Martian cratering 11. Utilizing decameter scale crater populations to study Martian history

New information has been obtained in recent years regarding formation rates and the production size‐frequency distribution (PSFD) of decameter‐scale primary Martian craters formed during recent orbiter missions. Here we compare the PSFD of the currently forming small primaries (P) with new data on the PSFD of the total small crater population that includes primaries and field secondaries (P + fS), which represents an average over longer time periods. The two data sets, if used in a combined manner, have extraordinary potential for clarifying not only the evolutionary history and resurfacing episodes of small Martian geological formations (as small as one or few km²) but also possible episodes of recent climatic change. In response to recent discussions of statistical methodologies, we point out that crater counts do not produce idealized statistics, and that inherent uncertainties limit improvements that can be made by more sophisticated statistical analyses. We propose three mutually supportive procedures for interpreting crater counts of small craters in this context. Applications of these procedures support suggestions that topographic features in upper meters of mid‐latitude ice‐rich areas date only from the last few periods of extreme Martian obliquity, and associated predicted climate excursions.     

Martian subsurface properties and crater formation processes inferred from fresh impact crater geometries

Abstract— The geometry of simple impact craters reflects the properties of the target materials, and the diverse range of fluidized morphologies observed in Martian ejecta blankets are controlled by the near‐surface composition and the climate at the time of impact. Using the Mars Orbiter Laser Altimeter (MOLA) data set, quantitative information about the strength of the upper crust and the dynamics of Martian ejecta blankets may be derived from crater geometry measurements. Here, we present the results from geometrical measurements of fresh craters 3–50 km in rim diameter in selected highland (Lunae and Solis Plana) and lowland (Acidalia, Isidis, and Utopia Planitiae) terrains. We find large, resolved differences between the geometrical properties of the freshest highland and lowland craters. Simple lowland craters are 1.5–2.0 times deeper (≥5s?_o difference) with >50% larger cavities (≥2s?_o) compared to highland craters of the same diameter. Rim heights and the volume of material above the preimpact surface are slightly greater in the lowlands over most of the size range studied. The different shapes of simple highland and lowland craters indicate that the upper ?6.5 km of the lowland study regions are significantly stronger than the upper crust of the highland plateaus. Lowland craters collapse to final volumes of 45–70% of their transient cavity volumes, while highland craters preserve only 25–50%. The effective yield strength of the upper crust in the lowland regions falls in the range of competent rock, approximately 9–12 MPa, and the highland plateaus may be weaker by a factor of 2 or more, consistent with heavily fractured Noachian layered deposits. The measured volumes of continuous ejecta blankets and uplifted surface materials exceed the predictions from standard crater scaling relationships and Maxwell's Z model of crater excavation by a factor of 3. The excess volume of fluidized ejecta blankets on Mars cannot be explained by concentration of ejecta through nonballistic emplacement processes and/or bulking. The observations require a modification of the scaling laws and are well fit using a scaling factor of ?1.4 between the transient crater surface diameter to the final crater rim diameter and excavation flow originating from one projectile diameter depth with Z = 2.7. The refined excavation model provides the first observationally constrained set of initial parameters for study of the formation of fluidized ejecta blankets on Mars.     

Martian rampart crater ejecta: Experiments and analysis of melt-water interaction

Viking images of Martian craters with rampart-bordered ejecta deposits reveal distinct impact ejecta morphology when compared to that associated with similar-sized craters on the Moon and Mercury. Topographic control of distribution, lobate and terraced margins, cross-cutting relationships, and multiple stratigraphic units are evidence for ejecta emplacement by surface flowage. It is suggested that target water explosively vaporized during impact alters initial ballistic trajectories of ejecta and produces surging flow emplacement. The dispersal of particulates during a series of controlled steam explosions generated by interaction of a thermite melt with water has been experimentally modeled. Preliminary results indicate that the mass ratio of water to melt and confining pressure control the degree of melt fragmentation (ejecta particle size) and the energy and mode of melt-ejecta dispersal. Study of terrestrial, lobate, volcanic ejecta produced by steam-blast explosions reveals that particle size and vapor to clast volume ratio are primary parameters characterizing the emplacement mechanism and deposit morphology. Martian crater ramparts are formed when ejecta surges lose fluidizing vapors and transported particles are deposited en masse. This deposition results from flow yield strength increasing above shear stress due to interparticle friction.     

Martian planetwide crater distributions: Implications for geologic history and surface processes

Population-density maps of craters in three size ranges (0.6 to 1.2 km, 4 to 10 km, and >20 km in diameter) were compiled for most of Mars from Mariner 9 imagery. These data provide: historical records of the eolian processes (0.6 to 1.2 km craters); stratigraphic, relative, and absolute timescales (4 to 10 km craters); and a history of the early postaccretional evolution of the uplands (> 20 km craters).Based on the distribution of large craters (>20 km diameters), Mars is divisible into two general classes of terrain, densely cratered and very lightly cratered—a division remarkably like the uplands-maria dichotomy of the moon. It is probable that this bimodal character in the density distribution of large craters arose from an abrupt transition in the impact flux rate from an early intense period associated with the tailing off of accretion to an extended quiescent epoch, not from a void in geological activity during much of Mars' history. Radio-isotope studies of Apollo lunar samples show that this transition occurred on the moon in a short time.The intermediate-sized craters (4 to 10 km diameter) and the small-sized craters (0.6 to 1.2 km diameter) appear to be genetically related. The smaller ones are apparently secondary impact craters generated by the former. Most of the craters in the larger of these two size classes appear fresh and uneroded, although many are partly buried by dust mantles. Poleward of the 40° parallels the small fresh craters are notably absent owing to these mantles. The density of small craters is highest in an irregular band centered at 20°S. This band coincides closely with (1) the zone of permanent low-albedo markings; (2) the “wind equator” (the latitude of zero net north or south transport at the surface); and (3) a band that includes a majority of the small dendritic channels. Situated in the southermost part of the equatorial unmantled terrain which extends from about 40°N to 40°S, this band is apparently devoid of even a thin mantle. Because this belt is also coincident with the latitutde of maximum solar insolation (periapsis occurs near summer solstice), we suggest that this band arises from the asymmetrical global wind patterns at the surface and that the band probably follows the latitude of maximum heating which migrates north and south from 25°N to 25°S within the unmantled terrain on a 50,000 year timescale.The population of intermediate-sized craters (4–10 km diameter) appears unaffected by the eolian mantles, at least within the ±45° latitudes. Hence the local density of these craters is probably a valid indicator of the relative age of surfaces generated during the period since the uplands were intensely bombarded and eroded. It now appears that the impact fluxes at Mars and the moon have been roughly the same over the last 4 b.y. because the oldest postaccretional, mare-like surfaces on Mars and the moon display about the same crater density. If so, the nearness of Mars to the asteroid belt has not generated a flux 10 to 25 times greater than the lunar flux. Whereas the lunar maria show a variation of about a factor of three in crater density from the oldest to the youngest major units, analogous surfaces on Mars show a variation between 30 and 50. This implies that periods of active eolian erosion, tectonic evolution, volcanic eruption, and possibly fluvial modification have been scattered throughout Martian history since the formation and degradation of the martian uplands and not confined to small, ancient or recent, epochs. These processes are surely active on the planet today.     

Martian cratering 12. Utilizing primary crater clusters to study crater populations and meteoroid properties

Images from Mars Global Surveyor and later images from Mars Reconnaissance Orbiter reveal that roughly half of the meteoroids striking Mars (at meter to few decameter crater diameters) fragment in the Martian atmosphere, producing small clusters of primary impact craters. Statistics of these “primary clusters” yield valuable information about important Martian phenomena and properties of interplanetary bodies, including meteoroid behavior in the Martian atmosphere, bulk strengths of bodies striking Mars, and the fraction of Martian “field secondary” craters, a datum that would improve crater count chronometry. Many Martian impactors fragment at altitudes significantly higher than 18 km above the mean surface of Mars, and we find that most bodies striking Mars and Earth have low bulk strengths, consistent with crumbly or highly fractured objects. Applying statistics of primary clusters at various elevations and independent diameter bins, we describe a technique to estimate the percentage of semirandomly scattered “field secondary” craters. Our provisional estimate of this percentage, in the diameter range ~250 m down to ~22 m, is ~40% to ~80% of the total impacts, with the higher percentages at smaller diameters. Our data argue against earlier suggestions of overwhelming dominance by either primaries or secondaries in this diameter range.     

Crater geometry and ejecta thickness of the Martian impact crater Tooting

Abstract— We use Mars Orbiter Laser Altimeter (MOLA) topographic data and Thermal Emission Imaging System (THEMIS) visible (VIS) images to study the cavity and the ejecta blanket of a very fresh Martian impact crater ?29 km in diameter, with the provisional International Astronomical Union (IAU) name Tooting crater. This crater is very young, as demonstrated by the large depth/diameter ratio (0.065), impact melt preserved on the walls and floor, an extensive secondary crater field, and only 13 superposed impact craters (all 54 to 234 meters in diameter) on the ?8120 km² ejecta blanket. Because the pre‐impact terrain was essentially flat, we can measure the volume of the crater cavity and ejecta deposits. Tooting crater has a rim height that has >500 m variation around the rim crest and a very large central peak (1052 m high and >9 km wide). Crater cavity volume (i.e., volume below the pre‐impact terrain) is ?380 km³ the volume of materials above the pre‐impact terrain is ?425 km³. The ejecta thickness is often very thin (<20 m) throughout much of the ejecta blanket. There is a pronounced asymmetry in the ejecta blanket, suggestive of an oblique impact, which has resulted in up to ?100 m of additional ejecta thickness being deposited down‐range compared to the up‐range value at the same radial distance from the rim crest. Distal ramparts are 60 to 125 m high, comparable to the heights of ramparts measured at other multi‐layered ejecta craters. Tooting crater serves as a fresh end‐member for the large impact craters on Mars formed in volcanic materials, and as such may be useful for comparison to fresh craters in other target materials.     

Determining proportions of lunar crater populations by fitting crater size distribution

We determine the proportions of two mixed crater populations distinguishable by size distributions on the Moon. A "multiple power-law" model is built to formulate crater size distribution N(D) ∝ D-αwhose slope α varies with crater diameter D. This model is then used to fit size distributions of lunar highland craters and Class 1 craters. The former is characterized by α = 1.17 ± 0.04, 1.88 ± 0.07,3.17 ± 0.10 and 1.40 ± 0.15 for D ranges ~ 10- 49, 49- 120, 120- 251 and ~ 251- 2500 km, while the latter has a single slope α = 1.96 ± 0.14 for about 10- 100 km. They are considered as Population 1 and2 crater size distributions, whose sum is then fitted to the global size distribution of lunar craters with D between 10 and 100 km. Estimated crater densities of Population 1 and 2 are 44 × 10-5and 5 × 10-5km-2respectively, leading to the proportion of the latter being 10%. This result underlines the need for more thoroughly investigating Population 1 craters and their related impactors, the primordial main-belt asteroids, which dominated the late heavy bombardment.     

Martian canyons and African rifts: Structural comparisons and implications

The resistant parts of the canyon walls of the Martian rift complex Valles Marineris have been used to infer an earlier, less eroded reconstruction of the major troughs. The individual canyons were then compared with individual rifts of East Africa. When measured in units of planetary radius, Martian canyons show a distribution of lengths nearly identical to those in Africa, both for individual rifts and for compound rift systems. A common mechanism which scales with planetary radius is suggested. Martian canyons are significantly wider than African rifts. This is consistent with the long-standing idea that rift width is related to crustal thickness: most evidence favors a crust on Mars at least 50% thicker than that of Africa. The overall pattern of the rift systems of Africa and Mars are quite different in that the African systems are composed of numerous small faults with highly variable trend. On Mars the trends are less variable; individual scarps are straighter for longer than on Earth. This is probably due to the difference in tectonic histories of the two planets: the complex history of the Earth and the resulting complicated basement structures influence the development of new rifts. The basement and lithosphere of Mars are inferred to be simple, reflecting a relatively inactive tectonic history prior to the formation of the canyonlands.     

Nature of the Martian uplands: Effect on Martian meteorite age distribution and secondary cratering

Abstract— Martian meteorites (MMs) have been launched from an estimated 5–9 sites on Mars within the last 20 Myr. Some 80–89% of these launch sites sampled igneous rock formations from only the last 29% of Martian time. We hypothesize that this imbalance arises not merely from poor statistics, but because the launch processes are dominated by two main phenomena: first, much of the older Martian surface is inefficient in launching rocks during impacts, and second, the volumetrically enormous reservoir of original cumulate crust enhances launch probability for 4.5 Gyr old rocks. There are four lines of evidence for the first point, not all of equal strength. First, impact theory implies that MM launch is favored by surface exposures of near‐surface coherent rock (≤10² m deep), whereas Noachian surfaces generally should have ≥10² m of loose or weakly cemented regolith with high ice content, reducing efficiency of rock launch. Second, similarly, both Mars Exploration Rovers found sedimentary strata, 1–2 orders of magnitude weaker than Martian igneous rocks, favoring low launch efficiency among some fluvial‐derived Hesperian and Noachian rocks. Even if launched, such rocks may be unrecognized as meteorites on Earth. Third, statistics of MM formation age versus cosmic‐ray exposure (CRE) age weakly suggest that older surfaces may need larger, deeper craters to launch rocks. Fourth, in direct confirmation, one of us (N. G. B.) has found that older surfaces need larger craters to produce secondary impact crater fields (cf. Barlow and Block 2004). In a survey of 200 craters, the smallest Noachian, Hesperian, and Amazonian craters with prominent fields of secondaries have diameters of ?45 km, ?19 km, and ?10 km, respectively. Because 40% of Mars is Noachian, and 74% is either Noachian or Hesperian, the subsurface geologic characteristics of the older areas probably affect statistics of recognized MMs and production rates of secondary crater populations, and the MM and secondary crater statistics may give us clues to those properties.     

Popigai crater: Origin and distribution of diamond-bearing impactites

Abstract— About three decades ago, a formerly unknown genetic type of natural diamonds was discovered in impact craters. Impact diamonds are currently known from a number of impact structures in Europe, Asia and North America, and it's likely that the number of finds will increase with time. The Popigai crater, Northern Siberia, where impact diamonds were first found, was specifically investigated in terms of geology, geophysics, petrography and mineralogy. Large resources of industrial impact diamonds were discovered, and these minerals were studied in detail. Authigenic impact diamonds occur in situ in shocked graphite-bearing gneisses that are found as inclusions in impact melt rocks: tagamites and suevites. According to the observed transformation of coexisting minerals, the lower estimated pressure of the coherent martensite transition of graphite to diamonds is 35 GPa. Impact diamonds inherit the original shape of graphite crystals and are composed of a polycrystalline structured aggregate of cubic and probably hexagonal carbon microcrystals 1–5 μm across. Numerous properties of diamonds depend on the high density of defects in the crystal lattice. Allothigenic impact diamonds occur in rocks produced by the homogenization and solidification of impact melt, which originated from the complete fusion of graphite-bearing precursor gneisses. These diamonds usually reflect the influence of the hot melt and are strongly corroded. Diamond-bearing tagamites and suevites in the Popigai crater interior occur as extended lens and sheets bodies and also as irregular small bodies. Diamond distribution depends on the original abundance of precursor graphite in the target rocks, on the superimposed shock-metamorphic zonation, and on the character of the ejection of shocked and melted material along different trajectories and azimuths. This has resulted in radial and concentric inhomogeneities in diamond distribution in the crater interior. On a second order, the distribution depends on the scale of melt contamination by clasts and fragments and by the duration of cooling of certain melt bodies and their constituents. Enrichment in diamonds at the margins of thick tagamite sheets is the result of rapid cooling, which prevents combustion of diamonds. A positive correlation between diamond content and the amount of phosphorus pentoxide in impactites indicates links to C and P probably in organic matter of the primary sedimentary rocks, which were subjected to granulitic metamorphism 2.4 Ga ago and melted at the time of impact 35.7 Ma ago.     

Areal distribution of Martian rampart craters

A survey of medium- and high-resolution Viking orbital imagery was carried out in order to characterize the areal distribution of Martian rampart craters. Such craters have been identified on nearly every major geologic unit on the planet, at all latitudes and longitudes, and over a wide range of altitudes. Rampart crater formation spans Martian geologic history from at least the formation of the Chryse channels to the present.     

Geophysical implications of the Martian Gravity Field

The undulations of the Martian gravitational potential indicate lateral density variations in the Mars interior. A gravitating and solid Martian model deforms under the influence of these variations, producing stress differences of about 125 bars at a depth of about 200 km. Introduction of a partially molten core of 1300 km radius does not affect the stress distribution in the mantle significantly, whereas the assumption of a partially molten asthenosphere umderlying a solid lithosphere of about 300 km increases the stress differences appreciably. A strong linear correlation of the gravitational potential and the surface topography indicates that the extensive volcanism at the Tharsis region is a recent phenomenon. The high stresses associated with this region imply that there has been no extensive molten region within the upper 300 km since the volcanism.     

GT-57633 catalogue of Martian impact craters developed for evaluation of crater detection algorithms

Crater detection algorithms (CDAs) are an important subject of the recent scientific research. A ground truth (GT) catalogue, which contains the locations and sizes of known craters, is important for the evaluation of CDAs in a wide range of CDA applications. Unfortunately, previous catalogues of craters by other authors cannot be easily used as GT. In this paper, we propose a method for integration of several existing catalogues to obtain a new craters catalogue. The methods developed and used during this work on the GT catalogue are: (1) initial screening of used catalogues; (2) evaluation of self-consistency of used catalogues; (3) initial registration from three different catalogues; (4) cross-evaluation of used catalogues; (5) additional registrations and registrations from additional catalogues; and (6) fine-tuning and registration with additional data-sets. During this process, all craters from all major currently available manually assembled catalogues were processed, including catalogues by Barlow, Rodionova, Boyce, Kuzmin, and our previous work. Each crater from the GT catalogue contains references to crater(s) that are used for its registration. This provides direct access to all properties assigned to craters from the used catalogues, which can be of interest even to those scientists that are not directly interested in CDAs. Having all these craters in a single catalogue also provides a good starting point for searching for craters still not catalogued manually, which is also expected to be one of the challenges of CDAs. The resulting new GT catalogue contains 57,633 craters, significantly more than any previous catalogue. From this point of view, GT-57633 catalogue is currently the most complete catalogue of large Martian impact craters. Additionally, each crater from the resulting GT-57633 catalogue is aligned with MOLA topography and, during the final review phase, additionally registered/aligned with 1/256° THEMIS-DIR, 1/256° MDIM and 1/256° MOC data-sets. Accordingly, the resulting GT-57633 catalogue can successfully be used as a part of the framework for evaluation of CDAs.     

Survival of microorganisms in smectite clays: implications for Martian exobiology

Manned exploration of Mars may result in the contamination of that planet with terrestrial microbes, a situation requiring assessment of the survival potential of possible contaminating organisms. In this study, the survival of Bacillius subtilis, Azotobacter chroococcum, and the enteric bacteriophage MS2 was examined in clays representing terrestrial (Wyoming type montmorillonite) or Martian (Fe(3+)-montmorillonite) soils exposed to terrestrial and Martian environmental conditions of temperature and atmospheric pressure and composition, but not to UV flux or oxidizing conditions. Survival of bacteria was determined by standard plate counts and biochemical and physiological measurements over 112 days. Extractable lipid phosphate was used to measure microbial biomass, and the rate of 14C-acetate incorporation into microbial lipids was used to determine physiological activity. MS2 survival was assayed by plaque counts. Both bacterial types survived terrestrial or Martian conditions in Wyoming montmorillonite better than Martian conditions in Fe(3+)-montmorillonite. Decreased survival may have been caused by the lower pH of the Fe(3+)-montmorillonite compared to Wyoming montmorillonite. MS2 survived simulated Mars conditions better than the terrestrial environment, likely due to stabilization of the virus caused by the cold and dry conditions of the simulated Martian environment. The survival of MS2 in the simulated Martian environment is the first published indication that viruses may be able to survive in Martian type soils. This work may have implications for planetary protection for future Mars missions.     

Morphologic classification of Martian craters and some implications

A computer data bank containing information on crater sizes, locations, and morphologies for all craters visible on Mariner 9 wide-angle mapping photography was used to construct a crater morphologic classification. Four general classes were constructed that can be interpreted to represent increasing degrees of crater degradation. Fresh class craters are nearly unmodified and consist of deep bowl-shaped craters and deep, flat-floored craters with terraced walls. The slightly modified class consists of deep flat-floored craters that usually have raised rims, but lack the terracing, central peaks, and hummocky floors indicative of unmodified impact crater morphology. Craters in the modified class are rimless and shallow and those in the ghost class are rimless and extremely shallow. Retention ages for fresh (i.e. unmodified) class craters on equatorial cratered terrain range from millions to billions of years, depending on the impact flux history used. If the trend is toward billions of years, then present degradation rates on Mars are low relative to earlier history and most craters in the degraded classes were probably modified in an early (>3.3 b.y.?) period.     

Martian northern polar cap: Layering and possible implications for radar sounding

The propagation of electromagnetic waves in the northern polar ice sheet of Mars is considered. It is shown that the dispersion and attenuation of radio waves in the polar sheet are regulated by two groups of factors: the physical and chemical composition of the ice, and the geometrical parameters of the layered structure of polar sheet. Both analytical and numerical simulations of ultra wide band chirp radar pulse propagating through the cap are performed. Wide variety of combinations of the physical and geometrical parameters of the ice sheet, consistent with previously published observational data, has been considered. The frequency bands of transparency and opacity of the northern ice sheet for radar signals were found. The side clutter for this particular region of the planet is studied.     

Investigating target versus impactor influences on Martian crater morphology at the simple‐complex transition

Comparing craters of identical diameter on a planet is an empirical method of studying the effects of different target and impactor properties while holding total impact energy nearly constant. We have analyzed the Martian crater population within a narrow diameter range (7 km < crater diameter < 9 km) at the simple‐complex crater transition using three approaches. We looked for correlations of morphology with surface geology using a global crater database and global geologic map. We examined selected regions in detail with high‐resolution images to further understand the relationship between crater morphology and bulk target properties. Finally, we examined craters in close proximity to each other in order to hold target properties constant, so that we could isolate impactor effects on crater morphology. We found a strong correlation between target properties and interior crater morphology, and we found little evidence that impactor properties (other than impact angle) affect crater appearance. Central uplift and wall slumping are enhanced for less consolidated targets. Layered targets affected both the excavation and modification stages of complex crater formation; the resulting craters have pseudoterraces, flat floors, and central pits.     

Numerical modelling of the impact crater depth-diameter dependence in an acoustically fluidized target

2D numerical modelling of impact cratering has been utilized to quantify an important depth-diameter relationship for different crater morphologies, simple and complex. It is generally accepted that the final crater shape is the result of a gravity-driven collapse of the transient crater, which is formed immediately after the impact. Numerical models allow a quantification of the formation of simple craters, which are bowl-shaped depressions with a lens of rock debris inside, and complex craters, which are characterized by a structural uplift. The computation of the cratering process starts with the first contact of the impactor and the planetary surface and ends with the morphology of the final crater. Using different rheological models for the sub-crater rocks, we quantify the influence on crater mechanics. To explain the formation of complex craters in accordance to the threshold diameter between simple and complex craters, we utilize the Acoustic Fluidization model. We carried out a series of simulations over a broad parameter range with the goal to fit the observed depth/diameter relationships as well as the observed threshold diameters on the Moon, Earth and Venus.