In article ,
Adam wrote:
Hi,

If our magnetic field protects our atmosphere from being blown away by
solar winds then how come venus can maintain an atmosphere 90x the
pressure of ours without a magnetic field?

Um, that'd be because you started with an incorrect premise.

The earth's magnetic field is not needed to retain its atmosphere.
It is useful, though, for keeping high energy particles from the
sun away from us at the surface.

I thought gravity except Venus is less dense and smaller than Earth so
I guess thats ruled out.

Can someone help? or point me in the right direction?

The thing you want to look up is gaseous escape from planetary
atmospheres. Most standard introductory astronomy books will
have a section on it. The main balance is between the temperature
of the gas, the molecular weight, and the escape velocity.

While Venus is slightly smaller than the earth, it's only slight.
It's hotter, so the molecules are moving faster. But the main
gas involved in Venus's atmosphere -- CO2 -- is sufficiently heavy
that there's no way any appreciable amount will be lost. What Venus
can't retain (neither can the earth) are hydrogen and helium. It's
too hot and small, as is the earth.

--
Robert Grumbine http://www.radix.net/~bobg/ Science faqs and amateur activities notes and links.
Sagredo (Galileo Galilei) "You present these recondite matters with too much
evidence and ease; this great facility makes them less appreciated than they
would be had they been presented in a more abstruse manner." Two New Sciences