Brighter Neptune Suggests a Planetary Change of Seasons

MADISON. A progressive increase in the brightness of the planet
Neptune suggests that, like Earth, the distant planet has seasons.

Observations of Neptune made during a six-year period with NASA’s
Hubble Space Telescope by a group of scientists from the University of
Wisconsin-Madison and NASA’s Jet Propulsion Laboratory (JPL) show that
the planet is exhibiting a significant increase in brightness. The
changes, observed mostly in the planet’s southern hemisphere, show a
distinct increase in the amount and brightness of the banded cloud
features that are a distinctive feature of the planet.

“Neptune’s cloud bands have been getting wider and brighter,” says
Lawrence A. Sromovsky, a senior scientist at UW-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, they obtained observations of a full
rotation of the planet. 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.

“In 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
spins on its axis and orbits the sun during 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 the sun as seen from the
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. In the hemisphere getting the most sunlight, we would expect
heating, which 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 UW-Madison’s Space Science and
Engineering Center, and Kevin H. Baines of NASA’s Jet Propulsion
Laboratory in Pasadena, Calif.

Terry Devitt, (608) 262-8282, trdevitt@facstaff.wisc.edu

CONTACT: Lawrence Sromovsky (608) 263-6785, lsromovsky@ssec.wisc.edu

NOTE TO EDITORS: High-resolution photos and a Quick Time movie are
available at

http://www.news.wisc.edu/newsphotos/neptune.html

More detailed information and pictures are available at
http://www.ssec.wisc.edu/media/Neptune2003.htm