A new mechanism for climatic change on Mars involving interactions between the atmosphere and a hypothesized ancient planetary magnetic field is suggested and discussed. An early dynamo for Mars is not inconsistent with either current data on the martian interior, models of planet origin, or dynamo theory. The Jacobs/Busse scaling law suggests a martian dynamo field would have effectively shielded the atmosphere from the solar wind. Cooling and/or solidification of the core would cause dynamo extinction and expose the martian atmosphere to the full solar and galactic particle flux. These events may have triggered atmospheric cooling and the present martian frozen surface; as suggested by: (1) climatic records of the earth, and (2) current models of nucleation and other relevant atmospheric processes.