A system of sensitive particle detectors on Mariner IV showed the presence of electrons of energy (E(e)) less than 40 kiloelectron volts out to a radial distance of 165,000 kilometers in the morning fringe of the earth's magnetosphere but failed to detect any such electrons during the close encounter with Mars on 14-15 July 1965, at the time when the minimum areocentric radial distance was 13,200 kilometers. This result can mean that the ratio of the magnetic dipole moment of Mars to that of the earth (M(M)/M(E)) is surely less than 0.001 and probably is less than 0.0005. The corresponding upper limits on the equatorial magnetic field at the surface of Mars are 200 and 100 gammas, respectively. It appears possible that the solar wind interacts directly with the Martian atmosphere.