What are the atmospheric constituents of the outer planets?

Using data from the Galileo Probe's plunge into the top cloud layers of Jupiter, Galileo has discovered that Jupiter has thunderstorms many times larger than Earth's. These storms result from the vertical circulation of water in the top layers, leaving large areas where air descends and becomes dry like the Sahara desert, and other areas where water rises to form the thunderstorms.

 

Despite the fact that there have been spacecraft sent to all the major planets except for Pluto, our detailed knowledge of the atmospheres of other planets is still not complete...and for the outer planets it is a patchwork of ground-based and satellite studies made at many different wavelengths. What is frustrating about studying planetary atmospheres is that it is usually the rare trace gases which cause many of the most dramatic atmospheric features such as on the planet Jupiter.

If you consult many textbooks on this subject, you will often find conflicting estimates at the level of several percent, for the inner planets, and 5-10 percent for the outer planets. There is also the ambiguity of whether the authors mean abundance by atom (nose counting) or abundance by mass.

I have used the 1989 book 'Origin and Evolution of Planetary and Satellite Atmospheres' by S. Atreya, J. Pollack and M. Matthews ( U. Arizona Press) as a guide, and this is the table I came up with after perusing the 870+ pages in this book! (The outer planet 'notes' are taken from page 566 of that book.). Also, the estimates for the abundance of the constituents of Mercury and Pluto are very uncertain.

Mercury

Sodium..............................90-100% ????
Other heavier elements.......1 - 10 % ????

Note: Mariner 10 detected about 100 atoms and 600 atoms/cc and ground-based studies have detected 30,000 atoms/cc of sodium. The H and He are probably from the solar wind because they cannot remain bound to the planet.

Venus

Carbon Dioxide .................96%
N2 ......................................3%
O2 ......................................<1%

Earth

N2 .....................................78%
O2 .....................................21%
H2O,CO2 ...........................2%

Mars

Carbon Dioxide ..................95%
N2,Argon ..........................4.5% ( about 3% N2)

Jupiter

Hydrogen ............................86% (92%)
Helium ................................ 7% (7%) (0.65xsolar)
Ammonia,N2,Methane,Ethane...Traces

Note: it has 2.3 times the Sun's abundance of carbon, and twice the Sun's abundance of nitrogen atoms.

Saturn

Hydrogen ............................92%
Helium ..................................7% (0.2x solar)
Methane, N2,Ammonia ................1%

Note: It has 5 times the Sun's abundance of carbon atoms; 3 times the solar abundance of phosporous, and 3 times the nitrogen abundance.

Uranus

Hydrogen ............................90%
Helium .................................10% (solar)

Note: it has 35 +/- 15 times the Sun's abundance of carbon atoms!

Neptune

Hydrogen .............................90%
Helium ...................................8%
Methane ................................2%
CO, HCN..............................trace

Note: it has 40 +/- 20 times the Sun's abundance of carbon atoms!

Pluto

N2 .........................................90%?
Methane ..................................<10%?

 

Along with the nine planets, the satellites that have a mentionable atmosphere are Io, Titan, and Triton, and to a lesser extent, the Moon, Europa and Ganymede.

The percentages for Uranus and Neptune are not known with very much precision and are assumed to be similar to Jupiter and Saturn. Pluto is known to have an atmosphere because it can be detected by methane absorption features, but it is not known if this is the dominant constituent or merely a trace gas.

For more information, visit Kurt Retherfords' Introduction to Planetary Atmospheres lecture notes pages at Johns Hopkind University.