{"id":4105,"date":"2016-09-16T16:08:34","date_gmt":"2016-09-16T15:08:34","guid":{"rendered":"https:\/\/ounews.co\/?p=4105"},"modified":"2016-09-16T16:08:34","modified_gmt":"2016-09-16T15:08:34","slug":"scientists-solve-puzzle-north-pole-charon","status":"publish","type":"post","link":"https:\/\/www.open.ac.uk\/blogs\/news\/science-mct\/space\/scientists-solve-puzzle-north-pole-charon\/","title":{"rendered":"Mordor mystery: scientists solve puzzle of the strange, dark north pole on Pluto\u2019s moon Charon"},"content":{"rendered":"

Had Pluto itself<\/a> not proved to be so spectacular when NASA\u2019s New Horizons<\/a> probe flew past last year, there can be no doubt that its large moon Charon<\/a> would have won more admirers.<\/p>\n

The remarkable moon has a mysterious dark-red stain over its north pole, called \u201cMordor Macula\u201d by the New Horizons team \u2013 where Macula means \u201cdark spot\u201d and Mordor refers to the \u201cblack land\u201d in Tolkien\u2019s The Lord of the Rings. While many bodies in the solar system have polar caps or hoods of some sort, these are typically bright, due to reflective ice or frost of some kind, rather than dark. So what\u2019s going on at Charon? A new study, published in Nature<\/a>, has proposed an answer.<\/p>\n

One of Charon\u2019s most interesting features is a vast chasm system, which cuts across the middle of Charon\u2019s Pluto-facing hemisphere and reaches a maximum depth of 7.5km. This speaks of an era of upheaval when slabs of Charon\u2019s icy crust were ruptured, tilted and then partially flooded by a type of lava produced by icy volcanism.<\/p>\n

\"\"<\/a>
Charon\u2019s vast equatorial chasms. The most obvious ones are unofficially named Macross Chasma (on the left) and Serenity Chasma (on the right).<\/span>
\nNASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute<\/span><\/span><\/figcaption><\/figure>\n

Charon\u2019s surface is mostly dirty, grey water-ice, and the red stain around the north pole looks to be a thin film, coating but not burying the underlying topographic features such as craters.<\/p>\n

Mordor Macula resembles the red-stained areas of Pluto. This led to early suggestions that they share a similar origin. However, Pluto\u2019s red stuff is understood to be tarry molecules called tholins<\/a>, which form in the atmosphere when sunlight strikes molecules and makes them sufficiently reactive to link together. This forms haze particles, which eventually settle to the ground. Unlike Pluto, however, Charon has no trace of an atmosphere, so how can tholins form there?<\/p>\n

\"\"<\/a>
Pluto\u2019s hazy atmosphere, made visible by scattered sunlight seen by New Horizons.<\/span>
\nNASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute<\/span><\/span><\/figcaption><\/figure>\n

The researchers behind the new study think they have the answer. The New Horizons probe detected two sorts of molecules leaking away from Pluto<\/a>: methane and nitrogen. Although nitrogen is the most abundant gas in Pluto\u2019s atmosphere, methane is lighter and is being lost about 500 times faster. Tholins and their precursors are far too heavy, and cannot escape Pluto in the same way, so Charon\u2019s tholins must be somehow made from methane arriving from Pluto.<\/p>\n

Charon orbits very close to Pluto, and quite a lot of Pluto\u2019s escaped methane falls onto its surface. If a methane molecule strikes the night side of Charon it will stick, particularly near the winter pole where the temperature is lowest (less than 30 degrees above absolute zero). This is because at lower temperatures surface molecules vibrate more slowly, and atmospheric molecules travel more slowly, so collisions are gentle. However, if a methane molecule hits a sunlit part of the surface, it will tend to bounce off and fly away into space because of Charon\u2019s weak gravity (which has long since allowed it to lose any atmosphere that it may once have possessed).<\/p>\n

\"\"<\/a>
Charon (left) and Pluto (right) at the same scale. Charon is 1,212 km in diameter, Pluto 2,370 km.<\/span>
\nNASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute<\/span><\/span><\/figcaption><\/figure>\n

Ultraviolet sunlight shining on methane molecules coating Charon\u2019s surface could stimulate photochemical reactions to link them into progressively longer molecular chains, to form tholins. However, the \u201cCatch 22\u201d is that a methane molecule on the cold night-side surface ought to be able to break free in the daytime, when the temperature rises to the giddy heights of -220\u00b0C \u2013 allowing it to float away into space and become permanently lost. And as there is no sunlight at night, methane molecules can\u2019t link together (making them too massive to float away) before the dawn of a new day.<\/p>\n

In a surprising result the new study shows that, even by night, Charon\u2019s poles receive enough ultraviolet light to allow methane molecules to link together. This is because interplanetary dust in the region scatters sunlight in all directions, including onto the night side of Charon. Once a molecule is a chain of two or three methanes linked together (with or without the occasional nitrogen for good measure) it is probably heavy enough to stay bound to the surface even during the daytime. Normal daytime sunlight can then take over to complete the process of forming tholins.<\/p>\n

\"\"
An example of a tholin molecule. This is a molecule proposed to form in the atmosphere of Saturn\u2019s moon Titan. Tholins on Charon probably have less nitrogen.<\/span>
\nafter Ehrenfreund et al., 1996<\/span><\/span><\/figcaption><\/figure>\n

But what about the south pole? Could Mordor Macula\u2019s location at Charon\u2019s north pole, where the temperature is low enough for cold-trapping of methane, be just a fluke? Apparently not. Although it was winter in Charon\u2019s southern hemisphere when New Horizon made its flyby, keeping its south pole in darkness, the researchers were able to study the south pole by \u201cPluto shine\u201d (sunlight reflected off Pluto). The spatial resolution of these data is poor, but it is sufficient to demonstrate that Charon\u2019s south pole has similarly dark red material at its surface. So there is not just one Mordor on Charon, but two.<\/p>\n

\"The<\/p>\n

David Rothery<\/a>, Professor of Planetary Geosciences, The Open University<\/a><\/em><\/p>\n

This article was originally published on The Conversation<\/a>. Read the original article<\/a>.<\/p>\n

Photo by Kevin M. Gill<\/a> <\/a><\/small><\/p>\n","protected":false},"excerpt":{"rendered":"

Had Pluto itself not proved to be so spectacular when NASA\u2019s New Horizons probe flew past last year, there can be no doubt that its large moon Charon would have won more admirers. The remarkable moon has a mysterious dark-red stain over its north pole, called \u201cMordor Macula\u201d by the New Horizons team \u2013 where […]<\/p>\n","protected":false},"author":19,"featured_media":4114,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[16],"tags":[421,963,1499,1736,1794,2084],"class_list":["post-4105","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-space","tag-charon","tag-geoscience","tag-nasa","tag-pluto","tag-professor-david-rothery","tag-space-science"],"_links":{"self":[{"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/posts\/4105","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/users\/19"}],"replies":[{"embeddable":true,"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/comments?post=4105"}],"version-history":[{"count":0,"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/posts\/4105\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/media\/4114"}],"wp:attachment":[{"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/media?parent=4105"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/categories?post=4105"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.open.ac.uk\/blogs\/news\/wp-json\/wp\/v2\/tags?post=4105"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}