3 billion miles (5 billion kilometers) from Earth.
The first color photos of Pluto’s atmosphere
trickled back from NASA’s New
Horizons spacecraft in October, giving
astronomers fresh evidence for how the
dwarf planet’s thin veil works.
“Who would have expected a blue sky in
the Kuiper Belt? It’s gorgeous,” says New
Horizons Principal Investigator Alan Stern of
the Southwest Research Institute (SwRI) in
Boulder, Colorado.
On Earth, our blue skies are caused by
light scattering off nitrogen and oxygen molecules
in the atmosphere. But on Pluto, scientists
suspect the Sun’s faint light scatters off
soot-like particles known as tholins, which
form as ultraviolet light breaks down and ionizes
molecules like methane and nitrogen.
The actual particulates are likely gray or red,
but the scattering makes them appear blue.
As these tholins fall to the surface, they grow
by interacting with volatile ices and ionized
molecules, eventually becoming red.
New Horizons data have already shown
Pluto has an unexpectedly low surface pressure
of just 1/100,000 that of Earth — about
half of the expected value. That indicates
much of its atmosphere already has collapsed
as Pluto moves out in its elliptical orbit.
In October, the spacecraft also found the
chemical fingerprints of water ice on the surface.
Water is abundant on Pluto, but its shell
is largely covered by nitrogen and methane.
“Understanding why water appears
exactly where it does, and not in other places,
is a challenge that we are digging into,” says
New Horizons scientist Jason Cook of SwRI.
Curiously, the regions richest in water ice
are also red. “We don’t yet understand the
relationship between water ice and the reddish
tholin colorants on Pluto’s surface,”
says Silvia Protopapa, of the University of
Maryland, College Park.
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