To be submitted to Icarus, January 2007 Electric currents and magnetic fields during planetary impacts Friedemann T. Freund 1,2,3 , Hollis H. Jones 4 , Shahid Aslam 4 , Bobby W.S. Lau 1,2 , and Akihiro Takeuchi 5 1 NASA Ames Research Center, Code SGE, Moffett Field, CA 94035-1000, USA, ffreund@mail.arc.nasa.gov 2 Carl-Sagan-Center, SETI Institute, 515 N. Whisman Rd., Mountain View, CA 94043, USA 3 San Jose State University, Department of Physics, San Jose, CA 95192-0106, USA 4 NASA Goddard Space Flight Center, Detectors Branch, Code 553, Greenbelt, MD 20771, USA 5 Niigata University, Department of Chemistry, Niigata 950-2181, Japan Abstract Models of lunar and martian magnetism need to explain the magnetic signatures observed in apparent association with impact structures. Until now most models start from the premise that the moon and Mars may have had early, short-lived dynamos and that impacts caused widespread demagnetization. We show that terrestrial igneous rocks, when subjected to deviatoric stress, can generate large electric currents. The charge carriers are associated with oxygen anions that have changed their valence from 2- to 1- (peroxy). An O - in the O 2- sublattice represents a defect electron, known as positive hole or phole for short. In unstressed rocks the O - exist in a dormant, electrically inactive form as O - pairs, chemically equivalent to peroxy links, O 3 X-OO-XO 3 with X = Si 4+ , Al 3+ etc. Stresses cause the peroxy links to break, turning the stressed rock volume into a battery from where electric currents can flow out. In the case of a gabbro, changing the stress rates from 2x10 -4 to 10 2 MPa/sec causes sustained currents, increasing from about 30,000 to 50,000 A/km 3 . At the highest stress rates an initial current spike develops exceeding 300,000 A/km 3 . Such large currents will cause large magnetic fields. The discovery of stress-activated electric currents points to the possibility that the magnetic signatures observed on the moon and on Mars were formed in the aftermath of impacts, during the flow of powerful, sustained currents. Keywords: Planetary magnetism; impact processes; stress activation; battery currents; magnetic fields; remanent magnetization. ++++++++++++++++++++++++++++++