In what way does Pluto differ from the other eight planets?
Inclination
The planets of our solarsystem are revolving around the sun in or near a plane, the ecliptic, which is the plane of the Earth's orbit.
Pluto's orbit compared to the ecliptic is extremly inclined in comparison to the other planets. In other words, Pluto's orbit is tilted.
Pluto's orbital inclination is approximately 17.148°.
The inclination of the other planets are:
| Name of planet | Inclination
|
| Mercury | 7.004°
|
|---|
| Venus | 3.394°
|
|---|
| Earth | 0.000°
|
|---|
| Mars | 1.850°
|
|---|
| Jupiter | 1.308°
|
|---|
| Saturn | 2.488°
|
|---|
| Uranus | 0.774°
|
|---|
| Neptune | 1.774°
|
|---|
| Pluto | 17.148°
|
|---|
Mercury also has an inclined orbit, but that is caused by the large influence of the suns gravity. Because of this influence Mercury doesn't orbit in the ecliptic but in the orbital plane of the sun, which is tilted 7 degrees relative to the ecliptic.
Schematic view of the planets orbits. (side view)
Obtained from http://www.seds.org/nineplanets/nineplanets/overview.html
Eccentricity
An other difference of Pluto's orbit is it's large eccentricity. The orbits of the other planets are allmost circles. In other words, their eccentricity is near zero.
Again Mercury is an exception, and again this is caused by the solar gravity.
Following table shows the eccentricities of the planets.
| Name of planet | Eccentricity
|
| Mercury | 0.2056
|
|---|
| Venus | 0.0068
|
|---|
| Earth | 0.0167
|
|---|
| Mars | 0.0934
|
|---|
| Jupiter | 0.0483
|
|---|
| Saturn | 0.0560
|
|---|
| Uranus | 0.0461
|
|---|
| Neptune | 0.0097
|
|---|
| Pluto | 0.2482
|
|---|
Schematic view of the planets orbit. (top view)
Showing Pluto's eccentricity
Obtained from http://dosxx.colorado.edu/Pluto/pluto3.html
Charon
On June 22, 1978 Pluto's satellite, Charon, was discovered.
James Christy of the US Naval Observatory accidentally discovered Charon when he was working on improving Pluto's orbit parameters.
To do this he had to study photographic plates of Pluto carefully. His close analization of the pictures turned up a new observation.
He noticed that the enlarged pictures of Pluto had an irregular 'blob' attached to its side.
At first it was thought that the pictures were defective or that the telescope had moved while taking the picture.
When Christy looked back at older photographs, though, he found that the bump on Pluto was at different positions.
A series of photographs taken in 1970 showed that the bump progressed around Pluto in about six days.
From the data gathered, Christy reasoned that Pluto must have a satellite.
The satellite was named Charon, the name of the ferryboat pilot in Greek mythology who takes the souls of the dead across the river Styx and into Pluto's realm.
Christy proposed this name, as it fit with Pluto.
The fact that Pluto has a satellite isn't strange, but Charon is very large compared to other satellites.
Plutos radius is 1160 km and Charons radius is 635 km. So the ratio of the radii is 635/1160 = 0,55. Compared to the other planets that is large.
In the next table the rates of the planets and their largest satellite are given.
| Name of planet | Name of satellite | Ratio (radius sat. / radius planet)
|
| Mercury | - | -
|
|---|
| Venus | - | -
|
|---|
| Earth | Moon | 0.27
|
|---|
| Mars | Phobos | 0.0040
|
|---|
| Jupiter | Ganymede | 0.037
|
|---|
| Saturn | Titan | 0.043
|
|---|
| Uranus | Titania | 0.031
|
|---|
| Neptune | Triton | 0.055
|
|---|
| Pluto | Charon | 0.55
|
|---|
Because of the very large satellite, for a long time science believed that the Pluto-Charon system was a dubble-planet. Nowadays they assume that Charon is a satellite, because models indicate that Charon was formed by a collision.
Composition
Spectroscopic observations show that Charon is largely (maybe completely) covered by frozen ice and some other unidentified materials (gray color, low albedo)
While the presence of H2O is certain, some other ices could also be present. Scientists found, through reflectance-models, large quantities of CO, CO2, CH4.
The presence of other volatile materials on both Pluto and Charon is of great interest in understanding the origin of these, and other, small bodies of the outer solar system.
The compositional relationship of this unique binary system to the neighburing Kuiper Belt concern the origin and chemical evolution of the comets,
the outer planets and their satellites. If we know what the composition is of the darker materials on Pluto and Charon we could learn if they are related to the organic materials
imported to the solar system from the nascent molecular cloud during formation, or if they are produced over time on the surface by cosmic-ray bombardments or photochemistry.
The facts about Pluto suggest an atmosphere wich is both varied and extreme in many ways.
They suggest large variations in the
surface temperature (consequence of the albedo). The atmospheric
structure near the surface is likely to exhibit large geographic
variations: horizontal temperature variations may be as large as
a factor of two. 10% is considered large on Earth. Vertically the temperature gradient
could be as steep as 20-30 K/km. Pluto has the most weakly bound atmosphere in the solar system
and consequently the atmosphere is lost rapidly, relative
to the atmospheric bulk.
Middle atmosphere:
The occultation is sensitive to the ratio of temperature to mean molecular weight T/µ, wich is directly proportional
to the atmospheric scale height. T/µ = 3.63 ± 0.33 above 1215 km.
N2 is the main ice on the surface. Because with the relevant temperature N2 has a vapor pressure an order of magnitude larger than CO and several orders of
magnitude larger than CH4. So it appears that N2 dominates the atmosphere and therefore the mean molecular mass should be 28.
This implies a atmosperic temperature of 102 ± 9 K.
Lower atmosphere:
The atmosphere below 1215 km is much more complicated and therefore less understood. Stransberry et. al. (1994) demonstrated
that a troposphere, i.e. a near-surface atmospheric region with a negative temperature gradient, up to 40 km deep would not produce noticeable effects in the occultation data.
Some models were produced about the lower atmosphere but they differ a lot from eachother and they all are poorly grounded, so the structure
of Pluto's lower atmosphere is an outstanding question.
In short Pluto is composed of:
• core of hydrated rock (70% of mass)
• mantle of water ice
• atmosphere containing methane ice (and possibly: N2, CO, CO2)
This differs a lot from the composition of the other outerplanets. These planets are composed of a small solid core and large mantle/atmosphere of gas.
Therefore Pluto's density is larger than the other outer planets.
| Name of planet | Density (kg/dm3)
|
| Mercury | 5.42
|
|---|
| Venus | 5.25
|
|---|
| Earth | 5.515
|
|---|
| Mars | 3.94
|
|---|
| Jupiter | 1.33
|
|---|
| Saturn | 0.69
|
|---|
| Uranus | 1.29
|
|---|
| Neptune | 1.64
|
|---|
| Pluto | 2.05
|
|---|
Mean density of planets vs. distance from sun
Obtained from: http://www.pha.edu/~akir/Seminar/seminar.html
Albedo
As the table below show Pluto's albedo isn't extraordinary. But what it doesn't shows is the fluctuation of it's albedo.
Next to Iapetus (moon of Saturn), Pluto has the largest global-scale contrast in the Solar system.
To ground the idea of variation of albedo it was necessary to find brightness variations in the rotational lightcurve.
Finally you will get a map with markings with the same albedo.
Models based on the assumption that the markings could be circles were developed by e.g. Marcialis in the early 80s.
When Charon started occulting Pluto it became possible to map the locations of albedo markings a lot more precisely,
but only Pluto's Charon-facing hemisphere, due to the tidal lock between them. Two models were computed and they both show a bright
south polar cap and a darker equatorial. (Buie (1992), Young and Binzel (1993))
This spreading of albedo is also found by the repaired HST. Stern found polar caps and large equatorial spots.
Charon on the other hand doesn't show a lot of variation. As function of wavelength it is constant, what isn't the case of Pluto.
Pluto's albedo namely increases with wavelength until the strong methane absorption. e.g. at a wavelength of µm Charon's reflectivity is about 40%,
although Pluto's varies from 40% to 60%. These variations on Pluto are found for large scales, but it is plausible that it fluctuates even more over smaller scales.
Another thing one can make of albedo maps is the composition of the planet. The regions where Pluto isn't covered by N2 have lower albedo.
As said before, the equatorial regions are darker and therefore composed of something different. Scientists don't know yet what kind of elements
occur around those regions, but generally they believe that it are:
•Organic solids produced by photochemistry of the molecules in the ice.
•Material accreted from outside sources.
•Products of cosmic ray bombardment and photochemical processes.
| Name of planet | Albedo
|
| Mercury | 0.10
|
|---|
| Venus | 0.65
|
|---|
| Earth | 0.37
|
|---|
| Mars | 0.15
|
|---|
| Jupiter | 0.52
|
|---|
| Saturn | 0.47
|
|---|
| Uranus | 0.51
|
|---|
| Neptune | 0.41
|
|---|
| Pluto | 0.5
|
|---|
Albedo map of Pluto
Obtained from: http://antwrp.gsfc.nasa.gov/apod/ap960311.html
Conclusion
Because of all the differences between Pluto and the other eight planets, it isn't hard to believe that Pluto belongs to another group
of objects. Pluto therefore might has been formed in a different way. Because the size and composition of Pluto resemble the sizes and composition
of Kuiper Belt Objects, science nowadays believes that Pluto might has been formed as a Kuiper Belt Object.
