Sky and Telescope - May 2018 - 53

N ASA / A M Y SIM ON (6)

gling to understand the steering forces
that could be causing it to move faster.
"The biggest factor is the surrounding winds," she explains, perhaps due
to very slight changes in the latitudes of
the adjacent zonal jets. But the effect is
hard to measure since the GRS deﬂects
those same winds. And the spot can be
buffeted by disturbances that churn
up the jets and other effects. "We have
hints," Simon says, "but nothing has
been conclusive."
The good news in all of this morphing is that the Red Spot has gotten
redder and darker over the past decade.
(Historically, the spot's color seems to
intensify when its westward drift accelerates.) The cause of the darkening isn't
clear, but Simon suspects it has to do
with conservation of angular momentum. As the spot shrinks, its internal
circulation should speed up - but it
hasn't. Instead, the swirling gas might
be forced to go to higher altitudes (and
there's evidence this is occurring), where
lower pressures and temperatures might
be driving "colorful" chemical reactions.
The exact chemical compound
responsible for the spot's ruddy hue is
still unknown. Some candidates, like
red phosphorous derived from phosphine (PH3), aren't wholly consistent
with the chemical makeup of the Jovian
atmosphere. In 2016 Robert Carlson
(Jet Propulsion Laboratory) and others suggested that the colorant results
from the reaction of ammonia (NH3)
and acetylene (C2H2). Recently a team
led by Lawrence Sromovsky (University
of Wisconsin at Madison) argued that
this same compound could explain the

hues of many other Jovian
cloud features.

Timing Is
Everything
Chemistry aside, the
Red Spot's now-deeper
July 1994
color makes this a great
time to try to spot it. But
don't just dash outside
with your scope - a little
preparation is called for.
Most critically, you'll
need to know when the
GRS is front and center on
Jupiter's rapidly spinning
July 2017
globe. Look at the table
on page 49 that lists when
the spot crosses the planet's central
meridian (the north-south line through
its poles). Don't forget to convert those
values from Universal Time to your
local time zone. Plan to look within an
hour of a listed time - otherwise the
spot will be close to the planet's limb
and difﬁcult to identify.
Ideally, chose a meridian-crossing
time that occurs when Jupiter is high
in your sky. You'll always see less detail
when the planet is close to the horizon,
both because of atmospheric turbulence
along your line of sight and also due to
the thick air's prism-like dispersion of
color in Jupiter's disk.
Next, use the Red Spot's color to your
advantage. A blue or green ﬁlter will
make it stand out more readily from the
surrounding clouds.
Finally, make sure your telescope is
big enough. S&T's Sean Walker suggests that you'll need at least 4 inches

t These violet-filtered Hubble
images accentuate the dramatic change in the GRS's size
and interior structure over a
23-year period.

(100 mm) of aperture
to spot the GRS reliably,
at least when observing from mid-northern
latitudes. David Arditti
(British Astronomical
Association) adds, "For
more experienced observers I have little doubt that
a 75-mm refractor would
be enough, under typical
conditions, with a magniﬁcation of 80×."
Former S&T editor Tony Flanders
notes that the task will be much easier
if the atmosphere is stable. "I ﬁnd the
variation in visibility from night to
night due to seeing swamps the variations depending on the GRS's appearance," he cautions. "On a half-decent
night, it should be prominent in a good
4-inch refractor or 6-inch reﬂector."
In fact, if you have two scopes, try
setting them up side by side at comparable magniﬁcations to see which one
most easily coaxes the GRS into view.
And if the spot itself isn't apparent,
look for the "Red Spot Hollow," a bright
indentation that the spot makes in the
broad, dark South Equatorial Belt to its
immediate north.
¢ KELLY BEATTY plans to be on the
lookout for the Great Red Spot all summer long.

Red
Spot
Hollow

April 2014

January 2015

February 2016

April 2017

p Natural-color Hubble Space Telescope images show how the Great Red Spot's color has darkened and deepened since 2014. The spot creates
pronounced turbulence in the westward flow of the South Equatorial Belt (lower right in each image).
s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 8

http://www.skyandtelescope.com

Sky and Telescope - May 2018

Table of Contents for the Digital Edition of Sky and Telescope - May 2018