Hi Emily,
That's an interesting question. As Christina said, Earth's atmosphere is probably safe, although solar activity can cause some minor changes to our atmosphere. However, the situation on Mars may be completely different. Earth's magnetosphere acts as a sort of magnetic shield that diverts the solar wind flow around our planet. Unlike Earth, Mars does not have a strong, global magnetic field or a magnetosphere that protects Mars from the solar wind. A few years ago, scientists studying data from the Mars Global Surveyor spacecraft found evidence to suggest that some of the atmosphere of Mars may have been blown away by the solar wind. You can read more here:
http://science.nasa.gov/headlines/y2001/ast31jan_1.htm
Kris