Rediscovering Pluto

Initial images and data from the NASA spacecraft New Horizons’ recent rendezvous with Pluto point to a surprisingly craterless mosaic of relatively ancient regions and very young places on the dwarf planet.

The side of Pluto that faces Charon, the largest of its five moons, photographed from New Horizons on July 11 as it made its closest approach in the climax of a decade-long journey to explore the dwarf planet.

REMINISCENT of the famous Raja Ravi Verma painting in which Shakuntala while pretending to remove a thorn from her foot is surreptitiously looking at her lover Dushyantha, the National Aeronautics and Space Administration’s (NASA) New Horizons spacecraft took a last peek at Pluto from a distance of 5.7 million kilometres in the wee hours of July 19 and brought to closure a week full of anticipation and excitement.

New Horizons, launched about nine and a half years ago, made its closest approach of Pluto on July 14 while cruising at a record speed of 58,500 kmph after travelling about five billion kilometres. At the time of the closest approach, 17:19 hrs IST (11:49 UTC [Coordinated Universal Time]), as the spacecraft passed 12,500 km above the surface of Pluto, a set of seven scientific instruments aboard the craft hungrily gathered data on the faraway world of Pluto and its five known moons.

This was the first ever foray by any space mission into the Kuiper Belt, the so-called “third region” beyond the terrestrial inner planets and the gaseous outer planets, swarming with teeny-weeny icy objects. New Horizons has indeed taken a cosmic step and as data from it, with tantalising images and surprising implications about Pluto and its five moons, begin to trickle in, our understanding of the weird world is all set to change radically. As one blogger opined, it is time to tear and throw away chapters on Pluto from every book that has ever been published. The world that the New Horizons mission paints is not at the perspective we had until now. We had known all along that Pluto was weird, but it is weirder than we imagined.

An odd ball Since its fortuitous discovery in 1930 by a 24-year-old farm boy–turned-astronomer, Clyde Tombaugh, Pluto has kept its secrets to itself: it has been an odd ball. Smaller than seven moons in our solar system, including our own moon, orbiting the sun at an astounding distance of 5.9 billion kilometres, a distance to which light would take five and a half hours to reach, Pluto was just a hazy grey blob through even the best of telescopes, hardly revealing any surface feature. As Pluto’s thin extended atmosphere came in the way, even determining its size was a challenge.

The discovery of its largest moon, Charon, in 1978 forced astronomers to revise the estimates of Pluto’s size. Previously, as Charon remained unknown, its mass was also attributed to Pluto. Two smaller moons, Nix and Hydra, were discovered by the sharp-eyed Hubble Space Telescope. A search for rings around Pluto by Hubble resulted in the discovery of Kerberos in 2011. The last of the known moons, Styx, was discovered in 2012 in the wake of the search for potential hazards for the New Horizons mission.

Pluto does not belong either to the rocky compact terrestrial inner planets (Mercury, Venus, the earth and Mars) or to the outer gas giant planets (Jupiter, Saturn, Uranus and Neptune). While the orbits of all these eight planets lie on a plane, Pluto’s orbit is tilted by about 17 degrees and is more elliptical than any other planet’s. The highly elongated orbit at times comes inside the orbit of Neptune.

At the time of its discovery, Pluto was celebrated as Planet-X, but as more and more information on it and its environs became available, astronomers were not comfortable about classifying it along with the other eight planets. Beyond the orbit of Neptune, in the Kuiper Belt, astronomers have discovered more than 1,300 Pluto-like celestial objects. In 2006, the year New Horizons was launched, the International Astronomical Union (IAU) decided that Pluto was not a full-blown planet and demoted it to the status of a dwarf planet, one among the estimated 7,00,000 objects in the Kuiper Belt.

What does it take to be a planet? The IAU has codified that the object must orbit the sun and be massive enough to assume “hydrostatic equilibrium”, that is, it should be spherical in shape. Further, crucially, it should have “cleared the neighbourhood” around its orbit. The first two criteria are simple and common sense, and the third has to do with our understanding of the evolution of planets not only around the sun but also around other stars. As a planet evolves, initially in an accretion disc around the proto-star, it gathers most of the materials in its orbit and grows in size or nudges the remaining away from its orbit.

Although Pluto goes around the sun and is nearly spherical, it is not the dominant body in its orbit. It possesses only 0.07 times the mass of the other objects in its orbit. Sure, other bodies are strewn along the earth’s orbit, but the mass of the earth is 1.7 million times the remaining mass in its own orbit. This implies that Pluto has not “cleared its neighbourhood” adequately. Such midway objects are classified by the IAU as “dwarf planets” and Pluto is the largest known so far.

The Kuiper Belt is thought to contain icy objects in pristine condition since the formation of the earth and its siblings many billions of years ago. It is hoped that peeking into this hitherto unexplored “third region” will shed more light on the evolution of the solar system.

We did it! As the spacecraft approached Pluto in April 2015, it was woken up from hibernation. One by one, the instruments were tested and kept ready for the encounter with Pluto and its moons. On the crucial day, when the spacecraft hurtled past Pluto and Charon, the spacecraft was turned so as to face Pluto. The feeble energy generated from plutonium in its on-board plutonium-oxide-based Radioisotope Thermoelectric Generator (RTG) and the finite computing power of the spacecraft were conserved for the immediate task of gathering data.

Three optical instruments, two plasma instruments, a dust sensor and a radio science receiver/radiometer on board the spacecraft are its eyes and ears. These instruments let astronomers investigate the global geology, surface composition and temperature, and the atmospheric pressure, temperature and escape rate of Pluto and its moons. They are so miniaturised that they collectively require less than 28 watts of power and can function even in the extreme cold conditions of deep space near Pluto and beyond.

Venetia Burney Student Dust Counter (SDC), aptly named after Venetia Burney, an 11-year-old girl who gave Pluto its name, is a unique instrument. The first-ever built and flown by a team of students, it will examine dust not only around Pluto but also during the entire course of its journey. The “eagle eyes” of New Horizons is LORRI (Long Range Reconnaissance Imager), a panchromatic high-magnification imager digital camera fitted with a large telephoto telescope. Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) is a lowest-power directional energetic particle spectrometer that can sniff ions escaping from Pluto’s atmosphere. Alice is a sensitive ultraviolet imaging spectrometer designed to probe the composition and structure of Pluto’s dynamic atmosphere. Ralph, consisting of three panchromatic and four colour imagers, give stereographic images that will enable astronomers to infer topographic relief. The Solar Wind Around Pluto (SWAP) measures the interactions of Pluto with solar wind. Using the occultation technique, Radio Science Experiment (REX) will probe Pluto’s atmosphere and also search for an atmosphere around Charon.

As these instruments were busy taking hundreds of high-resolution photographs, collecting spectral data, sampling the outer environment and measuring various parameters, the spacecraft was in radio silence, on autopilot mode and not in contact with the earth. The data that were being collected during this critical period of the mission were recorded on memory chips with backup for subsequent onward transmission. For about 21 hours during the close encounter—the radio silence phase —mission controllers could only hope that all was well with the spacecraft; there was no way to be certain. Ending hours of suspense, as anticipated, exactly at 6:22:03 a.m. IST on July 15, 2015, the first signal from the spacecraft touched base at the Deep Space Network Antenna (DSN) in Spain and streamed into the mission control centre at the Johns Hopkins University Applied Physics Laboratory. Alan Stern, principal investigator for the mission, triumphantly declared before the media: “I want to say to you just three words. We did it.” Paraphrasing Neil Amstrong’s famous quip, he added: “I’d like to characterise that DSN pass you just watched as one small step for New Horizons, and one giant leap for mankind.”

Face to a name The clarity of the images taken using LORRI are impressive, providing the first-ever images of surface features. Pluto was crystal clear in these images: ice caps; a mysterious elongated dark feature at the equator, dubbed “whale”; a region which looked like Bulls Eye on the face of Pluto permanently facing Charon; a large heart-shaped bright region measuring about 2,000 km across, subsequently dubbed “Tombaugh Regio”. Charon was also not disappointing, with a barren landscape of vast craters and chasms deeper than the earth’s own Grand Canyon. In a manner of speaking, we are able to put a face to the name we had known so long.

Close-up images of Tombaugh Regio indicate that it may be the eroding remains of a large crater caused by an impact at some point in the past; one side is smoother than the other, which researchers believe is filled with frozen gases from the atmosphere, including nitrogen, methane and carbon dioxide. Enlarged images reveal a vast crater-free plain, Sputnik Planum, estimated to be less than 100 million years old in Tombaugh Regio. This icy plain is criss-crossed with irregular polygon-shaped shallow troughs. These resemble mud cracks on the earth, and are either cracks caused by surface contraction or signs of convection, caused by the heat within the planet. The age of these patterns and what drives the process are yet to be determined. This region also shows dark hills and a pitted surface.

Pluto’s equator has youthful mountains rising as high as 3,500 metres above the rippled icy plains, named Norgay Montes, after Tenzing Norgay. This southern range of mountains is estimated to be younger than the Himalayas and perhaps is still in the process of being formed fully. Although one would be tempted to speculate that these are made up of frozen methane and nitrogen ice that abound on Pluto’s surface, simple computation shows that frozen methane and nitrogen will crumble under their own weight at those elevations. Hence, it is most likely that they are made up of water-ice, which would behave more like rock at the frigid cold temperature in Pluto.

Images of Charon returned in the days before the closest approach showed a dark, smooth region. Newer images showed a series of troughs and cliffs running across the moon for nearly 1,000 km, as well as a 10-kilometre-deep canyon on the limb.

As more and more razor-sharp images are received in due course, the terrain maps of Pluto and Charon will become clear.

Weird planet Pluto appears to have a reddish-brown hue in the close-up colour images. Scientists speculate that the colour is because of the presence of tholins in the atmosphere, which are complex molecules formed by the interaction of the sun’s ultraviolet rays with methane.

To everyone’s surprise, the PEPSSI instrument has detected nitrogen ions escaping from Pluto far upstream than anticipated as the spacecraft was hurtling towards Pluto, implying that perhaps Pluto is losing its atmosphere faster than predicted by earlier models. A tantalising look at Pluto’s plasma environment revealed by SWAP shows a region of cold, dense ionised gas tens of thousands of kilometres beyond Pluto clearly indicating that slowly the planet’s atmosphere is being stripped away by the solar wind and is being lost to space. The predominantly nitrogen-rich atmosphere of Pluto is ionised by solar ultraviolet rays and the ions are “picked up” by the solar wind and carried past Pluto to form the plasma tail. How fast is Pluto losing its atmosphere, the evolution of Pluto’s atmosphere, and the impact of solar wind and such other questions await crucial data collected by the Alice and the REX instruments, which are anticipated by August.

An hour after the closest approach, as the spacecraft receded behind Pluto, it positioned itself in a particular alignment to come right behind Pluto, keeping the sun on the far side. In this alignment, so to say, Pluto “eclipsed” the sun. As the spacecraft passed through Pluto’s shadow while the sun backlit Pluto’s atmosphere, the Alice imaging spectrograph carefully measured the extent of the atmosphere of Pluto. Even before the fly-by, given the weak gravity of Pluto, its thin atmosphere was expected to be proportionately larger than the earth’s. However, the preliminary result from Alice that Pluto’s nitrogen-rich atmosphere extends far beyond 1,600 kilometres exceeded any anticipation. More surprises may be in store when the full Alice occultation dataset is sent to the earth for analysis.

Why the dog did not bark? As the first few images were reconstructed, one had expected to see dead or docile Pluto, peppered all over with craters fossilised with remnants left over from the formation of the solar system some 4.6 billion years ago. Sure, some speculated that it had spectacular geysers and cryovolcanoes spewing ice and cold nitrogen, methane, carbon dioxide and water. However, what planetary scientists witnessed, as close-up pictures came up on their monitors, was baffling. The images showed hardly any craters on Pluto. There were copious amounts of methane frosting, water ice, polar caps and young mountains as tall as 3,000 metres. Chasms, mountain chains and a huge rift valley on Charon were unmistakable.

About four billion years ago, a period of intense comet and asteroid bombardment is thought to have dappled all the planets, including the earth, with craters. Many of the numerous craters dotting the moon and other bodies in the solar system are enduring testimonies of this cataclysmic event. Thus, believed to be asleep, Pluto was expected to carry the pockmarks from this period. However, to the astonishment of planetary geologists, Pluto appears to be a mosaic of relatively ancient regions and very young places that are currently undergoing geologic evolution.

Volcanism and tectonics require the planet to be of sufficient size to still have heat in its interior. Erosion requires sufficient atmosphere with winds to work efficiently. Only if a planet has an atmosphere and is still geologically active can the effects of impact cratering be erased.

On the earth, weathering and tectonic shifts have eroded and erased craters, except a handful, such as the famous one in Arizona in the United States, and the Lonar lake in Maharashtra, India. The earth has a sufficiently dense atmosphere for a weather system. Geophysicists believe that in addition to the primordial heat leftover from the formation of the planet, radioactive decay of naturally occurring radioactive materials (NORM) continuously generate about 20 TW (terawatt) of energy. Recently, an international team of scientists in Japan measured using the KamLAND detector the intensity of anti-electron-neutrinos coming from the interior of the earth to confirm that primeval heat combined with the heat generated by radioactive decay fuels the earth’s tectonic movements, which results in the laying of a fresh surface erasing the pockmarks from geologically earlier bombardments.

In contrast, Pluto is just one-sixth the size of the earth and has surface pressure ranging from 0.000006 to 0.000024 bar, meaning, nearly vacuum. Such a tiny planet was hardly expected to be geologically active. However, the evidence provided by New Horizons is clear. Pluto and Charon are alive and kicking. What fuels the geological activity? Where is it deriving energy from? Does it have a richer radioactive element interior than estimated? Or is it the ocean beneath it giving off energy as it contracts during winter. This is an enigma. Weeks have passed since the close fly-by. Yet, until now only about one gigabit of information, out of an estimated 50 gigabits of data and images captured by New Horizons, has been received. As New Horizons is way far deep into space, bandwidth is limited to 1 kbit/s even with the 70-metre DSN antenna, and as a result even a modest LORRI photo of 2.5 megabits takes 42 minutes to download. It will be months before all the data are received and analysed.

Yet, what we got to know about this flyspeck of a planet in the past few weeks is much more than what we had learnt since its discovery 85 years ago, way back in 1930. In the months to come, as New Horizons sends fresh data, planetary scientists will learn more about its composition and develop clues about the origins of Pluto and its five moons.

Not the last stop New Horizons’ visit to the Plutonian system, albeit brief, is the first step in exploring this new frontier in our solar system. Pluto is not the final destination but a stopover for New Horizons. The spacecraft is expected to be redirected to more distant and smaller Kuiper Belt Objects (KBOs) in 2019. Current telemetry data from the spacecraft indicate good health and adequate reserve of hydrazine propellant to perform a course correction manoeuvre to commence its journey towards its next target. NASA has so far identified two potential KBOs, 2014 MU69, a 40-70 km object, and 2014 PN70, a smaller 25 km object, for the next phase. However, the final call will be taken only during August-September 2016 after assessing all parameters. If all goes well, sometime in the end of next year, commands will be sent from the earth to New Horizons to thrust its rockets to tweak its trajectory for a possible fly-by of a second object in early 2019.

T.V. Venkateswaran is a Scientist with Vigyan Prasar, Department of Science and Technology, New Delhi.