Infrared
images of Uranus (1.6 and 2.2 microns) obtained on Aug. 6, 2014, with
adaptive optics on the 10-meter Keck II telescope. The white spot is an
extremely large storm that was brighter than any feature ever recorded
on the planet in the 2.2 micron band. The cloud rotating into view at
the lower-right limb grew into the large storm that was seen by amateur
astronomers at visible wavelengths. Credit: Imke de Pater (UC Berkeley) & W. M. Keck Observatory images.
Animation showing the movement of the bright spot as Uranus rotated over
a two hour period on Oct. 4, 2014. The images were taken at the Pic du
Midi telescope in the French Pyrénées. Credit: CLICK ON IMAGE TO SEE ANIMATED GIF
Courtesy of Marc Delcroix and F. Colas (S2P).
Optical
images of Uranus on Sept. 19 and Oct. 2, showing the dramatic
appearance of a bright storm on a planet that normally displays only a
diffuse bright polar region. Credit: Courtesy of Anthony Wesley, Murrumbateman, Australia.
MAUNA KEA, Hawaii —
The normally bland face of Uranus has become increasingly stormy, with enormous
cloud systems so bright that for the first time ever, amateur astronomers are
able to see details in the planet's hazy blue-green atmosphere.
"The weather on Uranus is incredibly active," said Imke de Pater,
professor and chair of astronomy at the University of California, Berkeley, and
leader of the team that first noticed the activity when observing the planet
with adaptive optics on the W. M. Keck Observatory in Hawaii.
"This type of activity would have been expected in 2007, when Uranus's
once every 42-year equinox occurred and the sun shined directly on the
equator,” noted co-investigator Heidi Hammel of the Association of Universities
for Research in Astronomy. “But we predicted that such activity would have died
down by now. Why we see these incredible storms now is beyond anybody's
guess."
In
all, de Pater, Hammel and their team detected eight large storms on Uranus’s
northern hemisphere when observing the planet with the Keck Observatory on
August 5 and 6. One was the brightest storm ever seen on Uranus at 2.2 microns,
a wavelength that senses clouds just below the tropopause, where the pressure
ranges from about 300 to 500 mbar, or half the pressure at Earth's surface. The
storm accounted for 30 percent of all light reflected by the rest of the planet
at this wavelength.
When amateur astronomers heard about the activity, they turned their telescopes
on the planet and were amazed to see a bright blotch on the surface of a
normally boring blue dot.
‘I got it!’
French amateur astronomer Marc Delcroix processed the amateur images and
confirmed the discovery of a bright spot on an image by French amateur Régis
De-Bénedictis, then in others taken by fellow amateurs in September and
October. He had his own chance on Oct. 3 and 4 to photograph it with the Pic du
Midi one-meter telescope, where on the second night, "I caught the feature
when it was transiting, and I thought, ‘Yes, I got it!’” said Delcroix.
“I was thrilled to see such activity on Uranus. Getting details on Mars,
Jupiter or Saturn is now routine, but seeing details on Uranus and Neptune are the
new frontiers for us amateurs and I did not want to miss that,” said Delcroix,
who works for an auto parts supplier in Toulouse and has been observing the
skies – Jupiter in particular - with his
backyard telescope since 2006 and, since 2012, occasionally with the Pic du
Midi telescope. “I was so happy to confirm myself these first amateur images on this
bright storm on Uranus, feeling I was living a very special moment for
planetary amateur astronomy.”Interestingly,
the extremely bright storm seen by the 10-meter Keck II telescope in the near
infrared is not the one seen by the amateurs, which is much deeper in the
atmosphere than the one that initially caused all the excitement. De Pater’s
colleague Larry Sromovsky, a planetary scientist at the University of Wisconsin, Madison, identified the amateur
spot as one of the few features on the Keck Observatory images from August 5
that was only seen at 1.6 microns, and not at 2.2 microns. The 1.6 micron light
is emitted from deeper in the atmosphere, which means that this feature is
below the uppermost cloud layer of methane-ice in Uranus’s atmosphere.
“The colors and morphology of this cloud complex suggests that the storm may be
tied to a vortex in the deeper atmosphere similar to two large cloud complexes
seen during the equinox,” Sromovsky said
Such vortices could be
anchored much deeper in the atmosphere and extend over large vertical
distances, as inferred from similar vortices on Jupiter, including its Great
Red Spot.
An expanded team of astronomers led by
Kunio M. Sayanagi, an Assistant Professor at Hampton University in Virginia,
leveraged the amateur observations to activate a “Target of Opportunity”
proposal on the Hubble Space Telescope, which imaged the entire planet on Oct.
14. Observing
at a variety of wavelengths, HST revealed multiple storm components extending
over a distance of more than 9,000 kilometers (5,760 miles) and clouds at a
variety of altitudes.
De
Pater, Sromovsky, Hammel and Pat Fry of the University of Wisconsin will report
the details of their observations on Nov. 12 at a meeting of the American
Astronomical Society’s Division of Planetary Sciences in Tucson, Ariz.
Ice giant
Uranus is an ice giant, about four times the diameter of Earth, with an
atmosphere of hydrogen and helium, with just a bit of methane to give it a blue
tint. Because it is so distant – 19 times farther from the sun than Earth –
astronomers were able to see little detail on its surface until adaptive optics
on both Keck Observatory telescopes revealed features much like those on
Jupiter.
De Pater and her colleagues have been following Uranus for more than a decade,
charting the weather on the planet, including bands of circulating clouds,
massive swirling storms and convective features at its north pole. Bright
clouds are probably caused by gases such as methane rising in the atmosphere
and condensing into highly reflective clouds of methane ice.
Because Uranus has no internal source of heat, its atmospheric activity was thought
to be driven solely by sunlight, which is now weak in the northern hemisphere.
Hence astronomers were surprised when these observations showed such intense
activity.
Observations taken with the
Keck II telescope by Christoph Baranec, an Assistant Professor at the
University of Hawaii on Manoa, revealed that the storm was still active, but
had a different morphology and possibly reduced intensity.
“If indeed these features are high-altitude clouds generated by flow
perturbations associated with a deeper vortex system, such drastic fluctuations
in intensity would indeed be possible,” Sromovsky added.
“These unexpected observations remind us keenly of how little we understand
about atmospheric dynamics in outer planet atmospheres,” the authors wrote in their
paper.
The W. M. Keck Observatory operates the
largest, most scientifically productive telescopes on Earth. The two, 10-meter
optical/infrared telescopes near the summit of Mauna Kea on the Island of
Hawaii feature a suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field spectographs and
world-leading laser guide star adaptive optics systems.
NIRC2
(the Near-Infrared Camera, second generation) works in combination with
the Keck II adaptive optics system to obtain very sharp images at
near-infrared wavelengths, achieving spatial resolutions comparable to
or better than those achieved by the Hubble Space Telescope at optical
wavelengths. NIRC2 is probably best known for helping to provide
definitive proof of a central massive black hole at the center of our
galaxy. Astronomers also use NIRC2 to map surface features of solar
system bodies, detect planets orbiting other stars, and study detailed
morphology of distant galaxies.
Keck Observatory is a private 501(c) 3
non-profit organization and a scientific partnership of the California
Institute of Technology, the University of California and NASA.
Imke de Pater
imke@berkeley.edu
Larry Sromovsky
larry.sromovsky@ssec.wisc.edu
Heidi Hammel
(203) 321-6929
hbhammel@aura-astronomy.org
Marc Delcroix
delcroix.marc@free.fr
MEDIA CONTACT:
Steve Jefferson
Communications Officer
W. M. Keck Observatory
808.881.3827
sjefferson@keck.hawaii.edu
Source: W. M. Keck Observatory
