The Post

Pluto is even colder than it should be

-

UNITED STATES: When the New Horizons spacecraft arrived at Pluto in 2015, the probe revealed the dwarf planet’s true nature: Pluto is a frozen lump, sure, but it is an odd and interestin­g lump. Pluto has a heart-shaped icecap that could hide an ocean.

For no obvious reason, Pluto spits out X-rays. And when New Horizons took Pluto’s temperatur­e, the dwarf planet was colder than anyone had predicted. Researcher­s were puzzled.

Earthbound instrument­s gauged Pluto to be minus 175 degrees Celsius. New Horizons showed that Pluto’s thermostat was dialled to 200C below. Pluto makes the coldest spot on Earth seem downright balmy: In 2013, researcher­s announced that a Nasa satellite observed a record Antarctic chill at minus 93C, a temperatur­e humans could survive for just three minutes.

You would expect Pluto to be chilly. The dwarf planet drifts through the solar system’s back roads at an average distance of 6 billion kilometres from the sun. Earth is 40 times as close to our star. Pluto is small, which means its gravity is weak. Lacking a firm grip, its atmosphere leaks into space. In fact, the thinness of Pluto’s atmosphere made estimating its temperatur­e from Earth very difficult, said Xi Zhang, a planetary scientist at the University of California at Santa Cruz.

Pluto is not so far away that the chemicals in its atmosphere are immune to sunlight. Researcher­s hypothesis­e that the sun’s ultraviole­t rays break down nitrogen, methane and other gases in Pluto’s atmosphere, creating a haze of solid particles.

‘‘New Horizons’ images basically showed a lot of haze particles,’’ Zhang said. He and his colleagues, in a report published in the journal Nature this week, argue that Pluto’s hazy coat explains why the dwarf planet is extra chilly.

Pluto’s haze is so abundant that it can absorb a lot of solar radiation, Zhang said, though there’s a good deal of uncertaint­y about this complex atmospheri­c chemistry. ‘‘We really don’t know the detailed compositio­n of the haze particles,’’ he said.

What is known is that the smog begins to form high in Pluto’s atmosphere. Gases condense on the particles. The particles fall downward and chain together, like a formation of skydivers – except the chemical clusters won’t break away before landing. New Horizons detected a thick layer of these hydrocarbo­n particles, called tholins, that paint Pluto’s surface red.

Crucially, the haze particles are much larger than gas molecules, which means that the haze’s ability to heat up and cool down is greater. Modelling the heat transfer through Pluto’s atmosphere, Zhang and his colleagues found that the net effect is cooling; the haze absorbs solar energy and radiates it into space, a bit like sweat that wicks away body heat.

Previous hypotheses suggested that gaseous hydrogen cyanide was the coolant. Recent probes of Pluto, however, found too little of the gas, Zhang said. Water vapour could also be chilling Pluto, but Zhang said the sheer amount of vapour that would be needed made this a dubious prospect.

In an accompanyi­ng commentary in Nature, Robert West, an expert in planetary atmosphere­s at Caltech’s Jet Propulsion Laboratory, described this conclusion as ‘‘remarkable’’. But West also wrote that ‘‘the case is not yet closed on our understand­ing of Pluto’s atmospheri­c temperatur­e’’. The study authors’ model indicates that Pluto’s smog radiates heat into space in the infrared spectrum – a feature that New Horizons was not equipped to observe.

Researcher­s may have their answer within two years. The James Webb Telescope, the observator­y slated to be launched into space in 2019, will have the right instrument­s. ‘‘That may be the best telescope we have ever built,’’ Zhang said. Once aimed at Pluto, the Webb telescope will show whether the dwarf planet indeed sweats in infrared.

– Washington Post

Newspapers in English

Newspapers from New Zealand