Global Electric Circuit of Mars
Even though several missions to Mars have passed through the Martian atmosphere and have had extended research programs on the surface, to date there have been no measurements of the vertical profiles of atmospheric conductivity, electric field, or current density. Therefore, any conclusions made concerning the global electric circuit of Mars must be made by using what is known of Earth's global electric circuit and applying it to Mars through analogy or by reproducing conditions found at Mars in a laboratory setting.
On Earth it is generally accepted that the global electric circuit is driven by thunderstorms [Wilson, 1920]. In this circuit model, thunderstorms act as electrical generators that drive currents upward. As a result, the upper atmosphere becomes positively charged with respect to Earth's surface. In the steady state, charge in the upper atmosphere leaks back to the ground through the finitely conducting atmosphere. Near Earth's surface, the atmospheric conductivity is large enough to dissipate any field on the order of minutes. Therefore, the average global electric field must be maintained by some almost continuous current source. For Earth, the dominant generator is believed to be thunderstorms [Krider and Roble, 1986]. Other sources also play a role in driving Earth's global electric circuit, but it is thought that thunderstorms are the dominant contributor.
How can knowledge of Earth's global electric circuit be applied to Mars? In order for a global electric circuit similar to Earth's to exist in the Martian atmosphere, a constant current source, or current generator must be located in a finitely conducting atmosphere. Over decades of visual observations by both orbiting spacecraft and landers, no thunderstorms have been detected on Mars. Most likely, the Martian environment is too dry and too cold for such phenomena to form. Therefore, alternate current sources must be found in order to drive the global electric circuit. Alternate current sources on Earth and their applications to the Martian environment will be investigated later. First, we will look at the possibility of the presence of a conducting atmosphere and some of the sources of atmospheric conductivity.