Press Release

Brighter Neptune Suggests a Planetary Change of Seasons

By SpaceRef Editor
May 15, 2003
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Brighter Neptune Suggests a Planetary Change of Seasons

Springtime is blooming on Neptune! This might sound like an oxymoron
because Neptune is the farthest and coldest of the major planets. But
NASA Hubble Space Telescope observations are revealing an increase in
Neptune’s brightness in the southern hemisphere, which is considered
a harbinger of seasonal change, say astronomers.

Observations of Neptune made over six years by a group of scientists
from the University of Wisconsin-Madison and NASA’s Jet Propulsion
Laboratory (JPL) show a distinct increase in the amount and brightness
of the banded cloud features located mostly in the planet’s southern
hemisphere.

“Neptune’s cloud bands have been getting wider and brighter,” says
Lawrence A. Sromovsky, a senior scientist at University of Wisconsin-
Madison’s Space Science and Engineering Center and a leading authority
on Neptune’s atmosphere. “This change seems to be a response to seasonal
variations in sunlight, like the seasonal changes we see on Earth.”

The findings are reported in the current issue (May, 2003) of Icarus,
a leading planetary science journal.

Neptune, the eighth planet from the Sun, is known for its weird and
violent weather. It has massive storm systems and ferocious winds that
sometimes gust to 900 miles per hour, but the new Hubble observations
are the first to suggest that the planet undergoes a change of seasons.

Using Hubble, the Wisconsin team made three sets of observations of
Neptune. In 1996, 1998, and 2002, observations of a full rotation of
the planet were obtained. The images showed progressively brighter bands
of clouds encircling the planet’s southern hemisphere. The findings are
consistent with observations made by G.W. Lockwood at the Lowell
Observatory, which show that Neptune has been gradually getting brighter
since 1980.

Neptune’s near-infrared brightness is much more sensitive to high
altitude clouds than its visible brightness. The recent trend of
increasing cloud activity on Neptune has been qualitatively confirmed at
near-infrared wavelengths with Keck Telescope observations from July
2000 to June 2001 by H. Hammel and co-workers. Near-infrared
observations at NASA’s Infrared Telescope Facility on Mauna Kea, Hawaii
are planned for this summer to further characterize changes in the
high-altitude cloud structure.

“In the 2002 images, Neptune is clearly brighter than it was in 1996 and
1998,” Sromovsky says, “and is dramatically brighter at near-infrared
wavelengths. The greatly increased cloud activity in 2002 continues a
trend first noticed in 1998.”

Like the Earth, Neptune would have four seasons: “Each hemisphere would
have a warm summer and a cold winter, with spring and fall being
transitional seasons, which may or may not have specific dynamical
features,” the Wisconsin scientist explains.

Unlike the Earth, however, the seasons of Neptune last for decades, not
months. A single season on the planet, which takes almost 165 years to
orbit the Sun, can last more than 40 years. If what scientists are
observing is truly seasonal change, the planet will continue to brighten
for another 20 years.

Also like Earth, Neptune spins on an axis that is tilted at an angle
toward the Sun. The tilt of the Earth, at a 23.5-degree inclination, is
the phenomenon responsible for the change of seasons. As the Earth
orbits the Sun over the course of a year, the planet is exposed to
patterns of solar radiation that mark the seasons. Similarly, Neptune is
inclined at a 29-degree angle and the northern and southern hemispheres
alternate in their positions relative to the Sun.

What is remarkable, according to Sromovsky, is that Neptune exhibits any
evidence of seasonal change at all, given that the Sun, as viewed from
the planet, is 900 times dimmer than it is from Earth. The amount of
solar energy a hemisphere receives at a given time is what determines
the season.

“When the Sun deposits heat energy into an atmosphere, it forces a
response. We would expect heating in the hemisphere getting the most
sunlight. This in turn could force rising motions, condensation and
increased cloud cover,” Sromovsky notes.

Bolstering the idea that the Hubble images are revealing a real increase
in Neptune’s cloud cover consistent with seasonal change is the apparent
absence of change in the planet’s low latitudes near its equator.

“Neptune’s nearly constant brightness at low latitudes gives us
confidence that what we are seeing is indeed seasonal change as those
changes would be minimal near the equator and most evident at high
latitudes where the seasons tend to be more pronounced.”

Despite the new insights into Neptune, the planet remains an enigma,
says Sromovsky. While Neptune has an internal heat source that may also
contribute to the planet’s apparent seasonal variations and blustery
weather, when that is combined with the amount of solar radiation the
planet receives, the total is so small that it is hard to understand the
dynamic nature of Neptune’s atmosphere.

There seems, Sromovsky says, to be a “trivial amount of energy available
to run the machine that is Neptune’s atmosphere. It must be a
well-lubricated machine that can create a lot of weather with very
little friction.”

In addition to Sromovsky, authors of the Icarus paper include Patrick M.
Fry and Sanjay S. Limaye, both of University of Wisconsin-Madison’s
Space Science and Engineering Center; and Kevin H. Baines of NASA’s Jet
Propulsion Laboratory in Pasadena, Calif.

Electronic images and additional information are available at
http://hubblesite.org/news/2003/17

SpaceRef staff editor.