Imaging the surface of Mercury using ground-based telescopes

We describe and compare two methods of short-exposure, high-definition ground-based imaging of the planet Mercury. Two teams have recorded images of Mercury on different dates, from different locations, and with different observational and data reduction techniques. Both groups have achieved spatial resolutions of <250 km, and the same albedo features and contrast levels appear where the two datasets overlap (longitudes 270–360°). Dark albedo regions appear as mare and correlate well with smooth terrain radar signatures. Bright albedo features agree optically, but less well with radar data. Such confirmations of state-of-the-art optical techniques introduce a new era of ground-based exploration of Mercury's surface and its atmosphere. They offer opportunities for synergistic, cooperative observations before and during the upcoming Messenger and BepiColombo missions to Mercury.     

Three‐dimensional shapes and Fe contents of Stardust impact tracks: A track formation model and estimation of comet Wild 2 coma dust particle densities

Abstract– We investigated three‐dimensional structures of comet Wild 2 coma particle impact tracks using synchrotron radiation (SR) X‐ray microtomography at SPring‐8 to elucidate the nature of comet Wild 2 coma dust particles captured in aerogel by understanding the capture process. All tracks have a similar entrance morphology, indicating a common track formation process near the entrance by impact shock propagation irrespective of impactor materials. Distributions of elements along the tracks were simultaneously measured using SR‐XRF. Iron is distributed throughout the tracks, but it tends to concentrate in the terminal grains and at the bottoms of bulbs. Based on these results, we propose an impact track formation process. We estimate the densities of cometary dust particles based on the hypothesis that the kinetic energy of impacting dust particles is proportional to the track volume. The density of 148 cometary dust particles we investigated ranges from 0.80 to 5.96 g cm^?3 with an average of 1.01 (±0.25) g cm^?3. Moreover, we suggest that less fragile crystalline particles account for approximately 5 vol% (20 wt%) of impacting particles. This value of crystalline particles corresponds to that of chondrules and CAIs, which were transported from the inner region of the solar system to the outer comet‐forming region. Our results also suggest the presence of volatile components, such as organic material and perhaps ice, in some bulbous tracks (type‐C).     

Wind-driven estuarine turbidity maxima in Mandovi Estuary,central west coast of India

Systematic studies on the suspended particulate matter (SPM) measured on a seasonal cycle in the Mandovi Estuary, Goa indicate that the average concentrations of SPM at the regular station are ∼20mg/l, 5mg/l, 19mg/l and 5mg/l for June–September, October–January, February–April and May, respectively. SPM exhibits low-to-moderate correlation with rainfall indicating that SPM is also influenced by other processes. Transect stations reveal that the SPM at sea-end stations of the estuary are at least two orders of magnitude greater than those at the river-end during the monsoon. Estuarine turbidity maximum (ETM) of nearly similar magnitude occurs at the same location in two periods, interrupted by a period with very low SPM concentrations. The ETM occurring in June–September is associated with low salinities; its formation is attributed to the interactions between strong southwesterly winds (5.1–5.6ms⁻¹) and wind-induced waves and tidal currents and, dominant easterly river flow at the mouth of the estuary. The ETM occurring in February–April is associated with high salinity and is conspicuous. The strong NW and SW winds (3.2–3.7ms⁻¹) and wind-driven waves and currents seem to have acted effectively at the mouth of the estuary in developing turbidity maximum. The impact of sea breeze appears nearly same as that of trade winds and cannot be underestimated in sediment resuspension and deposition     

Radar observations of asteroid 1998 ML14

Abstract— Goldstone and Arecibo delay‐Doppler radar imaging of asteroid 1998 ML 14 shortly after its discovery reveals a 1 km diameter spheroid with prominent topography on one side and subdued topography on the other. The object's radar and optical properties are typical for S‐class near‐Earth asteroids. The gravitational slopes of a shape model derived from the images and assumed to have a uniform density are shallow, exceeding 30° over only 4% of the surface. If 1998 ML14's density distribution is uniform, then its orbital environment is similar to a planetary body with a spheroidal gravitational field and is relatively stable. Integration of a radar‐refined orbit reveals that the 1998 apparition was the asteroid's closest approach to Earth since at least 1100 and until 2283, when it approaches to within 2.4 lunar distances. Outside of that time interval, orbit uncertainties based on the present set of observations preclude reliable prediction.     

Large-scale ionization fronts and the nature and distribution of light scattering particles in the orion nebula

Image-tube filter photographs calibrated against photoelectric filter photometry have been used to give maps of M42 in absolute flux units over the central 15 arc min of the nebula in Hα, [Nii] (λ 6584 Å), Hβ and continuum at λ 4700 Å. Maps of the ratios Hα/[Nii] and (for the first time) of continuum/Hβ have been produced with unprecedented spatial resolution. These show that the gas to dust ratio is high near the exciting stars and falls strongly in the vicinity of large scale ionization fronts marked by minima in the Hα/[Nii] ratio. These results are interpreted in terms of detailed shell models containing either ice or graphite or silicate scattering particles. In all models there must be a central hole in the distribution of scattering particles. The effect of neutral globules and intrusions is investigated. It is found that all types of grain are trapped inside neutral intrusions near the centre of the nebula by the pressure of the Lα light surrounding the globule, but in the early evolution of the nebula particles can escape into the ionized medium when fronts are R-type. Ice grains escaping at this time will be destroyed for distances to the exciting stars less than 1 pc. These results can explain both the central hole in dust and the underabundance of oxygen in the ionized gas observed earlier. Arguments depending on colour index of the scattered light indicate that mixtures of scattered light from ice in the globules and from ice in the ionized medium can explain the observations, but that the graphite and silicate particles fail. A schematic model of the Orion Nebula is presented to attempt to explain the large scale phenomena observed here. It demonstrates that simple shell models for this nebula are dubious.     

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

We present a preliminary analysis of CH₄ absorptions near 6800 Å 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 Å, 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.     

Observations of Jupiter at 26.3 MHz using a large array

A 640 element phase-steerable dipole array has been used to make highly sensitive observations of the planet Jupiter during the 1973 apparition. The satellite Io is found to have very little influence at the low flux levels, whereas the definition of sources A and B appears to be relatively flux independent. A two-dimensional analysis of the data in the Jupiter-Io plane has revealed considerable source B activity at low intensities which is not influenced by Io.     

Auroral recombination of N and O: A possible source for emission in the γ and δ bands of NO

Radiative recombination of N and O provides a significant source for auroral emission in the γ and δ bands of NO with selective population of vibrational levels in the A²Σ⁺ and C²Π states. This mechanism may account for emissions detected near 2150 Å. Models are derived for the auroral ionosphere and include estimates for the concentrations of N and NO. The concentration of NO is estimated to have a value of about 10⁸ cm^?1 near 140 km in an IBC III aurora. The corresponding density for N is about 5 × 10⁷cm^?3 and the concentration ratio $\text{NO}{}^{\mathrm{+}}\text{O}{}_2{}^{\mathrm{+}}$ has a value of about 5.5.