Microwaves from the Planets

Abstract

Study of the solar system has been enjoying a renaissance for several years. New observational techniques are making available a much wider spectrum of information than can be obtained with ground-based visual telescopes. These include radio telescopes on the ground and in orbit, X-ray, Llltraviolet and infrared sensors in orbit or on rozkets, and particle counters on space probes that sample the interplanetary matter directly. All these kinds of instruments can be put on probes that go near the planets, and ultimately, Ecientists hope, on spacecraft that will land on the planets or penetrate their atmospheres. Radio astronomy was the first of the new techniques to be applied to the planets, and there has probably been more observation of the planets by radio than by any technique except visible light. Nearly all the planetary radiations are thermal. Heating of the surface or atmosphere by the sun or some other source causes atomic and molecular vibrations that generate the microwaves. Planetary radio astronomy is now to the point where it compiles radio spectra of individual planets and makes theoretical models of their physical composition. Discrepancies between the radio observations and theoretical expectation about the surface of Mars and the atmospheres of the outer planets have aroused great interest, as exemplified in a series of articles in the current issue of ICARUS. The Martian data most discordant with theory concern radio wavelengths radiated by the planet's soil within a few centimeters of the surface. The theoretical prediction is derived from assumptions about the composition of the soil of Mars and the proposition that its temperature decreases with depth. This would be expected since the top layer of soil would be heated by the sun. Because shorter wavelengths come from nearer the surface, the brightness temperature, or apparent temperature of the source of radiation, should rise as observation passes from the centimeter to the millimeter range of wavelengths. It ultimately should approach the infrared average of 235 degrees K. (For comparison room temperature is about 300 degrees K; water freezes at 273 and boils at 373.) Unfortunately for the theory, the radio spectrum appears to remain flat or even turn down at short wavelengths. Since the brightness temperature depends on the radio emissivity of the substance radiating the waves as well as the true temperature, the presence of a substance with a lower emissivity in the outer part of the Martian surface account for the turndown. It would give a lower brightness temperature than the soil just below it even though its actual temperature were higher. The observed situation could be approximated, says Dr. Eugene E. Epstein of the Aerospace Corp. in Los Angeles, if the top few centimeters of the surface material contain some liquid water. This water might result from the partial melting of a permafrost layer. Drs. Carl Sagan and Joseph Veverka of Cornell University Itake up this idea more strongly. There's no question that liquid water will give such a curve, says Dr. Sagan. It's the only explanation. We tried everything else we think of. That is, he says, unless the data are in error. There is evidence for some water vapor in the Martian atmosphere, but there are arguments against liquid water on the surface. Dr. Andrew P. Ingersoll of the California Institute of Technology argues, in a paper published in SCIENCE a year ago, that under Martian conditions, atmospheric water vapor would condense to ice. The evaporation rate is so high, he says, that evaporation would keep the ice cooled below the freezing point regardless of any local heat sources. Pure ice would never melt, he says, and liquid water exist only in concentrated salt solutions. Dr. Ingersoll's argument notwithstanding, Drs. Sagan and Veverka find the idea of a melting permafrost layer attractive because their observations show that on the surface the temperature drops off as one looks toward the poles of Mars. This dependence of the temperature on latitude could be due to heat sinks related to phase changes at the boundary of such a layer, they say. Since the relevant electrical properties of ice are similar to those of rock, Jupiter/NASA