Three permanent magnet arrays were aboard each Viking lander: a strong array fixed on a photometric reference test chart (RTC) on top of the landers, and two other arrays, one strong and one weak, incorporated in the backhoe of the surface sampler. The RTC magnets on both landers have attracted magnetic particles from the dust cloud caused by the retro‐rockets on landing and by dust raised in connection with surface sample acquisition and delivery. A considerable amount of magnetic particles has been attracted to both backhoe magnets from the surface material as a result of sample acquisition. We judge that the loose Martian surface material contains from 1 to 7% highly magnetic mineral. Preliminary spectrophotometric analysis shows the material adhering to both weak and strong magnets to be identical and indistinguishable from the normal Martian surface material exposed in trenches. The highly magnetic mineral could be present as discrete grains of iron, magnetite, or pyrrhotite, each with a red ferric iron oxide coating, or as particles of maghemite, γFe2O3, with or without other red oxide. Alternatively, the magnetic mineral could be uniformly distributed as a subsidiary component of composite clay or other silicate mineral particles which constitute all or part of the surface material sampled. Mechanical mixtures of the above alternatives are also possible. Although none of the possibilities can be unambiguously excluded, the simplest explanation which fits the available data is that the red pigment in or on all surface particles consists, in part at least, of γFe2O3 and that it is the principal source of the magnetic susceptibility of the surface material.

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During the Viking extended mission the following were performed: (1) The backhoe magnets on Viking Lander (VL) 2 were successfully cleaned, followed by a test involving successive insertions of the cleaned backhoe into the surface. Rapid saturation of the magnets confirmed evidence from primary mission results that the magnetic mineral in the Martian surface is widely distributed, most probably in the form of composite particles of magnetic and nonmagnetic minerals. (2) An image of the VL 2 backhoe taken via the x4 magnifying mirror demonstrates the fine-grained nature of the attracted magnetic material. (3) The reference test chart magnet on VL 1, and by inference also on VL 2, continued to attract magnetic particles from the Martian atmosphere. The new results generally are in agreement with previous estimates of 1-7% of magnetic mineral, probably maghemite, in the surface material. The presence of maghemite and its occurrence as a pigment in, or a thin coating on, all mineral particles or as discrete, finely divided and widely distributed crystallites, are consistent with data from the inorganic analysis experiments and with laboratory simulations of results of the biology experiments on Mars. EXTENDED MISSION ACTIVITY Rate of Magnetic Mineral AcquisitionAttached to the front of each lander is a wire brush through which the backhoe can be passed in order to remove the attracted magnetic material. For a variety of reasons it was not possible to make use of this facility on either lander during the primary missions. However, a reasonably successful cleaning operation was carried out on Viking Lander (VL) 2 during the extended mission (sol 502; sol is Mars sidereal day, 24.623 hours) followed by a controlled experiment to determine the rate at which the magnetic phase accumulates on the backhoe magnets.The cleaning sequence consisted of three passes of the backhoe through the wire brush, the results of which are shown in Figures I and 2. Unfortunately, the clean backhoe image (Figure lb) showed only one (the strong) backhoe magnet, but this can be compared with an image of both magnets taken after a one-pass clean using identical surface sampler commands in a preliminary trial (Figure la). After the threepass clean, the strong magnet is obviously very clean, and the weak magnet can be assumed to have been at least partially cleaned.The Viking molecular analysis experiment has demonstrated the absence (within the detection limits which range from levels of parts per million to below parts per billion) of organic substances in the Martian surface soil at the two Viking landing sites. Laboratory experiments with sterile and nonsterile antarctic samples further demonstrate the capability and reliability of the instrument. The circumstances under which organic components could have escaped detection, such as inaccessibility or extreme thermal stability of organic polymers, are discussed but are found to be unlikely. The inability of the instrument to detect free oxygen evolved from soil samples is pointed out.

Previous theoretical work on the process of deposittonal remanent magnetization (DRM) is extended, and expressions are developed for the viscous damping and time constant of particle alignment in an applied field. Expected values of intensity of magnetization of sedimentary magnetic particles are used in the expressions and it is shown that damping is usually heavy and time constants low, leading to an efficient magnetizing process in small particles possessing a sufficiently high magnetic moment. The DRM of particles sufficiently small to be subject to Browntan motion of rotation is examined by analogy with Langevin's derivation of the susceptibility of an electron gas, and a weak magnetization is shown to be possible, although whether it could occur in nature is not clear. The bearing of the theoretical results on experimental work on DRM and on the magnetization of sediments is discussed.

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