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基本上讲到了鸡精里提到的点，有关lava在地球、月球和金星的比较。
大家可以阅读一下，希望有所帮助哈！
good luck guys!
Venusian Lavas
If Venusian lava is behaving differently to Earth-type lava, this is almost certainly due to its viscosity. Lava viscosity or mobility is known to depend chiefly on its chemical composition and temperature, but other factors affecting it include, pressure, gas content and density (MacDonald, 1972).
Viscosity is very dependent on the temperature of the magma (Figure 18). Both field and experimental data show that the viscosity of all magmas increases significantly on cooling (Williams & McBimey, 1979), partly due to crystallization.
However, at equivalent temperatures and pressures, different magmas have different viscosities, suggesting that compositional aspects are also important in determining their viscosities.
This close relationship between viscosity and temperature is vital in explaining Venusian lavas. The extent of the Venusian lava rivers in itself suggests very fluid lava exists on the surface of the planet. This idea is further enhanced by the high temperature (450^C) known to prevail there. This means the longer it takes for the lava to cool down, the longer it will maintain its low viscosity, and hence the further it will flow. Further to this, if the lava was initially very fluid and effusion rate high, it may travel quite fast and therefore travel some distance and still maintain its mobility, losing little heat on its way.
Volatile Content
Until recently (see Table 1), fluid lava existing at temperatures much below 800^C was unheard of, but an expedition to Tanzania, in Africa, changed this belief. The volcano, Longai, was expelling a quite mobile lava river, which melted at only 500°C, that is before it turned red-hot. This is indeed around the surface temperature of Venus.
This carbonatite lava was black in colour and looked like hot mud, but at night glowed red-hot. The resulting lava river was only small but had quite a meander and formed a lava pool in one section.
Carbonatite is widespread and locally abundant on Earth but is rare compared to basaltic and many other silicate magmas. Carbonatite volcanism can be found in certain shield areas, on hotspots, and along the East African Rift. Carbonite is mantle-derived and is almost exclusively associated with alkaline mafic and ultramafic silicate volcanic rocks such as alkali basalt, nephelinite and kimberlite. This fact is interesting because Venusian lavas tend to be highly alkaline, particularly from samples analyzed by the Venera 13 space probe, which closely resembles olivine nephelinite (Baker et al, 1992).
Quite clearly, a lava river on Venus like the one in Tanzania, would flow quite a considerable distance, and it would not cool at all below the critical viscosity- temperature point. This means the maximum length it could attain, assuming it remained channeled, would be determined only by the supply of lava at its source. Although the surface of Venus, in most cases can be regarded as being gently sloping, very fluid lavas, such as those in Hawaii, have attained the considerable speed of 30 or 40 km per hour on gentle slopes (MacDonald, 1972).
Thus, if a flow were to stay mobile for say 10 hours or more, not unreasonable under Venusian conditions as discussed, a lava river may attain a length in excess of 400 km. Again, not unreasonable, assuming there was an adequate supply of magma. This long flow period will also lead to channel excavation.
Lastly, if there are some very fluid lava on the surface of Venus, which certainly seems to be the case, the lava would need to be low in silica, that is, basaltic lava. In these sort of lavas, dissolved gases escape readily and typically extrude quietly from fissures and fractures (MacDonald, 1972).
Although fissure eruptions are beyond the scope of this thesis, it should be mentioned that if they are widespread on Venus, like the Earth, they would mask or hide with their lava flows, many areas such as crater chains or indentations.
Lunar rille
There are basically three types of lunar rilles.
The straight rilles are usually between one and five km wide and hundreds of km long, unrelated to surface topography, analogous to terrestrial fault grabens. Curved or arcuate rilles are variants of straight rilles, with similar dimensions, and form concentrically to major ring basins e.g. Mare Humorum. Sinuous and Meandering rilles, formed by thermal erosion by flowing lava. Hadley Rille, 1.2 km wide, 270 m deep, and 135 km long, visited by Apollo 15, is the type example.
Sinuous and straight rilles being the most common type on the Moon are equally as common on Venus and are believed to have been formed by similar processes. Favourable terrain, the availability of large amounts of lava and very low viscosity seem to be the most important factors in the large size and distribution of the sinuous rilles on the Moon, as was also the case on Venus.
Lunar rilles are similar in some regards to terrestrial rilles but the vast difference in size and the lack of mature meanders makes them more analogous with Venusian rilles, though the rilles on each planet are unique in their own right, and should be regarded as such.
Comparison of Lunar, Terrestrialand Venusian Channels
Comparisons between lava tubes and channels on the Earth and Venus have already been covered in some detail in this study. How these tie in with the lunar rilles will now be looked at.
Our earlier study revealed many similarities between the Earth and Venus in the way lava was transferred from one point to another. In fact, similar volcanic features such as volcanoes, domes, cinder cones, pit craters, and channels are found on both planets. However, there are notable differences, one important one being scale. Things seem to be so much bigger on Venus. Lava channels on Earth tens of km long dwarf those on Venus which may be thousands of km in extent.
It would seem that Venusian lava channels and tubes, at least in regard to size, have more in common with their lunar counterparts than with those on the Earth. This seems surprising when you consider the great contrasting surface conditions. Venus’s 90 atm. pressure and boiling temperature (450^C) as against the Moon’s near vacuum and much cooler conditions which can be as low as -170^0 (night).
Comparing lava channels on the Moon with those of the Earth is inherently difficult because of the difference in scale. For example, the sinuous Pahoehoe lava flow from Devil’s Garden in Oregon (U.S.A.) is similar in appearance to lunar meandering rilles, but only being a few km long, is on a much-reduced dimension to lunar rilles, which are hundreds of km long. Lunar rilles generally appear to be at least 100 times larger than the majority of terrestrial lava tubes and channels but interestingly, comparable in size to those on Venus.
Based upon calculations and allowing for the reduced gravity on the Moon, the theoretical width of a lunar collapsed lava tube could reach 385 m. But this could reach 500 m considering the greater vesicularity of the lunar lavas.
Furthermore, the degradation following the collapse of the vault could cause lateral slumping which might easily enlarge the sinuous channels by a factor of two, bringing the possible width to one km. The most developed terrestrial lava tubes, on the other hand, do not surpass widths of 30 m and lengths of 15 - 20 km.
Recall, some Venusian channels were also km wide, with the probable lava tubes candidates having an uncertain width, but hundreds of meters would appear quite possible.
Now since the theoretical limits for maximum lava tube width are exceeded on the Moon (Vallis Schroteri has an "upstream" section 9 km wide) and even more so on Venus (considering the higher gravity) than at least some of the lunar sinuous rilles and channels on Venus, rather than being collapsed lava tubes, must correspond to lava flow channels.
The appearance of Venusian channels hardly makes this conclusion surprising. These flow channels in fact, present at the outset a greater continuity and more sub parallel flanks. Furthermore, these can have a far greater width than the lava tubes.
Besides size, terrestrial lava tubes do not generally correspond to continuous furrows with parallel rims like those on the Moon but are discontinuous with "bridges" and irregular rims as was the case at Undara. Also, lunar lava tubes have wall slopes of about 30^ - 40^, whereas the terrestrial ones often have vertical walls and overhangs (Green & Short, 1971). Of course, the inclination of the lunar wall slopes could be caused by degradation, but this condition should be even more evident in the case of terrestrial walls where erosion is more rapid and pronounced. Terraced flooring which occurs in some lunar rilles has not been seen in Venusian lava channels but this may well be due to insufficient resolution. Also, channel branching appears rarer on the Moon and dendritic lava channel patterns seem to be unique to Venus (Photo 13).
As well, many lunar rilles has mature meanders and show a clear excavation of the channel. We saw this was also true of a number of Venusian channels (see Photos 10 and 32), however, terrestrial lava channels do not have mature meanders. Mature meanders only form when lava has flowed over a period of time and involve both excavation and deposition of material and are analogous to meandering terrestrial watercourses. Also, whereas some lunar rilles show signs of deposition few are clearly visible in Venusian channels at least at the Magellan radar resolution.
The similarities between lunar and Venusian sinuous rilles can also be seen by comparing the lunar rille in Figure 32 with the Venusian one in Photo 33. Both are similar in shape, comparable in size and show clear excavation of the channel.
This similarity between lunar and Venusian channels is probably due, at least in part, to the common properties of their lava. The behavior of lava in a vacuum is not well understood but lunar lava, like that of Venus, is believed to be very fluid. Some researchers (e.g.O’Keefe, 1962), believe that lunar lavas must be far more fluid than those on Earth. They base this assumption upon the supposed lack of scarps similar to those which mark terrestrial lava flows. This fluid lava, as was mentioned earlier with Venus, would cover large distances on a level terrain, especially if the effusion rate was high, as was the case at Undara.
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发表于 2018-1-25 12:14:08
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