Question:
Will the Mars ionosphere work the same way that Earth's
ionosphere does to reflect radio waves, so that one may communicate
via radio to points beyond the horizon?
Replies:
The same physics applies, though the sun is less effective at ionizing
the Martian atmosphere (because further away) and the atmosphere is
much thinner.
"Does the Mars ionosphere reflect radio waves of the same frequency
that the Earth's ionosphere does?"
No. The reflectivity depends on the ratio of the refractive index of
the plasma (ionized air) to that of the unionized air. The index of
refraction of a plasma depends on the ratio of the plasma frequency to
the frequency you're trying to reflect. The plasma frequency depends
on the density of charged particles in the plasma, and this will be
much smaller on Mars than on Earth. The net effect of all this is that
only radio frequencies near or lower than the plasma frequency are
reflected, and the plasma frequency of Mar's ionoshpere is much lower
than that of ours. Also, the reflectivity will be lower on Mars than
on Earth.
One more thing. The whole business works on Earth because there is
a layer of air below the ionosphere that is not ionized. Mars'
atmosphere may be thin enough to be uniformly ionized. If so,
there will be no reflected waves of any frequency.
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