IGPP Seminar Series

Comparative Planetology: Solar Wind Driven Mass Loss at Magnetized and Unmagnetized Planets

by Robert J. Strangeway


One of the mechanisms for atmospheric escape at planets such as Venus and Mars is through the direction interaction of the solar wind with the planetary atmosphere and ionosphere. Charge exchange of exospheric atoms and molecules within the solar wind provides one pathway for atmospheric escape, as does direct scavenging of the topside ionosphere by the solar wind. Contemporary planetary escape rates from Venus and Mars are typically of the order 10^24 particles per second. By way of contrast, the ionosphere and atmosphere of the Earth do not interact directly with the solar wind and it has been argued that Earth's magnetic field acts as a shield to the solar wind and the terrestrial ionosphere and atmosphere are protected from loss to the solar wind. This shielding has been invoked as a means for retention of water in the Earth's atmosphere, whereas Venus and Mars have very low water content in their atmospheres. It has been known for many years, however, that escape fluxes in the polar ionosphere give global rates that can be as large as 10^26 particles per second during times of high geomagnetic activity. During geomagnetically active intervals the Earth's polar ionosphere is heated by Joule dissipation and particle precipitation. Thus, even though the physical processes are different, the terrestrial planets seem to have similar atmospheric escape rates. Since the solar wind ultimately drives the escape processes, it is not clear that the relative escape rates would change significantly over the life of the solar system. Effects other than changes in the solar output may have to be considered to explain the loss of water from the atmospheres of Venus and Mars.
Tuesday, 09 February 2010
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM