M. J. S. Belton

A discussion of Martian atmospheric dynamics

Abstract We have computed radiative relaxation times for temperature perturbations in CO 2 with small admixtures of water vapor under conditions which are similar to those occurring in the martian atmosphere. Certain aspects of the influence of radiative transfer on dynamical phenomena in the martian atmosphere are discussed in terms of these radiative relaxation times. It is found that: (i) diurnal changes in the temperature profile will extend to considerably greater heights in the martian atmosphere than is the case on Earth; the diurnal wave will propagate more by radiative transfer than by turbulent diffusion; (ii) a non-negligible diurnal wind system exists on Mars; (iii) non-linear interactions between the motion field and the initial state of radiative equilibrium are much smaller on Mars than on Earth; (iv) radiative de-stabilization of regions with a negative lapse rate may be important in the upper martian atmosphere.

An interpretation of the near-ultraviolet absorption spectrum of SO2: Implications for Venus, Io, and laboratory measurements

Abstract The character of the line spectrum of SO 2 between 2000 and 3200 A is described, and a crude band model for its absorption characteristics longward of 2280 A is formulated. The model predicts planetary spectra that are substantially different from those predicted using a continuum model (Beers law) for SO 2 absorption, the latter model being widely used to interpret the IUE spectrum of Io and ground-based and Pioneer orbiter spectra of Venus. As a result, the SO 2 scale height and mixing ratio in the Venus stratosphere could be substantially different from the values that have been previously supposed, thereby changing the range of acceptable photochemical models. Also, it is found that the absence of the signature of SO 2 vapor absorption in Ios spectrum can be treated as a physically reasonable lower limit to the mean column abundance of SO 2 on the satellite as well as an upper limit. Thus a substantial SO 2 atmosphere may yet be found on Io. Laboratory measurements to establish accurate parameters for the band model are urgently required.

The periods of Neptune: Evidence for atmospheric motions

Abstract An extended photometric time series in the J and K bands of Neptune has a complex appearance which appears to require the simultaneous presence of three periodicities plus related harmonics in the ( J - K ) color. The most apparent of the fundamental periods is N1 = 17.73 hr. The two others are at N2 = 18.56 and N3 = 18.29 hr and may be the result of amplitude modulation of a previously reported period of 18.42 hr. We interpret the presence of multiple periodicity as indicating that distinct systems of zonal winds exist on the planet. We argue that these wind systems are probably confined to moderate or high latitudes on the basis of recent omages of the planet taken in a spectral region of strong CH 4 absorption, and, by analogy to the zonal wind systems that exist in Jupiters atmosphere, deduce a period of rotation for the body of the planet of 18.2 ± 0.4 hr. Zonal wind contrasts of up to 109 m sec −1 are implied in the atmosphere of Neptune by these observations.

Comments on the rotational state and non-gravitational forces of comet 46P/Wirtanen

Abstract Experience of modeling the rotational state and non-gravitational forces of comet 1P/Halley and other comets is applied to comet 46P/Wirtanen. While the paucity of physical data on 46P/Wirtanen makes this process somewhat speculative, this comets place as a target for the important Rosetta mission gives significance to such a study. The arguments are based on the summary of observational data provided by Jorda and Rickman (Planet. Space Sci. 43, 575, 1995) and a comparative study of the behavior of other periodic comets. It is found that 46P/Wirtanen has a level of surface activity relative to its mass that is dynamically more akin to that found in comet 1P/Halley than in a typical periodic comet. It is shown through an illustrative numerical example that this apparent fact should likely lead to an excited spin state for this comet and that significant changes in the spin period could occur in a single pass through perihelion. It is argued that the available observations are not sufficient to substantiate the claim of Jorda and Rickman (Planet. Space Sci. 43, 575, 1995) that the nucleus is undergoing retrograde rotation and it is possible that the rotation is prograde as well as retrograde. The substantial requirements that must be placed on any future observing program necessary to determine the precise rotational state are outlined. An extended (∼2 month) southern hemisphere observing campaign is advocated to determine the nuclear rotational state in 1996 if possible before activity turns on.

An estimate of the temperature and abundance of CH4 and other molecules in the atmosphere of Uranus

Abstract We present a preliminary analysis of CH4 absorptions near 6800 A in new high resolution spectra of Uranus. A curve of growth analysis of the data yields a rotational temperature near 100 K and a CH4/H2 ratio that is 1 to 3 times that expected for a solar type composition. The long pathlengths of CH4, apparently demanded by absorptions near 4700 A, are qualitatively shown to be the result of line formation in a deep, predominantly Rayleigh scattering atmosphere in which continuum absorption is a strong function of wavelength. The analysis of the CH4 also yields a minimum value for the effective pressure of line formation (∼ 2 atm). This value is shown to be twice that expected on Uranus if the atmosphere were predominantly H2. It is speculated that large amounts of some otherwise optically inert gas is present in the Uranus atmosphere. N2 is suggested as a possible candidate since there are cosmogonic reasons why Uranus should contain large amounts of N relative to C, He, and H, and also because the pressure-induced pure rotation spectrum of N2 could possibly account for the low brightness temperatures that have recently been observed at 33 and 350 μm. If N2 is present the planet probably possesses a surface at the 10–100 atmosphere level.

The distribution of CO2 on Mars: A spectroscopic determination of surface topography

Abstract A map of the distribution of CO 2 over the surface of Mars has been prepared from spectrophotometric observations of the ν 1 + 2 ν 2 + 3 ν 3 band of CO 2 at 1.05μ. The map covers the equatorial of the planet between latitudes −20° and +40°, stretches from 240°W through the meridian to 160°W and has a nominal precision of ±1 mb. Emphasis is placed on the details of the experimental method and reduction procedure. The pressure map is interpreted in terms of large-scale surface topography (resolution 800–1500 km). The dominance of the second harmonic in zonal topography is clearly evident by the existence of two broad (4000 km), high (12 km) ridges running approximately north-south across the planet seperated by about 180° in longitude. With one minor exception, all bright areas are high, although low areas can be both dark and bright. Syrtis Major is associated with a strong slope but the signiificance of this is doubtful because of the presence of a similar slope in a light area. The spectroscopic map is compared with data from other sources. Good agreement is found with the infrared spectrometer results of Herr et al . (1970), and the radio occultation data, both from Mariners 6 and 7. Comparison with ground-based radar data show good agreemenr at most longitudes but there is a considerable discrepancy between 80°W and 160°W. The most probable cause is found to be a systematic calibration error, incurred in normalizing spectroscopc data from different nughts to a common standard. (The problem is a closure error which can be avoided in the future.) A second map, normalized against radar data, is presented; it does not agree as well with the Mariner results. The physical bases of the radar and spectroscopic methods are compared, and the possibility is discussed of using the discrepancies to provide further inforation. The most promising prospects are the oblateness of the planet, gravitational anomalies, and violent Maatian weather.

Neptune's rotation period: A correction and a speculation on the difference between photometric and spectroscopic results

Abstract An error in the Hayes and Belton (1977) , Icarus 32 , 383–401) estimate of the rotation period of Neptune is corrected. If Neptune exhibits the same degree of limb darkening as Uranus near 4900 A, the rotation period is 15.4 ± 3 hr. This value is compatible with a recent spectroscopic determination of Munch and Hippelein (1979) who find a period of 11.2 −1.2 +1.8 hr. However, if, as indirect evidence suggests, the law of darkening on Neptune at these wavelengths is less pronounced than on Uranus, then the above estimates may need to be lengthened by several hours. Recent photometric data are independently analyzed and are found to admit several possible periods, none of which can be confidently assumed to be correct. The period of Neptune most probably falls somewhere in the range 15–20 hr. The Hayes-Belton estimate of the period of Uranus is essentially unaffected by the above-mentioned error and remains at 24 ± 4 hr. All observers agree that the rotation period of Uranus is longer than that of Neptune.

Why image Uranus

Abstract A review of visual and photographic data on the appearance of Uranus indicates that markings frequently occur on the planet. The featureless images obtained by the Stratoscope II balloon telescope are possibly the result of the broad spectral band that was used. Difference, or ratio, picture techniques which enhance color or polarization contrasts are proposed as the basis for Uranus imagery on the 79 MJU Mission. An attempt is made to predict the aspect of Uranus at high resolution on the basis of what is currently known about the Uranus atmosphere. The planet should have no visible surface, the tops of a thick NH 3 cloud layer should exist near the 3–4 bar level and there is a very uncertain possibility of a thin, broken CH 4 cloud layer near 300 mbar. It is proposed that if the choice of an MJU imaging system rests on Uranus objectives alone (i.e., excluding the satellites) then the system should emphasize photo-polarimetric observations between 5500 and 10 000 A. If, however, the total mission objectives are the basis of choice then a high resolution imaging system, based on the Mariner Jupiter-Saturn system, but including a solid state silicon array would be a more suitable choice. The performance of such a system at Uranus is analyzed.