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Mystery deepens after New Horizons’ encounter

Bernard Shaw’s witticism, “Science never solves a problem without creating ten more” is compelling in the light of New Horizons’ Pluto flyby. With just 2-3 per cent collected data received so far by the spacecraft, the picture that emerges raises more questions than it provides answers. NASA’s New Horizons mission has taken the cosmic step forward by unveiling the hitherto unexplored, mysterious and elusive Pluto, by providing detailed and stunning images of the dwarf planet and its five moons.
The mission has upturned several perspectives, dislodged speculations and compelled many models back to their workbenches.

Since its discovery by 24-year-old farm boy-turned-astronomer Clyde Tombaugh in 1930, Pluto has been shrouded in mystery. Orbiting the Sun at an average distance of 5.9 billion kilometres, a distance at which travelling at lightning 3,00,000 km per second light would take about 51/2 hours to reach, Pluto is just barely visible to ground-and space-based telescopes.

Although earlier one could estimate its gravity, orbital period and so on, we had no answers to even basic questions such as whether its surface is pocked by craters, what its atmosphere is like and does really cryovolcanoes or geysers often erupt throwing icy liquid all around Pluto?

Decades of space programme has explored the inner rocky planets—Mercury, Venus and Mars. More than one spacecraft have reached gas giants Jupiter, Saturn, Uranus, Neptune and some of their moons. NASA’s New Horizons spacecraft, after making its historic nine year and 5-billion kilometres journey, has been sending back, one by one, eye-catching, full-colour and first-ever high-resolution images of the far-flung dwarf planet.

The mission is also the first-ever foray into the “third region” of the solar system, the Kuiper Belt, containing thousands of icy worlds such as Pluto, Eris, Varuna and Makemake. The Multispectral Visible Imaging Camera (MVIC), working in visible and near-infrared conditions, is part of the Ralph payload with 250 metres per pixel resolution. This is complemented by a 50-metre high-resolution Long-Range Reconnaissance Imager (LORRI), thus providing us with the best-ever images of the faraway world of Pluto and its companions. The stereo images from Ralph help determine surface topography and a suite of scientific instruments provide data on plasma, dust and atmosphere on Pluto.

As the ultraviolet light rays from the sun hit the methane gas on the atmosphere of Pluto, complex molecules called tholins are generated, giving a reddish-brown hue to the dwarf planet. The data also indicates generation of methane and nitrogen snow as winter sets in, as Pluto drifts farther away from the Sun in its orbit. A close examination of the heart-shaped feature on Pluto’s surface, nicknamed Tombaugh Regio (after Clyde Tombaugh), shows it to be an icy plain devoid of any impact craters. High resolution images reveal icy mountains that are 11,000 feet tall, potential evidence of wind erosion and possible volcanic plumes on Pluto. These and other surface features that are relatively young, with little or no craters, indicate some sort of underlying geologic process that is generating fresh terrain and features with the passage of time.

However, geological activity requires some internal source of energy, and unlike Earth, Pluto appears to be too small to have large amount of radioactive materials left over from its creation. One of Jupiter’s moons, Europa, is also small and shows geological activity, but that can be easily explained by the “tidal heating” caused by the massive Jupiter it orbits. Dynamics of Pluto’s tectonic activity remains a big mystery at the moment.

New Horizons data has also shown that Pluto is slightly larger than previously thought (2,370+/-12 km), settling the decade-long debate on Pluto being the largest dwarf planet.

This implies that Eris discovered in 2003 is smaller in diameter than Pluto, but is about 27 per cent more massive, indicating perhaps a radically different composition and history of Pluto. Does this imply that we need to revisit Pluto’s demotion as a dwarf-planet? Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), aboard the spacecraft, detected nitrogen escaping from Pluto’s thin atmosphere at 6 million km rather than the predicted 1 to 2.5 million km. This could be either due to nitrogen escaping Pluto’s atmosphere faster or the concentration of nitrogen is many times stronger than the estimated models.

With just a 2.1-metre antenna pointed at Earth and signal travelling about 5 billion km, data from the spacecraft just trickles in. To capture such a weak signal, NASA has employed its Deep Space Network (DSN) antennas as wide as a football field (70 metres across). Even then at that distance, the downlink speed is just about 1,000 to 4,000 bits per second compared to 300 million bits per second of modern cable modems. It would take about 16 months for the entire data captured by New Horizons in a few weeks of close encounter with Pluto to be downloaded.

In years to come, additional data received from New Horizons will continue to fuel discovery and shed light on not only Pluto, but also on much broader class of objects—thousands of chunks of rocks and ice that orbit the sun in a region called the Kuiper belt beyond Neptune. DOWN TO EARTH
T V Venkateswaran is a scientist with Vigyan Prasar, New Delhi
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