552 Encyclopedia of the Solar System
lightcurve was seen in both immersion and emersion. It has
been suggested that the aerosols could be “photochemical
smog” similar to the aerosols discovered on Titan and Tri-
ton (and a distant cousin to the air pollution in the industrial
basins on Earth, such as Los Angeles).
In a second model, the sudden drop in the brightness of
starlight below 1215 km was caused by a change in the verti-
cal thermal structure of the atmosphere near the half-light
level. Such a gradient is not unexpected from theoretical
modeling (see earlier discussion) because atmospheric tem-
peratures are expected to be higher than surface tempera-
tures. Changes in atmospheric temperature cause a varia-
tion of refractivity with height in the atmosphere that could
be manifested as the accelerated diminution of starlight
seen in the occultation.
The haze layer and temperature gradient explanations
imply differences in the way that the color of starlight
changes during an occultation. Future occultations may
help decide between the two explanations if simultaneous
observations can be made at two or more well-separated
wavelengths. If the temperature gradient explanation is cor-
rect, Pluto’s surface radius is likely near 1206±11 km. If
the haze layer explanation is correct, Pluto’s surface radius
is more difficult to determine, but it is probably closer to
1180 km.
In either case, the occultation implies a radius that
is a few percent larger than the mutual event solution
(1151 km); in the case of the haze model, the radius cannot
be much less than 1180 km or else the haze would be so
thick as to completely obscure the surface. Clearly, there is
a discrepancy between the radii determined from the oc-
cultation and those derived from the mutual events, which
future research will have to resolve.
The subsequent well-observed occultation of a star by
Pluto occurred on 20 July 2002. Both large fixed telescopes
and small portable instruments observed the event. For-
tuitously, yet another event occurred on 21 August 2002,
which was successfully observed from large telescopes on
Mauna Kea. From these events, it was determined that the
“kink” or “knee” seen in the 1988 data is largely absent
from the 2002 data, implying that large changes in Pluto’s
atmospheric thermal structure, or its haze profile, or both,
occurred during the intervening interval.
Further analysis of the data reveals that the pressure in
Pluto’s atmosphere more than doubled between 1988 and
- This is likely due to pressure fluctuations associated
with seasonal change, and may even be related to insta-
bilities in the atmosphere prior to complete atmospheric
collapse. Further observations will be required to sort this
out.
Yet another occultation was observed on 12 June 2006.
This event showed that the lightcurve kink near the half-
light level remained less distinct than in 1988 and that the
turbulence level in Pluto’s lower atmosphere had increased.
Fortunately, Pluto is now moving through the dense star
fields of Sagittarius, and several more occultation events
are expected to be observed between 2007 and 2012.6.3 Atmospheric Escape
A particularly interesting feature of Pluto’s atmosphere is
the very rapid rate at which it escapes to space. Because of
Pluto’s low mass and consequently weak gravitational bind-
ing energy, combined with the 100 K gas temperature in
Pluto’s upper atmosphere, sufficiently energetic molecules
at the top of the atmosphere are able to escape the gravi-
tational pull entirely. This can result in a condition called
hydrodynamic escape, in which the high-altitude atmo-
sphere achieves an internal thermal energy greater than
the planetary gravitational potential energy acting on the
atmosphere.
The time-averaged rate of escape from Pluto’s atmo-
sphere is likely to be of order 1–5× 1027 molecules/second.
This corresponds to a total loss of up to several kilometers of
material from the surface over the age of the solar system.
Escape rate estimates also indicate that the present es-
cape rate may be so high that Pluto’s tenuous atmosphere
may be lost to the escape process (thus requiring replenish-
ment from sublimating surface ices) on timescales possibly
as short as a few hundred years. Relatively speaking, the
atmosphere of Pluto is escaping at a rate far greater than
any other planetary atmosphere in the solar system!
Another interesting feature of Pluto’s atmosphere is its
strong orbital variability. This is driven by the fact that the
strength of solar heating varies by a factor of almost 4 around
Pluto’s orbit, which in turn causes the vapor pressures of N 2 ,
CO, and CH 4 to vary by factors of hundreds to thousands.
Therefore, unlike any other planet, Pluto’s atmosphere is
thought to be essentially seasonal, with the perihelion pres-
sure being many many times the aphelion pressure. Indeed,
some models predict that between 2010 and 2020, just 2 or
3 decades after perihelion, Pluto’s atmosphere will largely
condense onto the surface, a condition called atmospheric
collapse.7. Charon
As previously described, Pluto’s largest satellite, Charon,
was discovered in 1978. Charon’s radius of≈604 km is about
half of Pluto’s, implying its mass is most likely between 10
and 14% of Pluto’s. By comparison, typical satellite:planet
mass ratios are 1000:1 or greater, and even the mass ratio
of the Moon to the Earth is only 81:1.
The Pluto–Charon mutual event observations resulted
in several key discoveries. These included the fact (1) that
Charon’s average visible surface albedo is 30–35%, much
lower than Pluto’s, and (2) that Charon’s visible surface color