Radar studies of the Martian surface at centimeter wavelengths: The 1975 opposition

The Goldstone radar system was operated at wavelengths of 3.5 and 12.6 cm to probe the Martian surface during the 1975 opposition. Regions studied in detail by range-Doppler techniques are Syrtis Major, Sinus Meridiani, and the crater Schiaparelli. Average rms slopes of 1.6° and 1.1° were measured in Syrtis Major at 3.5 and 12.6 cm, respectively, while the average reflectivity was 0.064 ± 0.02 at both wavelengths. No wavelength dependence of surface roughness was seen in Sinus Meridiani, where rms surface slopes averaged 1.8° and the reflectivity was 0.08 ± 0.02. The regions around Schiaparelli were probed at a 12.6-cm wavelength. The echo from the bottom of the crater was undetectable. Hence _ρ0C < 25, where _ρ0 is the reflectivity and C is the Hagfors roughness parameter. Operating at 3.5 cm during May and June of 1976, 149 continous-wave echo spectra were obtained near latitude 18°, sampling most longitudes including the early Viking landing sites A1 and A2. The average total radar cross section is 4.8% of the geometrical cross section. The diffuse component was estimated to be 1.9%, leaving 2.9% to the average quasi-specular component. The average rms slope is 4.1°. Six spectra obtained at site A1 indicate that rms slopes are 5 to 9° between latitudes 17 and 19°. Three spectra obtained at s site A2 indicate an rms slope of 3.9°.     

Radar measurement of small-scale surface texture: Syrtis major

Recent radar observations of Syrtis Major have shown it to be smooth and relatively homogeneous when sensed at centimeter wavelengths λ. There is a gradual decrease in surface roughness east to west across the basin, which correlates with an apparent decrease in small (< 1 km diameter) crater density. Root mean square surface slopes σ range from more than 1.5–2.0° in the east to less than 0.5° along the western margin at λ = 12.6 cm. The surface appears somewhat rougher at λ = 3.6 cm; a σ ∝ λ^?0.3 dependence is inferred. Radar reflectivity increases from about 5% to about 12% across Syrtis Major, being greatest near the western margin. High-resolution (8 to 20 m/pixel) Viking images suggest that an increasing amount of resurfacing has occurred in western Syrtis Major compared with the eastern parts. The radar, infrared, and optical imaing evidence are consistent with resurfacing by geologically recent, low-viscosity lavas which were subsequently mantled by thin layers of aeolian material. Each data set may be taken as a unique source of scale-dependent information on surface materials and properties. From comparison of radar-derived surface roughness with image-derived crater density curves we conclude that processes other than cratering control the surface texture on 0.03- to 100-m surface scales.     

Observations of Saturn's inner satellites and the orbit of Janus in 1980

Systematic observations of faint satellites were conducted at Pic-du-Midi with a focal coronograph from 1980 March 20 to 24, during which 150 exposures covering 17 hr were obtained; in addition, the 1966 discovery plates of satellites S10 Janus were reexamined together with other 1966 observations. Janus had its greatest eastern elongation on 1966 December 15.720 (±0.003) + light time, at a distance of 2.53R_eq. It is recognized that some of the observations thought to be 1966 S2 were in fact reobservations of Janus a few days after its discovery. Among the 1980 observations, differences in magnitudes indicate that is satellite 1980 S1 which corresponds to Janus; its greatest eastern elongation was observed on March 23.876 (±0.002) + light time. Subjected to corrections for librations, the mean period over the past 14 years has most probably one of three values: P₁ = 0.69458 day, P₂ = 06.9448 day or P₃ = 0.69438 day. The fainter satellite, S11, which is also 1980 S3, gravitates in an orbit similar to that of Janus and was leading it by +190° in March 1980; this difference of longitude was +224° in December 1966. An object of magnitude 15–16 was seen not detached from the ring; it could be a condensation in the external part of the rings or an additional faint inner satellite.     

Mariner 9 ultraviolet spectrometeer experiment: 1971 Mars' dust storm

The Mariner 9 ultraviolet spectrometer observed the brightness of a region on the south polar cap centered at approximately ?87°S, 10°W. Measurements taken at various incidence and emission angles (i and ?) show that the brightness increased with decreasing air mass, ≈(sec i + sec ?). The observed intensity consists primarily of a component reflected from the cap and twice-attenuated by the atmosphere and a component diffusely reflected from the atmosphere. The diffusely reflected component was determined from nearby observations of non-polar regions at the same incidence and emission angles and was substrated from the total intensity. Inversion of the intensity difference using a formula analogous to the Bouger-Langley law yielded the optical thickness of the atmosphere. The dust cloud over the polar cap was moderately thick between November 26 and December 2, 1971. At this time the optical thickness was near unity, and it decreased approximately linearly with time, reaching a value close to that of a Rayleigh atmosphere by mid-February. The optical thickness showed little dependence on the wavelength during the early orbital observations. As the dust storm cleared, the atmospheric optical thickness exhibited increasingly strong inverse wavelength dependence. Particles large compared with the wavelength dominated the Martian dust storm. These particles are estimated to have a mean radius of about 2 μm.     

Viking bistatic radar experiment: Summary of first-order results emphasizing north polar data

Bistatic radar observations of Mars' north polar region during 1977–1978 showed surface rms slope σ_β ranging from 1 to 6°; these values apply to horizontal scales of 1–100 m. Values of roughness tend to decrease with increasing latitude (especially over 65–80°N), but there are many exceptions. The smoothest surfaces (σ_β≤1°) appear to be inclusions within generally rougher (σ_β～3°) terrain, rather than broad expanses of very smooth material. The permanent north polar cap is relatively uniform with 2.5?σ_β?3.0°. Considerable structure has been found in echo spectra, indicating a heterogeneous and perhaps anisotropic scattering surface. Echo spectra obtained from the same region, but several months apart (1°<L_S<62°), show no significant differences in inferred roughness. Estimates of reflectivity and dielectric constant are systematically low in the polar region. This may indicate that surface material north of 65°N is less dense than that near the equator, but more study of these data is needed. Estimates of surface roughness and dielectric constant in the equatorial region are consistent with results from Earth-based measurements to the accuracy of our analysis.     

Comet C/2002 T7 (LINEAR): Polarimetric and photometric studies

We present the results of polarimetric and photometric observations of dynamically new comet C/2002 T7 (LINEAR) at phase angles from 6° to 26°. During the observations, the comet was at a distance of 2.7–1.3 AU from the Sun and 1.7–2.0 AU from the Earth. The aperture polarimetry was made with the 2.6-m Shain telescope and the 1.25-m AZT-11 telescope of the Crimean Astrophysical Observatory and with the 0.7-m telescope of the Astronomical Institute of the Kharkiv National University during the period from November 21, 2003, to February 21, 2004. The wideband UBVRI and WRC (λ7228/1142 Å) filters and the narrowband GC (λ5260/56 Å) filter were used. The photometric observations of the comet were carried out on February 21, 2004, with narrowband filters isolated the BC (λ4845/65 Å) and RC (λ6840/90 Å) continuum and the C₂ emission (λ5140/90 Å). The phase-angle dependence of linear polarization of the comet has been obtained, and its parameters, such as the minimal polarization P _min = ?1.63%, the phase angle of the minimal polarization α_min = 10.6°, the inversion angle α_inv = 22.7°, and the slope of the phase curve at the inversion angle h = 0.24% per degree, were found. From the photometric observations, the following quantities have been obtained: the column density of molecules C₂ in the line of sight logN (C₂) = ?9.15 mol/cm² and their production rate log Q (C₂) = 27.11 mol/s, the spectral gradient of reflectivity for the dust S′(BC, RC) ≈ 3%/1000 Å, and the dust production parameter Afρ equal to 371 and 273 cm for the blue and red continuum ranges, respectively. According to these results, the physical parameters of comet C/2002 T7 are close to the average characteristics of typical dusty comets.     

Spatially resolved infrared observations of Saturn III. 10- and 20-μm disk scans at B′ = −11°.8

Disk scans of Saturn at 10 and 20 μm were obtained when the Saturnicentric solar declination (B′) was ?11°.8. The scans show little change from scans obtained when B′ was ?16°.3, and this could result from the long radiative time constant of the Saturnian atmosphere. The observations at 20 μm, in the H₂ continuum, show positively that the temperature inversion at the south pole has a higher temperature than at any other point on the disk. In addition, the 12.1- and 20-μm scans indicate that the temperature of the inversion region is higher at the equator compared to the temperate zone. The data also suggest that enhanced 20-μm emission is correlated with the strength of the ultraviolet absorption.     

PGC 60020: a polar-ring galaxy

We present an analysis of new observations of a peculiar galaxy PGC60020, obtained with the 6-m BTA telescope of the SAO RAS with a multimode SCORPIO instrument. The observational data includes direct images in the B, V, R _c photometric bands and long-slit spectra in the red range (the H?? line spectral region). Based on the analysis of these data it was found that PGC60020 belongs to the type of classical polar-ring galaxies. Its main body is an S0 galaxy, around the major axis of which a disk of gas, dust and stars is rotating in the plane inclined at an angle of about 60° to the galactic plane. A loop-shaped structure stretches from the southern part of this disk (possibly, a tidal tail) towards the SDSS J 171745.58+404137.1 galaxy.     

Mars and Jupiter: Radio emission at 1.35 cm

We report observations of the radio disk temperatures of Mars and Jupiter made during October 1971, at a wavelength of 1.35 cm. The mean disk temperature of Jupiter is 136 ± 5°K, in good agreement with the value 139 ± 6°K obtained by Wrixon et al. (1971) with the same instrument three years earlier. The disk temperature of Mars is 181 ± 11°K, consistent with an essentially wavelength independent disk temperature for Mars at radio wavelengths. The ratio of the two disk temperatures, 1.33 ± .07, is largely free of the systematic uncertainties: antenna gain, pointing, and atmospheric extinction.     

Topography and radar scattering properties of Mars

High-resolution radar observations of Mars at a wavelength of 3.8 cm have been carried out at the Haystack Observatory for a period of about 6 months surrounding the 1971 opposition. The relative surface height variation with longitude over a band of Martian latitudes between about ?14° and ?22° has been derived from these observations with an error of about 75m in the most favorable cases. At ?14° latitude, the dielectric constant was found to increase steadily from about 1.7 at 70°W to about 5.0 at 110°W. Over the same interval the rms surface slopes vary from 2° to 1°. The mean equatorial radius of Mars as determined from the combined radar data of 1967, 1969 and 1971 was found to be 3394 ± 2km.     

Search for methane and upper limits to ethane and SO2 on Mars

The IRTF/CSHELL observations in February 2006 at L_S = 10° and 63–93°W show ～10 ppb of methane at 45°S to 7°N and ～3 ppb outside this region that covers the deepest canyon Valles Marineris. Observations in December 2009 at L_S = 20° and 0–30°W included spectra of the Moon at a similar airmass as a telluric calibrator. A technique for extraction of the martian methane line from a combination of the Mars and Moon spectra has been developed. The observations reveal no methane with an upper limit of 8 ppb. The results of both sessions agree with the observations by Mumma et al. (Mumma, M.J. et al. [2009]. Science 323, 1041–1045) at the same season in February 2006 and are smaller than those in the PFS and TES maps. Production and removal of the biological methane on Mars do not significantly change the redox state of the atmosphere and the balance of hydrogen. A search for ethane at 2977 cm^?1 results in an upper limit of 0.2 ppb. However, this limit does not help to establish the origin of methane on Mars. Reanalysis of our search for SO₂ using TEXES confirms the recently established upper limit of 0.3 ppb. Along with the lack of hot spots and gas vents with endogenic heat sources in the THEMIS observations, the very low upper limit to SO₂ on Mars does not favor geological methane that is less abundant than SO₂ in the outgassing from the terrestrial planets.     

Analysis of the orbit of cosmos 395 rocket (1971-13B) near 15th-order resonance

Cosmos 395 rocket (1971-13B) is moving in a near-circular orbit inclined at 74° to the equator. Its average height, near 540 km after launch in February 1971, slowly decreased under the action of air drag and on 24 March 1972 it experienced exact 15th-order resonance, with the successive equator crossings 24° apart in longitude. Its orbit has been determined at 21 epochs between September 1971 and September 1972 using 1100 observations, including 55 from the Malvern Hewitt camera: the mean S.D. in inclination is 0.001° and in eccentricity 0.00001.The variations in inclination i, eccentricity e, right ascension of the node $\text{Ω}$, and argument of perigee ω, near 15th-order resonance are analysed to determine values of lumped 15th-order harmonic coefficients in the geopotential. The inclination yields equations accurate to 4 per cent for coefficients of order 15 and degree 15,17,19..., which are in excellent agreement with those from Cosmos 387 (1970-111A) in an orbit of similar inclination but different resonant longitude. Analysis of the variations in e gives two pairs of equations for the coefficients of order 15 and degree 16, 18..., which are used to obtain tentative values of the (16,15) coefficients. For the first time the resonant variation of other elements ($\text{Ω}\text{and}\text{ω}$) has also been analysed with partial success.     

The rotation period and pole orientation of asteroid 4 Vesta

In 1971 asteroid Vesta was observed in a region of the sky in which it had never been observed before. Its photometric lightcurve had two distinct maxima. Those observations have been the only strong evidence to support a rotation period of about 10 hr 41 min. Lightcurves made in 1982, when Vesta was at the same aspect as 1971, do not show two different maxima. It is concluded that there was a systematic error in the 1971 observations. At this time a definitive statement cannot be made about the true period of Vesta, although the 5 hr 20 min period does appear more plausible. Radar echoes in 1988 and 1992 should resolve the problem. The shorter rotation period was assumed and the photometric astrometry method applied. The sidereal period is 5 hr 20 min 31.68 sec 0.2225889 ± 0.0000002 days, the rotation is prograde, and the coordinates of the north pole are 103° longitude and +43° latitude with an uncertainty of abour 6°.     

Far-infrared brightness temperature of Saturn's disk and rings

We present far-infrared observations of Saturn in the wavelength band 76–116 μm, using a balloon-borne 75-cm telescope launched on 10 December 1980 from Hyderabad, India, when B′, the Saturnicentric latitude of the Sun, was 4°.3. Normalizing with respect to Jupiter, we find the average brightness temperature of the disk-ring system to be 90 ± 3° K. Correcting for the contribution from rings using experimental information on the brightness temperature of rings at 20 μm, we find T_D, the brightness temperature of the disk, to be 96.9 ± 3.5° K. The systematic errors and the correction for the ring contribution are small for our observations. We, therefore, make use of our estimate of T_D and earlier observations of Saturn when contribution from the rings was large and find that for wavelengths greater than 50 μm, there is a small reduction in the ring brightness temperature as compared to that at 20 μm.     

Titan’s aerosol and stratospheric ice opacities between 18 and 500 μm: Vertical and spectral characteristics from Cassini CIRS

Vertical distributions and spectral characteristics of Titan’s photochemical aerosol and stratospheric ices are determined between 20 and 560 cm^?1 (500–18 μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined.In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ～3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC₃N), as inferred from a composite emission feature at ～160 cm^?1, appear to be located over a narrow altitude range in the stratosphere centered at ～90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S.There is some evidence of a second ice cloud layer at ～60 km altitude at 58°S associated with an emission feature at ～80 cm^?1. We speculate that the identify of this cloud may be due to C₂H₆ ice, which in the vapor phase is the most abundant hydrocarbon (next to CH₄) in the stratosphere of Titan.Unlike the highly restricted range of altitudes (50–100 km) associated with organic condensate clouds, Titan’s photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3–2.4 at about 160 km to 1.1–1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ～140 cm^?1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics.Finally, volume extinction coefficients Nχ_E derived from 15°S CIRS data at a wavelength of λ = 62.5 μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 μm. This comparison yields volume extinction coefficient ratios Nχ_E(1.583 μm)/Nχ_E(62.5 μm) of roughly 70 and 20, respectively, for Titan’s aerosol and stratospheric ices. The inferred particle cross-section ratios χ_E(1.583 μm)/χ_E(62.5 μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.     

Martian albedo feature variations with season: Data of 1971 and 1973

This paper is a qualitative and quantitative investigation of recent seasonal and secular albedo feature variations on Mars using the improved red-filter photography of the International Planetary Patrol Program obtained during the 1971 and 1973 apparitons. The boundaries of telescopic albedo features showing significant changes and uncommonly bright areas were measured with an image projection reader and plotted on Mars maps in order to compare changes with Mariner 9 variable feature data obtained in 1972. The Daedalia-Claritas darkening (90–130°W, 25°S) of 1973 was the most significant change observed on Mars since the Laocoontis-Alcyonius darkening of the 1950 decade. The 1973 apparition presented six excellent examples of normal seasonal changes.Photographic photometry of 21 albedo features located in three longitude regions was systematically done on 120 original red-filter negatives, about half representing the Martian southern spring season and the other half the southern summer season. These two sets of photographs differed about 90° in L_s but were nearly equal in viewing geometry. They were identical in image scale, emulsion, color response, and quality. The data, consisting of 3960 density measurements, were analyzed in terms of albedo ratios and their fluctuations. Several types of diagnostic graphs are presented.Seasonal differences of the mean brightness ratios for most of the features were found to be consistent with the contrast changes shown on photographic Mars maps. Seasonal variations of brightnesses with latitude and region were indicated, but not in the strict classical sense of a wave-of-darkening. Most albedo features were more variable during southern spring than they were in summer. Most light albedo features were more active than dark ones during both seasons. There was a general loss of relative contrast in southern summer. No systematic dependence on solar phase angle was detected within the range (38° > i > 16°) spanned by both sets of photographs.     

Atmospheric wave propagations in the mesopause region observed by the OH(8,3) band,NaD, O2A(8645Å) band and OI 5577 Å nightglow emissions

Simultaneous measurements of the upper mesospheric NaD and OH(8,3) band emissions by meridional scanning photometers, and the OI 5577 Å, O₂ Atmospheric band at 8645 Å, NaD and OH(8,3) band emissions by multi-channel tilting filter type zenith photometers have been carried out at Cachoeira Paulista (22.7°S, 45.0°W), Brazil. On two nights during the period May–August 1983, the meridional scanning observations showed horizontal intensity gradients and phase propagations. The nocturnal intensity variations on one of these occasions 13–14 June 1983, which was a magnetically disturbed night with 4 ?k_p? 8, also showed vertical phase propagation. In this paper, we present these observations and discuss the possible effects of the horizontal wind system and of gravity wave propagation.     

A theoretical model of the bow shock and the magnetosphere of Jupiter

Using the data obtained from the Pioneer 10 and 11 observations, a theoretical model is proposed for the bow shock and the magentosphere of Jupiter. This indicates that the distance of the magnetopause from Jupiter on the sunlit side is (50–55) × r_J (r_J: Jupiter radius, = 7 × 10⁹ cm) and that the ratio of the stand-off distance to this distance is about equal to or slightly larger than unity. Hence the Mach number of the solar wind seems to be less than 1.5 at Jupiter's orbit. This result necessarily leads to a blunt body model of the Jovian magnetosphere, the tail region of which is not as extended as observed in the Earth's case.     

Water vapor mapping on Mars using OMEGA/Mars Express

A systematic mapping of water vapor on Mars has been achieved using the imaging spectrometer OMEGA aboard the Mars Express spacecraft, using the depth of the 2.6 μm (ν₁, ν₃) band of H₂O. We report results obtained during two periods: (1) Ls=330–40° (January–June 2004), before and after the equinox, and (2) Ls=90–125°, which correspond to early northern summer. At low latitude, our results are globally consistent with previous measurements from ground-based and space (MAWD/Viking and TES/MGS) observations. However, at early northern summer and at high northern latitude (70–80 °N), the water vapor abundances, which we retrieved, appear to be weaker than MAWD and TES results. At the time of water sublimation during early northern summer, there is a maximum of water vapor content at latitudes 75–80°N and longitudes 210–24°E. This region is not far from the area where OMEGA identified a high abundance of calcium-rich sulfates, most likely gypsum. Our data provide the first high-resolution map of the martian water vapor content above the northern polar cap.     

Radiance factor calibration of near-infrared spectral images of Mars

We present radiometrically calibrated spectrophotometric images of Mars taken at the NASA Infrared Telescope Facility (IRTF) near and during the 1995 and 1999 oppositions. Absolute intensity and radiance factor (r_F = I/F) values have been calculated for approximately 95% of the surface over all longitudes between −70° to 90° latitude in the 1.5- to 4.1-μm spectral region at a spectral resolution (Δλ/λ) of 1.5%. Values of radiance factor range from r_F = 0.4 to 0.6 at 2.2 μm for the bright regions such as Moab and Arabia to r_F = 0.12 to 0.3 at 2.2 μm around the dark regions Syrtis Major and Acidalia Planitia. Variations are seen due to seasonal dust and/or condensate cloud cover and viewing geometry. Our results are generally consistent with the few reported previous radiance factor determinations for Mars. These data are unique among ground-based data in their relatively high spatial resolution (?200 km/pixel at the sub-Earth point) and coverage combined with their spectral resolution and coverage. These radiometrically calibrated observations can be used as input to studies focusing on spectral unmixing of surface and atmospheric components, radiative transfer modeling of disk and limb radiances, and photometric modeling of the martian phase function.