[meteorite-list] Cracks in Pluto's Moon Could Indicate it Once Had an Underground Ocean

[Ron Baalke]

http://www.nasa.gov/content/goddard/cracks-in-plutos-moon-could-indicate-it-once-had-an-underground-ocean/index.html

Cracks in Pluto's Moon Could Indicate it Once Had an Underground Ocean
Bill Steigerwald
NASA's Goddard Space Flight Center, Greenbelt, Maryland
June 13, 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 NASA-funded study.

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.

"Our model predicts different fracture patterns on the surface of Charon 
depending on the thickness of its surface ice, the structure of the moon's 
interior and how easily it deforms, and how its orbit evolved," said Alyssa 
Rhoden of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "By 
comparing the actual New Horizons observations of Charon to the various 
predictions, we can see what fits best and discover if Charon could have 
had a subsurface ocean in its past, driven by high eccentricity." Rhoden 
is lead author of a paper on this research now available online in the 
journal Icarus.

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.

"Depending on exactly how Charon's orbit evolved, particularly if it went 
through a high-eccentricity phase, there may have been enough heat from 
tidal deformation to maintain liquid water beneath the surface of Charon 
for some time," said Rhoden. "Using plausible interior structure models 
that include an ocean, we found it wouldn't have taken much eccentricity 
(less than 0.01) to generate surface fractures like we are seeing on Europa."

"Since it's so easy to get fractures, if we get to Charon and there are 
none, it puts a very strong constraint on how high the eccentricity could 
have been and how warm the interior ever could have been," adds Rhoden. 
"This research gives us a head start on the New Horizons arrival – what 
should we look for and what can we learn from it. We're going to Pluto 
and Pluto is fascinating, but Charon is also going to be fascinating."

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, according to Rhoden.

Since liquid water is a necessary ingredient for known forms of life, 
the oceans of Europa and Enceladus are considered to be places where extraterrestrial 
life might be found. However, life also requires a useable energy source 
and an ample supply of many key elements, such as carbon, nitrogen, and 
phosphorus. It is unknown if those oceans harbor these additional ingredients, 
or if they have existed long enough for life to form. The same questions 
would apply to any ancient ocean that may have existed beneath the icy 
crust of Charon.

This research was funded by the NASA Postdoctoral Program at the NASA 
Goddard Space Flight Center, administered by Oak Ridge Associated Universities, 
and NASA Headquarters through the Science Innovation Fund.