Without its magnetosphere, how has Venus held onto its atmosphere, when Mars lost its atmosphere and water when its magnetic field was turned off?
A lot of people assume that the magnetosphere of a planet is the main factor determining whether it can retain an atmosphere.
Long ago it was calculated that the escape velocity (not the surface gravity, the escape velocity) of a object was vital to how long it could retain an atmosphere.
As I remember, if a planet, moon, or other astronomical body has an escape velocity more than six times the average speed of molecules of a specific gas in the upper layers of its atmosphere, the escape of gases will be very, very, very slow and the astronomical body can retain that specific gas in its atmosphere for billions of years.
Having sufficiently high escape velocity is necessary, but not sufficient, for an astronomical body to retain its atmosphere.
And later it was discovered that the stellar wind of stars knocks molecules of gas out of the upper layers of atmospheres and causes them to escape. Thus having a strong enough magnetic field to deflect the stellar wind is also necessary, but not sufficient, for an astronomical body to retain its atmosphere.
No magnetosphere, no matter how strong, can enable an astronomical body with insufficient escape velocity to retain an atmosphere for billions of years.
And you might wonder how Titan has managed to retain an atmosphere, as well as how Venus has.