Primary Craters Research Articles

The role of ejecta in the small crater populations on the mid-sized saturnian satellites

We find evidence, by both observation and analysis, that primary crater ejecta play an important role in the small crater (less than a few km) populations on the saturnian satellites, and more broadly, on cratered surfaces throughout the Solar System. We measure crater populations in Cassini images of Enceladus, Rhea, and Mimas, focusing on image data with scales less than 500m/pixel. We use recent updates to crater scaling laws and their constants (Housen, K.R., Holsapple, K.A. [2011]. Icarus 211, 856–875) to estimate the amount of mass ejected in three different velocity ranges: (i) greater than escape velocity, (ii) less than escape velocity and faster than the minimum velocity required to make a secondary crater (vmin), and (iii) velocities less than vmin. Although the vast majority of mass on each satellite is ejected at speeds less than vmin, our calculations demonstrate that the differences in mass available in the other two categories should lead to observable differences in the small crater populations; the predictions are borne out by the measurements we have made to date. In particular, Rhea, Tethys, and Dione have sufficient surface gravities to retain ejecta moving fast enough to make secondary crater populations. The smaller satellites, such as Enceladus but especially Mimas, are expected to have little or no traditional secondary populations because their escape velocities are near the threshold velocity necessary to make a secondary crater. Our work clarifies why the Galilean satellites have extensive secondary crater populations relative to the saturnian satellites. The presence, extent, and sizes of sesquinary craters (craters formed by ejecta that escape into temporary orbits around Saturn before re-impacting the surface, see Dobrovolskis, A.R., Lissauer, J.J. [2004]. Icarus 169, 462–473; Alvarellos, J.L., Zahnle, K.J., Dobrovolskis, A.R., Hamill, P. [2005]. Icarus 178, 104–123; Zahnle, K., Alvarellos, J.L., Dobrovolskis, A.R., Hamill, P. [2008]. Icarus 194, 660–674) is not yet well understood. Finally, our work provides further evidence for a “shallow” size–frequency distribution (slope index of ∼2 for a differential power-law) for comets a few kilometers diameter and smaller.

Secondary crater fields from 24 large primary craters on Mars: Insights into nearby secondary crater production

[1] Crater statistics are used across a wide variety of applications on planetary surfaces, one of the most notable being estimating relative and absolute ages of those surfaces. This requires an assumed cratering rate over time and that craters be randomly distributed. Secondary craters - craters that form from the ejecta of an impact event - belie this assumption by creating greater crater density in a local area at a single time, significantly affecting crater statistics. There has been substantial debate over the relative importance of secondary craters, and our findings in this Mars study indicate that these events can be very significant and cannot be ignored when age-dating surfaces. We have analyzed secondary crater fields found close to 24 primary craters on Mars. Among other findings such as terrain control over secondary crater field characteristics, we conclude that a single large impact event (>100 km) can significantly affect crater statistics at the ∼1–5-km-diameter level over a non-trivial fraction of a planetary surface (minimum secondary crater diameters examined were ∼0.9 km; the minimum primary crater diameter was ∼20 km). We also suggest a potential way to avoid significant contamination by the majority of secondary craters that occur close to the primary impact event without the need to manually classify every crater as primary or secondary. Our findings are specific to Mars, but further work may show the patterns are applicable to other solid bodies.

Mercury crater statistics from MESSENGER flybys: Implications for stratigraphy and resurfacing history

The primary crater population on Mercury has been modified by volcanism and secondary craters. Two phases of volcanism are recognized. One volcanic episode that produced widespread intercrater plains occurred during the period of the Late Heavy Bombardment and markedly altered the surface in many areas. The second episode is typified by the smooth plains interior and exterior to the Caloris basin, both of which have a different crater size-frequency distribution than the intercrater plains, consistent with a cratering record dominated by a younger population of impactors. These two phases may have overlapped as parts of a continuous period of volcanism during which the volcanic flux tended to decrease with time. The youngest age of smooth plains volcanism cannot yet be determined, but at least small expanses of plains are substantially younger than the plains associated with the Caloris basin. The spatial and temporal variations of volcanic resurfacing events can be used to reconstruct Mercury's geologic history from images and compositional and topographic data to be acquired during the orbital phase of the MESSENGER mission.

Chandrayaan-1 observation of distant secondary craters of Copernicus exhibiting central mound morphology: Evidence for low velocity clustered impacts on the Moon

Analysis of the Chandrayaan-1 Terrain Mapping Camera image of a 20 km×27 km area in the Mare Imbrium region revealed a cluster of thousands of fresh and buried impact craters in the size range of 20–1300 m. A majority of the large fresh craters with diameter ranging from 160 to 1270 m exhibit near-circular mounds (30–335 m diameter and 10–40 m height) in the crater floor, and their size depends on the host crater size. The origin of this cluster of secondary craters may be traced to Copernicus crater, based on global lunar image and the analysis of Chandrayaan-1 Hyper Spectral Imager data. Our findings provide further evidence for secondary crater formation by low-velocity impact of a cloud of clustered fragments. The presence of central mounds can also distinguish the secondary craters from the primary craters and refine the chronology of lunar surface based on counting of small craters.

Constraints on Europa's surface properties from primary and secondary crater morphology

We investigate the near‐surface properties of Europa's ice shell by examining small crater morphology; we use both primary and secondary craters for our analysis. For primary craters, the simple‐to‐complex transition provides an estimate of 0.19–0.36 MPa for the cratering effective strength. Through the aggregate statistics of over 100 profiles, we find that secondary craters on Europa tend toward smaller depth‐to‐diameter (d/D) ratios than primary craters, consistent with observations on other cratered surfaces (the Moon and Mars). In addition, we find that adjacent secondaries tend to be more shallow than distant secondaries, also consistent with trends seen on the Moon and Mars. The presence of numerous and far‐flung secondaries requires a solid, competent surface for spallation that is inconsistent with weak ice. Although the effective strength evaluated by crater scaling laws does not have a direct quantitative relationship to lab‐based measurements of shear or tensile strength, comparison to other observationally derived effective strengths and geologic materials suggests that the mechanical properties of Europa's surface ice should be consistent with terrestrial measurements of cold ice.

Candidates for multiple impact craters?: Popigai and Chicxulub as seen by the global high resolution gravitational field model EGM2008

Abstract. In 2008 the new Earth Gravitational Model (EGM2008) was released. It contains a complete set of spherical harmonic coefficients of the Earth's gravitational potential (Stokes parameters) to degree 2190 and order 2159 and selected orders to degree 2190, that can be used for evaluation of various potential quantities with both the unprecedented accuracy and high spatial resolution. Two such quantities, the gravity anomaly and second-order radial derivative of the disturbing potential, were computed over selected areas with known impact craters. The displays of these derivatives for two such sites clearly show not only the strong circular-like features known to be associated with them but also other symmetrical structures which appear to make them multiple impact sites. At Popigai, Siberia, the series of circular features fall in a line from the "primary crater" in the southeast (SE) direction. At Chicxulub, Yucatán, there appears to be one more crater close to the "primary" in the northeast (NE) direction, as well as possibly others in the vicinity of the main crater (SW). Gravity information alone is not, however, proof of impact craters but it is useful in identifying candidate sites for further study, for examination by geologists and geophysicists. In the case of Chicxulub, a very recent single seismic profile suggests that a more likely explanation for the observed circular like gravity signal from EGM2008 NE of the "primary" is a pre-impact basin.

Geomorphic analysis of small rayed craters on Mars: Examining primary versus secondary impacts

Twenty confirmed impacts over a 7‐year time period on Mars were qualitatively and statistically compared to 287 secondary craters believed to originate from Zunil, an ∼500 ka, 10‐km diameter, primary crater. Our goal was to establish criteria to distinguish secondaries from primaries in the general crater population on the basis of their horizontal planforms. Recent primary impacts have extensive “air blast” zones, distal ray systems (>100 crater radii, R), and ephemeral ejecta. Recent primaries formed clusters of craters from atmospheric fragmentation of the meteoroid body. Secondary craters have ejecta blankets with shorter rays that are consistent with emplacement by low‐impact velocities (near 1 km/s). The mean extent of the continuous ejecta blankets was less distal for secondaries (5.38 ± 1.57R) versus primaries (18.07 ± 7.01R), though primary ejecta were less fractal (Fractal Dimension Index (FDI) < 1.30) and more circular on average (Circularity Ratio (CR) = 0.55 ± 0.25 versus 0.27 ± 0.13 for secondaries). Crater rims were remarkably circular (primaries CR = 0.97 ± 0.02, secondaries at 0.94 ± 0.05), though secondaries have the lowest values (CR < 0.9). Secondary crater rims were elongated toward or orthogonal to their primary of origin. Uprange source directions for most secondaries, determined by ejecta planform and crater rim ellipticity, point toward Zunil, although contamination from other primaries is considered in some areas. Ejecta blanket discrepancies between recent primaries and Zunil secondaries are attributable to differences in impact velocity and retention age. After removal of the ejecta blanket, crater rims are generally not diagnostic for determining crater origin. Fragmentation of primaries may play some role in steepening the size‐frequency distribution of crater diameters in the 5 m < D < 30 m range.

Theoretical analysis of secondary cratering on Mars and an image-based study on the Cerberus Plains

The issue of crater retention age estimates on planetary surfaces is discussed with an attempt to quantify the effect of overlapping primary and secondary impact crater populations in restricted crater diameter ranges. The approach to this problem is illustrated with a simple model production function where the secondary crater input is artificially enhanced. Extrapolation of such a secondary crater model distribution to a global record results in extraordinarily high crater frequencies that do not exist on Mars, and implies the need of detailed studies of the size-frequency distribution for remote secondary craters, to date poorly known. A key case, the martian crater Zunil and its secondary crater field, illustrate that reasonable predictions for the secondary crater size-frequency distribution at small (<100 m) crater diameters affected the standard model crater retention age for the Cerberus plains less than the statistical uncertainty. These observations show that age determination based on appropriate crater counting statistics is valid in a wide primary crater diameter range.

Mapping rays and secondary craters from the Martian crater Zunil

Zunil, a 10.1 km rayed crater in Elysium Planitia, Mars, produced more than 7 × 107 secondary craters ≥15 m in diameter. We mapped Zunil's rays from thermal IR THEMIS nighttime images up to 1700 km from the primary crater and mapped bright and dark ejecta craters (candidate secondaries) up to 3600 km range. Ray segments were mapped up to 450 km east of Zunil and up to 1700 km to the west. Both rays and bright ejecta craters are best detected over terrains with moderate thermal inertia, which are abundant west of but not east of Zunil. Nevertheless, our interpretation is that Zunil was created by a moderately oblique impact from the east. Zunil secondaries are abundant over all terrain types except the Medusae Fossae Formation (MFF). Given the likely age of Zunil (&lt;100 Ma), parts of the MFF must be eroding at ≥0.08 m/Ma. The size‐frequency distribution of the secondaries in Zunil's rays and probable distant secondaries (750–1700 km to the west of Zunil) have a cumulative power law exponent near –5, whereas secondaries between the major rays have an exponent near −3.4. We modeled the size‐velocity relationship for Zunil's ejected fragments; it is consistent with the predictions of Melosh (1984) for spallation of a strong surface layer and demonstrates that the inverse size‐velocity correlation continues up to at least 2 km/s.

Using lunar boulders to distinguish primary from distant secondary impact craters

A high‐resolution study of 18 lunar craters, including both primary and distant secondary craters, shows that the secondary craters produce larger ejecta fragments at a given crater size than do the primary craters. The maximum boulder diameter (B) increases with crater size (D) according to the power law B = KD2/3; for primary craters, when B and D are in meters, K is 0.29, whereas for secondary craters, we find that K is 0.46 (60% larger). Next we show that impact fracture theory predicts that secondary craters, because of their lower impact velocity, will produce larger ejecta fragments than primary craters. This result provides an opportunity for distinguishing between primary and secondary craters in high resolution planetary images. The ability to identify distant secondary craters will help constrain primary production rates of small craters and improve surface age determination of small areas based on small crater counts.

Martian cratering 9: Toward resolution of the controversy about small craters

The Malin et al. [Malin, M.C., Edgett, K., Posiolova, L., McColley, S., Noe Dobrea, E., 2006. Science 314, 1573–1577] report of ongoing production of decameter scale primary craters matches within an order of magnitude the crater production curves of Hartmann [Hartmann, W.K., 2005. Icarus 174, 294–320], and appears to contradict models that say our curves grossly overpredict primary production. Nonetheless, such models may still predict correct order-of-magnitude secondary production. The new observations, if correct, require geologically recent martian surface modification in some areas, on timescales comparable to the cycles of obliquity changes, suggesting episodic martian climatic/environmental change driven by the obliquity cycles.

THE IMPORTANCE OF SECONDARY CRATERING TO AGE CONSTRAINTS ON PLANETARY SURFACES

Small craters (less than one kilometer diameter) can be primary craters produced by impact of interplanetary debris, or they can be secondary craters produced by fallback of high-velocity ejecta blocks from much larger but infrequent primary impacts. The prevalent assumption over recent decades has been that primaries are most abundant, so most small craters are independent random events and can be used for dating. However, recent results from Europa and Mars support the early theory that distant secondaries globally dominate the number of small lunar craters; this would invalidate part of production functions that have been widely used for age dating. Crater excavation results in higher mean ejection velocities for smaller fragments, resulting in a steeper size-frequency distribution for secondary craters than is produced by the same size-frequency distribution of interplanetary debris. This review also discusses how small craters can sometimes be used to derive meaningful age constraints.

Secondary craters of Tycho: Size‐frequency distributions and estimated fragment size–velocity relationships

We examined the size‐frequency distribution and the spatial distribution of secondary craters around the lunar crater Tycho. Secondary crater diameters were found to range from 0.55 to 4.0 km, and their distance from Tycho to range from 130 to 370 km. The diameter and ejection velocity of the secondary‐forming fragments were also estimated from the crater size and the distance from the primary using the scaling relationships for cratering. The power law index of the size‐frequency distribution of the Tycho secondary craters ranges from 3.3 to 4.0, the same as the steep slope of the typical size‐frequency distribution for lunar craters of less than 4 km diameter. The decay index of secondary crater densities with distance from the primary craters is not constant and is small at large distances (less than 2.5 for r &gt; 2.5R). The total number of secondaries forming ejecta fragments is more than twice larger than for other lunar primaries previously studied. These results imply that there are many small secondary craters in fields far from the primary crater. A dominant contribution of secondaries to the size‐frequency distribution of all lunar craters is thus strongly suggested. The index range of 2.6 to 3.0 for estimated fragment size distributions is not only close to other natural and experimental craters, but also to those of blocks on asteroids. The relative fragment size distributions are independent of the ejection velocity. This result is the first observational confirmation of the theoretical assumption, and agrees with a recent result from laboratory experiments.

The rayed crater Zunil and interpretations of small impact craters on Mars

A 10-km diameter crater named Zunil in the Cerberus Plains of Mars created ∼ 10 7 secondary craters 10 to 200 m in diameter. Many of these secondary craters are concentrated in radial streaks that extend up to 1600 km from the primary crater, identical to lunar rays. Most of the larger Zunil secondaries are distinctive in both visible and thermal infrared imaging. MOC images of the secondary craters show sharp rims and bright ejecta and rays, but the craters are shallow and often noncircular, as expected for relatively low-velocity impacts. About 80% of the impact craters superimposed over the youngest surfaces in the Cerberus Plains, such as Athabasca Valles, have the distinctive characteristics of Zunil secondaries. We have not identified any other large (⩾10 km diameter) impact crater on Mars with such distinctive rays of young secondary craters, so the age of the crater may be less than a few Ma. Zunil formed in the apparently youngest (least cratered) large-scale lava plains on Mars, and may be an excellent example of how spallation of a competent surface layer can produce high-velocity ejecta (Melosh, 1984, Impact ejection, spallation, and the origin of meteorites, Icarus 59, 234–260). It could be the source crater for some of the basaltic shergottites, consistent with their crystallization and ejection ages, composition, and the fact that Zunil produced abundant high-velocity ejecta fragments. A 3D hydrodynamic simulation of the impact event produced 10 10 rock fragments ⩾10 cm diameter, leading to up to 10 9 secondary craters ⩾10 m diameter. Nearly all of the simulated secondary craters larger than 50 m are within 800 km of the impact site but the more abundant smaller (10–50 m) craters extend out to 3500 km. If Zunil is representative of large impact events on Mars, then secondaries should be more abundant than primaries at diameters a factor of ∼1000 smaller than that of the largest primary crater that contributed secondaries. As a result, most small craters on Mars could be secondaries. Depth/diameter ratios of 1300 small craters (10–500 m diameter) in Isidis Planitia and Gusev crater have a mean value of 0.08; the freshest of these craters give a ratio of 0.11, identical to that of fresh secondary craters on the Moon (Pike and Wilhelms, 1978, Secondary-impact craters on the Moon: topographic form and geologic process, Lunar Planet. Sci. IX, 907–909) and significantly less than the value of ∼0.2 or more expected for fresh primary craters of this size range. Several observations suggest that the production functions of Hartmann and Neukum (2001, Cratering chronology and the evolution of Mars, Space Sci. Rev. 96, 165–194) predict too many primary craters smaller than a few hundred meters in diameter. Fewer small, high-velocity impacts may explain why there appears to be little impact regolith over Amazonian terrains. Martian terrains dated by small craters could be older than reported in recent publications.

On estimating contributions of basin ejecta to regolith deposits at lunar sites

Abstract— We have developed a quantitative model for predicting characteristics of ejecta deposits that result from basin‐sized cratering events. This model is based on impact crater scaling equations (Housen, Schmitt, and Holsapple 1983; Holsapple 1993) and the concept of ballistic sedimentation (Oberbeck 1975), and takes into account the size distribution of the individual fragments ejected from the primary crater. Using the model, we can estimate, for an area centered at the chosen location of interest, the average distribution of thicknesses of basin ejecta deposits within the area and the fraction of primary ejecta contained within the deposits. Model estimates of ejecta deposit thicknesses are calibrated using those of the Orientale Basin (Moore, Hodges, and Scott 1974) and of the Ries Basin (Hörz, Ostertag, and Rainey 1983). Observed densities of secondary craters surrounding the Imbrium and Orientale Basins are much lower than the modeled densities. Similarly, crater counts for part of the northern half of the Copernicus secondary cratering field are much lower than the model predicts, and variation in crater densities with distance from Copernicus is less than expected. These results suggest that mutual obliteration erases essentially all secondary craters associated with the debris surge that arises from the impacting primary fragments during ballistic sedimentation; if so, a process other than ballistic sedimentation is needed to produce observable secondary craters. Regardless, our ejecta deposit model can be useful for suggesting provenances of sampled lunar materials, providing information complementary to photogeological and remote sensing interpretations, and as a tool for planning rover traverses (e.g., Haskin et al. 1995, 2002).

Testing hypotheses for the origin of steep slope of lunar size-frequency distribution for small craters

The crater size-frequency distribution of lunar maria is characterized by the change in slope of the population between 0.3 and 4 km in crater diameter. The origin of the steep segment in the distribution is not well understood. Nonetheless, craters smaller than a few km in diameter are widely used to estimate the crater retention age for areas so small that the number of larger craters is statistically insufficient. Future missions to the moon, which will obtain high resolution images, will provide a new, large data set of small craters. Thus it is important to review current hypotheses for their distributions before future missions are launched. We examine previous and new arguments and data bearing on the admixture of endogenic and secondary craters, horizontal heterogeneity of the substratum, and the size-frequency distribution of the primary production function. The endogenic crater and heterogeneous substratum hypotheses are seen to have little evidence in their favor, and can be eliminated. The primary production hypothesis fails to explain a wide variation of the size-frequency distribution of Apollo panoramic photographs. The secondary craters are likely the major source of the steepening of the distribution. It is ambiguous, however, which primary craters can produce sufficiently numerous secondary craters. The regional variation of the size-frequency distributions shows that few large impacts produce enough secondary craters to affect the distributions in the surrounding area. We emphasize that a crater size-frequency distribution of small craters on the moon should not be taken as an indication of the surface age. More data obtained from future lunar missions should be viewed in this context, and continued to be examined for further insight into the possible formation mechanism for secondary craters.

Quasistatic to hypervelocity impactor loading of glass: autodyn hydrocode and static testing configurations

Hypervelocity impact on glass leads to damage which involves both high pressure fluid dynamics (forming the “primary” crater) and comparatively low strain rate damage leading to spallation and extended platelet formation (conchoidal fracture). Conchoidal fracture may extend to much larger volumes than the primary crater and subsequently remove most or all of the primary crater. Understanding and modelling thus calls for knowledge of the equation of state, strength limits (both compressive and tensile) and, especially, crack propagation. We examine how a single improved hydrocode with Johnson Holmquist implementation in 2D and 3D can mimic both extremes of the impact process and look at how static test results compare to the hydrocode during the (conchoidal) stress relief by crack propagation at times very much greater than the final primary cratering phase.

Reconsideration of crater size-frequency distribution on the moon: effect of projectile population and secondary craters

The crater size-frequency distribution curve for the lunar surface has a bending point near 4 km in diameter. It is suggested that the steepening in the smaller diameter range is caused by the effects of secondary craters and/or size-frequency distribution of the impactors. In order to extract the fraction of the primary craters created by Near Earth Objects, among the primary-secondary mixture of km-size craters, we have examined the crater shape statistics using Clementine images of the lunar surface. We found that the size-frequency distribution of the craters with ellipticity smaller than 1.2 can be considered primary craters generated by impactors from interplanetary space.

A proposed clinical application of a model of CO2laser radiation induced damage craters

A method for a controlled generation of primary damage craters in methacrylate ester samples (Vedril C, Montedison) induced by continuous and long-term exposure to CO2 laser radiation is proposed. The results are compared with the experimental values obtained 'in vitro' with 10 W continuous output power of a CO2 laser beam and fg = 7.5 in focal length of the laser's focusing head, and this shows that the proposed model can explain the phenomenon related to the generation of the limiting crater in typical biological tissue. It may have applications in clinical procedures where long-term exposure of the tissue to a laser beam must be considered. Also, the method may possibly be used to create and update a computer library automatically collecting data regarding different thermodynamic characteristics and values of various biological media.

Variation in ejecta size with ejection velocity

A knowledge of the size‐velocity distribution of ejecta from impact craters is important for a number of processes that occur or may have occurred in the solar system, such as planetary accretion, collisional evolution of asteroids, the ejection of meteorites from parent bodies larger than asteroids (such as the lunar meteorites and the SNC meteorites, which may be from Mars), and massive biological extinctions on the Earth. In order to estimate the size‐velocity distribution of impact crater ejecta, the sizes and ranges from the primary crater were measured for the secondary craters of twelve large craters on Mercury, the Moon and Mars. The ballistic equation for spherical bodies was used to convert the ranges to velocities, and the velocities and crater sizes were used in the appropriate Schmidt‐Holsapple scaling relation to estimate ejecta sizes. Assuming that d ⧜ v− β, lines were fitted to the log (maximum size) vs. log (ejection velocity) using the least squares method to determine the velocity exponent. Certain problems with data collection, such as partial burial of some craters and interference from other craters and topographic features, made it impossible to determine a simple, unique relation between size and velocity. The velocity exponents are generally between −1 and −3, with an average value of approximately −1.9. The complicating effect of secondaries produced by clusters of impactors implies that the calculated lines are probably steeper than the slopes of the “true” fragment size‐velocity distributions.