Sky & Telescope - January 2024 - 53

DRAWING: CHARLES CAPEN; NEPTUNE PHOTO: NASA / ESA /
STSCI / M. H. WONG / L. A. SROMOVSKY / P. M. FRY
longstanding mystery that astronomers
may have only solved very recently.
The ultimate source of the difference
seems to arise from a deeper mystery.
Uranus radiates hardly any heat at all,
but Neptune radiates 2.6× more energy
than it receives from the Sun. As a
result, atmospheric circulation on Uranus
is sluggish, while Neptune's internal
heat drives weather of unimaginable
violence. The Voyager 2 spacecraft
clocked winds of 1,900 kilometers per
hour (1,200 mph) on Neptune compared
to 900 km per hour on Uranus -
the fiercest in the entire solar system.
Methane strongly absorbs red light
and is responsible for the overall cool
colors of both ice giants. The uppermost
layers of their atmospheres also
contain a haze of photochemical smog
produced by the interaction of solar
ultraviolet radiation with methane and
other hydrocarbons. Last year a team of
researchers led by Patrick Irwin, Professor
of Planetary Physics at the University
of Oxford, postulated that on both
planets, methane can freeze onto these
aerosol particles, causing them to fall as
snow. Because Neptune's atmosphere is
so turbulent, it's far more efficient than
the comparatively quiescent atmosphere
of Uranus at transporting methane
upward into the haze layers in order to
produce these haze-thinning snowfalls.
The presence of thicker atmospheric
hazes on Uranus appears to be responsible
for its paler hue.
Uranus is usually almost featureless
in visible light, although bright
storms are glimpsed on rare occasions.
In 1870, British amateur T. H. Buffham
detected " two round bright spots "
using a 9-inch reflector and was able to
follow their motion on two nights. In
1884, French astronomers Henri Perrotin
and Charles Trépied and British
astronomer Norman Lockyear independently
reported an elongated bright
spot near the planet's equator.
The appearance of two brilliant
spots in 1981 that persisted for several
weeks enabled the eagle-eyed observer
Stephen James O'Meara to accurately
determine the planet's rotation period
using Harvard Observatory's 9-inch
Clark refractor, perhaps the last great
planetary discovery by a visual observer
(S&T: Sept. 2012, p. 54). It's noteworthy
that in the four decades following
that feat O'Meara has never managed
to make out any Uranian features of
comparable prominence even with
far more powerful instruments. Low
expectations are the key to happiness
when observing Uranus, but I always
hope to be surprised by one of these rare
events whenever I point a telescope in
its direction.
Neptune would be a rewarding target
if it were only as distant as Jupiter. In
contrast to the bland appearance of
Uranus, Neptune's dynamic atmosphere
has dusky belts resembling those of
Jupiter and Saturn as well as huge dark
storms rivaling Jupiter's Great Red Spot
that persist for weeks. A handful of
observers wielding large telescopes have
caught sight of hints of these features.
While observing through the
26-inch Clark refractor of the U.S.
Naval Observatory on several nights
of superb seeing in 1899, astronomer
Thomas Jefferson Jackson See described
Neptune as " marbled and spotted " with
" extremely faint " and " granular " bands.
In 1959, Brazilian astronomer Ronaldo
Rogério de Freitas Mourão detected a
dusky spot near the center of Neptune's
disc using an 18-inch refractor.
Are any features on Neptune visible
through more modest instruments?
There is one compelling piece of
evidence that they are. In 1950, Lowell
Observatory astronomer Charles
F. Capen observed Neptune using a
12½-inch reflector at a magnification of
432×. His sketch depicts faint atmospheric
banding and at least one prominent
dusky spot. I find it uncannily
similar to Voyager 2 and recent Hubble
Space Telescope images of the planet.
Uranus reached opposition on
November 13th, so this month it's well
placed for observation in the constellation
Aries. Almost four months
removed from its September 19th opposition,
Neptune is still visible early in
the evening in the constellation Pisces.
Why not take advantage of the opportunity
to train your telescope on these
distant worlds?
¢Contributing Editor TOM DOBBINS has
never seen features on Neptune, but he
glimpsed a diffuse bright spot on Uranus
in 1995 through Lick Observatory's 36inch
refractor.
Features depicted in Charles F.
Capen's 1950 sketch of Neptune
(left) resemble those captured by
the Hubble Space Telescope in
2022 (right). The Hubble image
records a large dark storm at top,
a smaller storm to its lower right,
and a dusky polar region.
sk yand tele scope .o r g * JANUARY 2024 53
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