Pluto: an enigma in close-up

Only a month ago, Pluto was one of the most mysterious objects in the Solar System — so remote scientists weren’t even sure of its size. Today, we not only have a clear measure of its diameter (2,370 kilometres, give or take 20 kilometres), but we know it has a red-streaked polar cap, steep mountains nearly as tall as New Zealand’s Mt. Cook, and a surface so fresh and smooth that scientists think it might still be geologically alive. And it looks like the cold distant body will continue to surprise us. 

Furthermore, Pluto’s giant moon Charon turns out not to be a dead, cratered landscape like the Earth’s Moon, but has its own reddish polar cap, enormous canyons up to 10 kilometres deep, and another surface that suggests recent geological activity.

These findings were reported during an intense and at times emotional gathering at Johns Hopkins University’s Applied Physics Laboratory, command centre for NASA’s New Horizons spacecraft. The spaceship finally zoomed past Pluto early on 14 July. It had travelled 5 billion kilometres, reaching its destination nine years after leaving Cape Canavarel on January 19, 2006.

Adding to the drama, New Horizons was deliberately dropped out of contact with Earth during the 22 hours it was closest to Pluto. “Any second spent transmitting to Earth is a second spent not looking at Pluto,” explained NASA astrophysicist Kimberly Ennico-Smith. But it meant New Horizons  – which is about the size of a grand piano – was out of contact when it was most likely to collide with rock or ice debris.

Despite this anxiety, the mood during the wait for New Horizons to pass Pluto then phone home was festive. At 7:49 am local time, the time of closest encounter, hundreds of people packed a ballroom in APL’s conference centre, cheering and counting down the seconds.

Steve Squyres, a planetary scientist from Cornell University and principal investigator for the Spirit and Opportunity Mars rovers, noted the contrast with the rover landings. “At the time, two-thirds of the missions to Mars had failed, so there was an enormous amount of tension over whether we would land successfully,” he said. “This was much better.”

Some 13 hours after it flew closest to Pluto, New Horizons finally had a 15-minute window in which to beam back a status report. Scientists were already studying the data transmitted in the days before the flyby. This confirmed Pluto is the largest known body beyond Neptune. In many people’s eyes this has renewed the case for Pluto being classified as a planet – though it is only a handful of kilometres larger than its fellow Kuiper Belt object, Eris.

Giant mountains, probably made of rock-hard water ice, towered 3,300 metres above Pluto’s surface.

Scientists have also confirmed Pluto doesn’t have any more small moons than the five already identified by the Hubble space telescope. 

Most intriguing were the colour pictures taken on the last day of approach. They showed a remarkably complex Pluto with several distinct geological provinces.

Even a bright heart-shaped feature – now informally dubbed Tombaugh Regio in honour of American astronomer and Pluto’s discoverer Clyde Tombaugh – proved to be composed of two distinct regions, tinted white on the left and blue on the right. “From the ground we knew there were a lot of different colours on Pluto, but we never imagined anything like this,” said New Horizons scientist Cathy Olkin, from the Southwest Research Institute in Boulder, Colorado.

The first close-ups of Charon and Pluto were even more remarkable.

Giant mountains, probably made of rock-hard water ice, towered 3,300 metres above Pluto’s surface. Scientifically, the biggest shock so far is the complete lack of impact craters on the entire 250-by-150-kilometre section of Pluto's surface for which New Horizons has beamed home high resolution imagery.

Scientists had expected to find a world frozen and long-dead, thoroughly pockmarked by collisions with meteorites that had slammed into the dwarf planet over eons, like our cratered Moon. Instead, Pluto appears fresh-faced.

“This is not an ancient landscape,” said John Spencer, a planetary scientist also from Southwest Research Institute. “We think it has to be less than 100 million years old” – a blink of an eye, in geological terms.

How a cold, tiny world such as Pluto can have been recently geologically active – let alone active enough to form a smooth new crust over much of its surface, and produce mountains comparable in height to some of the Earth’s great ranges – is baffling. “This is a landscape that has been reshaped by forces from the interior of Pluto that we don't begin to understand,” Spencer said.

The conventional wisdom has been that a small icy world can only be active when there’s a massive planet such as a Jupiter or Saturn nearby, whose giant gravity exerts a tidal tug that distorts the tiny world and warms its insides through friction.

“But that can’t happen on Pluto,” he added. Charon might be big for a moon, but it isn’t capable of producing such gravitational energies in its present orbit. And Pluto is well out of the gas giants’ gravitational reach.

The discovery is “going to send a lot of geophysicists back to the drawing board to try to figure out jut how that can be”, Spencer said.

Scientists expect many more surprises in the countless images still to come. Information from New Horizons takes four and a half hours to reach the Earth, and is transmitted at about two kilobits per second, which is slower than internet dial-up speeds. At that rate, it will take 16 months for all New Horizons' flyby data to be received. In the meantime, Pluto has not disappointed. Alan Stern, principal investigator of the mission, said: “I don’t think any of us could have imagined that it was this good a toy store.”

This simulated flyover of Pluto’s Norgay Montes (Norgay Mountains) and Sputnik Planum (Sputnik Plain) was created from New Horizons closest-approach images. Norgay Montes have been informally named for Tenzing Norgay, one of the first two humans to reach the summit of Mount Everest. Sputnik Planum is informally named for Earth’s first artificial satellite. The images were acquired by New Horizons' Long Range Reconnaissance Imager on July 14 from a distance of 77,000 kilometres. Features as small as 1 kilometre across are visible.Credit: NASA/JHUAPL/SWRI