1. The dark areas of Mars show periodic variation of intensity following the cycle of the darkening element
  2. The average intensity of the dark area, not including the action of the darkening waves, increases from the poles toward the equator
  3. The action of each of the darkening waves decreases from the poles toward the equator. This decrease is balanced in the equatorial zone by the combined action of the two darkening waves alternately originating at the two poles. The mechanism of the darkness-generating element seems to be constant for all latitudes during the Martian year.

The variation in intensity has been explained recently by nonlife mechanisms for Depressio Hellespontica (an area showing one of the greatest seasonal changes) ([ref.2]). Similar nonlife mechanisms may be applicable to the other dark regions, and, thus, the "darkening" can be used only as circumstantial evidence in support of a Martian life form.

If inorganic interpretations of the seasonal albedo variation are accepted, then an inorganic interpretation must also be advanced for the polarization variation. Two possibilities can be suggested:

  1. A change in surface texture, caused by varying absorption of atmospheric constituents, causing both the albedo and polarization to change in the manner observed
  2. A change in surface texture, in which the surface material becomes rougher, which also explains the observed polarization data ([ref.5])

The third argument against the regenerative feature of the dark areas being a life process has been advanced by Kuiper ([ref.6]). It is based on atmospheric circulation causing dust, presumably lava, to be blown on the dark areas of Mars during the late summer, autumn, and winter, and then removed during the spring. Mamikunian and Moore have recently advanced the similar explanation that carbonaceous chondrites or asteroidal matter may induce the observed phenomenon if they are abundant on the planet's surface. The pulverized chondritic material will exhibit a high degree of opacity due to localization and, hence, a change in polarization characteristics and a decrease in polarization following mixing of the chondritic material with indigenous surface minerals.

The fourth observational argument, the Sinton bands ([ref.7]), has been shown to be at least doubtful. Rea, Belsky, and Calvin ([ref.8]) recorded infrared reflection spectra for a large number of inorganic and organic samples, including minerals and biological specimens, for the purpose of interpreting the 3µ-to-1µ spectrum of Mars. These authors state that a previous suggestion that the Martian "bands" be attributed solely to carbohydrates is not a required conclusion. At the same time they fail to present a satisfactory alternate explanation, and the problem remains unsolved. More recently, Rea et al. ([ref.9]) noted the similarity between the 3.58µ and 3.69µ minima in the Martian infrared spectra and those of D2O-HDO-H2O mixtures and, particularly, of HDO.

With all this marked disagreement in interpreting the observational data concerning Mars, it becomes clearly evident that an experimental approach to the detection of life on Mars should provide the maximum positive information possible. Some life-detection experiments developed with NASA support have been summarized by Quimby ([ref.10]).

The schema of the biological exploration of a planet is to conduct a series of complementary experiments proceeding from general to specific. The general experiments will examine gross characteristics of the planet's environment and surface for determining the probability of an active biota (life). Data from the general experiments will be significant in—

  1. Defining the nature of specific experiments in which life detection is the major objective; and
  2. Providing a high degree of confidence in undertaking specific experiments, since indications from the gross characterization of the planet in question will influence the choice and design of the specific experiments.

The biological exploration of planets is then to be defined as the search for those parameters relevant to the origin, development, sustenance, and degradation of life in a planetary environment. This definition will give rise to a critical question for each progressively specific and complex experiment to determine—