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The James Webb Space Telescope (JWST) has made a groundbreaking discovery: a wispy
sky above the icy dwarf planet Pluto is playing a vital role in regulating its
atmosphere. This Pluto haze not only cools the atmospheric temperature but also
propels methane and other organic compounds outward, some of which are captured by
Pluto’s close companion, Charon. These findings shed new light on Pluto’s atmosphere
and its interaction with its moon.
Haze Discovery Validates Previous Theories
Planetary scientist Xi Zhang, from the University of California, Santa Cruz,
originally predicted the presence of this haze back in 2017 to elucidate the
leakiness of Pluto’s tenuous atmosphere. Measurements taken by NASA’s New Horizons
spacecraft during its 2015 flyby of Pluto and Charon, led planetary scientist Will
Grundy at the Lowell Observatory in Arizona to calculate that Pluto’s atmosphere loses
approximately 1.3 kilograms (2.9 pounds) of methane every second. Intriguingly,
around 2.5% of this escaping methane is intercepted by Charon, tinting its poles red
through organic chemical reactions. The phenomenon of an atmosphere leaking onto a
neighboring celestial body is unique in our solar system.
The Role of Haze in Atmospheric Escape
The mechanism behind this atmospheric dissipation was previously unknown. However,
Zhang theorized that a haze layer within Pluto’s atmosphere would absorb trace
amounts of extreme ultraviolet radiation from the distant sun reaching Pluto. This
absorbed energy would then provide the necessary impetus for molecules to overcome
gravity and escape into space.
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Dual Effect: Heating and Cooling
Besides facilitating the escape of atmospheric molecules through heating, Zhang also
proposed that the haze could simultaneously exert a cooling influence on Pluto’s
atmosphere. This cooling effect had been previously observed in Pluto’s mesosphere,
the atmospheric layer situated above the virtually nonexistent troposphere and the
denser stratosphere.
Pluto’s mesosphere extends from approximately 20 to 40 kilometers (12.4 to 24.9
miles) in altitude, reaching a peak temperature of -163 degrees Celsius (110
Kelvin/-262 degrees Fahrenheit) before gradually cooling at a rate of 0.2 degrees
Celsius per kilometer, down to a minimum of -203 degrees Celsius (70 Kelvin/-334
degrees Fahrenheit).
Until the advent of the JWST, confirming the existence of this haze had remained
elusive.
JWST Pinpoints the Source of Thermal Emission
Zhang’s hypothesis suggested that any atmospheric cooling induced by the haze layer
would manifest as thermal emission at mid-infrared wavelengths. Past observations,
dating back to the Infrared Space Observatory in 1997, the Spitzer Space Telescope in
2004, and the Herschel Space Observatory in 2012, had detected mid-infrared
emissions from the Pluto-Charon system. However, the telescopes lacked the necessary
resolution to differentiate between Pluto and Charon, making it impossible to pinpoint
the source of the emission.
The JWST, equipped with its expansive 6.5-meter (21.4 feet) primary mirror and
Mid-Infrared Instrument (MIRI), can distinguish between Pluto and Charon. Zhang,
collaborating with a team led by Tanguy Bertrand of the Observatoire de Paris, leveraged
the JWST to successfully detect thermal mid-infrared emission originating from the
long-anticipated haze.
Bertrand explained that haze refers to layers of solid aerosols that are suspended high
in an atmosphere, scattering light and reducing visibility, creating a diffuse and
semi-transparent layer.
Composition and Dynamics of Pluto’s Atmosphere
Pluto’s atmosphere consists predominantly of nitrogen, with smaller quantities of
carbon dioxide and hydrocarbons, including methane, benzene, diacetylene, and hydrogen
cyanide. This atmosphere is remarkably thin, exhibiting a surface pressure of merely
13 microbars, compared to Earth’s surface pressure of approximately 1 bar (one million
microbars). Due to Pluto’s weak gravity, the upper atmosphere extends considerably from
the surface, spanning several Pluto radii (Pluto’s radius is 1,188.3 kilometers, or 737
miles). Minimal energy is required for molecules to escape the atmosphere, and that
energy is supplied by the sun.
Bertrand noted that a sizable portion of the incoming solar extreme ultraviolet
radiation is absorbed by the upper atmosphere, triggering heating that fuels
atmospheric mass loss. Atmospheric gases like nitrogen and methane are responsible for
absorbing radiation at these wavelengths.
Unraveling the Cooling and Heating Paradox
The ability of the haze to alternately induce atmospheric heating and cooling presents
an intriguing paradox.
Bertrand suggests that the cooling or heating hinges on the properties of the haze,
including particle size, shape, and composition (i.e., icy with hydrocarbon ice, or
non-icy), which are not well characterized yet. Advanced microphysical models are
being leveraged to examine this more closely.
The capacity of the haze to modulate atmospheric temperature implies that it exerts
control over the energy equilibrium within Pluto’s atmosphere. This, in turn,
influences global temperatures, atmospheric circulation, and the dwarf planet’s
climate. This climatic system is governed by sublimation and freezing cycles involving
molecular nitrogen, methane, and carbon monoxide, originating primarily from the vast
glacier within Sputnik Planitia, the heart-shaped feature on Pluto’s surface.
Seasonal Variations and Broader Implications
Elaborating on the energy balance, Zhang detailed that previous temperature data from
New Horizons indicated that gas heating exceeds gas cooling, thus, there is a net
radiative heating of the atmosphere. To sustain energy balance under these
circumstances, the haze must facilitate necessary net radiative cooling. The prevalence
of a net cooling effect during other seasons remains uncertain, given the considerable
variability of Pluto’s seasons!
These dramatic seasonal variations stem from Pluto’s highly elliptical orbit, causing
it to range from closer to the sun than Neptune to almost twice as far away. Even at
these extreme reaches of the solar system, variations in distance greatly influence the
degree of heating experienced by Pluto.
Pluto’s hazy atmosphere bears resemblance to the hydrocarbon-rich haze observed
on Saturn’s moon, Titan. Both hazes arise from photochemical reactions between solar
extreme ultraviolet radiation and molecules like nitrogen and methane. Even early Earth
may have possessed a hydrocarbon haze in its atmosphere, similar to Pluto, though much
denser. Consequently, studying Pluto’s atmosphere dynamics could offer insights
into the origins of our own planet.
The new study was published in the journal Nature Astronomy on June 2.