"Long before it's in the papers"
February 19, 2016


Pluto moon may have an ancient, frozen ocean, scientists say

Feb. 19, 2016
Courtesy of Johns Hopkins University 
Applied Physics Laboratory
and World Science staff

Pluto’s largest moon may have gotten too big for its own skin.

Im­ages from NASA’s New Hori­zons mis­sion sug­gest that the moon Char­on once had a sub­sur­face ocean that has long since fro­zen and ex­pand­ed, sci­en­tists say.

The ex­pan­sion, they sug­gest, pushed out on the moon’s sur­face and caused it to stretch and crack, not un­like the way the “In­cred­i­ble Hulk” tears his shirt as his mus­cles swell. The re­sult: chasms that dwarf the Grand Can­yon. 

This im­age was ob­tained by the Long-Range Re­con­nais­sance Im­ag­er on New Hori­zons. The res­o­lu­tion of the full im­age (click to ex­pand) is about 1,290 feet (394 me­ters) per pix­el. The im­age meas­ures 240 miles (386 km) long and 110 miles (175 km) wide. It was ob­tained at a range of about 48,900 miles (78,700 km) from Char­on, about an hour and 40 min­utes be­fore New Hori­zons’ clos­est ap­proach to Char­on on Ju­ly 14, 2015. (Cred­it: NA­SA/Johns Hop­kins U. Ap­plied Phys­ics Lab/South­west Re­search Inst.)

A July 2015 pho­to Char­on from the pass­ing New Hori­zons space­craft, ac­cord­ing to as­tro­no­mers, shows a sys­tem of “pull-apart” tec­ton­ic faults, which are ex­pressed as ridges, cliffs, and val­leys. The val­leys some­times reach more than 4 miles (6.5 km) deep. 

Char­on’s out­er lay­er is mainly wa­ter ice. When the moon was young this lay­er was warmed by the de­cay, or dis­in­tegra­t­ion, of ra­di­o­ac­t­ive el­e­ments, as well as Char­on’s own in­ter­nal heat from its form­a­tive time, ac­cord­ing to re­search­ers.

The sci­en­tists, with Johns Hop­kins Un­ivers­ity Ap­plied Phys­ics Lab­o­r­a­to­ry in Mar­y­land, which is par­ti­ci­pat­ing in the NASA mis­sion, ar­gue that this warmth could have melted the ice deep down. That would have cre­at­ed a sub­sur­face ocean. As Char­on later cooled, the ocean would have fro­zen and ex­pand­ed—as wa­ter does when it freezes—push­ing the sur­face out­ward and pro­duc­ing the mas­sive chasms we see to­day.

The im­age, plan­e­tary sci­en­tists ex­plain, fo­cus­es on a sec­tion of the fea­ture in­for­mally named Seren­ity Chasma, part of a vast equa­to­ri­al belt of chasms on Char­on. This sys­tem of chasms is one of the longest seen an­y­where in the so­lar sys­tem, run­ning at least 1,100 miles (a­bout 1,800 km) long and reach­ing 4.5 miles (7.5 km) deep. That’s around four times longer and deeper than the Grand Can­yon. Scientists also used a ver­sion of the im­age with color-coded to­pog­ra­phy to fur­ther re­veal the shapes.

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Images from NASA’s New Horizons mission suggest that Pluto’s Charon once had a subsurface ocean that has long since frozen and expanded, scientists say. The expansion, they suggest, pushed out on the moon’s surface and caused it to stretch and crack, not unlike the way the “Incredible Hulk” tears his shirt as his muscles swell. A July 2015 photo Charon from the passing New Horizons spacecraft, according to astronomers, shows a system of “pull-apart” tectonic faults, which are expressed as ridges, cliffs, and valleys. The valleys sometimes reach more than 4 miles (6.5 kilometers) deep. Charon’s outer layer is mainly water ice. When the moon was young this layer was warmed by the decay, or disintegration, of radioactive elements, as well as Charon’s own internal heat from its formative time, according to researchers. The scientists, with Johns Hopkins University Applied Physics Laboratory in Maryland, which is participating in the NASA mission, argue that this warmth could have melted the ice deep down. That would have created a subsurface ocean. But as Charon cooled, the ocean would have frozen and expanded—as water does when it freezes—pushing the surface outward and producing the massive chasms we see today. The image, planetary scientists explain, focuses on a section of the feature informally named Serenity Chasma, part of a vast equatorial belt of chasms on Charon. This system of chasms is one of the longest seen anywhere in the solar system, running at least 1,100 miles (about 1,800 km) long and reaching 4.5 miles (7.5 km) deep. By comparison, the Grand Canyon is 277 miles (446 km) long and just over a mile (1.6 km) deep. A version of the image using color-coded topography further reveal the shape of this feature, which tell scientists that Charon’s water-ice layer may have been at least partially liquid in its early history.