Radio Occultation Exploration of Mars

The radio occultation technique, consisting of the observation of changes in the phase, frequency, and amplitude of a radio signal from a spacecraft as it passes through the atmosphere of a planet before and after occultation, was first applied to measure the atmosphere of Mars with the Mariner IV spacecraft in 1965. The interpretation of these changes in terms of refraction of the radio beam by the neutral atmosphere and ionosphere of the planet provided the first direct and quantitative measurement of its vertical structure and established the surface atmospheric pressure of Mars as lying between 5 and 9 mb. The presence of a daytime ionosphere with a peak electron density of about 105 el cm−3 was also measured. The Mariner VI and VII spacecraft flew by Mars in 1969 and provided an additional four measurements of the atmosphere and surface radius of the planet. They confirmed the surface pressure values measured by Mariner IV and provided data for a crude estimate of the shape of the planet.By far the greatest volume of radio occultation information on the atmosphere and surface of Mars was returned by the Mariner IX orbiter which was placed in orbit about Mars in November of 1971. During three occultation episodes in November-December 1971, May-June 1972, and September-October 1972, the Mariner IX mission provided 260 successful radio occultation measurements.The early measurements, made at the time of the Martian dust storm of 1971, showed greatly reduced temperature gradients in the daytime troposphere, indicating the heating effect of the dust. The temperature gradients that were measured later in the mission, when the atmosphere was apparently free of dust, were still much lower than expected under conditions of radiative-convective balance, indicating that dynamics may play a large part in determining the temperature structure of the Martian troposphere. Temperatures taken at night near the winter poles were consistent with the condensation of carbon dioxide.The surface atmospheric pressure was observed to vary widely with topography ranging from about 1 mb at the summit of the Middle Spot volcano (Pavonis Mons) to over 10 mb in the North circumpolar region. In the South equatorial region the highest surface pressure of about 9 mb was measured at the bottom of the Hellas basin.The radius of the planet was measured with accuracies ranging from about 0.25 to about 2.1 km over latitudes ranging from 86° to −80°. These measurements have shown that Mars has pronounced equatorial and north-south asymmetries, which make it difficult to represent its shape by a simple triaxial figure.The daytime ionosphere measurements indicated that the main ionization peak was similar in behavior to a terrestrial F1 layer and is probably produced by photoionization of carbon dioxide by solar extreme ultraviolet. Comparison of the heights of the maximum between the early data taken in November-December, 1971, and the Extended Mission of May-June 1972, showed that the lower atmospheric temperatures decreased by about 25%, which is consistent with clearing of the atmosphere.The experience gained from Mars radio occultation experiments suggests that the quality of data can be significantly improved by such features of the spacecraft radio system as a stable oscillator, dual frequency downlink capability, and a steerable high-gain antenna.