Sky and Telescope - March 2016 - 21

NTrZ

February 2012

NTB
NEB

EZ
GRS

SEB
STrZ
STB

"Red Spot Jr."

First, the storm has gotten smaller not just in its longitudinal width but also in its north-south extent. In addition, it occasionally moves slightly northward or southward, aﬀecting which wind jet it deﬂects more. HST's
2014 measurements show the spot located near its normal
central latitude (22°° south), but both its north and south
edges contracted centerward as the entire system shrunk.
The westward jet still experiences signiﬁcant deﬂection around the GRS's northern edge, though perhaps
slightly less than previously. However, the eastward jet is
now barely deﬂected southward by the GRS. This means
that any entrained eddies in this region are not interacting closely enough to be caught in the spot's ﬂow.
We now strongly suspect that these jet interactions are
aﬀecting not only the color but also possibly the size of
the Great Red Spot. In time, our computer modeling of
the GRS's interaction with the nearby wind jets should
help us understand the energy exchange between them.

The Future
So what does the future hold for the most famous storm
in the solar system? That's diﬃcult to predict, as sudden
outbreaks could always alter the energy balance and
wind ﬂow near and within the GRS and change our view
completely. Atmospheric specialists describe the shape
of Jupiter's anticyclonic storms (high-pressure systems
like the GRS) by the ratio of their length to width, and
it's not unusual for this key aspect ratio to decrease in
such storms, particularly right after they form.
For example, the white ovals just south of the GRS
ﬁrst appeared in the late 1930s not as discrete spots but
rather as high-altitude clouds and haze that suddenly
inundated their latitude band, probably related to a large
convective event. The brightened zone then coalesced
into three distinct, oblong white storms, which observers designated BC, DE, and FA. By the Voyager ﬂybys in
1979, each had an aspect ratio of about 2. They continued

ALTERED BY
UPHEAVAL
Compare how Jupiter looked in 2010
(left) - with its dark
South Equatorial Belt
"missing" and its
GRS intensely colored
- with the view two
years later, after the
high-altitude clouds
obscuring the SEB had
dissipated and the GRS
had become paler.

DAMIAN PEACH

September 2010

to shrink until merging into a single oval in 2000, with
the new, combined oval BA having an aspect ratio of
1.25. In 2005, it turned red - earning the moniker "Red
Spot Jr." - and by 2015 its aspect ratio was 1.3. Meanwhile, the GRS's aspect ratio has changed from about 3
during the Voyager ﬂybys in 1979 to the 1.4 it has now.
Computer models suggest that Jupiter's anticyclonic
vortices should exhibit preferred aspect ratios, but those
depend somewhat on interactions with adjacent wind
jets, especially for a large storm that can deﬂect the jets.
So the GRS might eventually reach a shape that is "just
right," allowing it to stabilize.
Studies of its internal dynamics will help us to model
its fate, so we continue to make yearly Hubble observations. And amateurs' observations are critical for
monitoring the GRS. Their regular reports help us to ﬁll
in the gaps between studies with HST and professional
telescopes. They can also alert us to new happenings
that require rapid follow-up.
All things considered, we expect that the Great Red
Spot will continue to shrink for some time to come. We
don't expect it to disappear completely - but for now
what happens in the years ahead is anyone's guess. ✦
Amy Simon, an atmospheric dynamicist at NASA's Goddard Space Flight Center, leads the observing team for HST's
Outer Planet Atmospheres Legacy monitoring program.

G E T T O KN OW J U PITE R
For more information about the study of Jupiter's cloud
features and the Great Red Spot in particular, see The
Giant Planet Jupiter (Cambridge University Press, 1995),
authored by John Rogers of the British Astronomical Association. Also see page 48 for a general guide to observing
Jupiter and page 50 for a list of times when the GRS is
positioned best for telecopic viewing.

Sk yandTelescope.com March 2016

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Sky and Telescope - March 2016

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