Evidence of liquid water in mars' distant past suggests that it had a substantial atmosphere compared to what it has presently. What happened to its water vapor?.

The atmosphere has frozen out into the polar ice caps and permafrost as Mars has cooled.

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explanation:

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adding a(n) to an electric circuit would increase the magnetic field around a wire;

b. solenoid

a solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field.

if the purpose of the solenoid is instead to impede changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet.