1. What determines whether a planet has an atmosphere ? Why has Mercury no atmosphere ?

Answer 1

The value of gravity and hotness of the planet decide the existance of the atmosphere.
If the gravity is low,the escape velocity which is proprtional to the gravity become less.If the value of escape velocity becomes less than that of the molecules of air, then the molecules escape into space with passage of time and the planet may not have atmosphere.
Again,if the planet is very hot,the average speed of the molecules (which is propotional to the temperature of the planet) becomes more than that of the escape velocity at a particular stage and the molecules escape to the space leaving no atmosphere over the planet.
Mercury has no atmosphere as its surface temperature during day time is 430 degree celcius due to which the molecule velocity is more than the escape velocity resulting in the escape of molecules leaving no atmosphere.
Moreover the gravity may also be less due to small size of the planet.(gravity is proportional to radius).

Answer 2

Many factors determine whether or not a planet or moon will have an atmosphere. The size of the body in question has a lot to do with it, since the more gravitational pull an object has, the more atmosphere it can hold onto. An atmosphere may be released by volcanic action and transformed by chemical reactions or organisms that live on the planet, which is how Earth's atmosphere got its 21% oxygen. The effect of the solar wind can also strip an atmosphere away, which would have left Mercury without an atmosphere if it ever had a substantial one.

Answer 3

If there is an "Aoura" around it. Mercury's atmosphere burnt of long ago due to it's closeness to the sun.