The enhanced images of Pluto that were recently released by Nasa’s New Horizons team were truly dazzling, showing red and orange shades of the dwarf planet, flowing ice in its heart-shaped feature, some smooth plains and some mountains and craters. Pluto was observed with ground and space-based telescopes, and its best images — showing an immense dark band called the Whale, a bright heart-shaped feature, evenly spaced dark spots stretching for approximately 1,600 kilometres across the equator, and a recently found polygonal feature — were taken using the Hubble Space telescope.
K. Radhakrishnan But why is there such increased interest among scientists in Pluto, located 5.8 billion kilometers from the Sun and at the edge of the solar system?
Part of the excitement stems from the fact that space is an ideal laboratory for scientists. Its vastness enables fundamental experiments, orders of magnitude more accurate than on Earth, eliminating the influence of gravity, atmosphere, seismic noise and other interactions. These astronomical observations help us learn about the microscopic and macroscopic structure of the universe. Lunar and planetary missions, over the years, have provided extremely valuable scientific data on the formation and evolution of planets and their moons.
The Kuiper Belt That Pluto was discovered in 1930 as the ninth planet of the Solar system is well-known. Twenty years after its discovery, astronomers postulated the presence of the Kuiper Belt, comprising a vast collection of icy objects beyond the orbit of Neptune, in which Pluto itself was a member. The first Kuiper Belt Object (KBO) was discovered in 1992 — throwing doubt over Pluto’s status as planet — and since then observers have found more than 1,000 KBOs, with diameters ranging from 50 kms to almost 2,400 kms.
The International Astronomical Union in 2006 chose to classify Pluto and the recently discovered large Kuiper Belt Objects as dwarf planets. The Kuiper Belt contains a sizable supply of ancient, icy and organic material that are held in deep freeze, and that were left over from the birth pangs of the planets, containing evidences of the distant past. Because of this, planetary scientists are keen to learn more about Pluto and its moons, Charon (the largest), Styx, Nix, Kerberos and Hydra, and other objects in the Kuiper Belt.
The size of the Kuiper Belt and its shape and general nature appear to be much like belts seen around other stars. Additionally, when scientists used computer modelling to simulate the formation of KBO, as the solar system was coalescing from a whirling disk of gas and dust, they found that the ancient Kuiper Belt may have contained at least 10 times more mass than what it is today to give rise to Pluto and Charon and the other KBOs. It is estimated that there may have once been enough material to form another planet the size of Uranus or Neptune in the Kuiper Belt. Astronomers are keen to study the formation of Pluto and its moons. The prevailing theory of the formation of Pluto system is that Pluto collided with another large body, and much of the debris from this impact went into orbit around Pluto and eventually coalesced to form Charon. Pluto has a reflective surface with distinct markings that indicate polar caps. Charon’s surface is far less reflective, with indistinct markings. And while Pluto has an atmosphere, Charon is not known to have an atmosphere. There is a sharp contrast in the surface characteristics of Pluto and Charon, which is intriguing if they had a common birth. Interestingly, Pluto’s density, size, and surface composition are strikingly similar to Triton, Neptune’s largest satellite, considered to be a captured planet from the Kuiper Belt. A similar collision theory is also in place to explain the creation of Earth’s moon, and hence the study of Pluto and Charon could help scientists understand the history of our own planet.
Pluto is about 40 times farther away from the Sun than Earth, has a diameter of about 2,380 kms, and orbits the Sun once every 248 Earth years. It is a rocky icy planet with 35 per cent by mass being ice. Its atmosphere comprises mainly nitrogen, methane and carbon monoxide — that regularly transition between solid and gas state — and is extremely rarefied. Pluto’s surface gravity is about 6 per cent of Earth’s gravity and its estimated surface temperature is about (-) 233°C. Its atmosphere is also over 50,000 times less dense than Earth’s atmosphere. Pluto has a thin, tenuous atmosphere that expands when it comes closer to the Sun and collapses as it moves farther away — similar to a comet. Furthermore, Pluto’s surface temperature varies greatly because of the planet’s eccentric orbit and polar tilt. As the planet moves farther away and cools, the average surface temperature is expected to drop and most of the atmosphere will freeze out on the surface. As a result of this and also because the planet rotational axis is tilted by 28°, Pluto may have the most complex seasonal patterns in the solar system. Scientists believe that Pluto’s atmosphere loses a lot of mass into space. The thermal energy of typical molecules in the upper atmosphere is sufficient to escape Pluto’s gravitational hold, a process called hydrodynamic escape. The same may have been responsible for the rapid loss of hydrogen from Earth’s atmosphere early in our planet’s history, making Earth suitable for life. Pluto is the only place in the solar system where we can study hydrodynamic escape on a planetary scale today.
Another important connection between Pluto and life on Earth is the likely presence of organic compounds more complex than the frozen methane on Pluto’s surface and water ice inside the dwarf planet. Recent observations of smaller KBOs show that they are also most likely to harbour large amounts of ice and organic substances. Such objects are considered to have routinely strayed into the inner part of the solar system billions of years ago, collided with Earth, and helped to seed the young Earth with the building blocks of life. Given all these scientific motivations, it is understandable why there is increased scientific interest in Pluto and the Kuiper Belt.
NASA Mission The first dedicated spacecraft platform to explore Pluto at close quarters was NASA’s New Horizons, launched in 2006 from Florida. Its closest approach to Pluto was on July 14, 2015, the closest point being 12,500 km from Pluto’s surface at a velocity of 14 km/s. It has seven scientific instruments comprising an ultraviolet imaging spectrometer to probe atmospheric composition and surface structure, a visible and infrared camera/spectrometer to obtain high-resolution colour maps and surface composition maps, a long-range telescopic camera for high-resolution surface images, particle spectrometers to measure solar wind charged particles in and around Pluto’s atmosphere, a detector to measure masses of space-dust particles and two copies of a radio science experiment to examine atmospheric structure, surface thermal properties and the planet’s mass. The one-way time for communication is 4 hours 25 minutes.
NASA plans to turn around the spacecraft once it passes Pluto and try to map the planet’s night side, which will be softly illuminated by the moonlight from Charon. At this time, a powerful radio beam will be sent from Earth. This will aim to pass through Pluto’s atmosphere and reach the spacecraft’s. By measuring the effects of atmospheric refraction on the radio beam as it travels to the spacecraft, the temperature, density and composition profile of the atmosphere all the way to the surface can be estimated.
Valuable insights into the origin of the outer solar system and that of planet and satellite systems are expected to be discovered from the data sent by NASA’s New Horizons. This will raise scientific fervour all around.
(Dr. K. Radhakrishnan is former Chairman of the Indian Space Research Organisation.)
