White mountain peaks on Pluto not caused by snow
Images captured by NASA’s New Horizons spacecraft during its July 2015 Pluto flyby reveal the small planet to be the only place in the solar system other than Earth to have white-capped mountains. However, the processes by which the white peaks are created on the two worlds are very different.
Initially, scientists thought the white peaks on Pluto’s surface, located along its equator to the west of the iconic heart-shaped glacier known as Tombaugh Regio, in an area dubbed Cthulhu Regio, are whitened by methane snow.
While the peaks of the two-and-a-half-mile high mountaintops are composed of methane, the white methane frost is produced by condensation that takes place at higher, warmer altitudes in Pluto’s atmosphere.
The mountains are composed of water ice, which is rock hard on a world where surface temperatures can drop as low as minus 387 degrees Fahrenheit.
“Water ice on Pluto is so cold that it’s hard, just like rock on Earth. That’s why you can make mountains of water ice on Pluto,” explained Tanguy Bertrand of NASA’s Ames Research Center in California.
Bertrand, who took part in a study that used computer simulations to determine that Pluto’s white mountaintops are created differently than those on Earth, was fascinated by the fact that Pluto’s mountain peaks appear so similar to those on our planet.
The mountains themselves appear reddish brown due to particles that come from Pluto’s layered atmospheric haze.
“You have to imagine like a dark volcano on Earth covered by ashes, and on top, you still have these bright deposits, that really look like snow on Earth,” Bertrand said.
The different mechanisms that produce white-capped mountains on both worlds can be traced to variations in the structures of their atmospheres. In Earth’s atmosphere, temperatures decrease at higher altitudes, much like they do on the planet’s surface. As moist winds reach mountains, they travel upwards and cool down, forming clouds from which snow falls.
In Pluto’s atmosphere, temperatures get warmer at higher altitudes because they are heated by the Sun while surface temperature remains cold. The tall mountain peaks stretch into gaseous atmospheric methane, where their cold surfaces cause the methane to directly condense into frost.
“You have gaseous methane in the atmosphere, and this gaseous methane directly condenses to the surface, which is cold. You have no formation of clouds, and you have no formation of icy particles in the atmosphere. It’s not snowing on Pluto,” Bertrand emphasized.
Images returned by New Horizons show various mountains are different shades of red and brown. These variations may be caused by seasonal changes, which impact frost deposits on mountain peaks, Bertrand said. Mountains that have more of a reddish color could host frost that formed during previous seasons and subsequently protected their mountains’ surfaces from cosmic rays, which can give mountains a brown color.
No other location in the solar system is known to have white mountain peaks. Mountain frosts could potentially be present on Neptune’s moon Triton, forming much the same way they do on Pluto. However, Triton is a flatter world than Pluto and may not have enough mountains for this to occur.
A paper on these latest findings has been published in the journal Nature Communications.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.