Magnetic fields near Mars: first results

Abstract

BASED on the examples of the Earth and Moon, we expect that Mars has or once had an intrinsic planetary magnetic field. The Earth has a large magnetic moment, 8 x 10¹⁵ T m³, and the Moon's remanent magnetism in its surface rocks indicates that it once had a magnetic field¹. The radius of Mars (3,390 km) is intermediate between that of the Earth (6,371 km) and of the Moon (1,738 km). Its smaller size has led to a more rapid thermal evolution than Earth's but we do not know whether Mars still generates a magnetic field or has passed into a Moon-like state. The Mariner 4, and Mars 2, 3 and 5 missions have placed upper limits on the martian magnetic moment^2,3 in the range of 1–5 x 10¹² T m³ corresponding to an equatorial surface field of 25–125 nT. We have measured magnetic fields with high temporal resolution in the tail and down to an 850-km altitude. During four successive highly elliptical orbits we identified the position of the bow shock as well as of a transition layer, the 'planetopause'. Subsequent circular orbits at 6,000-km altitude provided the first high-resolution data in the planetary tail and indicate that the interplanetary magnetic field mainly controls the magnetic tail. We also detected magnetic turbulence when the spacecraft crossed the orbit of the moon Phobos, indicating the possible existence of a torus near the orbit of this moon.