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Byran

Feb 24 2009, 09:39 AM

ftp://ftp.lpi.usra.edu/pub/outgoing/lpsc2009/full254.pdf

A GLOBAL SUB-SURFACE ALKANIFER SYSTEM ON TITAN?

NUMBERS, DISTRIBUTION AND MORPHOLOGIES OF IMPACT CRATERS ON TITAN

Introduction: Titan has very few impact craters [1]. With more than 30% of the surface now imaged by Cassini Radar through data take T44 there are only seven certain impact structures known. Fifty additional possible craters also have been identified. The certain craters have two distinct morphologies, and the probable craters appear to mostly be more degraded version of these two types. The craters do not appear to be randomly distributed.

Crater Distribution: The distribution of Cassini Radar swaths is uneven, with little coverage south of 30°S, and more over the leading hemisphere (0-180°) than for the trailing. Presently, cartographic mapping is complete only through the T39 data take, which includes 49 craters. Based on those craters and the actual areal cover of data takes up through T39 we make the
following observations. Craters are found in proportion to the areas of coverage in equal area latitude bins, except there is a 17% deficiency between 42°N and 90°N. There is a 14% excess of craters between 42°N and 19.5°N. Again, these are deviations in the expected distribution of craters of all diameters based upon the actual areas of Radar coverage.
The paucity of impact craters in the northern polar region may be due to the abundance of lakes and seas which may submerge craters; indeed, a 10 km diameter crater is revealed by its circular rim rising above a lake surface. A large area of the surface north of 60° is also composed of circular and irregular depressions that may be of karstic origin [3]. In any case, few other types of landforms exist in these northern areas, implying that these surfaces may be relatively young.
Longitudinally, there is an excess of 20% of craters on the leading hemisphere compared to the trailing.
Although Titan has few craters, there are many areas with Radar coverage that lack any craters so this may be evidence for a significant leading-trailing hemispheric difference. Further statistical testing is required at the end of the mission when presumeably more craters will have been discovered and we will have detemined the areal coverage of all data takes.

Byran

Feb 24 2009, 10:01 AM

ftp://ftp.lpi.usra.edu/pub/outgoing/lpsc2009/full328.pdf

THE SURFACE AGE OF TITAN

Crater Frequency and Age Estimation: The area covered with sufficient resolutions by Cassini’s Radar to resolve circular features is about 20% of Titan’s surface. This is far from completion, but can be used to constrain the expected crater population. The distribution of so far identified impact craters, both confirmed and putative ones, is almost uniform over Titan’s surface with a slight increase on the trailing site (Fig.1). This observation appears to coincide with the impactor model of Korycansky and Zahnle (2005) [10] who suggest that the leading hemisphere should have a crater frequency about 5 times higher than the trailing side assuming Titan has been in synchronous rotation throughout its history. However, this observation may change in the course of the mission with increasing high-resolution coverage which is so far poorer on the leading site. The cumulative crater frequency is shown in Fig. 2 for both the confirmed five craters and the total of the putative craters. The overall shape of the frequency distribution is relatively flat compared to those of other icy satellites, especially at smaller crater diameters. However, the cumulative crater frequency for larger diameters remarkably fits that of the basins on Iapetus for craters down to about 80 km diameter (Fig. 2), although the number of craters is lower by about an order of magnitude (Fig. 2). The crater frequency at sizes < 80 km is far lower by about a factor of up to 200.
The similarity of the crater frequency with with large craters and those of older terrains on other icy Saturnian satellites indicates that the primary crust of Titan which holds the larger craters is old. The overall shape of Titan’s crater frequency distribution for craters < 80km is even shallower than what would be expected for atmospheric shielding [14,15]. Therefore, erosion must have played a major role in obliterating craters on Titan. Compared to the crater frequency distribution on Earth, Titan shows a similar shape (Fig. 2).
However, the density is about 5 to 10 times higher. Although the impactor population at Saturn might be different, this fact is mostly due to the lack of plate tectonics on Titan and to a lower heat flux driving erosion [16]. The absolute age model according to Neukum (1985) [17] and Neukum et al. (2005) [18] assumes a lunar-like impactor flux mainly of main-belt asteroids, whereas Zahnle et al. (2003) [19] and Korycansky and Zahnle (2005) [14] assume a constant impactor flux of cometary objects, either with a size distribution of Jupiter family comets (JFC) (case A), or with a size distribution of small comets in the Neptunian System ( case B ). In addition, Artemieva and Lunine (2005) [11] discuss a different model which, however, was derived from previous work of Zahnle [19]. According to the Neukum age model Titan's surface is as old as 3.9 Ga as derived from the largercrater (> 80 km) frequencies. The Zahnle model yields surface ages of 3.5 Ga in case A and 1.4 Ga in case B. According to the Artemieva and Lunine model, Titan’s surface appears as young as 500 – 100 Ma [15]. If only smaller craters, e.g. 10 km-sized craters, are taken into account for age determination, surface ages are 100 Ma according to the Neukum model, 8 Ma according to the Zahnle model case A, and 2 Ma for case B. Although the statistical precision of the Titan cratering results is not very high and cratering models for absolut ages are controversial, it is obvious that Titan's surface is partly as old as the other Saturnian satellites reflecting an early crust still preserved and has been partly modified and heavily resurfaced even in recent times.

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