Preliminary Results from the Viking Orbiter Imaging Experiment

One Mars Year: Viking Lander Imaging Observations

Throughout the complete Mars year during which they have been on the planet, the imaging systems aboard the two Viking landers have documented a variety of surface changes. Surface condensates, consisting of both solid H(2)O and CO(2), formed at the Viking 2 lander site during the winter. Additional observations suggest that surface erosion rates due to dust redistribution may be substantially less than those predicted on the basis of pre-Viking observations. The Viking 1 lander will continue to acquire and transmit a predetermined sequence of imaging and meteorology data as long as it is operative.

Mars and Earth: Origin and Abundance of Volatiles

Mars, like Earth, may have received its volatiles in the final stages of accretion, as a veneer of volatile-rich material similar to C3V carbonaceous chondrites. The high (40)Ar/(36)Ar ratio and low (36)Ar abundance on Mars, compared to data for other differentiated planets, suggest that Mars is depleted in volatiles relative to Earth-by a factor of 1.7 for K and 14 other moderately volatile elements and by a factor of 35 for (36)Ar and 15 other highly volatile elements. Using these two scaling factors, we have predicted martian abundances of 31 elements from terrestrial abundances. Comparison with the observed (36)Ar abundance suggests that outgassing on Mars has been about four times less complete than on Earth. Various predictions of the model can be checked against observation. The initial abundance of N, prior to escape, was about ten times the present value of 0.62 ppb, in good agreement with an independent estimate based on the observed enhancement in the martian (15)N/(14)N ratio (78,79). The initial water content corresponds to a 9-m layer, close to the value of >/=13 m inferred from the lack of an (18)O/(16)O fractionation (75). The predicted crustal Cl/S ratio of 0.23 agrees exactly with the value measured for martian dust (67); we estimate the thickness of this dust layer to be about 70 m. The predicted surface abundance of carbon, 290 g/cm(2), is 70 times greater than the atmospheric CO(2) value, but the CaCO(3) content inferred for martian dust (67) could account for at least one-quarter of the predicted value. The past atmospheric pressure, prior to formation of carbonates, could have been as high as 140 mbar, and possibly even 500 mbar. Finally, the predicted (129)Xe/(132)Xe ratio of 2.96 agrees fairly well with the observed value of 2.5(+2)(-1) (85). From the limited data available thus far, a curious dichotomy seems to be emerging among differentiated planets in the inner solar system. Two large planets (Earth and Venus) are fairly rich in volatiles, whereas three small planets (Mars, the moon, and the eucrite parent body-presumably the asteroid 4 Vesta) are poorer in volatiles by at least an order of magnitude. None of the obvious mechanisms seems capable of explaining this trend, and so we can only speculate that the same mechanism that stunted the growth of the smaller bodies prevented them from collecting their share of volatiles. But why then did the parent bodies of the chondrites and shergottites fare so much better? One of the driving forces behind the exploration of the solar system has always been the realization that these studies can provide essential clues to the intricate network of puzzles associated with the origin of life and its prevalence in the universe. In our own immediate neighborhood, Mars has always seemed to be the planet most likely to harbor extraterrestrial life, so the environment we have found in the vicinity of the two Viking landers is rather disappointing in this context. But the perspective we have gained through the present investigation suggests that this is not a necessary condition for planets at the distance of Mars from a solar-type central star. In other words, if it turns out that Mars is completely devoid of life, this does not mean that the zones around stars in which habitable planets can exist are much narrower than has been thought (114). Suppose Mars had been a larger planet-the size of Earth or Venus-and therefore had accumulated a thicker veneer and had also developed global tectonic activity on the scale exhibited by Earth. A much larger volatile reservoir would now be available, there would be repeated opportunities for tapping that reservoir, and the increased gravitational field would limit escape from the upper atmosphere. Such a planet could have produced and maintained a much thicker atmosphere, which should have permitted at least an intermittently clement climate to exist. How different would such a planet be from the present Mars? Could a stable, warm climate be maintained? It seems conceivable that an increase in the size of Mars might have compensated for its greater distance from the sun and that the life zone around our star would have been enlarged accordingly.

Occultation of ε Geminorum by Mars: Evidence for Atmospheric Tides?

Temperature profiles of the martian atmosphere have been derived from airborne observations of the 8 April 1976 occultation of epsilon Geminorum. Within the altitude range from 50 to 90 kilometers, these profiles show peak-to-peak variations of 35 degrees K with a vertical scale of 20 kilometers and represent evidence for strong tides in the martian atmosphere. However, more information is necessary to conclusively rule out a radiative explanation for the temperature variations.

Scientific Results of the Viking Missions

The two Viking missions to Mars have been extraordinarily successful. Thirteen scientific investigations yielded information about the atmosphere and surface. Two orbiters and landers operating for several months photographed the surface extensively from 1500 kilometers and directly on the surface. Measurements were made of the atmospheric composition, the surface elemental abundance, the atmospheric water vapor, temperature of the surface, and meteorological conditions; direct tests were made for organic material and living organisms. The question of life on Mars remains unanswered. The Viking spacecraft are designed to continue the investigations for at least one Mars year.

Viking Orbital Colorimetric Images of Mars: Preliminary Results

Color reconstruction and ratios of orbital images of Mars confirm Earth-based measurements showing red/violet ratios for bright areas to be roughly 1.5 times greater than dark areas. The new results show complex variation among dark materials; dark streaks emanating from craters in southern cratered terrains are much bluer than dark materials of the north equatorial plains on which Viking 1 landed.

The Viking Landing Sites: Selection and Certification

During the past several years the Viking project developed plans to use Viking orbiter instruments and Earth-based radar to certify the suitability of the landing sites selected as the safest and most scientifically rewarding using Mariner 9 data. During June and July 1976, the Earth-based radar and orbital spacecraft observations of some of the prime and backup sites were completed. The results of these combined observations indicated that the Viking 1 prime landing area in the Chryse region of Mars is geologically varied and possibly more hazardous than expected, and was not certifiable as a site for the Viking 1 landing. Consequently, the site certification effort had to be drastically modified and lengthened to search for a site that might be safe enough to attempt to land. The selected site considered at 47.5 degrees W, 22.4 degrees N represented a compromise between desirable characteristics observed with visual images and those inferred from Earth-based radar. It lies in the Chryse region about 900 kilometers northwest of the original site. Viking 1 landed successfully at this site on 20 July 1976.

The Surface of Mars: There View from the Viking 1 Lander

The first photographs ever returned from the surface of Mars were obtained by two facsimile cameras aboard the Viking 1 lander, including black-and-white and color, 0.12 degrees and 0.04 degrees resolution, and monoscopic and stereoscopic images. The surface, on the western slopes of Chtyse Planitia, is a boulder-strewn deeply reddish desert, with distant eminences-some of which may be the rims of impact craters-surmounted by a pink sky. Both impact and aeolian processes are evident. After dissipation of a small dust cloud stirred by the landing maneuvers, no subsequent signs of movement were detected on the landscape, and nothing has been observed that is indicative of macroscopic biology at this time and place.

Physical Properties of the Martian Surface from the Viking 1 Lander: Preliminary Results

The purpose of the physical properties experiment is to determine the characteristics of the martian "soil" based on the use of the Viking lander imaging system, the surface sampler, and engineering sensors. Viking 1 lander made physical contact with the surface of Mars at 11:53:07.1 hours on 20 July 1976 G.M.T. Twenty-five seconds later a high-resolution image sequence of the area around a footpad was started which contained the first information about surface conditions on Mars. The next image is a survey of the martian landscape in front of the lander, including a view of the top support of two of the landing legs. Each leg has a stroke gauge which extends from the top of the leg support an amount equal to the crushing experienced by the shock absorbers during touchdown. Subsequent images provided views of all three stroke gauges which, together with the knowledge of the impact velocity, allow determination of "soil" properties. In the images there is evidence of surface erosion from the engines. Several laboratory tests were carried out prior to the mission with a descent engine to determine what surface alterations might occur during a Mars landing. On sol 2 the shroud, which protected the surface sampler collector head from biological contamination, was ejected onto the surface. Later a cylindrical pin which dropped from the boom housing of the surface sampler during the modified unlatching sequence produced a crater (the second Mars penetrometer experiment). These two experiments provided further insight into the physical properties of the martian surface.