If Earth's magnetic field protects the atmosphere from being blown away by solar wind, why is Venus unaffected?

Mars atmosphere is more than 95% CO2. CO2 has a lot more mass than H2. Even though the solar wind has a higher velocity at Venus because Venus is closer to the Sun than Earth is. Because CO2 is more massive, CO2 also requires more velocity for the solar wind to blow it off into space, which IS happening, but the CO2 is also restored by volcanoes, just like volcanoes on Earth restore water and some CO2 and O2 to Earth's atmosphere. Lava is VERY wet. 80% OR MORE of Earth's water comes from volcanoes.

Actually, Venus does have a magnetic field, though it is much weaker than the one Earth has. But it takes more than solar wind to blow away an atmosphere. A planet's gravity plays a large part in holding the atmosphere in place.

Here's a quote from Wikipedia: "An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body."

The Earth's magnetic field is not entirely necessary to retain its atmosphere. It does help keep energetic particles from space from reaching the atmosphere and surface. But as on Venus, Earth's atmosphere is sustained by the planet's size and gravitation. Venus is nearly the same size as Earth and has very similar gravity. Mars may have had a thicker atmosphere at one time, but being half the size of Earth, and thus half the surface gravity, retaining it was much harder in the wake of such phenomena as solar wind.

Because the magnetic field isn't as important as it's cracked up to be (Earth loses it's magnetic field every so often when it reverse polarity).

Mars would've lost it's atmosphere regardless of whether or not it had a magnetic field.

DLM: Would you please stop repeating the crap about not having a magnetic field somehow allowing lots of dangerous radiation to the surface, that isn't something which would happen with any breathable atmosphere (on Mars even a pure oxygen atmosphere humans could breath would be at least 5 tonnes per square metre, enough to reduce cosmic ray exposure to acceptable levels and you would have probably at least 50% nitrogen doubling the mass per unit area).

It is my understanding that a magnetosphere prevents water vapor from escaping, not the atmosphere. Mars is subject to losing a terraformed atmosphere because of its low mass, in combination with other properties, it simply cannot hold on its atmosphere, the solar winds strip it away because, even at that greater distance, it cannot 'hold on' to it over time. This would explain why the tenuous atmosphere of Mercury has some water in it, Venus has practically none, despite a very dense atmosphere, and Earth has significant water present in the air.

What a lack of a magnetosphere does do to make terraforming Mars difficult, is the influx of cosmic and solar radiation can be downright harmful to biological life forms.

Someone will certainly correct me if I am mistaken. But that is how I understand the related topics to this question.

It's Earths gravity that holds on to the atmosphere.

earth is the only planet that exists