Pwyll Secondaries and Other Small Craters on Europa

Icarus 153, 264–276 (2001) doi:10.1006/icar.2001.6690, available online at http://www.idealibrary.com on

Pwyll Secondaries and Other Small Craters on Europa Edward B. Bierhaus Department of Aerospace Engineering Sciences, University of Colorado Boulder, Colorado 80309-0249 and Southwest Research Institute, 1050 Walnut Street, Suite 426, Boulder, Colorado 80302 E-mail: [email protected]

Clark R. Chapman and William J. Merline Southwest Research Institute, 1050 Walnut Street, Suite 426, Boulder, Colorado 80302

Shawn M. Brooks Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, Colorado 80309

and Erik Asphaug Earth Sciences Department, University of California, Santa Cruz, California 95064 Received February 2, 2001; revised June 15, 2001

We examine a population of Pwyll secondary craters, as well as several small crater populations seen in six high-resolution sequences of Europa, taken by the Galileo spacecraft. We conclude that post-Pwyll (the youngest large impact on Europa) endogenic surface activity occurred in the Conamara Chaos region. The Pwyll impact deposited a high-density secondary crater population in this area. The two terrains in Conamara Chaos show very different crater densities with one terrain containing anywhere from a few to 10 times higher density than the other. Surface activity within the one terrain, which degraded and erased part of that terrain’s crater population relative to the other terrain, best explains the density difference. Because Pwyll is a young impact crater, 18 million years old or younger, subsequent endogenic surface activity is consistent with Europa remaining active today. The steepness of the average differential power-law slope (−4.2) of the small-crater size distribution suggests that many of the small craters are secondaries. We demonstrate that the amount of mass ejected by Pwyll and the other large craters on Europa is potentially enough to create the majority of the small crater population via the secondary cratering process, allowing the possibility that most small craters on Europa are secondary craters. °c 2001 Academic Press Key Words: Europa; cratering; impact processes; satellite surfaces.

1. INTRODUCTION

The extensive imaging of Europa during the nominal Galileo mission, and then during the Galileo Europa Mission (GEM), revealed that Europa has a sparse crater population compared

with that of its neighbors Ganymede and Callisto. There are a few, but only a few (27 seen to date), craters with diameters greater than 4 km on Europa (Moore et al. 2001, Turtle et al. 1999). Pwyll, a 25-km-diameter crater, is probably the youngest crater larger than 20 km because its ray system (Fig. 1), which extends radially from Pwyll for over 1000 km, is more extensive and much brighter than that of any other large crater on Europa. Associated with the Pwyll rays are secondary craters— small craters made by material thrown from the initial impact. There are too few large craters (>10 km) on Europa to provide statistically meaningful crater density and age information on geological units, making a better understanding of the small crater population vital to surface age calculations. If secondary craters were mistakenly interpreted as primary craters, then the derived surface age would be greatly overestimated. We demonstrated (Bierhaus et al. 1998) that secondary craters dominate the population of small craters on certain areas of Europa, highlighting the need to understand their production and preservation so that we may derive more accurate surface ages. Understanding the small crater population also provides insight into the primary impacting population. One theory suggests that Jupiter family comets, derived from the Kuiper belt, constitute the majority of the impacting population in the jovian system (Zahnle et al. 1998). In this theory, Europa’s average surface age is less than 100 myr; further, it is possible that the jovian system has fewer small impactors relative to the impactor population in the inner Solar System (Chapman et al. 1997). In contrast, Neukum et al. (1997) believe that a single population, the asteroids, is

264 0019-1035/01 $35.00 c 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.

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ocean likely exists given the younger age, but may have frozen out given the older age. We must quantify the contribution of secondary craters before we can develop statistics for small primary craters. We address the issue of secondary craters in two main arguments: in Sections 2 and 3, we present complete measurements from a series of high-resolution images, which augment preliminary secondary crater measurements presented and discussed by Bierhaus et al. (1998); in Section 4, we demonstrate that the mass ejected from the primary craters larger than 10 km is enough to account for the majority of Europa’s small crater population, thus allowing the possibility that secondaries could account for most of Europa’s small craters. 2. HIGH-RESOLUTION MEASUREMENTS IN CONAMARA CHAOS

FIG. 1. High Sun, partial global mosaic of Europa taken by the Galileo spacecraft during the fourth orbit. Pwyll is the impact crater in the lower portion of the image, surrounded by bright rays that extend radially for over 1000 km. Part of a ray system overlaps Conamara Chaos, the dark region identified by the arrow.

primarily responsible for cratering throughout the Solar System. This model produces an age as old as 1 byr for Europa. These significantly different surface ages for Europa have implications for the existence of a possible subsurface ocean—the

Conamara Chaos is a region on Europa where the ice crust experienced a period of disruption, breaking the surface ice into large blocks that moved, rotated, tilted, and sometimes partially or fully submerged within what was once a mobile material. There are two terrain types in Conamara Chaos (Carr et al. 1998): (1) plates, which are the remaining sections of the displaced, preexisting ice crust, and (2) matrix, which is the interplate material, which is flat at large scales but has an irregular, lumpy topography at high resolution (Fig. 2). Additionally, there is a class of features on Europa, collectively called lenticulae, that includes pits, spots, and domes. As deduced from cross-cutting relationships, lenticulae are among the youngest features on Europa (Spaun et al. 1998). Numerous authors have discussed the terrain types and lenticulae in detail; see, for example, Greeley et al. (2000) and Head and Pappalardo (1999) for more information on chaos, Pappalardo et al. (1998) for discussion of lenticulae, and Figueredo and Greeley (2000) and Prockter et al. (1999) for discussion on stratigraphy.

FIG. 2. Mosaic of the five very high resolution (about 10 m/pix) frames that comprise the E12CHAOS01 sequence. Note the two types of terrains: the plates, which are remnants of the background ridged terrain (an example of which is labeled “Plate”), and the matrix, which is the interplate material. From left to right, the frames—as referred to in the text—are 71900, 71913, 71926, 71939, and 71952. Frame 71900 is in the middle of a Pwyll ray, while 71952 is well outside the ray.

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TABLE I Image Sequences Used to Map the Pwyll Rays in Conamara Chaos Sequence

Image number

Picture

E4GLOMAP01

s0374649000 s0374649013 s0374649026 s0374649039 s0374649052 s0383713700 s0383713713 s0383713726 s0383713739 s0383713752 s0383713765 s0383717500 s0383717513 s0383717526 s0383717539 s0426271900 s0426271913 s0426271926 s0426271939 s0426271952

ESE0001 ESE0002 ESE0003 ESE0004 ESE0005 ESE0020 ESE0021 ESE0022 ESE0023 ESE0024 ESE0025 ESE0050 ESE0051 ESE0052 ESE0053 ESE0038 ESE0039 ESE0040 ESE0041 ESE0042

E6DRKLIN01

E6BRTPLN01

E12CHAOS01

The Galileo spacecraft imaged the Conamara Chaos region (centered at about 9◦ north latitude, 274◦ west longitude), at three different resolutions during orbits 6 (at 200 and 55 m/pixel) and 12 (at 10 m/pixel). Earlier, global views provided low-resolution coverage of the same hemisphere (longitudes 250◦ –300◦ ). Using the global, regional, and local images, we mapped the extent of Pwyll rays across portions of Conamara Chaos. Coincident with these rays is a population of small craters. We present measurements for the craters within the Conamara Chaos sequences. Tables I and II summarize the sequences, resolutions, number of craters, and areas examined for each sequence. Note the dramatic increase in the number of craters per unit area found with each step in increasing resolution. 3. DISCUSSION

3.1. The Crater Plots Figure 3 shows log–log R-plots (as defined by the Crater Analysis Techniques Working Group, 1979) for the individual five frames of the E12CHAOS01 sequence, separated into the two terrains (plates and matrix): the circles represent measurements of craters on the plates, and the triangles represent measurements of craters in the matrix. Frame s0426271900 and the western edge of frame s0426271913 sit within the ray; the subsequent frames move farther away from the ray center (s0426271926 is at the edge of the ray, s0426271939 is outside the ray, and s0426271952 is “far” from the ray). For the rest of the paper, we shall refer to the images by the last five numbers only, e.g., 71900 or 71952. The crater diameters across the sequence range from just over 40 m (the completeness limit, typically three times the resolution of

the image examined) to almost 500 m. The largest diameter craters and the highest crater density are inside the ray. The peak R-value of between 0.05 and 0.06 is the highest we have yet measured on Europa, although it is still a factor of several below the empirical saturation R-value of around 0.2. We believe these craters are secondary craters, because the spatial correlation between the high crater density and bright albedo ray is very strong: the crater density drops off by a factor of several to over an order of magnitude between the ray center and outside the ray. Further, our measurements on surrounding terrain reveal only a few scattered craters outside the ray. 3.2. Crater Densities on the Plates vs the Matrix The R-plots of the individual frames show that the plates generally have a higher density of craters than the matrix, often by a factor of several. The plots also show that the density of craters is much higher within the ray (frame 71900) than outside the ray (frame 71952). Based on these observations, we separated the measurements in two ways: first we compared the measurements between frames in terms of distance from the ray (Fig. 4), and second we compared the measurements between the plates and matrix terrains (Fig. 5). The trend from the first comparison is clear—the crater density falls off dramatically with increasing distance from the Pwyll ray. The second trend is similarly clear—crater densities on the plates are higher at all crater diameters than on the matrix. The two trends present a paradox. If the Pwyll impact occurred before the formation of the chaos region, then would we expect to see the fall off of crater densities outside the ray on just the plates terrain, because the matrix terrain would not yet have existed and so would not have been exposed to the Pwyll secondaries. The matrix terrain would only possess the small to perhaps nonexistent population of background primaries collected after the matrix formation. If the Pwyll impact occurred after the formation of the chaos, then we would expect to see similar crater densities on both the plates and matrix terrains, because both would have been exposed to the ejecta launched by the Pwyll impact. Therefore we should see either one trend or the other, but not both. Somehow we must reconcile these seemingly incompatible observations. 3.2.1. Biased detection. One contributing factor to the density difference between the terrains is that detection of the craters within the matrix is inherently more difficult, because many of the craters are similar in size to the bumps and depressions that TABLE II Overview of Crater Measurement in and near Conamara Chaos No. of craters, Nc Sequence

Res. [m /pixel]

Plates

Matrix

Area [km2 ]

Nc /km2

E6DRKLIN01 E6BRTPLN01 E12CHAOS01

200 55 10

12 138 1111

0 15 638

9.0 × 105 5114 387

1.0 × 10−5 3.0 × 10−2 4.5

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FIG. 3. R-plot for each frame in the E12CHAOS01 sequence. There are two interesting characteristics: (i) the plates terrain has a higher crater density relative to the matrix and (ii) the density on both terrains decreases with increasing distance from the ray.

comprise the matrix terrain; craters are easier to detect on the plates. To test the effect of the matrix topography on crater detection, we also studied crater densities found in the Europa E6BRTPLN01 sequence, in which the matrix topography appears more smooth due to the lower resolution (10 m/pixel for E12CHAOS01 vs 55 m/pixel for E6BRTPLN01). This sequence covers more area of Conamara Chaos and contains larger diameter craters less obscured by the small-scale matrix topography. Restricting our measurements to the boundaries of Conamara

Chaos, we made distinctions similar to those in E12CHAOS01, i.e., we separated the crater measurements into plates and matrix terrains (Fig. 6). Because there are fewer visible craters in the lower resolution images (Table II), we must be cautious when deriving conclusions based on the statistics of small numbers. Still, the crater density on the plates is again many times higher than that found on the matrix; Table III lists the crater density (Nc = number of craters/km2 ) for the plates and matrix terrain in each image for the two sequences. Indeed, the ratios of crater

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FIG. 4. R-plot of E12CHAOS01 craters, divided into craters within the ray, craters near the ray, and craters outside the ray. On both the plate and matrix terrains, the crater density is much lower outside the ray than inside the ray.

FIG. 6. R-plot of the craters found within the E6BRTPLN01 sequence. Note that (i) the plates possess a higher density, as seen in the E12CHAOS01 sequence (see Fig. 5), and (ii) despite the increase in area, there still are no craters above about 1-km diameter seen.

densities (Nc , plates/Nc , matrix) are higher in E6BRTPLN01 than in E12CHAOS01. We conclude that crater densities are lower in the matrix even when the confusing topography of the matrix is smaller than the image resolution can resolve. Also, we note that the rollover on the size-distribution plot due to incompleteness (a function of the finite resolution in an image) occurs at the same diameter on both the plates and matrix. Figure 5 is an R-plot that compares the combined crater measurements on the plates with the combined crater measurements on the matrix for the E12CHAOS01 sequence. Unlike the other R-plots in this paper, we display the bins at small sizes that suffer from incompleteness (i.e., not all the craters in the smallest size

TABLE III Crater Densities (number/km2 ) on the Plates and Matrix

FIG. 5. R-plot of E12CHAOS01 craters, divided into plate and matrix terrains. The plate terrain has a higher density of craters than the matrix terrain at all crater diameters. The vertical dashed line marks the transition to incompleteness. The decrease in density to the left of this line does not reflect the actual crater population; instead it is a function of the finite resolution of the image. Below this size (3–4 pixel diameter, about 0.04 km for the resolution of this sequence) it is difficult to distinguish true craters. Because this transition occurs at the same diameter for both terrains in Conamara Chaos, we believe there is not a significant bias to detect craters on one terrain over the other.

Sequence

Image

Nc , Plates

Nc , Matrix

Nc ,Plates Nc ,Matrix

E12CHAOS01

s0426271900 s0426271913 s0426271926 s0426271939 s0426271952 s0383717513 s0383717526 s0383717539

16.61 11.92 5.23 4.97 1.31 0.050 0.030 0.080

4.14 6.85 1.69 1.04 0.81 0.009 0.005 0.016

4.01 1.74 3.09 4.78 1.62 5.56 6.0 5.0

E6BRTPLN01

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bins could be measured because the crater diameters approach the limit of the resolution), demonstrated by the rapid decrease in crater density at small diameters. Both terrains show a general increase in crater density down to roughly 0.06 km, below which the density rapidly decreases due to incompleteness. If the matrix topography significantly inhibited crater identification, then the rollover due to incompleteness for the matrix would happen at a larger diameter; yet this is not the case. Because the crater density on the plates is higher regardless of the resolution of the image examined, and because the density decrease at small diameters due to incompleteness occurs at the same diameter for both terrains, we conclude that the confusing topography within the matrix can only partly explain the crater density difference, and only for the smallest crater diameters. The plate terrain truly possesses a higher crater density than the matrix terrain. 3.2.2. Post-Pwyll activity. Bierhaus et al. (1998) proposed that the crater density differences could be explained by crater formation and/or retention variation between the two terrains. We now believe that the density difference results primarily from retention variation, which reflects ongoing surface activity. We propose that post-Pwyll endogenic activity within Conamara Chaos subdued and partly erased a portion of the crater population within the matrix terrain, reducing the crater density relative to the plate terrain. We discuss the evidence for endogenic activity below, but first we address why the density difference is likely not due to differences in crater formation. Suppose the crater density disparity arose from formation differences caused by differences in strength or rheology between the two terrains. Because the two terrains were exposed to the same secondary crater population, the shape of the crater curves should be similar for the two terrains, but with one curve shifted horizontally on the R-plot relative to the other because of differences in crater formation. For example, if the matrix is composed of fresh, unprocessed and newly frozen ice, and the plates are largely composed of heavily processed, fractured ice, then one could imagine that the same impactor would make a smaller size crater on the matrix than on the plates. The crater curve for the matrix would be shifted horizontally to smaller diameters relative to the crater curve for the plates. However, this is not the case. In Fig. 5, the plates and the matrix have similar crater curves that are unshifted relative to one another, suggesting that crater formation is essentially the same on the two terrains. If the difference in crater density is not a result of formation, then it must be a result of retention. Variation in retention could result from different responses to external (exogenic) or internal (endogenic) effects. The only external effect that we are aware of that could significantly erode features (such as craters) with sizes and heights tens of meters and larger is additional impact cratering. It is unreasonable to think that additional cratering somehow eroded a crater population on one terrain but not on another immediately adjacent terrain. Rather, because Europa is covered by areas of youthful geological activity, it is more reasonable to expect that such endogenic activity is responsible

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for the partial removal of the crater population within the matrix. Indeed, as we discuss below, it appears that Conamara Chaos has experienced endogenic activity beyond its initial formation. If all the craters in Conamara Chaos are secondaries, then they formed at essentially the same time; if they erode at the same rate they should also express similar states of degradation. However, when studying the craters on the plates, we noticed that the morphology of the craters follows the morphology of the plates they appear on; that is, fresh craters appear on relatively unmodified plates, while degraded craters appear on degraded plates. It is unlikely that the craters found on the degraded plates appear degraded because they formed on a surface less pristine than the craters on relatively unmodified plates. Craters formed in sand or other materials with weak strength still exhibit a spectrum of degradation: from fresh, bowl craters with well-defined rims and slopes to craters barely visible as softly undulating depressions. The craters on the modified plates exhibit characteristics of degradation such as modified rims and shallow depths relative to craters of similar size on the “fresh” plates (Fig. 7). It appears that there was a period of activity within Conamara Chaos, after its initial formation, that caused the degradation of some plates and the crater population on those plates. We propose that this subsequent activity not only caused crater degradation on the plates but also caused crater degradation and erasure within the matrix. (Perhaps the plates are more resilient to endogenic activity than the matrix because they survived the initial chaos formation; thus they would merely suffer degradation while the matrix suffers degradation and erasure.) Before continuing our discussion of post-Pwyll activity further, we present the caveat that we consider surface activity only as a means to resolve what otherwise seems a paradox within the cratering record of a region on Europa. Invoking surface activity successfully resolves the paradox, but we do not address the nature of the surface activity; to do so would be beyond the scope

FIG. 7. Two plates in the Conamara Chaos region. A is plate seen in Frame 71900 (see Fig. 2) that lies inside the Pwyll ray. This plate is covered with fresh secondaries from Pwyll. In contrast, B shows a plate (frame 71939) outside the ray containing a significantly less dense secondary crater population. Further, the craters on this plate are degraded relative to the craters in A. The arrows point to craters of similar size on each plate; note that the crater in A has a clearly defined rim and bowl, while the crater in B is a barely visible circular depression. Because the craters on both plates are likely secondaries from Pwyll, and thus formed at the same time, some processing must have occurred to the plate in B to degrade its craters relative to those on the plate in A.

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FIG. 8. Three craters in the middle of the Pwyll ray. A shows a 260-m crater in the matrix that experienced significant infilling to create a flat floor. B shows another matrix crater (330 m) that underwent some sort of collapse that formed a ridge inside the crater floor. C shows a 240-m crater on an immediately adjacent plate that, other than some rim modification by other secondary craters, appears fresh. Because these three craters all formed at the same time, the matrix must have undergone some activity to degrade craters A and B relative to the crater on the neighboring plate crater C.

of this paper, which focuses primarily on Europa’s potential secondary crater population. The nature of resurfacing on Europa is currently the topic of much debate (see Pappalardo and Head (2001) and Greenberg et al. (2001) and the references in each). It is sufficient for our purposes to note that surface activity has taken place without knowledge of the details of the mechanisms behind the surface activity. Other researchers have looked for current and recent surface activity on Europa via a plume search and change detection (Phillips et al. 2000); others (Riley et al. 2000, and the references contained therein) have noted examples of multiple episodes of surface activity within the same region. Figure 8 shows three craters of similar size that appear within a Pwyll ray, one fresh crater (240-m diameter) on a plate and two craters (260- and 330-m diameter) on the matrix. Other than its rim being partly modified by the impact of surrounding secondaries, the crater on the plate (Fig. 8C) has a fresh, relatively unmodified appearance. The neighboring craters on the same plate appear similarly fresh. However, the craters in the immediately adjacent matrix bear signs of substantial modification. One (Fig. 8A) has a noticeably flat floor, perhaps caused by infilling, and another (Fig. 8B) shows signs of slumping, forming a small ridge inside the crater bowl. The bright Pwyll ray is visible over the darker matrix material in Conamara Chaos (Fig. 9) and thus post dates the chaos, but there are at least two regions where portions of the bright ray appear to have been destroyed by continuing activity in the chaos. These examples appear in the regional-resolution E6DRKLIN01 frame s0383713726 (Fig. 10). The image shows the bright albedo ray covering the western margin of Conamara Chaos. However, the southern boundary of Conamara Chaos truncates the bright albedo ray; indeed it almost appears that

material associated with the Chaos continues to encroach upon the ray and onto the background ridged terrains (see arrows in Fig. 10). We must be cautious with this interpretation because there is a case on Ganymede where a catena (a chain of similarly sized and evenly spaced craters formed by the impact of a disrupted comet such as Shoemaker–Levy 9) crosses the boundary of light and dark terrain. The ejecta rays are visible on the bright terrain but are not visible on the dark terrain. The dark terrain presumably did not experience recent activity to erase the rays (the dark terrain is heavily cratered and universally considered the oldest terrain on Ganymede); the rays simply are not visible on that surface. We do not believe this is the case on Europa in the Conamara Chaos region, because the Pwyll ray is visible on both terrains, and so any truncation of the ray potentially represents erasure by activity subsequent to the ray formation. Definitive evidence of surface activity after the matrix formation can be seen in Fig. 2; a crack cuts through the matrix terrain in frames 71913, 71926, and 71939. Such continuing activity is not unique to Conamara Chaos. Riley et al. (2000) indicate that modification of chaos terrain by tectonics or subsequent chaos formation is common. Head et al. (1998) see regions where embayment by some relatively fluid flow is the most recent activity. The combination of these observations (a fresh and degraded population of Pwyll secondaries, possible destruction of the bright Pwyll ray, a tectonic feature in Conamara Chaos that must postdate the original formation of the chaos, and other clear examples of continuing activity within chaos regions) lead us to believe that post-Pwyll surface activity occurred within the Conamara Chaos region; this activity caused the degradation and partial destruction of the secondary crater population on the matrix terrain. This explains the crater density difference between the plates and the matrix terrains seen in Conamara Chaos. 3.3. Ages The matrix terrain is at least marginally younger than the plates terrain, because the matrix terrain formed by destroying the plates terrain. The extremely low density of background craters, which represent the primary production function, makes a relative age determination between the plates and the matrix impossible. Determining a relative age between the two terrains becomes even more muddled when evidence exists that evolution of Conamara Chaos continued after the deposition of Pwyll secondaries—we cannot be sure of the time scale associated with the initial formation or subsequent activity. However, we can estimate an upper limit for the age of this region assuming that surface activity took place after the Pwyll impact, which we believe to be the youngest large crater on Europa. Zahnle et al. (1998) indicate that 20-km craters should form about once every 1.4 myr; Levison et al. (2000) refined those calculations and suggest that the average time between impacts should be increased by a factor of four. These figures are based on numerically integrating the migration of Kuiper belt objects (KBOs) into the jovian system. They suffer from two main uncertainties:

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FIG. 9. An enhanced color image of part of the Conamara Chaos region. The bright white/blue patches are part of the Pwyll ray system, and high small-crater densities are spatially correlated with the ray. The ray is visible on both the plates and on the matrix terrains.

(1) knowing how to scale the absolute magnitude of active comets to their diameters and (2) the ratio of active to inactive comets. If we take Pwyll as the youngest crater in their 20-km category, then Pwyll is less than 6 myr old. The uncertainty of the Zahnle and Levison calculations could change this value by a factor of several, and new results from Bottke et al. (2001) indicate that that the rate needs to be reduced by another factor of three (based on scaling the numbers of inactive comets, rather than active comets), making Pwyll less than 18 myr old; although Zahnle’s (2001) most recent estimate is back down to 3.3 myr. These numbers are uncertain, by a factor of a few, but the point is that, relative to geological time scales, Pwyll is young. The post-Pwyll surface activity in the Conamara Chaos region must be younger still, meaning that it is plausible that Europa’s surface continues to be active today. 4. HOW MANY SMALL CRATERS MIGHT BE SECONDARIES?

Pwyll is not the only large crater on Europa that retains its ray system; however, Pwyll’s ray system is brighter and much more extensive than that of any other crater. Further, the im-

age data described in previous sections are the only examples of high-resolution coverage that clearly transect a crater ray. However, when we examined the small crater distribution in other high-resolution sequences, we noticed two characteristics that suggest the presence of secondary craters, despite the absence of obvious high-albedo rays. First, many small craters appear in clusters or clumps (Bierhaus et al. 2001). Because primary cratering is a random process, one expects to see a population of primary craters distributed randomly over a surface, not in spatially distinct clusters. However, our understanding of the production and distribution of secondary craters (Melosh 1989, Vickery 1986, 1987) suggests that material ejected from a primary crater as a spall-plate or loose clump will disperse slightly “in-flight,” making a cluster of craters when it lands. Another characteristic that suggests a secondary origin for much of Europa’s small crater population is the steep-sloped size distribution of the small craters. We have measured the size distribution of six high-resolution sequences, listed in Table IV, with an average differential slope index of −4.2. These steep slopes are reminiscent of those measured on the Moon at small diameters, which are likely due to secondary craters (Shoemaker 1963).

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FIG. 10. A highly contrasted portion of E6DRKLIN01 frame s0383713726 showing how the southern portion of the matrix terrain apparently “eats” into the bright Pwyll ray on the southern portion of Conamara Chaos. The arrows indicate where matrix terrain seems to extrude onto the background plains (right), or where dark material embays onto the background plains (left). Also note that the plates in this area of proposed recent activity (letters B and C) are darker and less well defined than the plates that appear inside the approximate outlines of the ray boundary (letter A). The resolution is about 200 m/pix, north is up, and the Sun is from the right.

On the basis of these two observations—the frequency of small craters appearing in clusters and the measured steep-sloped size distribution—we became intrigued by the possibility that many or most of the small craters on Europa are in fact secondary craters. A primary diagnostic constraint for this hypothesis lies TABLE IV Image Sequences and Their Slope Indexes Image sequence

Slope

E11MORPHY01 E12CHAOS01 E14DRKSPT01 E17LIBLIN01 E17THRACE01 E17THYLIN01 Average

−4.9 −3.6 −4.1 −4.0 −4.1 −4.5 −4.2

in the “mass balance,” i.e., whether or not there is enough material ejected from the few large primary craters to account for the total mass necessary to create the measured population of small craters. To address this issue, we estimated the amount of mass ejected from Europa’s primary craters and the amount of mass necessary to create the small crater population assuming that all small craters, <2-km diameter, are secondary craters. We use an upper limit of 4 km based on the argument that secondaries are no more than 5% the size of the primary (Melosh (1989)); because the largest primary on Europa is the 40-km Tyre, the largest secondary would be 2 km. Rather than attempting to determine absolute quantities for the masses involved, we performed an order-of-magnitude sequence of calculations to bracket plausibility. The details of these calculations follow. 4.1. Mass Ejected from Primary Craters To determine the mass ejected from a primary crater, we first translate the measured crater diameter back to the transient

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crater diameter and excavation depth, which better represent the shape—and thus the volume and ejected mass—of the excavated crater. Late in the formation process of large craters, the shape of the crater cavity is unable to support itself against gravity, leading to collapse and slumping. This tends to increase the crater diameter and decrease the crater depth, making the final crater larger in diameter and more shallow than the initial, transient crater. Turtle et al. (1999) measured the crater diameters of all known craters larger than 4 km on Europa and estimated the transient crater diameters and excavation depths. For transient crater diameter, they used the relation developed by McKinnon and Schenk (1995), D f ∼ 1.176Dtr1.108 , where D f is the measured crater diameter and Dtr is the transient crater diameter; for excavation depths they use Hexc ∼ 13 Dtr (Melosh 1989). Further, Turtle et al. (1999) list upper and lower values that encompass the range of error in both the initial crater diameter measurement and the scaling of the crater diameter. From their values for the transient crater diameter and excavation depth, we calculate the mass ejected from each crater assuming a density ρice = 919.7 kg/m3 and paraboloid of revolution, V = (π Hexc Dtr2 )/8, for the crater volume. For this calculation, we use only those primary craters larger than 10 km, which account for the vast majority of the total ejected mass—a 10-km crater contributes only 2% as much as a 40-km crater (e.g., Tyre). The resulting ejected mass from the primary craters, calculated using the lower and upper limits listed in the Turtle et al. (1999) table, is (3–13) × 1015 kg. This is the total mass ejected, not all of which produces distant secondary craters. Based on an analysis of lunar and martian craters, Melosh (1989) indicates that the fraction of ejected mass that produces distant secondary craters is uncertain (a few to 30%), but it is likely to be only a few percent. We choose a somewhat arbitrary but reasonable value of 5%, making the mass available to form distant secondary craters (m ej ) (1–7) × 1014 kg. 4.2. Mass Required to Create “Secondary” Craters We make several assumptions to derive the mass necessary to create the small craters. Most important, we assume that all small craters (less than 2 km) are secondaries, which establishes an upper limit on the required mass. Next we derive a global surface density for small craters by using the average values from the six measured high-resolution sequences listed in Table IV. Unfortunately, these images represent only about 0.001% of Europa’s surface, and so a global average derived from these sequences could be substantially in error. However, we note that these sequences are from widely separated locations on Europa’s surface, and the relative similarity (Fig. 11) of their crater densities at least suggests that these areas are not atypical. To find the number of craters in each size bin on Europa’s surface, we multiply the average differential value of each bin (# of craters/ (area·bin width)) by the surface area of Europa, 3.078× 107 km2 , and then sum the bins.

FIG. 11. An R-plot for the six measured high-resolution sequences. There is variation among the data, but we believe the slopes (see Table IV) and densities are similar enough that their average is an appropriate Europa global average for our order-of-magnitude calculations.

To calculate the mass necessary to form the small craters we use the technique developed by Vickery (1987) (also used by Alpert and Melosh 1999), which requires knowing the measured crater diameter (Dobs ) and the distance from the primary crater (R). The distance R is used to estimate the ejection velocity of each fragment that forms a crater by the equation (Vickery 1986, Eq. 1) vej2 =

g R E tan φ , cos2 θ tan φ + sin θ cos θ

where vej is the ejection velocity, g is the surface gravity of Europa (1.31 m/s2 ), R E is the radius of Europa (1565 km), φ = R/(2R E ) is the half-angular distance of travel, and θ is the ejection angle (assumed 45◦ ). Then, using Dobs and vej we can find the size, and thus mass (assuming a spherical shape), of the ejected fragments using Vickery’s scaling relationships. The appropriate scaling relationship depends upon the conditions of formation, i.e., whether the crater forms in the strength regime or the gravity regime, and the conditions of the surface. Because the transition between strength scaling and gravity scaling likely occurs within the size range of the craters under consideration, 40 m to 2 km, we use both gravity- and strength-scaling relationships. For completeness within the gravity regime, we also use both the porous and nonporous gravity scaling relations. These relationships are (Vickery 1986, Eqs. 9–11)

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à −2/3

d = 0.593Dobs (sin θ)

à 1.2 (sin θ )−0.4 d = 0.76Dobs

Y 2 δ · vimp

g

TABLE VI Ratio of Mass Ejected to Mass Necessary to Form the Small Craters

(strength regime),

2 vimp

à 1.28 (sin θ)−1/3 d = 0.752Dobs

!0.2

!0.236

(gravity regime, porous surface), !0.277 g 2 vimp

(gravity regime, nonporous surface), where d is the fragment diameter, Y is the yield strength of ice (7.6 × 106 Pa, McKinnon and Schenk 1995), δ is the impactor density (taken to be 919.7 kg/m3 because the impactors are flying chunks of Europa’s surface), and vimp is the impact velocity, which we assume is equal to the ejection velocity (vej ) because Europa has a negligible atmosphere. We know the small crater diameters (Dobs ) from our measurements; to find the distance between a crater and its primary R is slightly more problematic. Ultimately we wish to tie obvious crater clusters on Europa with their parent primaries, but currently we have not completed that analysis, meaning we do not know where the ejecta originated that formed the craters we measured. To find a reasonable value for R that is appropriate for our order-of-magnitude calculation, we assume that the 15 large craters (used in the calculation in the previous section) are distributed evenly over Europa’s surface and that an average distance between the primaries and the “secondaries” is halfway between the evenly distributed primary craters. The distance between the primaries and secondaries is therefore taken to be 716 km. (This is not an unreasonable number because we see crater clusters hundreds of kilometers away from the nearest large primary—indeed the craters in Conamara Chaos are around 1000 km from Pwyll, and Pwyll rays extend even further. Based on these observations, we would not expect that clusters appear primarily within a few crater radii of their parent primary.) We then use Vickery’s equations to calculate the fragment mass necessary for each size bin, multiply that mass by the number of craters in each bin, and finally sum those products for the total mass. Table V summarizes the results.

TABLE V Mass Required to Form “Secondaries” Scaling relationship

Resultant mass [kg]

Gravity, porous Gravity, nonporous Strength

400 × 1013 60 × 1013 0.005 × 1013

Scaling relationship

Mass ratio (m ej /m re )

Gravity, porous Gravity, nonporous Strength

0.03–0.18 0.17–1.67 1900–13,000

4.3. Comparison of Results We calculated the ratio of the mass available to make distant secondary craters (5% of the ejected mass), m ej , to the mass required to make all small craters via the secondary cratering process, m re . A value of 0.5 or above means that there is enough ejected mass for secondaries to account for at least 50% of the measured small craters. Table VI lists the ratios. The resultant ratios span several orders of magnitude (from 0.03 to 13,000), although there are several factors to consider that can better constrain the likely value. First, Europa’s highly cracked and fractured surface suggests a nonporous, competent surface. Second, the smallest measured craters (less than a few hundred meters in diameter) probably formed in the strength regime, while the larger craters were formed in the gravity regime. These considerations lead us to believe that the ratio lies within the nonporous gravity range, with some portion of the very smallest craters within the strength regime. Moreover, Vickery derived her scaling laws from experiments that focused on simulating the lunar environment, which is composed of rock, not ice. The strength-scaling result is vastly different from either of the gravity-scaling values and, unlike the gravity scaling, depends on the density of the impactor. We do not know what the state of the ejecta is when it re-impacts the surface, but it is at least competent enough to make a crater. Even if we drop the density of the impactor (i.e., the ejecta) to 0.5 kg/m3 , m re becomes 1 × 1013 kg, making the ratio of m ej /m re 10–70, still significantly above the critical value of 0.5. Based on this analysis, it is reasonable that secondary craters may dominate the small crater population, because there is potentially enough mass ejected from the primary craters to account for all of the small craters. 4.4. Uncertainties This is an order-of-magnitude calculation and thus we are not concerned with precise values of m ej and m re ; we wish to determine only whether they are potentially similar amounts. However, it is useful to examine our assumptions and scaling relationships to see how realistic our answer may be. The major sources of uncertainty for m ej , from most to least significant, are (1) translation from measured crater diameter to transient crater diameter, (2) completeness of primary craters over 10 km, and (3) the fraction of mass ejected that creates secondary craters. The first source results from our limited understanding of the mechanics of crater collapse. However, we have already

EUROPA’S SMALL CRATERS

considered this uncertainty in our calculation, as described in Section 4.1 (i.e., Turtle et al. (1999) provide a range of values for the transient crater diameter and excavation depth; in our calculations this subsequently becomes a range of mass ejected). Completeness is a source of uncertainty, because it is likely that we have not completely sampled Europa’s crater population for diameters >10 km. It is unlikely that we are missing any craters >30 km, and those are the craters that dominate the available m ej . However, because only 42% of Europa is sampled at 1 km/pixel or better (Cynthia Phillips, personal communication), it is possible that we are missing several 10-km craters, and perhaps even a few 20-km craters. It is important to note, however, that this incompleteness further supports our argument; if we have almost enough m ej to create Europa’s small craters even with the incompletely sampled large primary craters, additional large primary craters will simply provide more ejected mass to create secondary craters. Finally, we do not know what percentage of the mass ejected creates distant secondary craters. Assuming that some combination of the strength and nonporous gravity scalings is appropriate, our argument would still be valid even if the amount of mass ejected that creates distant secondary craters were only 1–2%, rather than the 5% we initially chose. There are three major uncertainties associated with m re , the mass required to form the secondaries. The first involves deriving a global small-crater density average from just six measured high-resolution sequence. Because these images cover a tiny fraction of Europa’s surface, we cannot be sure that these counts are representative of the average small crater population. However, we note that these images sample a variety of widely separated locations on Europa, and because they sample such a tiny fraction of Europa’s surface, it is unlikely that they all contain an anomalous crater population. Figure 11 is an R-plot of the six measured high-resolution sequences. The data do show variation but are similar enough in slope and density that we feel their average potentially reflects a global average. Although Europa does have two major terrain types, ridges and chaos, these are tightly interspersed with one another, and the general paucity of large craters suggests that, on average, Europa’s surface is young. This is in contrast to the Moon and Mars, both of which have two major terrain types with different crater densities, i.e., the maria vs the highlands on the Moon, and the northern lowlands vs the southern highlands on Mars. It would be misleading to average the crater densities from those different terrain types with one another, whereas we can average crater measurements on Europa, because no one part of Europa can have a vastly different age than another part. The second set of uncertainties associated with m re comprises scaling uncertainties from crater diameter to fragment (impactor) diameter. These we address in part by considering three different scaling regimes (strength, porous gravity, and nonporous gravity). The remaining uncertainties in the crater scaling do not drastically shift our order-of-magnitude calculation, because the most important factor is the impact velocity, and

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that is fairly well constrained between Europa’s escape velocity (about 2 km/s) and the ballistic trajectory necessary to travel at least a few hundred kilometers (1 km/s in case of the Pwyll secondaries in Conamara Chaos; about 1000 km from Pwyll). Third, there are uncertainties associated with our measurements of the size distribution of small craters, the error of which √ is usually taken to be N , where N is the number of craters. This error represents the Poisson counting statistics only; it does not reflect other systematic errors, such as measuring small craters near the resolution limit or the influence of confusing terrain (for example, Section 3.2.1 of this paper). To ensure reasonable completeness of our measurements, the images were examined at multiple resolutions and contrasts on at least two occasions, separated by at least one day. We would need to revise our estimates only if we vastly undercounted the small craters; if we overcounted (identified features as craters that are in fact something else), then the amount of mass needed to create the actual crater population is smaller. Our goal is to explain the craters that are visible; we cannot speculate on craters that may or may not exist at sizes unresolved in the current images. Because we believe we have not significantly undercounted the small crater population, we believe that this is an appropriate order-of-magnitude argument. 4.5. Implications There are two immediate consequences if secondary craters indeed dominate Europa’s small crater population. First, the possible scarcity of small primary craters suggests that the impacting population is deficient in small objects. This is inconsistent with the Neukum et al. (1997) model in which the production function has a steep slope for projectiles that make small craters. On the other hand, Zahnle et al. (1998) predict that Jupiter family comets are the dominant impactors in the jovian system, with no restrictions on the size distribution at small diameters; few small primary craters would then indicate a paucity of small comets. Thus, formation and/or evolutionary models must explain why small comets do not form, or why their flux into the jovian system is so low. The second consequence centers on calculating relative ages for different geological units on Europa: if small impactors are not abundant, then extreme care must be taken when using small craters to date specific regions on Europa, either in absolute or relative terms. A higher crater density may not necessarily reflect an older age, but rather an exposure to a population of secondary craters. Similarly, a sparsely cratered surface would be older than one might initially expect, because fewer small impactors will hit Europa less frequently. In general terms, the possible preponderance of secondaries on Europa is grounds to reconsider the origin of small craters on other planetary surfaces. It is known that secondaries appear on the Moon, Mercury, and Mars and that they contribute to the steep-sloped size distributions of craters at small diameters. How much they contribute is still unknown, though if results in this study apply to rocky surfaces as well, then the number of secondaries could be significantly higher than previously thought. Alternatively, the response of rock and ice to the high shock

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values during an impact may differ enough that ejecta fragment production and distribution on Europa cannot be compared with, for example, the Moon. Future work should re-examine secondaries on the Moon to see if there, too, secondaries significantly contribute to the small crater population. 5. SUMMARY

We present crater densities in the Conamara Chaos region, based on measurements from images in the E12CHAOS01, E6BRTPLN01, and other high-resolution sequences that demonstrate a number of characteristics of small-scale cratering on Europa: far-flung secondaries can dominate local small cratering; the crater density can drop as much as an order of magnitude between in-ray and adjacent regions; recent activity in Conamara Chaos destroyed a portion of the Pwyll secondary crater population within the matrix terrain, indicating endogenic surface activity likely occurring within the last 18 myr; several populations of small craters on Europa have steep size distributions, which is indicative of a secondary origin; and finally, the few large primary craters on Europa could produce enough mass to account for the bulk of the small crater population, allowing the possibility that secondary craters dominate Europa’s small crater population. ACKNOWLEDGMENTS E.B.B. thanks his coauthors for thoughtful discussions and comments and also Louise Prockter and Jeff Moore for their thorough and insightful reviews of the manuscript. Finally, the Galileo SSI Team and Staff deserve many thanks for their continuing vigilance that keeps the spacecraft alive and returning invaluable data. This work is supported by grants from NASA’s Jovian System Data Analysis Program to C. R. Chapman (NAG5-8896) and W. J. Merline (NAG5-8090).

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