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What Have We Learned About Pluto?

This month (March 2016), in the journal Science, New Horizons scientists have authored the first comprehensive set of papers describing results from last summer’s Pluto system flyby. These detailed papers completely transform our view of Pluto and reveal the former “astronomer’s planet” to be a real world with diverse and active geology, exotic surface chemistry, a complex atmosphere, puzzling interaction with the sun and an intriguing system of small moons.

Here’s a breakdown of what we’ve learned about Pluto:

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1. Pluto has been geologically active throughout the past 4 billion years. The age-dating of Pluto’s surface through crater counts has revealed that Pluto has been geologically active throughout the past 4 billion years. Further, the surface of Pluto’s informally-named Sputnik Planum, a massive ice plain larger than Texas, is devoid of any detectable craters and estimated to be geologically young – no more than 10 million years old.

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2. Pluto’s moon Charon has been discovered to have an ancient surface. As an example, the great expanse of smooth plains on Charon is likely a vast cryovolcanic flow or flows that erupted onto Charon’s surface about 4 billion years ago. These flows are likely related to the freezing of an internal ocean that globally ruptured Charon’s crust.

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3. Pluto’s surface has many types of terrain. The distribution of compositional units on Pluto’s surface – from nitrogen-rich, to methane-rich, to water-rich – has been found to be surprisingly complex, creating puzzles for understanding Pluto’s climate and geologic history. The variations in surface composition on Pluto are unprecedented elsewhere in the outer solar system.

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4. Pluto’s atmosphere is colder than we thought. Pluto’s upper atmospheric temperature has been found to be much colder (by about 70 degrees Fahrenheit) than had been thought from Earth-based studies, with important implications for its atmospheric escape rate. Why the atmosphere is colder is a mystery. 

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5. We know what Pluto’s atmosphere is made of. The New Horizon spacecraft made observations of sunlight passing through Pluto’s atmosphere. We see absorption features that indicate an atmosphere made up of nitrogen (like Earth’s) with methane, acetylene and ethylene as minor constituents.

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6. We might have an idea for how Pluto’s haze formed. For first time, a plausible mechanism for forming Pluto’s atmospheric haze layers has been found. This mechanism involves the concentration of haze particles by atmospheric buoyancy waves, created by winds blowing over Pluto’s mountainous topography. Pluto’s haze extends hundreds of kilometers into space, and embedded within it are over 20 very thin, but far brighter, layers.

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7. There isn’t much dust around Pluto. Before the flyby, there was concern that a small piece of debris (even the size of a grain of sand) could cause great damage to (or even destroy) the spacecraft. But the Venetia Burney Student Dust Counter (an instrument on the New Horizons spacecraft) only counted a single dust particle within five days of the flyby. This is similar to the density of dust particles in free space in the outer solar system – about 6 particles per cubic mile – showing that the region around Pluto is, in fact, not filled with debris.

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8. Pluto’s atmosphere is smaller than we expected. The uppermost region of Pluto’s atmosphere is slowly escaping to space. The hotter the upper atmosphere, the more rapid the gasses escape. The lower the planet’s mass, the lower the gravity, and the faster the atmospheric loss. As molecules escape, they are ionized by solar ultraviolet light. Once ionized, the charged molecules are carried away by the solar wind. As more Pluto-genic material is picked up by the solar wind, the more the solar wind is slowed down and deflected around Pluto. So – the net result is a region (the interaction region), which is like a blunt cone pointed toward the sun, where the escaping ionized gasses interact with the solar wind. The cone extends to a distance about 6 Pluto radii from Pluto toward the sun, but extend behind Pluto at least 400 Pluto radii behind Pluto – like a wake behind the dwarf planet.

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9. Pluto’s moons are brighter than we thought. The high albedos (reflectiveness) of Pluto’s small satellites (moons) – about 50 to 80 percent – are entirely different from the much lower reflectiveness of the small bodies in the general Kuiper Belt population, which range from about 5 to 20 percent. This difference lends further support to the idea that these moons were not captured from the general Kuiper Belt population, but instead formed by the collection of material produced in the aftermath of the giant collision that created the entire Pluto satellite system.  

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What Have We Learned About Pluto?

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