These unpaired electrons are the cause for the high chemical reactivity of free radicals.

由于这些不配对电子，这些分子或原子常具有高度的化学活性。

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And it's believed that the high chemical reactivity of the surface is due to the deposition of chemically active gases from the atmosphere onto the surface, gases like hydrogen peroxide and ozone.

NPR: Revisiting the Viking Missions to Mars