Page 2 December 7, 2017
Looking Up
Hydrocarbon Haze Keeps Dwarf Planet Colder than Expected
The result is an atmospheric temperature of
about 70 Kelvin (minus 203 degrees Celsius,
or minus 333 degrees Fahrenheit), instead of
the predicted 100 Kelvin (minus 173 Celsius,
or minus 280 degrees Fahrenheit).
According to Zhang, the excess infrared
radiation from haze particles in Pluto’s
atmosphere should be detectable by the
James Webb Space Telescope, allowing
confirmation of his team’s hypothesis after
the telescope’s planned launch in 2019.
Extensive layers of atmospheric haze can
be seen in images of Pluto taken by New
Horizons. The haze results from chemical
reactions in the upper atmosphere, where
ultraviolet radiation from the Sun ionizes
nitrogen and methane, which react to form
tiny hydrocarbon particles tens of nanometers
in diameter. As these tiny particles sink down
through the atmosphere, they stick together
to form aggregates that grow larger as they
descend, eventually settling onto the surface.
“We believe these hydrocarbon particles
are related to the reddish and brownish
stuff seen in images of Pluto’s surface,”
Zhang said.
The researchers are interested in studying
the effects of haze particles on the atmospheric
energy balance of other planetary bodies,
such as Neptune’s moon Triton and Saturn’s
moon Titan. Their findings may also be
relevant to investigations of exoplanets with
hazy atmospheres.
New Horizons passed within 7,800 miles of
Pluto, with this closest approach on July 14,
2015. New Horizons had a relative velocity
of 30,800 mph at its closest approach, and
came as close as 17,900 miles to Charon.
Starting 3.2 days before the closest approach,
long-range imaging included the mapping
of Pluto and Charon to 25 miles resolution.
Because of Pluto’s tilt, portions of the northern
hemisphere were in shadow at all times.
Meanwhile, instruments on board New
Horizons studied the atmosphere, both
by emissions of atmospheric molecules
(airglow), and by dimming of background
stars as they pass behind Pluto (occultation).
During and after closest approach, other
instruments sampled the planet’s high
atmosphere and its effects on the solar
wind. They also searched for dust. A
communications dish on Earth measured
the disappearance and reappearance of the
radio signal as the probe flew by behind
Pluto. The results resolved Pluto’s diameter
(by their timing) and atmospheric density
and composition (by their weakening and
strengthening pattern). •
Based on a Press Release from UC
Santa Cruz, Provided by Bob Eklund
New analysis of Pluto’s atmosphere explains
why New Horizons spacecraft measured
temperatures much colder than predicted.
The gas composition of a planet’s
atmosphere generally determines how much
heat gets trapped in the atmosphere. For the
dwarf planet Pluto, however, the predicted
temperature based on the composition of
its atmosphere was much higher than actual
measurements taken by NASA’s New Horizons
spacecraft in 2015.
A new study published November 16 in
Nature proposes a novel cooling mechanism
controlled by haze particles to account for
Pluto’s frigid atmosphere.
“It’s been a mystery since we first got the
temperature data from New Horizons,” said
first author Xi Zhang, assistant professor of
Earth and planetary sciences at UC Santa
Cruz. “Pluto is the first planetary body we
know of where the atmospheric energy budget
is dominated by solid-phase haze particles
instead of by gases.”
The cooling mechanism involves the
absorption of heat by the haze particles,
which then emit infrared radiation, cooling
the atmosphere by radiating energy into space.
Pluto’s haze layer is blue in this image taken by the New Horizons Ralph/Multispectral Visible Imaging Camera and computer generated to
replicate true color. Haze is produced by sunlight-initiated chemical reactions of nitrogen and methane, leading to small particles that grow
and settle toward the surface. (Image credit: NASA/JHUAPL/SwRI)
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