Styles of Cratering on Europa

1. Styles of Cratering on Europa Abstract #2005, 33rd LPSC, March 2002 Clark R. Chapman, Beau Bierhaus, and William J. Merline Southwest Research Institute, Boulder CO Secondary Craters on Europa: Bierhaus and his colleagues [cf. 4] have embarked on a multifaceted approach of studying secondary craters on Europa, which are potentially the chief alternative to primary craters for the smaller craters seen on Europa's surface. He has found that (a) smaller craters are concentrated in a Pwyll ray in the vicinity of Conamara Chaos and decrease away from the ray; (b) statistical tests show that small craters are non-randomly distributed on typical Galileo frames; and (c) that simple modelling (based on mass arguments) of the number of secondary fragments from the few large primary craters on Europa suggests that as many as all of the smaller craters could be secondaries. Of course, there must be some smaller primaries, and the issue is to determine what fraction are primaries and, if possible, to determine which particular ones are the primaries so that they can be used to date units. Most recently, clustering algorithms have been used to determine that as many as 80% of small craters on typical Europan units (not proximate to big primaries) are spatially clustered (but showing no correlation with geological unit boundaries), implying that they are secondaries [5]. In this poster we show counts of small craters around Tyre, one of the largest ring structures on Europa, as well as in another region, near a strike-slip fault, far from a large primary crater. Each dot indicates the location of a small crater. It can be seen that the distribution of small craters is asymmetric in the near-field of Tyre; as before, far-field craters are distinctly clustered. The accompanying R-plots of frequency as a function of size show the steep slopes (-4 to -5.5 differential), another characteristic of secondary cratering. Introduction: Since Voyager imaging, Europa has been known to have a relatively youthful surface. What were then thought to be poorly resolved impact craters ~10 km in diameter are now understood to be "pits", part of a suite of endogenic features generally known as lenticulae. From the ~12-20 large (>20 km diameter), primary impact craters on Europa and the (imperfectly) known flux of small bodies (chiefly Jupiter family comets), the average age of Europa's surface is ~50 Myr, within factors of a few [1]. During this comparatively recent epoch, the style of resurfacing of Europa's surface has undergone a dramatic change [2], whether it be a single change from ridged plains to lenticulae/chaos or only the final stage of a cyclical phenomenon. In order to study the duration of phases of this evolving style, and to determine whether or not it happened contemporaneously over the entire globe or evolved region-by-region (e.g. perhaps more rapidly at the poles and more slowly at the equator), it would be useful to determine -- from crater counts -- the relative ages of smaller geological units (or at least regions) on Europa. However, the numbers of smaller craters are too few to permit statistically significant counts in smaller units. For example, craters 1 km in diameter are down nearly 3 orders of magnitude from empirical saturation, with only a couple showing up in a typical medium resolution Galileo frame; that is a distribution far too coarse to permit relative age-dating by crater counts. While craters <<1 km diameter become quite numerous on Europa, at least in certain locales, their spatial densities and size distributions are markedly non-uniform, raising questions about whether they are primary craters. Indeed, Chapman [3] suggested, from preliminary Galileo observations of small crater densities on several Galilean satellites, that the numbers of primary craters might be unexpectedly small, suggesting a depletion (relative to expectations of a collisional size distribution) for small comets. This locale on Europa (E17 STRSLP01) illustrates the kind of clustering of small craters evident in most high-resolution frames. While the craters are somewhat easier to detect on the smoother terrains, biases in sampling are small. The middle panel shows the center positions (not sizes) of all identified craters, showing the strikingly non-random distribution. The righthand panel represents the results of a run of one clustering algorithm, which replaces the center dot with a cluster sequence number (0 = not in any cluster). It is evident that a rather small proportion of craters are outside of identified clusters. We believe that clustering is a strong indicator of secondary cratering. Another indicator is the steep slope of the size distribution of the identified craters, shown in an R-plot to the right (data for small craters, believed to be subject to incompleteness, are omitted from the plot. Take-Away Message: Secondary cratering dominates the small-crater populations on Europa, even far from the few large primaries. If this is true on other bodies as well (where crowding renders clustering of distant secondaries less apparent), secondary cratering could be a more important process than has been believed, especially in the outer solar system, where small comets may be few. Discussion: It is beginning to appear that only a very small fraction of smaller craters on Europa can be primaries. Depending on the numbers of very distant secondaries, plus any possible Jupiter orbital debris that impacts Europa, the more randomly distributed subset of small craters could be primaries. Allowing for possible leading/trailing side differences, the minimum crater densities may eventually be identified permitting us to estimate the primary contribution. Of course, at least crudely, secondary craters may also be used to determine relative ages of units, since some secondary crater fields are undoubtedly older than others. The very youthful surface of Europa is probably the best place to study the physics of secondary cratering. Because of the few primaries, secondary crater fields should not generally overlap on Europa, yet ejecta are clearly emplaced far from primaries in order to account for clusters of small craters seen in all but one of many dozens of frames examined. If similar numbers of secondaries are generated on other icy and rocky bodies, they could be more important contributors to small-crater populations on other bodies than had been thought…especially in the outer solar system, where comets should dominate the primary impactor population, but even in the inner solar system, as well. Although details are difficult to grasp from just looking at this mosaic of Tyre, plots of crater center positions reveal a striking pattern of secondaries. Although there is a general decrease in secondary crater density with distance from the center of Tyre, significant asymmetries are evident. The R-plot above represents the size-frequency relation for the identified craters. The slope is very steep for sizes down to 1 km diameter. The two data points to the left roll off into incompleteness, although it is possible that second point is valid and that there is a true preference for near-field secondaries about 1 km in diameter. References: [1] Zahnle K. (2001) LPSC XXXII, #1699. [2] Spaun N.A. (2001) PhD. thesis, Brown Univ. [3] Chapman C.R. (1997) MAPS, 32, A27. [4] Bierhaus E.B. et al. (2001) Icarus, 153, 264-276. [5] Bierhaus E.B. et al. (2001) Bull. Am. Astron. Soc., 33, 1106-1107.