Using data from NASA's New Horizons spacecraft and two
telescopes at Earth, an international
team of scientists has found that one of the solar system's
largest and newest storms — Jupiter's
Little Red Spot, or LRS — has some of the highest
wind speeds ever detected on any planet.
New Horizons researchers combined observations from
their Pluto-bound spacecraft, which
flew past Jupiter in February 2007; data from the Hubble Space
Telescope orbiting Earth; and the
European Southern Observatory's Very Large Telescope,
perched on an Atacama Desert mountain in
Chile. This is the first time that high-resolution,
close-up imaging of the Little Red Spot has been
combined with powerful Earth-orbital and ground-based
imagery made at ultraviolet through mid-
infrared wavelengths.
Jupiter's LRS is an anticyclone, a storm whose winds
circulate in the direction opposite to that
of a cyclone — counterclockwise, in this case. It is
nearly the size of Earth and as red as the similar but
larger and more-well-known Great Red Spot, or GRS. The
dramatic evolution of the LRS began with
the merger of three smaller white storms that had been
observed since the 1930s. Two of these
storms coalesced in 1998, and the combined pair merged with
a third major Jovian storm in 2000. In
late 2005 — for reasons still unknown — the
combined storm turned red.
The new observations confirm that wind speeds in the
LRS have increased substantially over
those in the precursor storms, which had been observed in
past decades by NASA's Voyager and
Galileo missions. Researchers measured the latest wind
speeds and directions using two image mosaics
from New Horizons' telescopic Long Range Reconnaissance
Imager, taken 30 minutes apart, in order to
track the motion of cloud features. New Horizons obtained
the images from a distance of
approximately 1.5 million miles from Jupiter at a
resolution of 8.9 miles per pixel. The LRS' maximum
wind speeds of about 384 mph far exceed the 156 mph
threshold that would make it a Category 5
storm on Earth.
"This storm is still developing, and some of the
changes remain mysterious," said Andrew Cheng
of Johns Hopkins' Applied
Physics Laboratory, who led the study team. "This
unique set of
observations is giving us hints about the storm's structure
and makeup; from this, we expect to learn
much more about how these large atmospheric disturbances
form on worlds across the solar system."
Jupiter's venerable Great Red Spot has decreased
steadily in size over the past several
decades. In addition, a rare "global upheaval" in Jupiter's
atmosphere began before New Horizons
visited last year. This upheaval involved the disappearance
of activity in the South Equatorial Belt
(which left the GRS as an isolated storm), the appearance
of a south tropical disturbance north of the
Little Red Spot and other spectacular cloud changes.
"This was a rare opportunity to combine observations
from a powerful suite of instruments, as
Jupiter will not be visited again by a spacecraft until
2016 at the earliest," said Cheng, whose team
publishes its work in the June Astronomical
Journal.
Scientists combined Long Range Reconnaissance Imager
maps of cloud motions with visible-color
images from Hubble and mid-infrared images from the Very
Large Telescope. The latter technique
allows scientists to "see" thermal structure and dynamics
beneath the visible cloud layers because
thermal infrared wavelengths (indicating heat) can pass
through the higher clouds.
"The new observations confirm that the thermal
structures, wind speeds and cloud features of
the LRS are very similar to those of the GRS," said Hal
Weaver, a member of the study team from
APL and the New Horizons project scientist. "Both the LRS
and the GRS extend into the
stratosphere, to far higher altitudes than for the smaller
storms on Jupiter."
The observations offer clues to the mystery of why the
GRS, and now also the LRS, may be so
red. The wind speeds and overall strength of the LRS
increased substantially in the seven years
between the Galileo and New Horizons observations, during
which the storm became red. "This
supports the idea that a common dynamical mechanism
explains the reddening of the two largest
anticyclonic systems on Jupiter, one possibility of which
is that storm winds dredge up material from
below," said Amy Simon-Miller of NASA's Goddard Space
Flight Center.
In their report, the scientists also wonder about the
future of Jupiter's two giant storms. The
LRS already rivals the steadily shrinking GRS in size and
wind speed. The new thermal and wind field
observations hint at an interaction between the south
tropical disturbance, the Little Red Spot and a
warm cyclonic region south of the LRS, forming a complex
that could dwarf the Great Red Spot.
"The Great Red Spot may not always be the largest and
strongest storm on Jupiter," said Glenn
Orton of NASA's Jet Propulsion Laboratory. "Continued
monitoring of Jupiter's constantly evolving
atmosphere will surely yield more surprises."
New Horizons is the first mission in NASA's New
Frontiers Program of medium-class spacecraft
exploration projects. Alan Stern leads the mission and
science team as principal investigator; APL
manages the mission for NASA's Science Mission
Directorate.