HR: 15:25h
AN: GP22A-08    [PDF]
TI: Evidence for Demagnetization of the Utopia Impact Basin on Mars
AU: * Mitchell, D L
EM: mitchell@ssl.berkeley.edu
AF: Space Sciences Laboratory, Univ. of Calif., Berkeley, CA 94720 United States
AU: Lillis, R
EM: rlillis@ssl.berkeley.edu
AF: Space Sciences Laboratory, Univ. of Calif., Berkeley, CA 94720 United States
AU: Lin, R P
EM: boblin@ssl.berkeley.edu
AF: Space Sciences Laboratory, Univ. of Calif., Berkeley, CA 94720 United States
AU: Connerney, J
EM: jec@lepmom.gsfc.nasa.gov
AF: NASA, GSFC, Greenbelt, MD 20771 United States
AU: Acuna, M
EM: mha@lepmom.gsfc.nasa.gov
AF: NASA, GSFC, Greenbelt, MD 20771 United States
AB: One of the great surprises of the Mars Global Surveyor mission was the discovery of intensely magnetized crust [Acu\~{n}a et al., Science 284, 790, 1999; Acu\~{n}a et al., JGR 106, 23403, 2001]. The crustal fields are predominantly associated with the southern highlands, and impact demagnetization of the Hellas, Argyre, and Isidis basins indicates that Mars' crustal magnetic fields are among the oldest preserved geologic features on the planet. Much of the northern lowlands appears to be non-magnetic, except for relatively weak north polar anomalies and a few magnetic sources adjacent to the dichotomy boundary. The MGS Magnetometer measured crustal magnetic fields at altitudes from 100 to over 1000 km. Because of the long duration in the mapping orbit, the crustal field map at 400 km altitude is fully sampled on both the day and night hemispheres; however, the sampling is sparse at lower altitudes, and nearly all of those data were obtained on the sunlit hemisphere, where the solar wind distorts crustal fields and obscures those weaker than ~30 nT. Thus, it is possible that weak crustal fields exist in the northern lowlands that have so far escaped detection. We have systematically searched for crustal magnetic fields in the northern lowlands using electron reflection magnetometry, which has a sensitivity similar to that of a magnetometer orbiting at 200 km altitude. We have detected several new magnetic features in the northern lowlands. Some of these appear to be northward extensions of previously identified magnetic sources along the dichotomy boundary. A group of magnetic sources form a partial ring around the Utopia basin perimeter, but no sources were detected within the basin itself. These observations suggest that the Utopia impact resulted in crustal demagnetization, and that the relatively weak magnetic sources in the northern lowlands are as old as those in the southern highlands.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 5420 Impact phenomena (includes cratering)
DE: 5440 Magnetic fields and magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting