Sky and Telescope - November 2016 - 19

South pole
facing Sun

Meanwhile, prior to the spacecraft's arrival, the scientiﬁc consensus held that Pluto had too little gravity to
hold onto its nitrogen gas and thus must be irreversibly
losing it to space at a rapid rate, roughly 1027 molecules
per second. That's a few thousand tons per day, enough
for researchers to question where it's all coming from
and whether it's been leaving Pluto so rapidly throughout solar system history. Some had even speculated that
Pluto's atmosphere extended all the way to Charon, some
18,500 km away.

Reality Check
So imagine the team's surprise - shock, really - to learn
that Pluto's upper atmosphere is far colder than expected
(roughly 70K instead of the anticipated 100K), and consequently that it hugs the surface much more compactly
than anyone realized. Decades of ground-based observations, especially stellar occultations, had suggested that
the exobase (essentially the level at which atoms can ﬂy
away freely into space) was roughly 7,500 km up - seven
or eight times Pluto's radius. But instead, Young reports,
it's more like 2½ times. "From an Earth-based perspective," admits principal investigator Alan Stern (Southwest
Research Institute), "we got it all wrong."
Because it's so close to Pluto, the thin air is held in
place far more strongly by gravity than it would be if it
were puﬀed up higher. Consequently, the solar wind
can't strip it away easily. In fact, PEPSSI didn't detect any
interaction with the solar wind until the spacecraft got to
within about 7,000 km of Pluto. Instead, the escape rate of
nitrogen must be only 1023 molecules per second - a mere
1/10,000 (0.01%) of the pre-arrival prediction. What little gas
does leak away is mostly methane. "There's essentially no
nitrogen escaping from the upper atmosphere of Pluto,"
admits Michael Summers (George Mason University).
This key ﬁnding means that Pluto's atmosphere is
there to stay. "We expected [an] escape rate rapid enough
to lose the equivalent of a half-mile-thick layer of surface
ice over 4½ billion years," Young explains, "but now it's
more like a half foot."
As the New Horizons investigators detail in one of
ﬁve articles published in March 18th's Science, the reasons for the unexpectedly cold upper atmosphere aren't
clear. Perhaps some other compound is radiating away
heat to space. The simple organic molecule hydrogen
cyanide (HCN) synthesizes easily in the upper atmosphere and would be a plausible candidate. But observations with the ALMA radio-telescope array in Chile,
acquired by Emmanuel Lellouch (Paris Observatory) and
others weeks before the ﬂyby, suggest that there's not
nearly enough HCN present to do the job.
Soon after the ﬂyby, investigators announced that
they'd seen a number of discrete haze layers suspended
high above the surface. The idea of hazes wasn't new -
they'd been implicated for decades as a way to explain

N
S

Perihelion
(1989); N
equator
facing S
Sun

Sun

N
S

Aphelion
(1866);
equator
facing
Sun

Current S
location

N
S

North pole
facing Sun

NEAR AND FAR Pluto's highly eccentric orbit means that this
little world is nearly twice as distant from the Sun at aphelion as it
is at perihelion - apparently with major climatic consequences.
S&T: LEAH TISCIONE, SOURCE: NASA / JHU-APL / SWRI

quirks in ground-based occultation data. Besides, once
sunlight breaks down methane, the molecular fragments
readily recombine to form tiny aerosol particles of heavier
compounds like acetylene (C2H2), ethylene (C2H4), and
ethane (C2H6). The spacecraft detected all of these.
The puzzle was ﬁnding so many layers - a score
of them - each a few kilometers thick and situated at
roughly regular intervals that extend to 200 km above
the surface. The ones high up can't be stable, because
temperatures there are warm enough (by Plutonian standards) to vaporize those candidate organic aerosols.
An important clue comes from the haze layers'
quasi-regular spacing. As Randy Gladstone (Southwest
Research Institute) and colleagues explain in one of the
Science papers, gentle surface-level winds that transport heat from warmer to cooler regions can trigger the
formation of gravity waves while ﬂowing over Pluto's
rather substantial mountain ranges. The waves propagate upward, alternately compressing and rarefying haze
particles with just the right spacing. "It's almost like
the atmosphere of Pluto is 'ringing' in a radial direction," Summers explains. Some other process might be
involved, but gravity waves oﬀer the best explanation.

Hints of a Far Denser Atmosphere
The recording of New Horizons' radio signal as it ducked
behind Pluto and then reappeared revealed surface pressures of 11 and 10 microbars, respectively - only 0.001%

GR AVIT Y VS. GR AVITATIONAL WAVE
Gravitational waves are ripples in spacetime, created by accelerating masses. Gravity waves form when buoyancy pushes
a ﬂuid up and gravity pulls it back down (such as ﬂow over a
mountain) in an attempt to preserve equilibrium.
Sk yandTelescope.com November 2016

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Sky and Telescope - November 2016

Table of Contents for the Digital Edition of Sky and Telescope - November 2016