Although the Phobos spacecraft will make some limited in situ measurements of Mars' upper ionosphere during its transfer orbits, the major part of the mission will be limited to remote sensing by radio occultation and topside sounding methods. To 'calibrate' the former as a means of studying the ionosphere and solar wind interaction, the Pioneer Venus Orbiter radio occultation experiment results are examined in light of what is known from in situ plasma and magnetic field data. This calibration can be used to reassess the data from previous Mars missions and to provide a basis for interpreting data from the upcoming Phobos mission.

Luhmann, J. G.↗

NASA NTRS1982

The Mars reference atmosphere

A nominal atmospheric model of Mars is presented, with detailed consideration being given to the various sources of variability in the Martian atmosphere. A basic model for the midlatitude summer is outlined for the northern and southern hemispheres. Attention is given to meteorological variability due to winds, diurnal and seasonal pressure variations, temperature changes, and the effects of dust opacity, particularly on temperature stratification. Viking lander IR thermal mapper data are examined in terms of diurnal and latitudinal temperature variations, and cloud and haze formation and locations are discussed. Mass spectrometry of the atmosphere is used to describe the molecular abundances, and water vapor measurements are reviewed. Finally, radio occultation, UV airglow, and mass spectrometry of the Martian upper atmosphere are investigated, along with the temperature structure of the upper atmosphere.

Kliore, A.↗

NASA NTRS1976

The occultation of Mariner 10 by Mercury

Analysis of 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 deg north latitude and 67.4 deg east longitude on the night side of the planet, gave a value for the radius of 2439.5 + or - 1 km. Egress near 67.6 deg north latitude and 258.4 deg east longitude on the sunlit side yielded a radius of 2439.0 + or - 1 km. The atmospheric measurements showed the electron density to be less than 1000 per cu cm on both sides of the planet. From the latter result one may infer an upper limit to the dayside surface gas density of 1 million molecules per cu cm.

Fjeldbo, G.↗

NASA NTRS1976

The Pioneer 11 radio occultation measurements of the Jovian ionosphere

Radio occultation data obtained with the Pioneer 11 spacecraft are utilized to study Jupiter's ionosphere. The ingress measurements, which were conducted by using a stable earth-based frequency reference for the tracking link, yielded ionospheric data near the morning terminator at about 79 deg south latitude. Data were also taken during egress on the evening side near 20 deg north latitude. The latter measurements were conducted in the one-way mode; i.e., an on-board crystal oscillator was employed as a frequency reference for the downlink (spacecraft-to-earth) signal. These data confirm previous results obtained with Pioneer 10 and show that Jupiter has a multilayered ionosphere extending over an altitude range of more than 3000 km. The topside scale height near 79 deg south latitude was 540 + or - 60 km. Assuming a topside electron, H(+) distribution controlled by diffusion yields a plasma temperature of 850 + or - 100 K in this region. The radio data indicate that the upper atmosphere is either warmer or more dissociated into atomic hydrogen than previously anticipated.

Fjeldbo, G.↗

NASA NTRS1975

Effect of the Jovian oblateness on Pioneer 10/11 radio occultations

The oblateness of Jupiter, which was neglected in the standard inversion technique for radio occultation data, has a substantial effect on the refractivity profiles of the neutral Jovian atmosphere derived from the Pioneer 10/11 occultations. The geometry of the Pioneer 10 entry and exit occultations has the effect that oblateness should account for much of the discrepancy of the results with plausible atmospheric models. The effect of the oblateness on Pioneer 11 entry is smaller, where much of the discrepancy could be due to other effects, such as cycle-slip in the spacecraft transponder. Theory indicates that the Pioneer 11 exit occultation should be affected by the oblateness in the opposite sense from the first three occultations; and, indeed, the data which had not been invertible under the spherical assumption produced reasonable results when an oblateness correction was applied.

Hubbard, W. B.↗

NASA NTRS1975

Atmosphere of Jupiter from the Pioneer 11 S-band occultation experiment - Preliminary results

Two additional radio occultation measurements of the atmosphere of Jupiter were obtained with Pioneer 11. The entry measurement leads to a temperature profile that is substantially in agreement with those obtained with Pioneer 10, showing temperatures much higher than those derived from other observations. The exit measurement is not usable because of the discontinuous drift of the spacecraft auxiliary oscillator, presumably due to the trapped radiation belts of Jupiter. The combination of two Pioneer 10 measurements and one Pioneer 11 measurement yields an oblateness of 0.06496 at 1 millibar and 0.06547 at 160 millibars. Measurements in the Jovian ionosphere indicate a number of layers distributed over about 3000 kilometers, with a topside temperature of about 750 K.

Kliore, A.↗

NASA NTRS1975

The Pioneer 10 radio occultation measurements of the ionosphere of Jupiter

Data from the Pioneer 10 radio occultation measurements are utilized to study the vertical electron number density distribution in the Jovian ionosphere. The immersion measurements were made at 26 North latitude in the late afternoon local time. The solar zenith angle in this region was 81 deg. Emersion measurements were made at 58 North latitude near the morning terminator where the solar zenith angle was 95 deg. The detectable portion of the Jovian ionosphere consists of a number of layers distributed over an altitude range of more than 3000 km. The maximum density appears to be on the order of 30,000 electrons per cu cm. Assuming that H(+) is the principal ion in the upper portion of the ionosphere yields a topside plasma temperature of 900 plus or minus 400 K.

Fjeldbo, G.↗

NASA NTRS1974

Pioneer 10 Jupiter atmospheric definition results: A summary

The various entry probes for measuring outer planetary atmospheric compositions are discussed. Considered are chemical components and physical accumulation processes observable by spectroscopic studies, as well as pressure gauges, temperature gauges, accelerometers, nephelometers, and visible and infrared sensors for determining abundances.

Wolfe, J.↗

NASA NTRS1974

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 experiment has revealed the presence of an ionosphere on the Jovian satellite Io having an electron density peak of about 60,000 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 10 billion to one trillion per cubic centimeter, corresponding to an atmospheric surface pressure of between .1 and 10 nanobar, 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.

Kliore, A.↗

NASA NTRS1972

Bistatic radar measurements of the surface of Mars with Mariner 1969.

The detection of echoes produced by oblique reflection of the RF (2300 MHz) spacecraft carrier from the Martian surface as Mariner 6 and 7 flew behind Mars in 1969, is described. Changes in echo center frequency and bandwidth are utilized to study the radius and roughness of the surface along a quasi-specular radar track that led from an optically dark and densely cratered region of Meridiani Sinus over into a smoother and brighter looking area of Thymiamata. A 3 to 1 decrease in surface roughness of large size compared to the wavelength (13 cm) was observed as the reflecting zone moved across the boundary between these two regions. The average radius obtained along the track was 3393 plus or minus 3 km. Due to large angles of incidence (86 to 90 deg), and surface shadowing, the data are not suitable for mapping the reflection coefficient of the surface material.

Fjeldbo, G.↗