General Description of the Experiment

At the time when the earliest spectroscopic data for the Martian atmospheric pressure were beginning to appear, preparations were in hand for launching a Mariner spacecraft to Mars (ch. XIII). The trajectory of Mariner IV, which passed close to Mars on July 14, 1965, was deliberately chosen so that information could be obtained about the atmosphere of the planet. The spacecraft carried a transmitter which generated radio waves at a constant frequency (2300 million cps). These waves were normally utilized for tracking purposes and for telemetering information from the spacecraft instruments to receivers on Earth.

As Mariner IV passed close to Mars, the radio signal was transmitted first through the upper levels (ionosphere) of the atmosphere of Mars on its way to Earth (fig. 5.6). In due course, the signal passed successively through the lower layers of the Martian atmosphere, until the spacecraft went behind the planet, as viewed from Earth, and the signal was cut off. Mariner IV was then said to be occulted by Mars.

FIGURE 5.6. Occultation of Mariner IV by Mars.

The occultation lasted for almost 54 minutes, after which the signal from Mariner IV was again received on Earth. At first the signal passed through those layers of the Martian atmosphere closest to the surface. The later signals were transmitted through the upper layers on their way to Earth. Before occultation, Mariner IV was on the daylight side of Mars, and the radio signals passed through the sunlit atmosphere of the planet. After occultation, the part of the atmosphere through which the transmission occurred was on the night side of Mars (fig. 5.6).

The radio signal from Mariner IV in its normal transmission to Earth—that is, when it was not passing through the Martian atmosphere—was affected by two factors. These are the motion of the spacecraft with respect to Earth and passage of the radio wave through Earth's atmosphere to the receiver. A radio signal passing close to Mars, however, is changed considerably because the path of the radio waves is bent (or refracted) by the Martian atmosphere (fig. 5.7). Furthermore, the extent of the bending (refractive index) of the path of the radio signal varies with the altitude at which it passes through the atmosphere of Mars on its way to Earth.

Results of the Occultation Experiment

From very precise measurements, made by receivers on Earth, on the signals transmitted by Mariner IV, both well before and after occultation by Mars, the characteristic signals expected in the interval were calculated. By subtracting these ideal theoretical values from the observations, the effect of the Martian atmosphere on the radio waves could be determined. The results, given as a change in the equivalent, or phase, path, expressed in cycles or wavelengths as a function of time, are reproduced in figure 5.8.

It is seen that at about 02:29:20 (Greenwich mean time), the electrically charged ionosphere of Mars is just beginning to affect the radio transmission. At this time, the signal was passing through a level in the atmosphere estimated to be at an altitude of about 250 kilometers (150 miles). Subse

FIGURE 5.7. Refraction of radio signal by the atmosphere of Mars.

Atmosphere

FIGURE 5.7. Refraction of radio signal by the atmosphere of Mars.

quently, this negative effect increased and attained its maximum magnitude indicated by the minimum in the curve at 02:30:12. The altitude of the level then being probed by the signal was roughly 120 kilometers (74 miles) from the surface. There is a small bend in the curve at 02:30:22, which is thought to be caused by passage of the radio signal through a lower level, just below 100 kilometers (62 miles) altitude, in the ionosphere.

The effect of the neutral, lower atmosphere on the transmitted signal commenced around 02:30:50 at an altitude of roughly 50

of 20 cn c

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