Pluto’s cloud components verified

Clouds in Pluto’s atmosphere may be composed of tiny frozen spherules of nitrogen or carbon monoxide, rather than snowflake-like clumps of tiny particles as previous research had suggested, new analyses suggest.

Information about Pluto’s atmosphere is, like that atmosphere itself, exceedingly thin because no space probes have yet visited there. So most speculations about the dwarf planet’s atmosphere stem from analyses of light passing through that tenuous shroud on the rare occasions when Pluto passes in front of a distant star, says Pascal Rannou, a planetary scientist at the University of Reims in France.

The dwarf planet’s tenuous atmosphere contains suspended particles, not just gas. Previous research suggested that those aerosols are about 200 nanometers across — larger than expected and therefore probably raspberry-like clumps or snowflake-like aggregates of tiny particles that had frozen together when they collided aloft, he notes.

But a new model by Rannou and university colleague Georges Durry, reported online and in an upcoming Journal of Geophysical Research–Planets, hints that individual particles in the clouds may simply be single spherical droplets of frozen nitrogen and carbon monoxide.  

Data the team analyzed comes from observations made by several ground-based telescopes in August 2002. As Pluto eclipsed a star in the constellation Ophiuchus, clouds in the atmosphere blocked the light, says Rannou. But different wavelengths of light were blocked to different degrees, a trend that provides hints about the size and composition of the particles in those clouds. The team’s analyses suggest that individual particles are no more than 200 nanometers across, about one-twentieth the size of a fog droplet in Earth’s atmosphere. If the particles were much larger, Rannou and Durry contend, the light would have been extinguished almost equally at all wavelengths.

Because solar radiation is so feeble at Pluto’s great distance from the sun, haze formed in the atmosphere by light-driven chemical reactions probably isn’t a major component of the clouds. Support for this notion comes from recent observations by the Cassini probe of haze formation rates on Titan, a large moon of Saturn, says Rannou.

Instead of haze, the researchers note, the particles are likely frozen condensation composed of either nitrogen — the gas that makes up about 99.5 percent of the dwarf planet’s atmosphere — or carbon monoxide, a substance found there only in trace amounts.

“It’s welcome to see detailed models of haze and condensation in Pluto’s atmosphere,” says Leslie A. Young, a planetary scientist at Southwest Research Institute in Boulder, Colo. “This is a huge step forward,” she notes.

NASA’s New Horizons mission, which will swing by Pluto in 2015, is designed to make detailed observations of Pluto and its atmosphere. The best data about the dwarf planet’s haze and clouds will be gleaned as the probe passes behind Pluto and looks back through the atmosphere toward the sun, she notes.

Pluto’s clouds are likely made of material that had sublimed, or evaporated directly from a solid, from the dark and relatively warm areas on the icy surface, says Darrell Strobel, a physicist at Johns Hopkins University . When that gas drifted into cooler areas, it condensed and refroze into particles, some of which were small enough to stay aloft as icy clouds or fog.