Images
of Neptune taken during twilight observing revealed an extremely large
bright storm system near Neptune’s equator (labeled ‘cloud complex’ in
the upper figure), a region where astronomers have never seen a bright
cloud. The center of the storm complex is ~9,000 km across, about 3/4
the size of Earth, or 1/3 of Neptune’s radius. The storm brightened
considerably between June 26 and July 2, as noted in the logarithmic
scale of the images taken on July 2. Credit: N.Molter/I. de Pater, UC Berkeley/C. Alvarez, W.M. Keck Observatory
Extremely large, bright storm system caught on camera at W. M. Keck Observatory
Maunakea, Hawaii – Striking images of a storm system nearly the size of Earth have astronomers
doing a double-take after pinpointing its location near Neptune’s equator, a
region where no bright cloud has ever been seen before.
“Seeing a storm this
bright at such a low latitude is extremely surprising,” said Ned Molter, a
graduate student at University of California, Berkeley, who spotted the storm
complex during a test run of twilight observing at W. M. Keck Observatory on
Maunakea, Hawaii. “Normally, this area is really quiet and we only see bright
clouds in the mid-latitude bands, so to have such an enormous cloud sitting
right at the equator is spectacular.”
This massive storm
system is about 9,000 kilometers in length, or 1/3 the size of Neptune’s
radius, spanning at least 30 degrees in both latitude and longitude. Molter
observed it getting much brighter between June 26 and July 2.
“Historically, very
bright clouds have occasionally been seen on Neptune, but usually at latitudes
closer to the poles, around 15 to 60 degrees north or south,” said Molter’s
advisor, Professor Imke de Pater of UC Berkeley’s Astronomy Department. “Never
before has a cloud been seen at, or so close to the equator, nor has one ever
been this bright.”
At first, de Pater (pictured above) thought it was the same Northern Cloud Complex seen by the Hubble Space
Telescope in 1994, after the iconic Great Dark Spot, imaged by Voyager 2 in 1989,
had disappeared. But de Pater says measurements of its locale do not match,
signaling that this cloud complex is different from the one Hubble first saw
more than two decades ago.
A huge, high-pressure, dark vortex system anchored
deep in Neptune’s atmosphere may be what’s causing the colossal cloud cover. As
gases rise up in a vortex, they cool down. When its temperature drops below the
condensation temperature of a condensable gas, that gas condenses out and forms
clouds, just like water on Earth. On Neptune we expect methane clouds to form.
As with every planet, winds in Neptune’s atmosphere vary
drastically with latitude, so if there is a big bright cloud system that spans
many latitudes, something must hold it together, such as a dark vortex.
Otherwise, the clouds would shear apart.
“This big vortex is sitting in a region where the
air, overall, is subsiding rather than rising,” said de Pater. “Moreover, a
long-lasting vortex right at the equator would be hard to explain physically.”
If it is not tied to a vortex, the system may be a
huge convective cloud, similar to those seen occasionally on other planets like
the huge storm on Saturn that was detected in 2010. Although
one would also then expect the storm to have smeared
out considerably over a week’s time.
“This shows that there are extremely drastic changes
in the dynamics of Neptune’s atmosphere, and perhaps this is a seasonal weather
event that may happen every few decades or so,” said de Pater.
Neptune is the windiest planet in our solar system,
with the fastest observed wind speeds at the equator reaching up to a violent 1000
miles per hour. To put this into perspective, a Category 5 hurricane has wind
speeds of 156 miles per hour. Neptune orbits the Sun every 160 years, and one
season is about 40 years.
Keck visiting scholars program
The discovery of Neptune’s mysterious equatorial
cloud complex was made possible by the Keck Visiting Scholars Program, a new
program launched this summer that gives graduate students and post-doctoral researchers
experience working at the telescope, while contributing to Keck Observatory and
its scientific community.
“This result by Imke and her first year graduate
student, Ned, is a perfect example of what we’re trying to accomplish with the
Keck Visiting Scholars Program,” said Anne Kinney, chief scientist at Keck Observatory.
“Ned is our first visiting scholar, and his incredible work is a testament to
the value of this program. It’s just been an outrageous success.”
Molter (pictured to the left with Kinney) is one of eight scholars accepted into the
program this year. His assignment during his six-week stay at the Observatory
was to develop a more efficient method for twilight observing, making use of
time that otherwise might not be used. Most observers in the Keck Observatory
community peer deep into the night sky and cannot observe their targets during
the ‘Twilight Zone.’
“Ned had never observed before, and he’s very bright,
so when Anne told me about the program, I knew he would be the perfect student
for it,” said de Pater. “Now that we’ve discovered this interesting cloud
complex in Neptune, Ned has a running start on a nice paper for his PhD
thesis.”
“I loved being at Keck. Everyone was extremely
friendly and I had a ton of personal interaction with the support astronomers
and observing assistants,” said Molter. Being able to go behind the scenes to see
how they run the telescopes and instruments every day, getting 10 nights of
observing and engineering time on the telescopes, and going up to the summit
twice to see the incredible engineering behind this gigantic machine has turned
me from a student to an actual observer. It was an incredible opportunity.”
The Keck Visiting Scholars Program is generously
sponsored by Roy and Frances Simperman, with major contributions from the M. R.
and Evelyn Hudson Foundation, William M. Keck, Jr. Foundation, Edge of Space, Inc.,
Thomas McIntyre, and Jeff and Rebecca Steele.
Next Steps
Molter and De Pater will continue to analyze their
data and propose for more twilight observing time at Keck Observatory this fall
so they can learn more about the nature of this storm and get an idea of what it
will be doing over time.
Having a better understanding of Neptune’s atmosphere
will help give astronomers a clearer picture of this icy giant’s global
circulation. This has become increasingly more important in the exoplanet
realm, as a majority of exoplanets found so far are nearly the size of Neptune.
While scientists can calculate their size and mass, not much is currently known
about exoplanets’ atmosphere.
About W. M. Keck Observatory
The W. M. Keck Observatory
operates the most scientifically productive telescopes on Earth. The two,
10-meter optical/infrared telescopes on the summit of Maunakea on the Island of
Hawaii feature a suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field spectrometers, and
world-leading laser guide star adaptive optics systems. The 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.
Source: W.M. Keck Observatory
