B. L. Seidel

The atmosphere of Mars from mariner 9 radio occultation measurements

Abstract The Mariner 9 spacecraft was used to perform 160 radio occultation measurements in orbit about Mars during November and December of 1971. At that time, Mars was experiencing a severely obscuring global dust storm. The effect of dust in the atmosphere was reflected in the reduced temperature gradients that were measured in the daytime near-equatorial atmosphere, indicating heating of the atmosphere by solar radiation being absorbed by dust and a simultaneous cooling of the surface. The temperature gradients increased somewhat with time, possibly indicating a gradual clearing of the atmosphere. Measurements made at 65° latitude near the morning terminator showed atmospheric temperatures consistent with condensation of carbon dioxide at low altitude. The surface pressures in the near equatorial regions ranged from a high of 8.9 mbar in Hellas to a low of 2.8 mbar in the Claritas and Tharsis areas, with a mean pressure of 4.95 mbar. The pressures deduced from measurements at 65° latitude ranged from 7.2 to 10.3 mbar, with a mean of 8.9 mbar. The pressure altitudes, referred to a pressure level of 6.1 mbar, show a range in the equatorial regions from a low of −4.4 km in Hellas to a high of 9.6 km in Claritas, with a net excursion of 14.0 km and a mean altitude of 2.7 km. In contrast, the region at 65° longitude shows uniformly negative altitudes, with a mean of −2.6 km. This disparity in pressures, which is also reflected in the measured radii between the near-equatorial and 65° latitude measurements, strongly suggests that the physical shape of Mars is more oblate than the shape of its gravitational equipotential surface, leading to higher atmospheric pressures near the poles than at the equator. However, more measurements at high latitudes are necessary to support this hypothesis. A daytime ionosphere having a peak density of about 1.5 − 1.7 × 10 5 el/cm 3 at an altitude of 140-134 km over a range of solar zenith angles of 56-47° was measured, showing some correlation between the variations in the peak density and the solar flux measured from the earth. The average topside plasma scale height was 38.5 km, showing little correlation with solar flux and solar zenith angle.

Preliminary Results on the Atmospheres of Io and Jupiter from the Pioneer 10 S-Band Occultation Experiment

The preliminary analysis of data from the Pioneer 10 S-band radio occultation experinment has revealed the presence of an ionosphere on the Jovian satellite Io (JI) having an electron density peak of about 6 x 104 electrons per cubic centimeter at an altitude of approximately 60 to 140 kilometers. This suggests the presence of an atmosphere having a surface number density of about 1010 to 1012 per cubic centimeter, corresponding to an atmospheric surface pressure of between 10-8 and 10-10 bar, at or below the detection threshold of the Beta Scorpii stellar occultation. A measurement of the atmosphere of Jupiter was obtained down to the level of about 80 millibars, indicating a large temperature increase at about the 20 millibar level, which cannot be explained by the absorption of solar radiation by methane alone and can possibly be due to absorption by particulate matter.

Venus - Mass, gravity field, atmosphere, and ionosphere as measured by the Mariner 10 dual-frequency radio system

Analysis of the Doppler tracking data near encounter yields a value for the ratio of the mass of the sun to that of Venus of 408,523.9 � 1.2, which is in good agreement with prior determinations based on data from Mariner 2 and Mariner 5. Preliminary analysis indicates that the magnitudes of the fractional differences in the principal moments of inertia of Venus are no larger than 10-4, given that the effects of gravity-field harmonics higher than the second are negligible. Additional analysis is needed to determine the influence of the higher order harmonics on this bound. Four distinct temperature inversions exist at altitudes of 56, 58, 61, and 63 kilometers. The X-band signal was much more rapidly attenuated than the S-band signal and disappeared completely at 52-kilometer altitude. The nightside ionosphere consists of two layers having a peak density of 104 electrons per cubic centimeter at altitudes of 140 and 120 kilometers. The dayside ionosphere has a peak density of 3 X 105 electrons per cubic centimeter at an altitude of 145 kilometers. The electron number density observed at higher altitudes was ten times less than that observed by Mariner 5, and no strong evidence for a well-defined plasmapause was found.

The atmosphere of Io from Pioneer 10 radio occultation measurements

Abstract The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S -band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 10 4 elcm −3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na + and 495 K if N 2 + is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 10 3 elcm −3 at an altitude of 50 km. The topside plasma temperature is 160 K for N 2 − and 131 K for Na + . If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10 11 −10 12 cm 3 , corresponding to a surface pressure of less than 10 −8 to 10 −9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation.

Pioneer 6: Measurement of Transient Faraday Rotation Phenomena Observed during Solar Occultation

Pioneer 6, which was launched into orbit around the sun on 16 December 1965, was occulted by the sun in the last half of November 1968. During the period in which the spacecraft was occulted by the solar corona, the S-band telemetry carrier underwent Faraday rotation as a result of this anisotropic plasma. The NASA-Jet Propulsion Laboratory 210-foot (64-meter) antenna of the Deep Space Network at Barstow, California, which was equipped with an automatic polarization tracking system, was used to measure this effect. Three large-scale transient phenomena were observed. The measurement of these phenomena indicated that Faraday rotation on the order of 40 degrees occurred. The duration of each phenomenon was approximately 2 hours. These phenomena appear to be correlated with observations of solar radio bursts with wavelengths in the dekametric region.

Mariners 6 and 7: Radio Occultation Measurements of the Atmosphere of Mars

Radio occultation measurements with Mariners 6 and 7 provided refractivity data in the atmosphiere of Mars at four points above its surface. For an atmosphere consisting predominantly of carbon dioxide, surface pressures between 6 and 7 millibars are obtained at three of the points of measurement, and 3.8 at the fourth, indicating an elevation of 5 to 6 kilometers. The temperature profile measured by Mariner 6 near the equator in the daytime indicates temperatures in the stratosphere about 100�K warmer than those predicted by theory. The measurements of Mariner 6 taken at 79�N at the beginning of polar night indicate that conditions are favorable for the condensation of carbon dioxide at almost all altitudes. Mariner 7 measurements taken at 58�S in daytime and 38�N at night also show that carbon dioxide condensation is possible at altitudes above about 25 kilometers. Measurements of the electron density in the ionosphere show that the upper atmosphere is substantially warmer than it was in 1965, possibly because of increased solar activity and closer proximity to the sun.

Mariner 9 S-Band Martian Occultation Experiment: Initial Results on the Atmosphere and Topography of Mars

A preliminary analysis of 15 radio occultation measurements taken on the day side of Mars between 40�S and 33�S has revealed that the temperature in the lower 15 to 20 kilometers of the atmosphere of Mars is essentially isothermal and warmer than expected. This result, which is also confirmed by the increased altitude of the ionization peak of the ionosphere, can possibly be caused by the absorption of solar radiation by fine particles of dust suspended in the lower atmosphere. The measurements also revealed elevation differences of 13 kilometers and a range of surface pressures between 2.9 and 8.3 millibars. The floor of the classical bright area of Hellas was found to be about 6 kilometers below its western rim and 4 kilometers below the mean radius of Mars at that latitude. The region between Mare Sirenum and Solis Lacus was found to be relatively high, lying 5 to 8 kilometers above the mean radius. The maximum electron density in the ionosphere (about 1.5 x 105 electrons per cubic centimeter), which was found to be remarkably constant, was somewhat lower than that observed in 1969 but higher than that observed in 1965.

Microwave antenna holography

The application of the holography technique to the newly constructed NASA/JPL Deep Space Network (DSN) 34-m beam-waveguide antenna, resulting in 4.1-dB performance improvement at 32 GHz by reducing the main reflector RMS surface error at 0.43 mm, is described. The improved antenna performance was verified by additional holographic measurements and efficiency measurements at X-band (8.45 GHz). Microwave holography has been demonstrated to be a required tool for achieving antenna aperture efficiency of 52% at Ka-band (32 GHz) and is probably necessary for maintaining an operational DSN Ka-band ground antenna capability. >

The occultation of Mariner 10 by Mercury

Abstract An analysis of the Mariner 10 dual frequency radio occultation recordings has yielded new information on the radius and atmosphere of Mercury. The ingress measurements which were conducted near 1.1° North latitude and 67.4° East longitude on the night side of the planet, gave a value for the radius of 2439.5 ± 1 km. Egress near 67.6° North latitide and 258.4° East longitude in the sunlit side yielded a radius of 2439.0 ± 1 km. The atmospheric measurements showed the electron density to be less than 10 3 cm −3 on both sides of the planet. From the latter result one may infer an upper limit to the dayside surface gas density of 10 6 molecules per cm 3 .

The shape of Mars from the Mariner 9 occultations

Abstract The extinction time of the radio signal, as the Mariner 9 spacecraft was occultated by Mars, together with an accurate ephemeris of the spacecraft were used to determine radii from the mass center to the occulting feature. Similarly estimations were made of the radius to a point where the pressure reached a certain fixed value. Several simple models were proposed to fit both sets of radii data.