have lava flows on its surface. The planet is so close to its star, the same side of the planet always faces the star, such that the planet has permanent day and night sides.

Jupiter's North Pole Unlike Anything Encountered in Solar System

Juno spacecraft has sent back the first-ever images of Jupiter's north pole, taken during the spacecraft's first flyby of the planet with its instruments switched on.