Science

Pluto

by Duncan Lunan

Source: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

Percival Lowell, famous or infamous for the ‘canals’ on Mars, also started the search which led to the discovery of Pluto.  Venetia Phair, the only person to have named a planet in the 20th century, died on April 30th 2012, aged 90, after a long and full life, in which she gained qualifications in maths and chartered accounting, married in 1947 and is survived by a son.  But she was quietly famous throughout her life because of a conversation in Oxford on March 14th, 1930, when as Venetia Burney she was 11 years old, about the planet newly discovered by Clyde Tombaugh at the Percival Lowell Observatory in Arizona.

Venetia Burney

Over breakfast with her grandfather, Falconer Madan, she suggested that the new planet might be named after Pluto, the Roman god of the Underworld, which in their mythology was cold and dark.   Madan was retired Librarian of the Bodleian, and in 1878 his uncle Henry Madan, a science master at Eton, had successfully suggested naming the moons of Mars ‘Phobos’ and ‘Deimos’ – which knocks on the head Velikovsky’s suggestion that they were visible in Homer’s time.  Madan passed on the idea to the Professor of Astronomy at Oxford, Herbert Hall Turner, and he in turn passed it on to Vesto Slipher, the Director of the Lowell Observatory, who backed the name because it incorporated Lowell’s initials.  Madan rewarded Venetia with £5, no small sum in 1930.  (Obituary, ‘Venetia Phair’, Daily Telegraph, May 6th, 2009.)  In H.P. Lovecraft’s story ‘The Whisperer in Darkness’, the narrator writes, “I wish, for reasons I shall soon make clear, that the new planet beyond Neptune had not been discovered… Astronomers, with a hideous appropriateness they little suspect, have named this thing Pluto.”   Supposedly it was the base for interstellar travelling monsters such as Cthulhu, now asleep on Earth but soon to be awakened.  

But although ‘Pluto’ was internationally accepted, the perception was that the naming of planets couldn’t be left to children and the International Astronomical Unionwas organised in haste to take responsibility for naming everything in the sky from then on.  In August 2006, the IAU decided that Pluto should no longer be considered as a planet but as a dwarf planet, along with Ceres and the object in the Kuiper Belt which was bigger than Pluto, then renamed ‘Eris’ after the goddess of discord.  The discord continued regardless, because Pluto is the only planet to be discovered by an American and the US delegation had already left when the vote was taken.  Kuiper Belt objects have since been re-renamed ‘Plutoids’ as a compromise, but the basic argument rumbles on.

When I go to talk in primary schools, I am often asked why Pluto is no longer a planet – but it’s still nearly as common to be asked why it’s named after Mickey Mouse’s dog.  The answer, of course, is that Walt Disney was at the start of his career and the dog is named after the planet, because it was in the news.

Pluto and Mercury are the only two planets with sufficient orbital inclination to appear occasionally outside the band of Zodiacal constellations.  Since the 1960s Pluto has moved through Ophiucus into Serpens, outside the Zodiac  (only Mercury and Pluto can do that).  Although, at age 60 in 2006, I hadn’t yet noticed the policemen looking younger, it was slightly scary that I had lived long enough to see Pluto change constellations.  

Pluto satellite system

But Pluto’s 248-year period had meanwhile caused another anomaly.  Pluto’s orbit is eccentric enough to bring it within Neptune’s for part of Pluto’s year, and from 1989 to 1999 Neptune was in reality the planet furthest from the Sun.  Pluto’s methane atmosphere had ceased to be “frozen on the rocks”, as so often suggested, and become gaseous.   With ‘Ice’, then ‘Pluto Express’ and finally ‘New Horizons’, the priority was to get images of Pluto before the atmosphere freezes back on to the rocks, because it will be around 2237 AD before we see it that way again.  Nobody knew how soon that would happen, though the atmospheric density dropped by 50% in the last year before the flyby and haze layers were forming as temperatures dropped.   New Horizons photographed these as it looked back at Pluto in crescent phase, and they proved to be more extensive, numerous and complicated than anyone had anticipated.  The final collapse took longer than expected, but by 2021 the thermal inertia of the atmosphere has run out and there has been a further sharp drop in atmospheric pressure there this year.

To travel beyond the orbit of Mars the spacecraft required Radioisotope Thermal Generators  (RTGs)  for power, and environmentalist groups lodged objections to the launch;  nevertheless the White House and Congress cleared it and the period for lodging objections closed in June 2005, by which time the spacecraft was already at Cape Canaveral.  The probe was successfully launched by Atlas 5 booster in January 2006. 

Pluto is so small, and so far from the Sun, that a 12-inch aperture telescope at least is needed to be able to see it.  Throughout my life, each time Pluto occulted a star the estimates of its diameter have been getting smaller, making it less likely to be the gas giant planet Lowell and Tombaugh were searching for.  Even at the time of my New Worlds for Old in 1979 it was possible that it could be Earth-sized, though it would have to have a very shiny surface on which we saw only the reflection of the Sun.  By 2012 it was known to be only 1200 miles in diameter, smaller than Earth’s Moon, but we still had only the vaguest maps of features on its surface, obtained during a series of mutual eclipses by Pluto and its major moon Charon, observed by the Hubble Space Telescope in 1994,  2002-2003 and 2010.  In Arthur C. Clarke’s classic novel Childhood’s End, at a key point Jan Rodricks looks at the Moon and imagines he can see ‘the main base of the Overlords, lying within the ramparts of Plato’.  The misprint ‘Pluto’ was in the 1961 Pan edition, if not earlier, and continued throughout the decade.  If Jan could have seen that with the naked eye, his eyesight would have been really good.

Pluto Charon Moon Earth Comparison

Pluto’s major satellite Charon wasn’t discovered until July 1978.  Strictly, Pluto and Charon are a double planet, keeping the same faces towards each other as they rotate and revolve with a locked period of 6.38 days.  It’s possible, though currently looking less likely, that they are the fragments of a satellite of Neptune which was fragmented and expelled in a catastrophic encounter with a tenth planet in the early history of the Solar System  (Duncan Lunan, ‘Shadows on the Milky Way’, Honourable Mention, 1985 Griffith/Hughes Aircraft Essay Competition, Griffith Observer, December 1986).  That was one of the assumptions of ‘Out of the Ecliptic’, my tenth planet story for Sydney Jordan’s Lance McLane strip  (Daily Record, 1987).  

Charon-red, CH4 from Pluto frozen during 100-yr night, tholins prod by sunlight

Charon has no atmosphere, as far as we know.   But in July 2007 strong evidence emerged for its having ice geysers, like those on Saturn’s moon Enceladus, which feed into the planet’s E-ring.   The Gemini Observatory’s adaptive optics system on Mauna Kea in Hawaii detected large deposits of ammonia hydrates and water crystals spread out across the surface of the icy moon, perhaps due to interactions between water and ice below the surface.  This action could happen in a few hours or even days, but over 100,000 years the process could give Charon a new surface one millimetre thick.  The discoverers believed there was a dynamic process going on because Charon’s surface didn’t appear to be “primordial ice”; ice that was created during the formation of the Solar System.    Instead, it’s much more crystalline in appearance, and must have formed recently.  (‘Pluto’s Moon Charon has Geysers Too’, Gemini Observatory Press Release, July 17th, 2007.)

‘floating hills’, water icebergs miles across

The Pioneer 11 flyby of Saturn in 1979 was intended as a pathfinder for the Voyagers  (see ‘Saturn and its Moons’), and when Pioneer’s Titan data was lost due to a tracking station failure, it was decided to send both Voyagers to Titan rather than sending Voyager 1 to Pluto.  NASA repeatedly tried to mount a mission to Pluto, at various times called ‘Ice’, then ‘Pluto Express’ and now ‘New Horizons’.  Like Pioneers 10 and 11, New Horizons crossed the Moon’s orbit in nine hours after launch in early 2006, and passed Jupiter on February 28th 2007.   The Student Dust Counter experiment on the spacecraft was named ‘Venetia’, much to her delight, but due to age she was unable to accept an invitation to the launch, and sadly now will not see the planetary encounter either.     

New Horizons payload inc Venetia student dust counter

The existence of Charon had been known since the 1970s, but after New Horizons was launched in 2006 two more moons, Nix and Hydra, were discovered, whose orbits and reflectivity suggested that they had been blown off Charon by big impacts.  That would not have been expected in a two-planet system because tidal forces should make their orbits unstable;  but further examination suggests that all three moons were formed by a collision in the early history of the Solar System.  The discovery of two more moons made it clear that they have settled, long ago, into a stable system of mutual resonances – a miniature Solar System, like the major moons of Jupiter and the inner moons of Saturn.

The Solar System has its Asteroid Belt and Kuiper Belt, though, and both Jupiter and Saturn have rings.  Even if the Pluto moons aren’t debris from recent impacts, it still seemed likely that there would also be rings in the same plane, which could cause problems for New Horizons on its planned trajectory.  (Kulviner Singh Chadha, ‘New Moons Found at Pluto’, ‘Planet Pluto Has Rings’, Astronomy Now, December 2005 and April 2006).  Spacecraft and ring planes make unhappy companions.  Proposals to send the Cassini probe through the Cassini division in Saturn’s rings were quietly dropped after the Voyagers found lots of material in there, and even so, Cassini took thousands of hits from microparticles when crossing the ring plane further out from the planet.  

After passing Pluto, New Horizons had to spend the next fourteen months transmitting its findings back to Earth.  Mission controllers had to find ‘safe haven’ paths along which the spacecraft could fly with a low probability of crossing rings, and the trajectory commitment had to be made by early 2015.  Most of the options involved passing further out from the planet than originally intended, but one exciting one  (not selected)  would take it on a very close pass over Charon and its possible geysers. 

Charon map July 2017

In the event however the flyby was a complete success, and even the first photographs revealed that both Pluto and Charon have complex landscapes, with mountains of water ice, chasms suggesting liquid below, and plains of frozen methane, nitrogen and carbon monoxide, displaying large-scale chemical reactions at temperatures less than 30 degrees above absolute zero.  There is evidence on both mini-worlds for internal heating, which can’t readily be explained either by tidal forces or radioactive decay, and it’s possible that the large plain of frozen nitrogen now called ‘Sputnik Planum’ may overlie a basin which is still filled with liquid water after the impact which formed it.  The surrounding mountains are water ice, like Titan’s, and the hexagonal nitrogen cells are rotating over liquid of some sort, while water icebergs migrate along the cracks like the ‘moving rocks’ of the USA’s Death Valley.  The ice mountains south of it have been named after Sir Edmund Hillary and Tenzing Norgay, and the dark region south of them after Lovecraft’s Cthulhu, while another to the east is now ‘Balrog’ after the demon in The Lord of the Rings.  Hills within and around the plain have been named after spacecraft, and the chasm bordering it on the northeast honours Jacques Cousteau.  Lowell’s name has been given to another dark plain further west.  ‘Bladed terrain’ east of Sputnik Planum, which lies within Tombaugh Regio, may feature ‘penitentes’, narrow spires of ice which are found in the Andes and in Antarctica.  The identification of ‘cyrovolcanoes’ on Titan remains controversial, but they’re unquestionably found at Wright and Picard Montes, south of Sputnik.

Charon st names new horizons, official 2017

Further north, towards the pole, ammonia ice was found;  while tinges of red in the landscape hint at  more complex chemistry, possibly the ‘tholins’ which are thought to form under solar ultraviolet rays and to be responsible for the redness found in the outer regions of the Asteroid Belt, as well as in the Kuiper Belt, beyond Neptune, of which Pluto is the largest member.  Many more features have been discovered:  clouds, chasms, geological faults, pits up to 2 miles deep with frozen nitrogen floors, possible ‘chaotic terrain’ opposite the Sputnik impact… And there’s still an entire hemisphere of Pluto which remains almost unexplored:  due to the speed of the spacecraft and to Pluto’s slow rotation  (about 6.4 days), the other side of it could not be seen from much closer than 2 million miles, during approach.  Hopes that moonlight from Charon might reveal more were not fulfilled, because Pluto and Charon have mutually trapped rotations and the poorly seen hemisphere was on the Farside from Charon, where the moon would never be visible.

Pluto Sputnik Planum from over Cthulhu

Charon itself failed to show active geysers as predicted  (with the images still under study, ‘as yet’ should be added), but there were major signs of activity on and below the surface.  Most striking at first sight were a huge dark red area in the north, named ‘Mordor Macula’, and possibly tholins, formed by ultraviolet action on methane deposited from Pluto.  Below the equator there’s a vast canyon system like the ones on the moons of Uranus, now thought to be the products of subsurace water.  Provisionally its sections were named after fictional spaceships including Serenity, Nostromo and TARDIS.  Serenity, four times the length and depth of the Grand Canyon, is fringed with ridges up to 4 miles tall, the source of landslides 200 metres thick.  Dark-floored craters are distinctive features, suggesing upwelling of material from below;  Charon may even have a full subsurrface ocean.  East of Serenity’s northern end, there’s a crater which is almost filled by a strangely large central peak, and further east an equally large feature was projecting up into sunlight from beyond the sunset line.  Adjacent craters is the north  (Skywalker and Organa)  have similar ejecta patterns, but one is dominated by water ice and the other by ammonia.  And so on… a catalogue of mysteries and features formed by activity on a moon that we once expected to be frozen solid and inert.

After the encounter, the spacecraft team started a ‘planethood for Pluto’ campaign to restore it to full status in the planetary community, so far without success.   Another entertaining development was the decision to visit Ultima Thule, now called Arrokoth, another Kuiper Belt object, only 19 miles across, which was the most promising target lying near New Horizons’ ongoing flight path.   Four rocket burns were required in October 2015 to put the probe on course for it, but under the strange rules governing NASA’s activities as a government agency, enshrined in the US Constitution and dating back to the 18th century, authoristion for the extended mission could not be granted till 2016.  The controllers at the Jet Propulsion Laboratory took a chance on it and the encounter was scheduled for January 2019 – more on that next time.

You can find all of Duncan Lunan’s excellent series for Beginner’s Astronomy and of the planets in our archives.