Unmanned flights are the best way to discover the beauty of our cosmic neighbourhoodby Philip Ball / July 23, 2015 / Leave a comment
“Pluto is a real place, with incredibly complex geology. It is beautiful and it is strange.” © Nasa Read more: Is Nasa about to discover alien life? As David Cameron discovered on 8th May, success is all the more euphoric when your expectations start low. NASA’s New Horizons mission to Pluto and Charon (the largest of its five known moons) had been under the scientific radar for most of its nine-year journey to what used to be considered the edge of the solar system. Those who remembered the lonely spacecraft could have been forgiven for secretly thinking it wouldn’t find much of note when it arrived at the dwarf planet. A NASA mission to Pluto was proposed in the 1990s, but was dismissed as too unimportant. Then in 2006, only months after New Horizons set off, Pluto was demoted from planet status by the decision of the International Astronomical Union (IAU). New Horizons could have seemed like a voyage of obligation rather than necessity, filling in the gaps in our understanding of this obscure object, which is smaller than our own moon. How wrong we were. New Horizon’s Pluto Flyby has easily been the most eye-opening scientific story of the year. The astonished expression of its leader, the planetary scientist Alan Stern, as the latest images hit the screens of mission control at Johns Hopkins University in Baltimore, summed up everyone’s fascination as Pluto’s surface came into focus during July. Some scientists had imagined it to be an inert ball of icy rock pocked with craters that, as on our Moon, testify to a steady, tedious rain of meteors over billions of years onto a static surface. But on the contrary, Pluto has a complex surface with smooth ice plains, some fractured like the intricate craquelure on an Old Master painting and, most extraordinarily, mountains the size of the Rockies, made of ice and weirdly out of scale on a body this small. The most striking implication is that Pluto has tectonic activity: its surface is moving and being renewed, rather like Earth’s. That’s the only explanation for why there aren’t impact craters all over it—they get swallowed back up into the crust, or buried by fresh material. But where, on a “planet” this tiny and this remote from the Sun, does the energy come from for that geological activity? One thing’s for sure: as Stern said, “It’s going to send a lot of geophysicists back to the drawing board.” Why isn’t Pluto a true planet? Because it fulfills only two of the three criteria stipulated by the IAU: it orbits the Sun (albeit in a peculiar orbit that sits at a tilt to the plane of all the other planets) and it’s massive enough for gravity to pull it into a sphere, but it is too small to have swept up all the other smaller objects that lie along its orbit by drawing them into its gravitational field. The fact is that Pluto is just one of a vast population of ice-and-rock bodies lying beyond the orbit of Neptune, called the Kuiper Belt—rather like the asteroid belt between Mars and Jupiter, but wider and more remote, stretching from about 30 to 50 times the Earth-Sun distance. Pluto is the largest of these known bodies, although an object of similar size but greater mass, called Eris, lies within another overlapping belt of objects called the scattered disk. Both Pluto and Eris are now designated dwarf planets. For many, one of the biggest shocks of the New Horizons mission is how ignorant we were previously about Pluto. At an average distance of 4,400m to 7,400m kilometres from the Sun (and 4,300m from Earth on closest approach), it was too remote even for the Hubble Space Telescope to have offered more in the way of snapshots than a fuzzy blur. There were some light yellowish bits and some dark brownish bits, and that’s all we could see. It was known to have a core of dense rocky material about 1,700 km in diameter, with the rest of the 2,370km diameter being made of ice, along with a veneer of frozen nitrogen, methane and carbon monoxide. The size of Pluto’s mountains, made of water ice, suggest that the more volatile layer of nitrogen ice on its surface must be very thin and the ice beneath the surface must be very strong to support such an immense weight—at such frigid temperatures ice becomes as hard as rock. The mission scientists have also been fascinated by the diversity of this surface, with a bright heart-shaped plain now called the Tombaugh Regio after Pluto’s discoverer Clyde Tombaugh (who spotted it as a pinprick among the stars in 1930) surrounded by darker patches and streaks. These visual differences might be due to the greater accumulation of ice in colder parts, but some dark reddish regions might be covered with a complex carbon-based material called tholin, formed from chemical reactions of nitrogen, methane and carbon monoxide that are triggered by sunlight and cosmic rays. In any event, the lack of cratering in many areas (such as Regio Tombaugh) implies that the surface is being remolded. On Earth, tectonic activity is driven by heat from the molten core and from decay of radioactive elements in the mantle. On Pluto, radioactivity seems the only plausible energy source, but it would be surprising that a body as small as Pluto’s rocky core would have enough undecayed radioactive elements left this long after its formation. As Ellen Stofan, NASA’s chief scientist, told Nature, “Pluto is a real place, with incredibly complex geology. It is beautiful and it is strange.” The same could be said of Charon, which measures an impressive 1,200 km across, and was probably formed when a giant impact split a proto-Pluto apart in the early days of the solar system. It’s not clear if Charon should be regarded as a moon at all: both it and Pluto rotate once every 6.4 Earth days around a centre of mass that lies outside both of them, and some have argued that Pluto-Charon is more like a binary dwarf-planet system. But the profound differences between the two have been a big surprise. Charon has more cratering than Pluto, but also some bizarre grooves winding across much of its surface: chasms bounded by immense cliffs, deeper than the Grand Canyon. And there’s a dark cap of reddish material at its pole, currently dubbed Mordor, which might also be tholin—perhaps gusted there from Pluto, implying that the two bodies share an atmosphere. “There’s all kinds of crazy stuff on here,” planetary scientist Alyssa Rhoden of Arizona State University in Tempe told Nature. After Pluto and Charon, New Horizon will head out further into the Kuiper Belt to examine more of its icy mini-worlds. Few now would be willing to guess what strange riches it might find there. What with the Cassini-Huygens mission to Saturn and its moon Titan in 2004-5, the continued operation of the Opportunity and Curiosity rovers on Mars, and the dramatic landing of the Rosetta Philae spacecraft on a comet last November, this seems to be a golden age of planetary exploration to rival the wonders of the Voyager missions in the 1970s. With the United States currently lacking any homegrown means of sending people beyond the Earth’s atmosphere, and some ill-starred efforts by private spaceflight ventures, the message could hardly be plainer: manned spaceflight might retain its public popularity and glamour, but unmanned missions are for the foreseeable future the way to do real science, and to discover the true beauty and wonder of our cosmic neighbourhood.