Hey, Mars, you’ve got company. Looks like there’s a second “red planet”
in the Solar System — Pluto. Color images returned from NASA’s New Horizons spacecraft,
now just 10 days from its encounter with the dwarf planet, show a
distinctly ruddy surface with patchy markings that strongly resemble
Mars’ appearance in a small telescope.
On Mars, iron oxide or rust colors the planet’s soil, while Pluto’s coloration is likely caused by hydrocarbon molecules called tholins
that are formed when cosmic rays and solar ultraviolet light interact
with methane in Pluto’s atmosphere and on its surface. Airborne tholins
fall out of the atmosphere and coat the surface with a reddish gunk.
A particular color or wavelength of UV light called Lyman-alpha
is most effective at stimulating the chemical reactions that build
hydrocarbons at Pluto. Recent measurements with New Horizons’ Alice instrument reveal the diffuse glow of Lyman-alpha light all around the dwarf planet coming from all directions of space, not just the Sun.
Since one of the main sources of Lyman-alpha light besides the Sun
are regions of vigorous star formation in young galaxies, Pluto’s
cosmetic rouge may originate in events happening millions of light years
away.
Pluto’s reddish color has been known for decades, but New Horizons is
now allowing us to correlate the color of different places on the
surface with their geology and soon, with their compositions.
Tholins have been found on other bodies in the outer Solar System,
including Titan and Triton, the largest moons of Saturn and Neptune,
respectively, and made in laboratory experiments that simulate the
atmospheres of those bodies
As you study the photos, you’ll notice that Pluto’s
largest dark spot is redder than the most of the surface; you also can’
help but wonder what’s going on with those four evenly-spaced dark
streaks in the equatorial zone. When I first saw them, my reaction was
“no way!” They look so neatly lined up I assumed it was an image
artifact, but after seeing the rotating movie, maybe not. It’s more
likely that low resolution enhances the appearance of alignment.
But what are they? Located as they are on the Charon-facing side of
Pluto, they may be related to long-ago tidal stresses induced by each
body on the other as they slowly settled into their current
tidally-locked embrace or something as current as seasonal change.
Voyager 2 photographed cyrovolcanos at Triton
during its 1989 flyby of the Neptune system. Nitrogen geysers and
plumes of gas and ice as high as 5 miles (8 km) were seen erupting from
active volcanoes, leaving dark streaks on its icy surface.
Today, New Horizons lies just 7.4 million miles (11.9 million km)
from its target. Sharpness and detail visible will rapidly improve in
just a few days.
“Even at this resolution, Pluto looks like no other world in our Solar System.
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