The evidence suggests that comets are highly negatively charged with
respect to the Sun. As they rush toward the Sun, the voltage increases
until at some point the comet nucleus begins to discharge. Electrons are
stripped from a few points on the comet surface where the electric field
is strongest. These "spark discharges" finely machine rocky material from
the surface to form a "cathode jet" of negatively charged dust together
with surface matter that has been torn apart to release ionized atoms and
molecules, including oxygen.

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