Gunnar Fjeldbo

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.

The Mariner 10 Radio Occultation Measurements of the Ionosphere of Venus

Abstract Data from the Mariner 10 radio occultation experiment have been utilized to determine the vertical electron density distribution in the ionosphere of Venus. The ingress measurements, which were made at latitude 1.3°N on the nightside of the planet, show two distinct layers. The main layer was located at 142 km altitude and had a peak density of 9 × 103 electrons cm−3. A secondary layer with a peak density of 7 × 103 cm−3 was detected at 124 km altitude. During egress, the ionosphere was probed at latitude 56.0°S on the dayside of Venus. The solar zenith angle in this region was 67.0°. The dayside ionosphere consisted of a main layer with a peak density of 2.9 × 106 cm−3 at 142 km altitude and several minor layers. At the top of the dayside ionosphere, the measurements showed an abrupt drop in the density from 2000 cm−3 at 335 km attitude to below the level of detectability, i.e., less than 200 cm−3, at 360 km altitude. This abrupt density change may be the ionopause where the solar wind plasma in...

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.

Radio science investigations with Voyager

The planned radio science investigations during the Voyager missions to the outer planets involve: (1) the use of the radio links to and from the spacecraft for occultation measurements of planetary and satellite atmospheres and ionospheres, the rings of Saturn, the solar corona, and the general-relativistic time delay for radiowave propagation through the Suns gravity field; (2) radio link measurements of true or apparent spacecraft motion caused by the gravity fields of the planets, the masses of their larger satellites, and characteristics of the interplanetary medium; and (3) related measurements which could provide results in other areas, including the possible detection of long-wavelength gravitational radiation propagating through the Solar System. The measurements will be used to study: atmospheric and ionospheric structure, constituents, and dynamics; the sizes, radial distribution, total mass, and other characteristics of the particles in the rings of Saturn; interior models for the major planets and the mean density and bulk composition of a number of their satellites; the plasma density and dynamics of the solar corona and interplanetary medium; and certain fundamental questions involving gravitation and relativity. The instrumentation for these experiments is the same ground-based and spacecraft radio systems as will be used for tracking and communicating with the Voyager spacecraft, although several important features of these systems have been provided primarily for the radio science investigations.

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 .

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.