Atmosphere of Celestial Bodies

An atmosphere is the gaseous envelope surrounding a star or any other celestial body. The most important requirement is the gravitational attraction of the body. It must be strong enough to hold on to the gases.

Another factor that impacts an atmosphere is proximity to the parent star (the Sun in our case). Any object close the Sun will be cooked by intense sunlight and blasted by coronal mass ejections. Either of these can carry off atmospheric molecules and deplete an atmosphere over time. Thus Titan, which circles distant Saturn, has retained its atmosphere while the planet Mercury (a similar body with a diameter of 3,031 miles) has lost its atmosphere. Our Moon looses on both counts: it is smaller and much closer to the Sun than Titan.

• Moon

  Atmosphere: essentially non-existent
  Oceans: none (the mares are lava filled basins)  
  Diameter: 2,160 miles (3,476 km)
  Gravity 0.166 of Earth 

• Titan (Saturn’s largest moon)

   (The only moon in the Solar system with a true atmosphere)
   Atmosphere: Nitrogen and Methane
   Oceans (liquid methane)
    Diameter: 3,200 miles (5,150 km)
    Gravity: 0.28 of earth (estimate)

• Earth

  Atmosphere: nitrogen 78%, oxygen 21%, and carbon dioxide 450PPM 
  Oceans (water)
  Diameter: 7,926 miles (12,756 km)

• Mars

   Atmosphere: CO2 (95%) and inert gasses
   Atmospheric pressure <1% of Earth’s
   Oceans: none at present (Water oceans may once have existed there.)
   Diameter: 4,217 miles (6,786 km)
   Gravity 0.38 of Earth’s
 

References

1. 1. The Universe Explained - The Earth-Dwellers Guide to the Mysteries of Space, Colin A. Ronan, C 1994, Henry Holt & Company

1. 2. The Atlas of the Universe, Patrick Moore, Rand McNally & Co., C1970