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Pluto is the biggest Dwarf Planet. The largest object beyond Neptune in the Kuiper Belt. Though Kuiper Belt contains hundreds of thousands of objects over 100 kilometers in diameter, their total mass is estimated at just 2% of Earth’s mass.

Pluto in the Kuiper Belt. Image Credit: NASA

Objects beyond Neptune are difficult to observe, as they are small and very far away. But in 2015 the New Horizons fly by turned Pluto from a blurry image into a unique world.

Tenzing Montes rising 6 km above the surrounding surface in the foreground and Hillary Montes in the horizon (named after Edmund Hillary and Tenzing Norgay who first climbed Everest). The mountains on Pluto are made of water ice. At a surface temperature of -230 ° C ice is as hard as rock here on Earth. To their right is Sputnik Planitia, a flat area of icy nitrogen with no craters. Pluto’s thin atmosphere can also be seen in layers above the planet. This Plutonian landscape is just 380 kilometers across, Pluto is a small planet. Image Credit: NASA, Johns Hopkins Univ./APL, Southwest Research Institute

As New Horizons approached, one of the most important features scientists were anticipating was cratering. By counting the number of craters we can get an estimate of the age of a surface. But when the first images came back the result was shocking. Scientists expected Pluto to look a bit like the Moon, with craters densely packed on the surface. Instead we saw places with few or no craters at all. Large areas without craters mean they are new, just a hundred million years old or younger. That’s very recent in geological terms. That in turn means that parts of Pluto have been resurfaced, but what process could do that on cold, frozen Pluto?

Pluto – the white heart-shaped area is Tombaugh Regio after Clyde Tombaugh who discovered Pluto. The left part is Sputnik Planitia, a flat basin. Ανοίξτε σε νέα καρτέλα για απίθανη ανάλυση – Cretid NASA/JHUAPL/SwRI

Το κύριο χαρακτηριστικό κοιτάζοντας τον πλανήτη είναι μία λευκή περιοχή σε σχήμα καρδιάς. Η περιοχή ονομάζεται Tombaugh Regio από τον Clyde Tombaugh ο οποίος ανακάλυψε τον Πλούτωνα. Η αριστερή πλευρά της ονομάζεται Sputnik Planitia από τον πρώτο τεχνητό δορυφόρο της Γης. Είναι μια επίπεδη περιοχή χωρίς καθόλου κρατήρες με ηλικία μόλις 10 εκατομμυρίων ετών!

Η Sputnik Planitia είναι πιθανότατα μια λεκάνη πρόσκρουσης, 2,5 χιλιόμετρα κάτω από το μέσο υψόμετρο του Πλούτωνα, που σχηματίστηκε από την πρόσκρουση ενός σώματος διαμέτρου 200 χιλιομέτρων. Στην επιφάνεια της λεκάνης παρατηρούμε ένα κέλυφος παγωμένου αζώτου που καλύπτει ένα πιο ρευστό εσωτερικό. Κάνοντας ζουμ στην πολύ επίπεδη αυτή περιοχή διακρίνουμε κερματισμένες πλάκες που μοιάζουν με κύτταρα. Οι πλάκες είναι οι κορυφές στηλών μεταφοράς θερμότητας, όπου θερμό υλικό ωθείται προς τα πάνω στο κέντρο των πλακών και ψυχρό υλικό βυθίζεται στις άκρες. Κάποια πηγή θερμότητας βαθιά μέσα στον Πλούτωνα προκαλεί ρεύματα μεταφοράς κάτω από την επιφάνεια, με άνοδο θερμού ρευστού και βύθιση ψυχρότερου υλικού. Το υλικό δεν πρέπει να είναι ακριβώς υγρό μα παχύρρευστο, σαν οδοντόκρεμα για παράδειγμα. Ποια είναι όμως η πηγή θερμότητας; Ίσως βαθιά μέσα στον Πλούτωνα υπάρχει μια μικρή ποσότητα ραδιενεργών υλικών, τα οποία καθώς διασπώνται θερμαίνουν το περιβάλλον τους.

Sputnik Planitia – segmented plates on ice are the tops cells of convection, with warm stuff pushing up in the center of the plates and cooler material sinking around the edges. Cretid: NASA/JHUAPL/SwRI

Sputnik Planitia. Huge cracks at the left indicate the existence of an underground ocean. As the water freezes it expands, forming these cracks on the surface. Sputnik Planitia itself may have been created by a massive collision that stirred up this underground ocean Image Credit: NASA, Johns Hopkins U./APL, Southwest Research Inst.

Underground Ocean: The presence of various large cracks on the surface of Pluto suggest the existence of a global underground ocean of salt water. As the water at the outer limit of this ocean freezes, it expands causing the outer surface to expand, forming these huge cracks.

Huge cracks hundreds of kilometers long and 2-4 kilometers deep, on the surface of Pluto. Credit: NASA / JHUAPL / SwRI 

An underground ocean more than 100 km thick below the icy surface of Pluto. Credit: Wikimedia Commons

An underground ocean so far from the Sun in the Kuiper belt increases the places in the solar system where liquid water exists and can possibly host life. We have discovered underground oceans on several moons of Jupiter and Saturn, underground lakes on Mars, and there are possible evidence of underground oceans on Uranus and Neptune’s moons and on the dwarf planet Ceres.

Wright Mons cryovolcano in the center, is 150 kilometers across and 4 kilometers high. Because we can see only one crater on its slopes, we estimate that it was active until recently in the geological history of Pluto. Credit: NASA/JHUAPL/SwRI

Cryovolcanoes – Pluto has no tectonic plates, geological activity on the planet may be created by the underground ocean. In addition to the mountains and huge cracks, we have also observed two cryovolcanoes. A cryovolcano instead of lava (molten rock) erupts molten ice from the interior from material such as water, ammonia or methane.

Methane snow – At the top of the mountains we have observed a layer of methane ice, which may have fallen like snow just as water falls like snow in the mountains of the Earth.

Snow covered mountains on Pluto – Methane fells like snow just as water falls like snow in the mountains of the Earth. Credit: NASA/JHUAPL/SwRI

Pluto’s blue atmosphere backlit by the sun. Credit: NASA/JHUAPL/SwRI

Pluto’s blue sky, similar to Earth’s blue sky, is caused from scattering of sunlight by very small particles, which allow longer wavelength redder light to pass right by them, but scatter shorter wavelength blue light in every direction. On Earth, these particles are very tiny nitrogen molecules. On Pluto they appear to be larger — but still relatively small — soot-like particles we call tholins.

Ultraviolet light from the distant Sun breaks down methane and other molecules, which then reorganize themselves into a complex mixture of hydrocarbons such as acetylene and ethylene. These hydrocarbons accumulate into small particles — a fraction of a micrometer in size — tholins.

Tholins create a haze in the atmosphere of Pluto, organised in multiple layers as seen in the picture below. The haze particles themselves are likely gray or red, and are responsible for the dark red color areas seen on Pluto.

Pluto’s atmosphere in layers. Credit: NASA, Johns Hopkins Univ./APL, Southwest Research Institute

Pluto’s thin atmosphere has a surface pressure 1/100,000 of Earth’s atmospheric pressure, as measured by New Horizons in 2015. It consists mainly of nitrogen (like Earth’s air), with minor amounts of methane and carbon monoxide, all of which are vaporized from their ices on Pluto’s surface. In Pluto’s weak gravity (1/16th as compared to Earth’s) the atmosphere reach much higher altitudes than our thicker atmosphere on Earth. The layers we see extends more than 200 km above the surface of the planet.

Pluto sparse craters reveal a layered subsurface. Credit: NASA/JHUAPL/SwRI

Pluto has five known moons, one really big and four small ones. Charon’s limb is at the bottom, to scale. Credit: NASA/JHUAPL/SwRI

Charon, Pluto’s biggest moon, is about half the size of Pluto – the largest known moon in relation to the size of its host planet -we may as well consider Pluto and Charon as a two dwarf planet system.. Credit: NASA/JHUAPL/SwRI

New Horizons also captured Pluto’s moon Charon, revealing some surprising geology. Charon has two distinct terrain types: an immense, southward-stretching plain officially called Vulcan Planitia, and a rugged terrain called Oz Terra that stretches northward to Charon’s north pole. Both seem to have formed from the freezing and expansion of an ancient ocean beneath Charon’s crust. Vulcan Planitia is thought to be a giant cryoflow that covered the entire region early in Charon’s history. In the northern rugged terrain we can see chasms up to 10 km deep, while the tallest features reach 5-6 km high. So there’s an elevation swing more than 15 km across the moon!

Pluto and Charon though small are unique worlds. To think of them in scale, Pluto is 2.377 km in diameter (smaller than Earth’s Moon) and Charon half that size. The difference in mass is even more pronounced. Pluto has only 0.2% the mass of the Earth, or 18% the mass of the Moon. And Charon’s mass is 12% of Pluto’s mass.

Kostantinos Sakkas