This is a tough question as it is still being
investigated by scientists at NASA. I did not know
the answer and did some digging around to find
some sources that might be useful.
To start, gravity itself plays a large role in
keeping the atmosphere in place, I think much
larger than the Earth's magnetic field does to
protect against solar wind. There has been some
research on Mars about how much of the atmosphere
gets taken into space from solar wind. Mars does
not have a magnetic field now, but it used to.
Researchers and scientists are using this
information to determine the rate at which the
magnetic field disappeared and how the planet is
currently affected by solar winds. Studying the
environment on Mars and other planets may help to
predict what would happen to Earth if it were to
lose its magnetic field.
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