In recent years humanity has learned that oceans are not unique to Earth. Discoveries of them on Saturn’s moons Enceladus and Titan and Jupiter’s moon Europa indicate that oceans may actually be ubiquitous in our solar system.
Now, it seems, it is possible that an ocean once existed on a satellite of the dwarf planet Pluto.
NASA said Friday that images obtained by the New Horizons probe suggest that Charon once had a subsurface sea that expanded outward as it froze, cracking the surface of the moon.
The photographs show tectonic faults, including some valleys more than 6.5 kilometers deep, that indicate an expansion of Charon sometime in the past.
Here’s how it likely happened:
The outer-most layer of Charon, including its surface, is now water ice. But the satellite’s internal heat would have caused that ice to melt deep under the surface, which would have resulted in a subsurface ocean. As the decay of radioactive elements that constitute Charon’s structure subsided, and as the rocky object cooled as the time since its formation passed, that ocean would have frozen. Just as the change of water’s state from liquid to solid on Earth causes an increase in the the compound’s volume, ice formation under the surface of Charon would have pushed the surface outward, resulting in the valleys and also numerous ridges and scarps.
The New Horizons spacecraft won’t arrive in the vicinity of Pluto until 2015, but it is close enough now to obtain images of some Kuiper Belt objects.
NASA has released images of Charon, Pluto’s largest moon, taken by New Horizons earlier this month. The images of Charon are somewhat fuzzy because the spacecraft is still about 550 million miles (900 million kilometers) away from Pluto, which is farther away from the dwarf planet than Earth is from Jupiter. From the perspective of the spacecraft, Charon’s location is about 0.01 degrees from Pluto.
Nevertheless, the detail they provide about Charon is a significant improvement over that obtained from Earth-based observation.
“The image itself might not look very impressive to the untrained eye, but compared to the discovery images of Charon from Earth, these ‘discovery’ images from New Horizons look great!”, Hal Weaver, a New Horizons project scientist affiliated with The Johns Hopkins University Applied Physics Laboratory said. “We’re very excited to see Pluto and Charon as separate objects for the first time from New Horizons.”
The photographs were obtained by the spacecraft’s Long Range Reconnaissance Imager in less than one second.
Charon is not Pluto’s only satellite. However, the images obtained by LORRI (an acronym for New Horizons’ photographic apparatus) indicate just how tiny the other moons of Pluto are. Pluto is about 2,300 kilometers across and Charon is about half that large (1,200 kilometers). None of the other moons of Pluto exceed about 170 kilometers across. Some may be as small as 10 kilometers (that’s about 6.2 miles) across!
This composite image of Charon is an amalgamation of six images. Pluto is the bright object near the center of the frame, while Charon is the dimmer object to the upper left of Pluto. Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
New Horizons is beyond the orbit of Uranus and is traveling at a velocity of about 15 kilometers per second. That’s about 32,000 miles per hour! Despite that amazing speed, the spacecraft won’t arrive at Pluto until July 2015. By the time it gets there it will have been traveling in the cosmos for about nine and one-half years.
If that causes you to wonder just how big the solar system is, it should. Think of it this way: the solar system is so enormous that even the two Voyager spacecraft that were launched 36 years ago, which are still moving as fast as New Horizons is, have not yet left it.