Subsolar Latitude Research Articles

The wavelength dependence of Triton's light curve

Using Voyager observations, we demonstrate that Triton's orbital light curve is strongly wavelength dependent, a characteristic which readily explains some of the apparent discrepancies among pre‐Voyager telescopic measurements. Specifically, we find a light curve amplitude (peak to peak) that decreases systematically with increasing wavelength from about 0.08 magnitude (peak to peak) near 200 nm to less than 0.02 magnitude near 1000 nm. Peak brightness occurs near 90° orbital longitude (leading hemisphere). The brightness variation across this hemisphere is close to sinusoidal; the variation across the darker hemisphere is more complex. The decrease in light curve amplitude with increasing wavelength appears to be due to a decrease in contrast among surface markings, rather than to atmospheric obscuration. Our model also explains the observed decrease in the amplitude of Triton's light curve at visible wavelengths over the past decade, a decrease related to the current migration of the subsolar latitude toward the south pole: we predict that this trend will continue into the 1990s.

Subsurface Energy Storage and Transport for Solar-Powered Geysers on Triton

The location of active geyser-like eruptions and related features close to the current subsolar latitude on Triton suggests a solar energy source for these phenomena. Solidstate greenhouse calculations have shown that sunlight can generate substantially elevated subsurface temperatures. A variety of models for the storage of solar energy in a sub-greenhouse layer and for the supply of gas and energy to a geyser are examined. "Leaky greenhouse" models with only vertical gas transport are inconsistent with the observed upper limit on geyser radius of approximately 1.5 kilometers. However, lateral transport of energy by gas flow in a porous N(2) layer with a block size on the order of a meter can supply the required amount of gas to a source region approximately 1 kilometer in radius. The decline of gas output to steady state may occur over a period comparable with the inferred active geyser lifetime of five Earth years. The required subsurface permeability may be maintained by thermal fracturing of the residual N2 polar cap. A lower limit on geyser source radius of approximately 50 to 100 meters predicted by a theory of negatively buoyant jets is not readily attained.

Albedo patterns on Triton

A model for seasonal transport of volatiles based on Trafton [1984] is coupled to considerations of methane photochemistry to infer albedo patterns on Triton's surface at the time of the Voyager encounter. The relatively large southern subsolar latitude at present is predicted to cause the deposition of an extensive fresh frost layer covering approximately the northern half of the planet. The region between 10° and −35° latitude is expected to be devoid of surface frosts and will probably be covered with organic compounds resulting from methane dissociative chemistry. These substances are thought to be rather low in albedo and red in color. Triton's south polar cap is expected to extend from −35° latitude to the southern pole. Sublimation of Triton's south polar ices been occurring since 1930, but has been preferentially occurring near the pole. The sublimation of ices in this region may be concentrating dark organic matter on the surface of the ice or exposing layers of this material which have built up in the ice over many seasonal cycles. The south polar cap may be distinctly darker near the pole than around the equatorward edge.

Systematic biases in radiometric diameter determinations

Radiometric diameters and albedos of asteroids and other Solar System bodies have generally been determined with the aid of a standard thermal model, which assumes a smooth nonrotating surface and relies on an empirical beaming factor to adjust model fluxes to the values observed for objects of known radius. It has been assumed that the need for a correction factor is generally due largely to the effects of roughness on the thermal emission from real surfaces. We show that in many cases the effects of rotation are also important and must be considered in radiometric diameter determinations. The thermal effect of rotation depends not only on the object's thermal inertia, rotation rate, and pole orientation, but also on its temperature: colder objects with constant rotation rate and thermal inertia radiate proportionally less of their heat on the day hemisphere and more on the night hemisphere. If the asteroids Ceres and Pallas have lunarlike thermal inertias the effect of rotation on their thermal emission is very important: in particular, large variations in thermal emission correlated with variations in subsolar latitude around their orbits are expected. Such variations are not observed, and we therefore suggest that these and other main-belt asteroids have thermal inertias ≤1.5 × 10 4 erg cm −2 sec −1 2 K −1 , ≤30% that of the Moon. We determine a disk-integrated beaming parameter of 0.72 for the Moon and use this value to correct empirically for roughness effects in our thermophysical models. As a result of the influence of temperature on the thermal effects of rotation, the standard thermal model systematically underestimates the diameters (and overestimates the albedos) of cold objects. For example, assuming a thermal inertia 20% of the lunar value and the Sun in the equatorial plane, a Trojan asteroid would be 11% larger than the diameter determined radiometrically at 10 μm, and if it had a lunar thermal inertia, it would be 67% larger than determined radiometrically at 10 μm. Errors at 20 μm are smaller. The error in radiometric diameter varies with the body's albedo, thermal inertia, and subsolar latitude in ways that we describe.

Polar warming in the middle atmosphere of Mars

We report ground-based laser heterodyne spectroscopy of non-thermal emission in the cores of the 10.33- μm R(8) and 10.72- μm P(32) lines of 12C 16O 2, obtained at 23 locations on the disk of Mars during the 1984 opposition, at L s = 130°. The data were obtained at a sub-Doppler spectral resolution, and the temperature of the middle Martian atmosphere (50–85 km) is derived from the frequency width and intensity of the R(8) emission, and from the total intensity of the P(32) emission. We find that the temperature of the middle Martian atmosphere varies with latitude. Near the subsolar latitude, the average 50- to 85-km temperature is close to the radiative equilibrium value for a CO 2 atmosphere. However, at high latitudes in both the northern (summer) and southern (winter) hemispheres the 50- to 85-km temperature exceeds the CO 2 radiative equilibrium value; a meridional gradient in the range of 0.4 – 0.9°K per degree of latitude is indicated by our data. The highest temperatures are seen at high latitudes in the winter hemisphere, reminiscent of the seasonal effects seen at the Earth's mesopause. As in the terrestrial case, this winter polar warming in the Martian middle atmosphere necessitates departures from radiative equilibrium; dynamical heating of order 4 × 10 2 ergs g −1 sec −1 is required at the edge of the winter polar night. A comparison with 2-D circulation models shows that the presence of atmospheric dust may enhance this dynamical heating at high winter latitudes, and may also account for heating at high latitudes in the summer hemisphere.

Thermal infrared properties of the Martian atmosphere: 2. The 15‐μm band measurements

Viking infrared thermal mapper observations of Mars in the 15‐μm CO2 band reveal global atmospheric thermal behavior at the 0.3‐ to 0.6‐mbar level. Dust entrained by storms produces major modification of diurnal and latitudinal structure in the brightness temperature T15. In the dust‐laden atmosphere of southern spring and summer 1977, T15 was a maximum in late afternoon at a latitude well south of the subsolar latitude. Diurnal amplitude was as great as 30 K, while diurnal mean temperatures exceeded 220 K. Over the northern winter polar cap, T15 increased dramatically following the second global dust storm of 1977; even in regions of polar night the change was up to 80 K. Inversions of similar magnitude resulted, and the change in downward radiance was sufficient to modify substantially the rate of CO2 condensation at the surface.

Atmospheric density above 158 kilometers inferred from magnetron and drag data from the satellite OV1-15 (1968-059A)

A cold-cathode magnetron pressure gage and a C-band radar beacon were part of the complement of experiments aboard the polar-orbiting atmospheric research satellite OV1-15 (1968-059A). Over the period July 13–15, 1968, when the perigee of the satellite was near the equator at about local noon, the atmospheric density height-profiles were repeatedly measured down to 158 km. The profiles agreed in the northern and southern hemispheres below 200 km, but above 200 km the northern profile was greater than the southern, in approximate agreement with the Jacchia model that includes a symmetric density bulge at the subsolar latitude at 1400-hr local time. During a moderate magnetic disturbance the atmospheric density at latitudes below about 40° increased with an amplitude and lag that are adequately described by the Jacchia model. The duration of the density enhancement was observed to be several hours longer than the duration of the geomagnetic disturbance. At latitudes above 40° (which were available to us only in the southern hemisphere) the density enhancement decreased markedly with increasing latitude. The drag-determined density near perigee was the same, within the estimated errors, as that determined from the gage measurement, in contrast to the factor-of-2 discrepancy found in a similar comparison of data from the satellite Explorer 17.

Tidal Forces and the Formation of Hurricanes

MAY I raise the following question: “Do tidal forces affect the formation of hurricanes?” If one examines the changing direction of the tidal force due to the sun and a moon near new (or full) on an August day, say, as this force acts on a small element of air located at a point south of the sub-solar latitude and north of the equator, one finds the tidal force would tend to make the atmospheric element move in a counter-clockwise direction as viewed from above. Furthermore, as the sun moves southward in August and early September, the latitudes where the highest atmospheric tides would be expected (here considered semi-diurnal), would fall alternately north and south of the equator, the successive points on either side approaching the equator. Might there not be one time at which the separation of these two latitudes would correspond to a natural frequency of oscillation of the atmosphere? If these arguments have any validity, the hurricanes in the North Atlantic should show an increase in number shortly before September 23 and a decrease shortly after that date.