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Puzzling Seasons and Signs of Wind Found on Pluto

July 2003
Seasonal change on Pluto is causing the planet to warm up even as it moves away from the Sun, according to two studies that also detected the first firm signs of weather on the tiny planet.

In a deeper analysis of data first announced in October, researchers now say Pluto's atmospheric pressure doubled since 1988. They say the average global temperature must have climbed, too, by about 2 degrees Fahrenheit (1 degree Celsius).

The diminutive planet's tenuous atmosphere is still utterly wispy, at best. Air pressure there is just a few millionths of what envelops a human. And astronomers expect it will eventually freeze out entirely as Pluto moves farther from the Sun during its long "year."

Perhaps most interesting, the new studies uncovered small spikes in temperature and density that are the first observational hints of strong winds astronomers expected would occur in Pluto's thin air.

The results will be published the July 10 issue of the journal Nature.

More heat from less sunlight

Pluto rounds the Sun every 248 Earth-years. Its orbit is highly elongated. From 1979 to 1999, it was inside the orbit of Neptune. Now Pluto is again the ninth planet from the Sun, moving farther outward every moment.

Logic might suggest the planet would cool as it receives less sunlight each day.

Bruno Sicardy, an Observatoire de Paris astronomer who led the one of the investigations, suggests how simple seasonal change -- owing to the fact that Pluto's axis is tilted, as is Earth's -- might be responsible for the surprise warming trend.

Following a winter in the southern hemisphere that lasted more than 120 years, Sicardy explained via e-mail, spring came in 1987.

"Since then the south polar cap has been warming up and sublimating, a little bit like the ice pack melting at spring around the polar region on Earth," Sicardy said. "This sublimation would then cause the increase of the atmospheric pressure, and thus explain the doubling that we observe."

Eccentricity: Pluto's distance from the Sun varies by 25 percent. In 1999, Pluto crossed the orbital path of Neptune to become, once again, the most distant planet in the solar system.

Sublimation occurs when ice turns directly to gas, skipping the liquid phase.

If this seasonal scenario is right, it can't go on forever. The north pole entered permanent darkness in 1987, Sicardy points out. It should be cooling down. Nitrogen, the primary component of the atmosphere, would freeze out there and be trapped.

Drastic effects that can't last

Pluto's odd tilt and trajectory are important factors in the change, if the change indeed is seasonally related. The planet's orbit is more elliptical than any other in the solar system, and its rotational axis is tipped 118 degrees (Earth's axis is tilted 23.5 degrees).

Both factors could contribute to drastic seasonal changes, according to the other research team, led by MIT astronomer James Elliot.

It is not certain, though, that seasons alone are behind the noted shifts in atmospheric pressure and temperature.

"Seasonal change is a possible but not proven explanation," Elliot told "Another possibility would be some longer term change, analogous to long-term climatic changes on Earth."

Elliot has also speculated that some sort of as-yet-unknown volcanic activity could contribute, but he said only a mission to Pluto would show if that's the case. Sicardy agreed it's possible, but does not think volcanism would be the sole culprit.

Strong winds and knowledge as thin as air

Meanwhile, the new studies reveal what appear to be the first signs of weather on Pluto, small fluctuations of air density and temperature. Sicardy's team figures the changes, seen as spikes in the data, are caused "either by strong winds between the lit and dark hemispheres of the planet, or by convection near the surface of Pluto."

Scientists have long suspected that pressure difference in the tenuous atmosphere, created by stark temperature differences from the day side to the night side, would fuel brisk breezes.

The researchers did not attempt to estimate the strength of Pluto's apparent winds.

Pluto gives up its secrets slower than any planet.

In fact, compared to what's known about the other eight planets, observations of Pluto are as thin as its air. Pluto is too far away to be studied normally or regularly. Even with the most powerful telescopes -- Hubble included -- the planet appears as no more than a few blurry pixels.

The most recent observations of Pluto's atmosphere -- in 1988 and 2002 -- were accomplished as it passed in front of a star. Tiny changes in the amount of starlight detected at Earth, as the star's light first passed through Plutonian air and then was blocked out entirely, revealed clues to the height, density and composition of the atmosphere.

More star occultations, as they are called, should help astronomers unravel Pluto's mysteries.

But truly detailed observations of the distant world will require a robotic mission, scientists agree. They will have to wait until at least 2015, when NASA's New Horizon's mission is slated to fly past Pluto.

New Horizons was approved just this year and is scheduled for launch in 2006. Despite NASA's unwillingness to commit to the mission, astronomers had long argued for it, citing the urgent need to get to Pluto before the atmosphere presumably freezes out.

The new studies do not provide evidence for if or when the freeze-out might occur, Elliot said.

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