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  What are the most interesting things about Pluto? What are the most unique aspects of Pluto?

Some of our interest in Pluto is that of studying an planet of extremes.

  1. Most distant planet (on average).
  2. Smallest planet.
  3. Pluto-Charon binary system are the most nearly equal in mass.
  4. Very cold and thin atmosphere.
  5. Surface covered in exotic frosts, methane (CH4), nitrogen (N2), carbon monoxide (CO).
  6. Pluto's surface may well have some of the brightest and darkest suface areas in the solar system. Bright from frost and ice, dark from organic molecules.
  7. May have an "active" surface from winds and frost migration coupled with large change in its distance from the Sun.
  8. Some models predict that Pluto's atmosphere will disappear completely when it is furthest from the Sun. No other planet or satellite boasts this large of a seasonal change. Only comets beat Pluto in this respect.
  9. Its internal composition provides important clues as to planetary building blocks in the outer solar system. We've studied the inner solar system (mostly the Earth and the Moon) and to fully understand the formation of planets we'd like to know what is inside Pluto.

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What could possibly be gleaned from HST pictures? What could give students the most satisfaction from this observation? Is Pluto changing? If so what could we find from the changes?

A limited number of pictures of Pluto have already been taken with HST. These pictures show albedo patterns on the surface with very little computer processing.

The most exciting thing we will get from a set of Pluto pictures is a chance to discover changes in these albedo markings. These markings are the result of the distribution of frost overlying darker regions. If winds scour a region free of frost it would appear darker. If frosts collect on a region it will become brighter. Also, as Pluto moves away from the Sun, the solar energy input begins to diminish leaving the possibility of having some of the atmosphere freeze out on the surface. Not only is Pluto moving away from the sun, it is also moving toward winter in its southern hemisphere. Already, the 15 degrees of latitude closest to the south pole is in permanent night that will last for about 120 years. Without an atmosphere, the south pole could get very, very cold. However, the atmosphere can help transport energy from the lit pole to the unlit pole and would keep the pole from getting a lot colder. The observable effect of the atmospheric flow would be to systematically deposit frosts on the cold pole and deplete the frosts from the lit pole, thus making it darker. The rate at which the surface darkens will tell us about the global atmospheric circulation rates on Pluto.

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Apparantly the HST is going to be of great use to students, what makes Pluto the most worthy of this project?

Of the four projects, this one has the most potential for seeing something no one else has seen (changes on the surface). Will we see it for sure? No one knows. It could look the same as it did 3 years ago when we last looked. It could look totally different (which would be a very big surprise). It could also be very slightly different which would show up as subtle changes from the last pictures.

No matter what, the pictures will show some structure on the surface and this structure can be turned into a map. If we look at Pluto just once, we get only one side. If we look at Pluto twice (opposite hemispheres), we would get most of the surface. With two pictures, portions of the surface will be highly foreshortened on the limb and we won't be able to say much about those areas. Three pictures are the absolute minimum to get a global mosaic of pictures covering the entire surface. More is always better but even our professional efforts 3 years ago netted us just 4 pictures, 90 degrees apart from one another. As always with science, one must trade off different goals. 3 separate orbits on Pluto will yield the best data on Pluto but you can't look at anything else. If you cut back to two (or one) picture on Pluto, you won't get as good of a map. So, everyone needs to decide if a picture of another planet is as important as filling in a complete map of Pluto.

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What has been learned recently about Pluto?

Just about everything we know about Pluto has been learned since 1980. Prior to that time all we really knew was where to look to find it and how long its day is (rotation period). Astronomers have finally been able to figure out quite a few things more recently. Some of these are:

  1. Size of Pluto and Charon: we know these number to within about 20 kilometers or so.
  2. Surface composition of Pluto (methane, nitrogen, CO, probably dark organic compounds).
  3. Surface composition of Charon (water ice).
  4. Bulk densities of Pluto+Charon 2.0 g/cc (we don't quite know the individual densities yet, there's still some debates raging over this).
  5. Atmospheric composition and structure of Pluto. Mostly nitrogen with trace methane. Warm upper atmosphere (100 K) and gets cold (~40 K) very fast within 10 km of the surface.

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"Is it too cold to have volcanos? It's not too cold for Jupiter's moon, why not Pluto? Does it have any clouds or wind or any atmosphere at all? What kind of land is there? Is there going to be anything interesting on it at all? There probably isn't any life because it's so cold, but it'd be neat if we found something."

Pluto may be cold, but that doesn't mean there couldn't be things happening on the surface. Remember the Voyager spacecraft pictures of Neptune's moon, Triton? A couple of the pictures should very clearly an active geyser or plume rising from the surface. This isn't evidence for volcanos like on the earth or Jupiter's moon, Io. It's more like "Old Faithful" in Yellowstone National Park. Old Faithful is caused by subsurface geothermal heating causing a buildup of steam pressure until it releases and spews out of the ground. In the case of Triton, one theory says that there is nitrogen ice being heated underground and the sublimating gas builds up pressure high enough to break free and causing the plume we see.

Could the same thing happen on Pluto? Well, sure. They are similar in size, surface composition, distance from the Sun, just about everything is the same. Is it happening? We don't know. Could we see these plumes from HST? No, not directly. These plumes are very, very small and our images would be very coarse. However, if a plume went off and deposited black gunk on the surface over a very large area, we could see that that area had gotten darker.

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