Clyde Tombaugh saw a dim star shining at a feeble magnitude of +15.2 at the Lowell Observatory in early 1930 while blinking a pair of star-filled photographic plates. That dim star moved at just the correct amount to indicate it was far away on the edge of the Solar System. The night he discovered it was on February 18, and it was announced to the world on March 13. On May 1, 1930, the dim little star became known as the planet Pluto, named by a young English girl, Venetia Burney. Longtime favorites, Mars and Saturn, had to make way for tiny, remote Pluto as Earth quickly fell in love with it.

The love for Pluto remains true to this very day as demonstrated by the inhabitants of Earth who defended its planetary status fiercely in 2006 when it failed to keep its orbit clean and got demoted to a dwarf planet. At least it is round, so it kept some of its planetary dignity, but in 2015 Pluto gave Mars a challenge for the favorite planet, dwarf or not. The mystery of Pluto fired the imaginations of scientists and children alike. It easy to remember facts about Pluto: 3600 miles in diameter, no moons, orbits over three billion miles from the Sun, a cratered, icy world. The best telescopes back in the early years could only reveal Pluto as a tiny, featureless disc with a pale-yellow hue. Its orbit is inclined 17.1º to the orbital plane of the rest of the planets. Even more bizarre is that Pluto crosses Neptune’s orbit and is closer to the Sun than Neptune for 20 years. Nobody knew why but its planetary status was not questioned. Pluto started to reveal more about itself in 1976 when methane ice was discovered on its surface. Then a large moon, Charon, was discovered by James Christy in 1978 and Pluto turned out to be tipped 122º on its axis, more than Uranus. An atmosphere was discovered in 1988 and occultations between Pluto and Charon during 1985-1990 revealed large bright and dark patches on Pluto, but a bland Charon. Pluto was no larger than 1450 miles across while Charon was about half as large. The Hubble Space Telescope confirmed the varying patches on Pluto, and while it could not resolve them, it did prove that Pluto was as interesting as Mars.

Starting in 1992, small, icy worlds, now called Kuiperoids, were discovered in a region beyond Pluto. They shared similar characteristics with Pluto, which eventually lead to it being reclassified in 2006 as a Kuiper Belt Object along with being a dwarf planet. This angered many and broke many hearts, but an unknown giant heart was out there, unbroken, bold, bright, stronger than rock and waiting to be discovered.

The motivation to go to Pluto goes back to 1989 and Triton receives full credit. Voyager 2 flew past Neptune and its large moon, Triton, in August 1989 and discovered geysers on Triton where temperatures plunge to -392ºF. Triton and Pluto were thought to be similar and scientists wanted to get a close-up look at Pluto, especially since Triton was known to be geologically active. After a tough decade of politics, design issues, and a near-cancellation, a cost-conscious mission was finally approved in November 2001; the New Horizons Spacecraft was born thanks to Johns Hopkins University/Applied Physics Laboratory (JHU/APL). Everything came together for a perfect launch on January 19, 2006. Everyone had July 14, 2015, marked on their calendars as the day New Horizons would fly closest to Pluto and Charon and finally reveal what they look like, although February 14 would have been a more fitting date but they didn’t know it at the time.

As New Horizons neared Pluto, the large bright area that was fuzzily seen from Earth came into focus as a huge, heart-shaped region of smooth nitrogen ice covered with methane and carbon monoxide ices. This is a vast, 400,000 square-mile nitrogen glacier called Sputnik Planitia that forms the western lobe of Pluto’s icy heart feature. There is nothing like it elsewhere in the Solar System. Several features seen on this glacier make it exotic: the cellular structure on its surface indicating convective motion in the ice, recharge zones along its edges, hundreds of mile-wide subduction pits caused by nitrogen ice turning directly into gas, obvious evidence of glacial flow along the edges and adjacent surrounding mountains. The lack of craters indicates that this massive glacier is constantly renewing itself even though it is one of the coldest spots on Pluto at nearly -400ºF! Clusters of hills up to a mile across dot the surface of Sputnik Planitia and appear as if they were bobbing on the glacier. These are most likely chunks of water ice that flow along and collect near the edges where polygon slabs meet. High resolution images from New Horizons show that the nitrogen ice is indeed flowing into Sputnik Planitia from higher elevations. They pass through narrow canyons and carry along dark ribbons of material that look like debris streams and moraines created by terrestrial glaciers. Dunes of solid methane ice also cover Sputnik Planitia probably caused by icy grains deposited by winds descending from the mountains! This vast glacier looks like a lava lamp in slow motion where nitrogen ice from beneath is warmed, rises to the surface, cools, and eventually flows back into the cracks. The polygons are the tops of these bubbles percolating from below and are probably up to three miles deep. There is evidence of a vast reservoir of water ice beneath the glacier and, if confirmed, this would classify Pluto as an Ocean World along with Europa, Ganymede, Callisto, Enceladus, Titan, Mimas, Triton, and Ceres.

Another unique feature is the bladed terrain within the eastern lobe of the heart. These are very long, 1000-feet high linear ridges made of methane ice that are also not seen elsewhere in the Solar System. Pluto has dramatic icy mountains up to 11,000 feet high near Sputnik Planitia, but a pair of even taller mountains have a strange texture that appear to have been built up in layers and have deep central pits. These are most likely giant cryovolcanoes that were built up from an erupting mixture of water ice, methane, and nitrogen like a thick slurry of toothpaste and tower 2½ and 3½ miles high! There are several ranges of dark mountains that have peaks capped in bright methane ice. Pluto also revealed a frozen nitrogen lake to add to the fascination of this awesome world. Cthulhu Regio is a very dark band along Pluto’s equator and wraps almost completely around it. It is not the actual rocky surface, but most likely a thick deposit of reddish-brown hydrocarbons called tholins created by solar radiation interacting with methane. This ice-free belt corresponds to a region along the equator that receives a lot of sunlight.

New Horizons also discovered an extensive haze above Pluto extending up to 120 miles above the surface. The atmosphere is very thin, about 60,000 times less than Earth’s, and lacks oxygen, consisting of 98% nitrogen and traces of methane, carbon monoxide, and cyanide for a very poisonous atmosphere indeed! When New Horizons departed Pluto, it could see up to 20 haze layers, most of them being ½ - 1 mile thick, which were probably produced when the wind blows over the mountains creating waves. Color imaging shows Pluto’s atmosphere is blue, but from the surface the sky is still black with perhaps a band of fine haze along the horizon. New Horizons saw distant mountains casting long shadows into the haze. This haze must create beautiful sunrises, sunsets, and twilight.

Pluto’s largest moon, Charon, also has unique terrain. Even though it is nearly the same size as the other icy moons of the outer planets with similar composition and density, it has two types of surface features not seen elsewhere. One is the dark reddish polar stain that may be formed when gases escaping from Pluto condense on Charon’s cold poles, and then chemically altered by solar radiation. The other unique feature is the moated mountains which are surrounded by a trench as if something flowed and surrounded them, but the exact cause remains a mystery. The theory is that Charon was created by a collision between Pluto and another small planet which launched material into orbit around Pluto and accumulated to form Charon. One clue is the large size of Charon compared to Pluto. Another was the discovery of the Kuiper Belt in the 1990s, which could have been the source of the impactor. The discovery of four small moons, Hydra, Nix, Styx, and Kerberos, in the same orbital plane with Charon by the Hubble Space Telescope strengthened this theory. New Horizons added even more evidence by discovering that Nix and Hydra are both covered in water ice, which is what impactors would produce during a giant impact, plus both moons are the same age as Charon.

The next phase in the exploration of Pluto is a dedicated orbiter mission known as Persephone that will orbit Pluto for several years perhaps by October 2058 if approved for funding and could launch in February 2031. Pluto, a dynamic, geologically alive world brought into focus by New Horizons, has stolen our hearts again. It is obvious that everyone loves Pluto!