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This answer on Space Exploration to a question about Mars says that one reason Mars has such a thin atmosphere is because it lacks a magnetic field to protect it from the effect of double solar winds.

Here MBR explains that Venus does not have a magnetic field.

Venus-atmopshere

Image credit: ESA

If this is the case, then why has Venus's atmosphere not been stripped away by solar wind like that of Mars?

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up vote 26 down vote accepted

Venus has a strong ionosphere that protects it against violent solar winds. So, even though Venus has no intrisic magnetic field, it has an effective, induced magnetic field due to the interaction between the solar winds and the atmosphere, that protects it against solar winds.

Venus atmosphere is thick enough to have a consequent ionosphere, that would be the difference between Mars and Venus (and Venus was able to keep a thicker atmosphere due to its greater mass, contrary to Mars).

Sources:

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I believe I read somewhere that the character of Venus's atmosphere has been changed over the eons. Water in the atmosphere was split into separate hydrogen and oxygen atoms, with the hydrogen being blown into space by the solar winds. This makes Venus not only hot, but dry. I wonder if Earth might have benefited by capturing some of the hydrogen and matching it up with free oxygen in our atmosphere. – Howard Miller Jun 19 at 23:27

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