Triton and Pluto

We will look at the fact sheets for Triton, the largest satellite of Neptune, and for Pluto.  Following its discovery in 1930, Pluto was originally classified as the ninth planet.  However, as we have learned more about Pluto and similar objects, this original classification appears to be a mistake.  It makes sense to discuss the satellite Triton and the (non-planet) Pluto together because, as we will see, they are very similar objects.  Pluto is the only (former) planet discovered from the USA (from Arizona, actually!).  See http://en.wikipedia.org/wiki/Percival_Lowell

Both of these objects are ice-rich and have cold, tenuous, nitrogen-rich atmospheres.  They are very similar in size and composition, and some scientists think that Triton may have been captured by Neptune from a cloud of Pluto-like objects.  This is one of the reasons that Pluto has been demoted from planethood.  It is now termed a dwarf planet.

The strange retrograde orbit of Triton (click here for a movie of Triton's orbit; requires QuickTime plugin) suggests that it may have been captured late in the history of Neptune.

The atmospheres of both Triton and Pluto are very cold, cold enough for methane to freeze solid and for even nitrogen to condense:

Triton has been visited only once, in 1989 by the Voyager 2 spacecraft. Pluto has never been visited, but the New Horizons spacecraft is on the way and will fly by Pluto in July 2015.  Nevertheless, we know a great deal about these objects from Earthbased observations.  Triton has been observed several times during stellar occultations, and Pluto has also occulted stars.  Pluto's satellite Charon underwent a series of eclipses with Pluto around 1988, which gave us detailed information about both bodies.

Recall:
A typical radio occultation of a spacecraft looks like this:

This works just like seismology.  The radio link between the spacecraft and Earth bends in the planet's atmosphere, because radio signals travel more slowly in an atmosphere than they do in empty space, and they bend in accordance with Fermat's principle.

We don't even need a spacecraft to use this technique.  We can use light beams from stars to make similar measurements:

Here is some information about a Triton occultation observed by a UA team in 1997:

Six hours later, the occultation star Tr176 had moved to the other side of Neptune and was almost directly behind Triton.  Here is a movie of the dimming of the star as it went behind Triton:

Tr176 movie

Here is a diagram showing the difference between a primary and a secondary atmosphere:

Triton geyser movie on YouTube

artist's conception of Triton geysers

Small bodies

Recall the discussion of Lagrangian points early in the semester.  Here are some real examples of these types of orbits in the Saturn system.  The satellites involved are Dione plus Helene (the lagrangian satellite with Dione), and Tethys plus Telesto and Calypso (lagrangian satellites with Tethys; movie only shows Tethys and Calypso).  By now, satellites have been discovered at both the L4 and L5 points of Dione and Tethys.

Co-orbiting bodies at either the L4 or L5 point are generically called trojans.

We also have horseshoe orbits, as shown by Janus and Epimetheus, which exchange orbits around Saturn:

Both Pluto and Triton have nitrogen-rich atmospheres, similar in composition to Earth's, but much more tenuous (surface pressure only a few microbars).  Their atmospheres are secondary, like Earth's.  In the case of Triton, we can see the escape of the secondary atmosphere.

Determination of the mass of Pluto and Charon

Pluto is actually a double planet like Earth.  It has a massive satellite named Charon.  In addition, it has four more (much smaller) satellites:

How does this affect the argument about whether Pluto is a planet?

Although Pluto hasn't yet been visited by a spacecraft, we already know both its mass and Charon's mass by using the general form of Kepler's third law:

P² = 4(pi)²a³ / G(M_Pluto+M_Charon)

Knowing P and a, we can determine the sum of the masses.  The separate masses are determined from the motions with respect to the system barycenter (center of mass):

Here is a link to a movie showing motion about a barycenter (we'll discuss this further when we talk about extrasolar planets).