Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..222b&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
mostly based on the analysis of data acquired by the Magellan mission: SAR images with 100-200 m resolution and the maps of topography, surface radar reflectivity, emissivity, roughness and gravity anomalies [1]. After initial analysis of the data summarized in [2, 3] several groups of researchers continued to study the geology and geophysics of the planet, resulting in numerous publications, some of which are referenced below. Very important for the studies emphasizing the geologic history of Venus was, and still is, a program of 1:5,000,000 geologic mapping coordinated by the US Geological Survey [4]. A recent summary of these studies can be found in [5]. Observations and analysis: All researchers in this study area analyze the same data sets and follow the same guidelines [4, 6] so geologic units identified by them and their time sequences are generally similar, although different researchers may name the same units differently and may interpret differently some details of local time sequences. Figure 1 shows a time sequence of geologic units suggested by [7, 8]: materials of tessera terrain (tt), densely fractured plains (pdf), fractured and ridged plains (pfr), shield plains (psh), plains with wrinkle ridges (pwr), lobate (pl) and smooth (ps) plains as well as materials of radar-dark craterassociated parabolas (cdp). These are material units. In addition, some researchers identify and map structural units. In Figure 1 examples of these are fracture belts (fb) and rifted terrain (rt). synchronous on a global scale. The first option can be visualized with Figure 1, suggesting that it is applicable for Venus globally. This option was suggested by Basilevsky and Head [e.g., 7, 8] as well as by Ivanov and Head [e.g., 9]. The second option, first clearly formulated by [10], can be visualized by the upper part of Figure 2 showing the situation in three different hypothetical geologic provinces on Venus. In these provinces the unit time sequences are the same: tt => pdf => pfr/RB => pwr, but morphologically similar units, for example, units pwr, are not synchronous between them: unit pwr in province 1 is generally synchronous with unit tt in province 2 and with unit pfr/RB in province 3. As it was mentioned in [7, 8], if geologic analysis and mapping are being done within spatially separated geologic provinces, the synchronous vs. not synchronous alternative cannot be resolved. But if the geologic analysis and mapping are done within large areas, which include several geologic provinces with nonsynchronous units sequences, then at the boundaries of the geologic provinces one should see contradictions in the units' age relations. These contradictions are visualized in the lower part of Figure 2: Tessera massif at the boundary between geologic provinces 1 and 2, from the province 1 side, should be formed as a result of tessera-forming deformation of material unit pwr, while from the province 2 side it is embayed by the unit pwr. Similar contradictions are observed in relations between tessera and unit pfr/RB in provinces 2 and 3. We have mentioned in several publications [e.g. 11] that very large (more than half) regions of the planet have been mapped and such contradictions were not met by us and not reported by other researchers. So we stated that this favored the synchronous option, but that time we could not global geologic mapping of Venus has been recently completed [12] and such contradictions have not been met, we can say that this mapping has proved that morphologically similar units occupying similar positions in the local time sequences are globally synchronous. Of course, each of units considered had been formed not instantaneously, but within some period of time. So we refer to the general synchroneity and minor overlapping in absolute time of formation between stratigraphically neighboring units as certainly possible. This global mapping of [12] led to the identification of geologic units and their time sequence that is very similar to those identified by [7, 8]; this allows us to return to that model of regional and global stratigraphy of Venus (Figure 3). This figure is almost identical to Figure 22 in [8] and differs only in the estimate of absolute age of the boundary between the Fortunian and Guineverian periods (1.2T here vs. 1.4T in [8]). The question of the estimation of absolute ages of geologic units is difficult for Venus because the atmosphere is too massive to allow craters smaller than a few kilometers in diameter to be formed on its surface. As a result, the total number of impact craters on Venus is only about 1,000, and this makes it possible to estimate more or less reliably only the mean surface age of Venus, and less reliably the mean ages of several large globally observed geologic units such as pwr, tt or pl. Crater count techniques used for other planetary bodies, which permits absolute dating and time correlations of units occupying relatively small areas, can not be used in this way on Venus. The existing estimates of mean absolute ages of the larger Venusian geologic units, such as pwr, tt or pl [e.g., 13-15], were obtained by counting craters on areally separated outcrops of these units and normalizing sums of crater numbers by the total areas of the unit outcrops. Although the results of such an approach were consistent with stratigraphies based on geologic analysis [e.g., 13-15] this was keeping in mind the possibility that in different part of the planet absolute ages of the same units may be significantly different. Now with the completion of the global geologic mapping of Venus, such inconsistencies are excluded. Another problem in crater-count-based estimations of absolute ages of the geological formations on Venus is related to uncertainties of a number of parameters crucial for reliable modeling to transition from number of craters to millions and billions of years: e.g., 1) the meteoroid flux in the vicinity of the planet Venus, 2) the physics of passing of meteoroids through the dense Venus atmosphere, and 3) cratering under the high atmospheric pressure. As a result of these uncertainties, even statistically reliable estimates of the mean surface age of Venus are not very certain: ~750 m.y., but any values between 300 m.y. and 1 b.y. are considered possible [16]. This is why researchers using crater statistics to estimate absolute ages of individual geologic units use fractions or multiples of the mean surface age of Venus, instead of millions or billions of years, designating it as T [e.g., 14, 15]. The new estimates based on the global geologic mapping of [12] suggest that ages of selected units (in T with 2 σ error bars) are: tt, 1.09 ± 0.17; psh, 1.04 ± 0.18; pwr, 1.05 ± 0.12; pl, 0.54 ± 0.19; rt, 0.63 ± 0.26; confirming a new and more reliable basis for earlier estimates [13-15, 17-24]. This returns us to the conclusion made in [11]: …the earlier suite of units (from heavily deformed tesserae through slightly deformed regional plains) occurred during a time period an order of magnitude shorter than the subsequent period (from the end of emplacement of the wrinkle-ridge network until the present). These results imply high global rates of endogenic (volcanic) activity during the first era (comparable to that of mid-oceanic-ridge volcanism of Earth) and much lower global rates of endogenic activity (by two orders of magnitude) for the second period (page 1015, abstract). Conclusions: As it follows from the above consideration, tectonic and volcanic processes in the beginning of the morphologically recognizable part of the geologic history of Venus (since tessera time) were rather active and resurfaced the entire planet. But then, after about 10-20% of the total duration of this part of history they rather sharply occurred sporadically and in separate spots and zones and affected only 15-20% of the Venus. For better understanding the rates of tectonic and volcanic processes on Venus, knowledge of which is crucial for working out reliable geodynamic models of the evolution of this planet, we need to have isotopic dating for absolute ages of
Basilevsky Alexander T.
Head James W.
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