Pluto-Charon Binary System
Alan Stern proposed to compare a tiny sized dog to one of a different breed but bigger in size and concluded that it is a good way to view the matter. Most of the sites speaking about Alan Stern and his position about Pluto refer to the Star Trek test that he coined which defines our intuitive way to judge if a celestial object is to be recognised by us as a planet. In a sense it is true, the best way to convince yourself if Pluto is a planet is to revisit it. If you have not already done so, the images that New Horizon transmitted to Earth allow you to discover how Pluto is not only a Planet but a spectacular one. If pedantically speaking we can say that Jupiter is in a binary relation with the sun, then Pluto and her moon Charon are in a proper binary planet system relationship and so far, this relationship is the only one known in the solar system. Charon’s barycentre lies around 960 kilometres above the surface of Pluto and orbits at a distance of some 19,640 kilometres from it.
Charon is enormous compared to its parent planet Pluto being about half of Pluto’s size with a diameter of 1200 kilometres and if Charon is seen as a standalone object orbiting the sun, it would have been considered a Dwarf planet. Charon is even thought in the past to have been able to sustain a subsurface ocean and possesses some extraordinary features such as a canyon four times the length of the famous Grand Canyon in the United States as this scar stretches for 1600 kilometres across the surface. At a distance of only 19,640 kilometres from Pluto, the system is sometimes referred to as a double dwarf planet. Because both objects truly orbit the other, and Charon has 12.2% the mass of Pluto, it has been argued that Charon should be considered to be part of a binary system with Pluto. The International Astronomical Union (IAU) states that Charon is considered to be just a satellite of Pluto, but the idea that Charon might be classified as a dwarf planet in its own right may be considered at a later date.
As in a chapter of a science fiction book describing an alien world, there is an unusual transfer of material from Pluto to Charon. Observing Charon when flying at its closest to it at a distance of 27,000 kilometres, the New Horizons probe noticed a red formation spread over the North Pole; surprisingly the material was composed of nitrogen and carbon dioxide and comes from the tenuous atmosphere of Pluto which escapes it. Some of this material crosses the space between the two planets and accumulates around the North Pole region. Charon always faces the same side to Pluto as there is only one side continuously visible of Pluto from Charon because the Double Dwarf planet is locked in what is called an extreme mutual tidal locking. Compared to the system Earth/Moon, it differs by the fact that though we see only one side of the moon from Earth, all the sides of our planet are visible from our planetary sized satellite, the Moon. Once Alan Stern explained that if you can blot out the full moon with your thumb if you extend your arm standing on Earth, you will however need your full fist to blot out Charon in the sky of Pluto.
Pluto the dwarf planet with disputed status is orbited by four other small moons, Styx, Nix, Kerberos, and Hydra. ranging from 7 to 55 kilometres on their longest axis.
Speaking of size, Pluto is very often referred to as tiny, small and the adjective little is often attached to it. Small is however relative, for one to figure out how big Pluto is and make sense of it, let’s speak about the surface area of Pluto. Earth’s surface area is some 510 million square kilometres, Venus which is about 90 percent of Earth’s size has a surface area of 460 million square kilometres, but on earth only 29 percent of the planet is covered by land meaning that the total land surface available has a value of 149 million square kilometres. Mars for instance with 144.8 million square kilometres of surface area despite being smaller, would have almost the same land area available as on Earth due to the lack of large bodies of liquid occupying the surface .Our Planetary sized moon sports an honest 38 million square kilometres which could easily contain the Australian continent, in fact the moon is 6.5 million square kilometres short to equate to the surface area of Asia. Pluto with 16.7 million square kilometres is almost the same size as Russia and Russia is a vast territory with one ninth of the world’s land mass and 11 time zones and of course contains a variety of geological features like mountains, rivers, lakes etc.
With a diameter of 2373.6 kilometres, Pluto is the first dwarf planet of the Kuiper belt whose inner edge starts at the orbit of Neptune situated at 30 AU from the Sun. Much more dynamic than ever expected, Pluto has peaks towering 3500 kilometres high. Though a sub surface ocean was suspected, cracks found on the ‘Dark’ side of the planet reveal that contrary to assumptions that the ocean of Pluto had a ‘cold start’ which means that the ocean started frozen at the beginning and then gradually heated up with radioactive decay is not correct. The cracks found show evidence of expansion and not contraction that only happen when the crust is moving apart from each other. If Pluto had a hot start, the following question is where the heat came from? It could be that Impacts caused by the multitude of icy bodies present in the region where Pluto was forming, generated the heat. If that is the case, Pluto could have had a surface ocean far back in her history and evidence to support this assertion is a frozen lake which seems to have channels nearby it and is pictured on the images from New Horizons. It would mean then that in a distant past, the atmosphere of the Planet could have been 40 times denser than the atmosphere of Mars.
Today’s atmosphere on Pluto is very tenuous and as we saw, is not even able to retain the particles escaping the planet on their journey to the Moon Charon. The main constituents of this atmosphere are nitrogen, methane and carbon monoxide and Pluto’s atmosphere undergoes a cycle where its density varies. This phenomenon is due to the sublimation of ice directly into gas which replenishes the atmosphere when Pluto is at its closest point to the sun. The process reverses when Pluto finds itself at its furthest point to the sun. This atmospheric yoyo game has led some to present Pluto as a gigantic comet but Pluto retains an atmosphere during all the time of her revolution around the sun. The atmosphere is even capable to generating winds. Clouds may have been spotted on the images of New Horizon as per an announcement of the NH team in 2016. Methane which is a green house gas, induces the upper atmosphere which extends 1600 kilometres above the surface to be warmer than the surface temperature. No one speaks about Pluto without mentioning the heart of Pluto. This region’s official name is the Tombaugh region and this unique feature in the solar system has the structure in the shape of a heart and is 1590 kilometres across. The western and eastern parts of the lobe have a different geological story.
The western lobe of the Tombaugh region is named Sputnik Planitia and is an Ice filled basin that covers an area of 1050 kilometres by 850 kilometres in size. It is devoid of impact craters which imply that the formation could not be more than ten million years old which is relatively young on the geological time scale. Some models based on sublimation give an approximate surface age ranging from 140,000 years ago to 270,000 years ago.
Though there are alternative propositions regarding its formation, the most accepted explanation is that the basin was probably created by an impact caused by a hypothetical impactor of a size ranging from 150 to 300 kilometres. The Impact basin would have collected the ice volatiles slowly filling the depression,. Sputnik Planitia is an effective cold trap, the colder air strengthens while circulating in the basin and the higher pressure noted at the surface is partly due to the accumulation of kilometres (some estimates put a depth of 3 kilometres but at least 1kilometre) of ice in the basin. It seems that Sputnik Planitia adds more complexity than otherwise suspected to the atmospheric dynamism of the dwarf world. A thin layer of the nitrogen Ice covering Sputnik Planitia sublimates passing without transition to its gaseous state, then during the night condensation occurs thus causing a cycle of nitrogen winds around the planet. Not content to play the role of atmospheric regulator, the center of the formation is situated very close to the tidal axis (an imaginary line linking Pluto to Charon) which could have played an important role in the extreme mutual tidal locking between Pluto and Charon.
Another mind boggling feature which occurs on Pluto is a belt of sharp shreds, evenly spaced apart by a few kilometres sometimes as lengthy as 30 kilometres, made of nitrogen ice which go around the whole planet, some of them cumulating at 1 kilometre high and rising in the sky as drawn blades. Pluto is simply a festival of geological features and some are on a gigantic scale and more importantly, it has active geological features like cryovolcanism.
 Pluto Facts