Re: What celestial body has an atmosphere closest to the earths?

Joe,
     After checking all the known planets with atmospheres (including 
several moons, notably Titan and Triton), it seems that there are two 
planets that have atmospheres closest to that of the Earth.  They are 
also the planets that have a similar overall composition and origin to 
our planet:  Venus and Mars.
    Neither one is a dead match, however.  Venus' atmosphere is 
about 100 times denser than the Earth's, and Mars' atmosphere is 
about 100 times less dense than the Earth's.  The things that these 
two planets' atmospheres have in common with the Earth are the 
gases that are in them.
     They all have Carbon Dioxide(making up around 95% of Mars' and 
Venus' atmospheres and only .00035% of Earth's) Nitrogen,  Argon 
and water vapor in differing amounts.  However, neither Venus or 
Mars have the abundance of free oxygen that is the hallmark of the 
Earth's atmosphere- owing largely to the presence of plant life.
    I hope I answered your question Joe.  For more information, there 
is a great in depth discussion of this topic in chapter 13 of the 4th 
edition of  "The New Solar System" edited by J. Kelly Beatty, Carolyn 
Collins Petersen and Andrew Chaikin, copyright 1999.  Nice 
straightforward question!
                                       -Paul  Barrett
                                        Lead Science Demonstrator
                                        Pacific Science Center
                                        paul_barrett@pacsci.org