Martian atmospheric erosion rates

Mars was once wet but is now dry, and the fate of its ancient carbon dioxide atmosphere is one of the biggest puzzles in martian planetology. We have measured the current loss rate due to the solar wind interaction for different species: Q(O+) = 1.6.10(23) per second = 4 grams per second (g s(-1)), Q(O+2) = 1.5.10(23) s(-1) = 8 g s(-1), and Q(CO+2) = 8.10(22) s(-1) = 6 g s(-1) in the energy range of 30 to 30,000 electron volts per charge. These rates can be propagated backward over a period of 3.5 billion years, resulting in the total removal of 0.2 to 4 millibar of carbon dioxide and a few centimeters of water. The escape rate is low, and thus one has to continue searching for water reservoirs and carbon dioxide stores on or beneath the planetary surface and investigate other escape channels.     

Martian topography: large-scale variations

The variation in carbon dioxide abundances detected in the 1.05-micron band over small, discrete areas on Mars indicates that larger-scale topographical differences are present than had previously been believed. Spectroscopic mapping of the surface also indicates no apparent correlation between albedo and height; the results are in good agreement with topographical data derived from the range-gated rodar scan along +21 degrees N. High and low areas are found in both the major equatorial maria and the bright deserts in the northern hemisphere.     

Martian craters: number density

The incremental frequency distribution of Martian crater diameters larger than 20 to 30 kilometers follows an inverse-square law, with density equal to that of craters on the lunar continents. This finding accords with the prediction that lunar continents and the Martian surface carry an equilibrium density (saturation) of craters originating in meteoroidal impact. Therefore crater statistics alone cannot be used for estimation of the age of the Martian surface.     

Coupled ferric oxides and sulfates on the Martian surface

The Mars Exploration Rover (MER), Opportunity, showed that layered sulfate deposits in Meridiani Planum formed during a period of rising acidic ground water. Crystalline hematite spherules formed in the deposits as a consequence of aqueous alteration and were concentrated on the surface as a lag deposit as wind eroded the softer sulfate rocks. On the basis of Mars Express Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) orbital data, we demonstrate that crystalline hematite deposits are associated with layered sulfates in other areas on Mars, implying that Meridiani-like ground water systems were indeed widespread and representative of an extensive acid sulfate aqueous system.     

Martian relief and the coming opposition

In the report "Martian relief and the coming opposition" (3 Mar., p. 1100), D. H. Harris stated that, "... the reduced contrast with decreasing (terminator distance) just balances the increase in visibility due to shadow length." This is obviously erroneous. A more careful examination of the problem shows that for favorable values of the Aerocentric EarthSun Sun angle, visibility of relief increases toward the terminator, clouds not withstanding.     

Martian relief and the coming opposition

Ground-based observations of Martian relief may be possible when the Earth-Mars geometry is optimum and the observing conditions are near perfect. There is some evidence that detectable relief is present. Groundbased observations of Martian relief during the coming opposition will be optimum between 1 February and 1 March and between 1 June and 15 August 1967.     

Martian dust storm: its depth on 25 november 1971

Carbon dioxide absorption on Mars was observed by Earth-based spectroscopy at 10,500 angstroms. Its abundance was equivalent to a partial pressure of 2.0 millibars instead of the normal 5.5 millibars. The dust layer was therefore opaque to a height of about 11 kilometers. On 10 December, a pressure of 5.8 millibars was found.     

Martian craters: comparison of statistical counts

Comparisons of the various crater counts obtained from the Mariner IV photographs indicate that, out of 300 to 600 possible craters mapped by different investigators, only about 120 craters can be reliably identified. Thus, only a lower limit for the Martian crater density can be established from the Mariner IV data.     

Martian surface paleotemperatures from thermochronology of meteorites

The temporal evolution of past martian surface temperatures is poorly known. We used thermochronology and published noble gas and petrographic data to constrain the temperature histories of the nakhlites and martian meteorite ALH84001. We found that the nakhlites have not been heated to more than 350 degrees C since they formed. Our calculations also suggest that for most of the past 4 billion years, ambient near-surface temperatures on Mars are unlikely to have been much higher than the present cold (<0 degrees C) state.     

Martian surface materials: effect of particle size on spectral behavior

The presence of abundant limonite on Mars has long been the subject of controversy. Some advocates of abundant limonite also suggest that the albedo differences between Martian light and dark areas are caused by different sizes of particles in those areas. We show that the relative albedo is reversed from the blue to the red for samples of limonite with particles of different sizes. Observations of Mars reveal no blue-red albedo reversal between the light and dark areas. Consequently, the hypothesis of particle size control of albedo is incompatible with the presence of abundant limonite on Mars.     

几种坡度的台阶式溢洪道消能特性试验研究

对3种斜坡(5.7°,19°和30°)的台阶式溢洪道进行了模型试验研究。结果表明,当堰上相对临界水深yc/h≤2.5时,滑行水流、过渡水流和跌落水流消能率的大小不受堰上相对临界水深的限制,而只是相对坝高Hdam/yc和斜坡角度的函数,即随坡度变缓或相对坝高Hdam/yc增加,消能率增大;当坝坡相同坝高相等时,滑行水流、过渡水流和跌落水流之间的消能率差异很小,并且其消能率大小与台阶个数无关;此外,文中还给出了计算消能率的经验公式。     

Morphology and origins of sedimentary structures on submarine slopes

Submarine slopes in deep water, such as continental slopes, are often indented by valleys or channels and made uneven by ridges or levees. The origins of many of these features are unknown or disputed. Morphologically, however, there is often great similarity between forms on deep slopes and forms on shallow slopes or on land. Structurally the slopes in deep water are less well explored, but several observations reveal features, such as lamination and crossbedding, that are known from shallow water also. Measurements of current indicate that periodically the movement of water near the bottom is fast enough to move particles of sediment from time to time. Morphology, fine structure, and currents suggest that internal waves and associated currents, as well as gravity, may control the shape of deep submarine slopes analogously to the shaping by surface waves of slopes in shallow water.