This extract from a global map of Jupiter was made from observations performed with NASA’s Hubble Space Telescope.
Credit: NASA's Goddard Space Flight Center
Scientists have noticed that Jupiter's Great Red
Spot has been getting smaller over time. Now, there's evidence the storm
is actually growing taller as it shrinks. Credits: NASA's Goddard Space Flight Center. This video is public domain and can be downloaded from NASA's Scientific Visualization Studio.
Though once big enough to swallow three Earths with room to spare,
Jupiter’s Great Red Spot has been shrinking for a century and a half.
Nobody is sure how long the storm will continue to contract or whether
it will disappear altogether.
A new study suggests that it hasn’t all been downhill, though. The
storm seems to have increased in area at least once along the way, and
it’s growing taller as it gets smaller.
“Storms are dynamic, and that’s what we see with the Great Red Spot.
It’s constantly changing in size and shape, and its winds shift, as
well,” said Amy Simon, an expert in planetary atmospheres at NASA’s
Goddard Space Flight Center in Greenbelt, Maryland, and lead author of
the new paper, published in the Astronomical Journal.
Observations of Jupiter date back centuries, but the first confirmed
sighting of the Great Red Spot was in 1831. (Researchers aren’t certain
whether earlier observers who saw a red spot on Jupiter were looking at
the same storm.)
Keen observers have long been able to measure the size and drift of
the Great Red Spot by fitting their telescopes with an eyepiece scored
with crosshairs. A continuous record of at least one observation of this
kind per year dates back to 1878.
Simon and her colleagues drew on this rich archive of historical
observations and combined them with data from NASA spacecraft, starting
with the two Voyager missions in 1979. In particular, the group relied
on a series of annual observations of Jupiter that team members have
been conducting with NASA’s Hubble Space Telescope as part of the Outer
Planets Atmospheres Legacy, or OPAL, project. The OPAL team scientists
are based at Goddard, the University of California at Berkeley, and
NASA’s Jet Propulsion Laboratory in Pasadena, California.
The team traced the evolution of the Great Red Spot, analyzing its
size, shape, color and drift rate. They also looked at the storm’s
internal wind speeds, when that information was available from
spacecraft.
The new findings indicate that the Great Red Spot recently started to
drift westward faster than before. The storm always stays at the same
latitude, held there by jet streams to the north and south, but it
circles the globe in the opposite direction relative to the planet’s
eastward rotation. Historically, it’s been assumed that this drift is
more or less constant, but in recent observations, the team found the
spot is zooming along much faster.
The study confirms that the storm has been decreasing in length
overall since 1878 and is big enough to accommodate just over one Earth
at this point. But the historical record indicates the area of the spot
grew temporarily in the 1920s.
“There is evidence in the archived observations that the Great Red
Spot has grown and shrunk over time,” said co-author Reta Beebe, an
emeritus professor at New Mexico State University in Las Cruces.
“However, the storm is quite small now, and it’s been a long time since
it last grew.”
Because the storm has been contracting, the researchers expected to
find the already-powerful internal winds becoming even stronger, like an
ice skater who spins faster as she pulls in her arms.
Instead of spinning faster, the storm appears to be forced to stretch
up. It’s almost like clay being shaped on a potter’s wheel. As the
wheel spins, an artist can transform a short, round lump into a tall,
thin vase by pushing inward with his hands. The smaller he makes the
base, the taller the vessel will grow.
In the case of the Great Red Spot, the change in height is small
relative to the area that the storm covers, but it’s still noticeable.
The Great Red Spot’s color has been deepening, too, becoming
intensely orange since 2014. Researchers aren’t sure why that’s
happening, but it’s possible that the chemicals which color the storm
are being carried higher into the atmosphere as the spot stretches up.
At higher altitudes, the chemicals would be subjected to more UV
radiation and would take on a deeper color.
In some ways, the mystery of the Great Red Spot only seems to deepen
as the iconic storm contracts. Researchers don’t know whether the spot
will shrink a bit more and then stabilize, or break apart completely.
“If the trends we see in the Great Red Spot continue, the next five
to 10 years could be very interesting from a dynamical point of view,”
said Goddard co-author Rick Cosentino. “We could see rapid changes in
the storm’s physical appearance and behavior, and maybe the red spot
will end up being not so great after all.”
By Elizabeth Zubritsky
NASA’s Goddard Space Flight Center in Greenbelt, Md.
Editor: Karl Hille
