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New Horizons: NASA's Pluto-Kuiper Belt Mission
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Science Overview
Everything About Pluto, Charon and the Kuiper Belt
 • The Basics
 • Pluto's Orbit
 • Discovery of Pluto
 • Discovery of Charon
  • Discovery of Nix, Hydra and 'P4"
 • What's in the Names?
 • Pluto & System: Family Album
 • What Is a Binary Planet?
 • Making Maps
 • Surface Composition
 • Pluto's Atmosphere
 • Colossal Cousin to a Comet?
 • The Inside Story
 • Is Triton Pluto's Twin?
 • The Kuiper Belt
 • Comparative Planetology
 • Is Pluto a Planet?
 • Phases & Seasons
Data Collection
Science Operations Center
Science FAQs
Glossary

 

   
Colossal Cousin to a Comet?

Colossal Cousin to a Comet?

Pluto's very small gravitational acceleration (approximately 1/16 of Earth's gravity) leads scientists to think that carbon monoxide, methane and molecular nitrogen are escaping from Pluto's atmosphere at a rate of approximately 100 to 1,000 pounds (about 50-500 kilograms or 2-20 x 1027 molecules) per second.

The behavior is similar to that of a comet, though Pluto is more than 100 times larger than a typical comet nucleus and materials blow away from comets at rates 100 to 1,000 times faster.


Solar Wind Interaction with Pluto

We do not know how Pluto interacts with its surroundings or the solar wind — the streams of charged particles that speed from the Sun into space — and New Horizons seeks to find that out. The two critical quantities that define the nature of Pluto's interaction with the solar wind are the atmospheric escape rate, and whether the planet has a magnetic field.

While Pluto likely doesn't have an active internal magnetic dynamo — the source of Earth's global magnetic field — some remnant magnetization could exist. Because the solar wind is so weak out at 30 AUえーゆー, even weak magnetization could produce an appreciable magnetosphere, or magnetic "bubble." If Pluto's atmospheric escape rate is significantly greater than 100 pounds (about 50 kilograms, or 2 x 1027 molecules) per second then Pluto will resemble a comet. If the escape flux is significantly less, scientists expect the solar wind to be at least partially deflected around Pluto's electrically-conducting ionosphere, similar to the situation at Mars or Venus.

Diagram of solar wind interaction with Pluto
In any case, any atmospheric gases that escape will be broken up into atoms and ionized (forming a cometary plasma) by the Sun's ultraviolet light. They will then be carried away from Pluto in a long tail - as pick-up ions - by the solar wind.

Thinking about possible variations in Pluto's behavior over time, strong temporal variations are expected in the nature of the interaction on time scales of days (due to changes in the solar wind) and over the 248 years of Pluto's orbit (due to changes in Pluto's atmosphere as it moves farther and closer to the Sun).


What Would New Horizons Measure?

In any case - mini-comet or mini-Venus - the Solar Wind Around Pluto (SWAP) instrument on New Horizons is designed to measure how Pluto perturbs the solar wind. The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument will detect the material that escapes from Pluto's atmosphere, becomes ionized and streams away from Pluto.


Charon and the Solar Wind

Charon likely lacks enough atmosphere or electrical conductivity to affect the solar wind that surrounds it. Like Earth's Moon, Charon probably just absorbs the solar wind that bombards it. Bombardment of surface ices by energetic particles can often change the ices' chemistry - sometimes making larger molecules or breaking up larger molecules embedded in the ice. Over a long period of time this process - called radiolysis - may change the surface properties of bodies in the outer solar system.
 
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