Martian impact craters: Continuing analysis of lobate ejecta sinuosity

The lobate ejecta morphology surrounding most fresh Martian impact craters can be quantitatively analyzed to determine variations in ejecta sinuosity with diameter, latitude, longitude, and terrain. The results of such studies provide another clue to the question of how these morphologies formed: are they the results of vaporization of subsurface volatiles or caused by ejecta entrainment in atmospheric gases. Kargel provided a simple expression to determine the degree of non-circularity of an ejecta blanket. This measure of sinuosity, called 'lobateness', is given by the ratio of the ejecta perimeter to the perimeter of a circle with the same area as that of the ejecta. The Kargel study of 538 rampart craters in selected areas of Mars led to the suggestion that lobateness increased with increasing diameter, decreased at higher latitude, and showed no dependence on elevation or geologic unit. Major problems with the Kargel analysis are the limited size and distribution of the data set and the lack of discrimination among the different types of lobate ejecta morphologies. Bridges and Barlow undertook a new lobateness study of 1582 single lobe (SL) and 251 double lobe (DL) craters. The results are summarized. These results agree with the finding of Kargel that lobateness increases with increasing diameter, but found no indication of a latitude dependence for SL craters. The Bridges and Barlow study has now been extended to multiple lobe (ML) craters. Three hundred and eighty ML craters located across the entire Martian surface were studied. ML craters provide more complications to lobateness studies than do SL and DL craters - in particular, the ejecta lobes surrounding the crater are often incomplete. Since the lobateness formula compares the perimeter of the ejecta lobe to that of a circle, the analysis was restricted only to complete lobes. The lobes are defined sequentially starting with the outermost lobe and moving inward

Revision of the Martian relative age chronology

This study has provided a more detailed chronology than currently exists in the literature and has created some changes to the currently accepted geological evolutionary sequence of Mars. The period of heavy bombardment, although dominated by impact processes, experienced many forms of volcanic activity and at least one episode of intense fracturing. Most small volcanic constructs and the ridged plains regions are found to date from this early period, contrary to common belief. The fracturing and dissection of the highlands helps to provide further constraints on the timing of events such as the formation of the hemispheric dichotomy and the formation of the Tharsis Bulge. The northern plains are found to consist of a number of differently aged regions. The difference in age between the chaotic terrain and the outflow channels together with differences in the distribution curves among craters of various erosional states found on the channels support the theory of episodic periods of flooding

The ridged plains as a possible landing site for the Mars sample return mission

Differences in the shape and density of crater size-frequency distribution curves have been interpreted as indicators of different impactor populations. Within the inner solar system two production populations are seen. The signature of the first is recorded in the heavily cratered regions of the Moon, Mercury, and Mars and displays a multi-sloped distribution curve which cannot be described by a power law function at all crater diameters. The signature of the second population is seen in the lightly cratered lunar and Martian plains, where the size-frequency distribution curve can be approximated by a power law function of -3 differential slope in the 8- to 70-km diameter range. Based on data obtained from the Apollo lunar samples and crater flux estimates, the first population is believed to have been emplaced during the period of heavy bombardment which, at least on the Moon, ended about 3.8 BY ago. The second population has dominated the cratering record since that time and is commonly assumed to be due to comets and asteroids

Martian crater counts on Elysium Mons

Without returned samples from the Martian surface, relative age chronologies and stratigraphic relationships provide the best information for determining the ages of geomorphic features and surface regions. Crater-size frequency distributions of six recently mapped geological units of Elysium Mons were measured to establish their relative ages. Most of the craters on Elysium Mons and the adjacent plains units are between 500 and 1000 meters in diameter. However, only craters 1 km in diameter or larger were used because of inadequate spatial resolution of some of the Viking images and to reduce probability of counting secondary craters. The six geologic units include all of the Elysium Mons construct and a portion of the plains units west of the volcano. The surface area of the units studied is approximately 128,000 sq km. Four of the geologic units were used to create crater distribution curves. There are no craters larger than 1 km within the Elysium Mons caldera. Craters that lacked raised rims, were irregularly shaped, or were arranged in a linear pattern were assumed to be endogenic in origin and not counted. A crater frequency distribution analysis is presented

Constraints on early events in martian history as derived from the cratering record

The shapes and densities of crater size-frequency distribution curves are used to constrain two major events early in Martian history: termination of high obliteration rates and viability of the multiple impact origin of the crustal dichotomy. Distribution curves of fresh craters superposed on uplands, intercrater plains, and ridged plains display shapes and densities indicative of formation prior to the end of heavy bombardment. This observation correlates with other geologic evidence, suggesting a major change in the erosional regime following the last major basin size impact (i.e., Argrye). In addition, the multisloped nature of the curves supports the idea that the downturn in the crater size-frequency distribution curves reflects the size-frequency distribution of the impactors rather than being the result of erosion. The crustal dichotomy formed prior to the heavy bombardment intermediate epoch based on distribution curves of knobby terrain; if the dichotomy resulted from a single gigantic impact, this observation places constraints on when this event happened. An alternate theory for dichotomy formation, the multiple-impact basin idea, is questioned: since distribution curves of large basins as well as heavy bombardment era units are not represented by a −3 differential power law function, this study finds fewer basins missing on Mars compare to the Moon and Mercury than previously reported. The area covered by these missing basins is less than that covered the northern plains

Martian subsurface volatile concentrations as a function of time: Clues from layered ejecta craters

Martian layered ejecta morphologies are characterized using a new preservation classification system and through measurement of ejecta mobility (EM) ratios. EM, the ratio of ejecta extent to crater radius, is believed to provide information about ejecta material fluidity during emplacement. This study compares EM and preservation classification to d etermine if subs urface volatile concentrations have changed measurably over time. Results from both regional and local analyses suggest that concentrations of subsurface volatiles have remained approximately constant at the depths and over the time periods recorded by these craters

Martian impact crater ejecta morphologies as indicators of the distribution of subsurface volatiles

Fresh Martian impact craters display a variety of ejecta blanket morphologies. The fluidized appearance of most fresh ejecta types is commonly ascribed to heating and vaporization of subsurface volatiles during crater formation. We have conducted a study of the distribution of the three dominant fluidized ejecta morphologies (single layer ejecta (SLE), double layer ejecta (DLE), and multiple layer ejecta (MLE)) within the ±60° latitude zone on Mars. We have subdivided this region into 5° x 5° latitude-longitude boxes and have computed the following for each box: (1) percentage of craters showing any ejecta morphology as a function of total number of craters, (2) percentage of SLE craters as a function of craters with an ejecta morphology, (3) percentage of DLE craters as a function of craters with an ejecta morphology, and (4) percentage of MLE craters as a function of craters with an ejecta morphology. We confirm previous reports that the SLE morphology is the most common ejecta type within the study area, constituting >70% of all ejecta morphologies over most of the study area. The DLE and MLE morphologies are much less common, but these morphologies are concentrated in localized regions of the planet. Using these results, we discuss how subsurface volatile reservoirs may be distributed across the planet. The regional variations found in this study generally correlate with the proposed locations of near-surface H2O reservoirs detected by Mars Odyssey

Martian impact crater ejecta morphologies as indicators of the distribution of subsurface volatiles

[1] Fresh Martian impact craters display a variety of ejecta blanket morphologies. The fluidized appearance of most fresh ejecta types is commonly ascribed to heating and vaporization of subsurface volatiles during crater formation. We have conducted a study of the distribution of the three dominant fluidized ejecta morphologies ( single layer ejecta (SLE), double layer ejecta (DLE), and multiple layer ejecta (MLE)) within the +/- 60degrees latitude zone on Mars. We have subdivided this region into 5degrees x 5degrees latitude-longitude boxes and have computed the following for each box: (1) percentage of craters showing any ejecta morphology as a function of total number of craters, (2) percentage of SLE craters as a function of craters with an ejecta morphology, (3) percentage of DLE craters as a function of craters with an ejecta morphology, and (4) percentage of MLE craters as a function of craters with an ejecta morphology. We confirm previous reports that the SLE morphology is the most common ejecta type within the study area, constituting \u3e 70% of all ejecta morphologies over most of the study area. The DLE and MLE morphologies are much less common, but these morphologies are concentrated in localized regions of the planet. Using these results, we discuss how subsurface volatile reservoirs may be distributed across the planet. The regional variations found in this study generally correlate with the proposed locations of near-surface H2O reservoirs detected by Mars Odyssey

Sinuosity of Martian rampart ejecta deposits

The sinuosities of 2213 Martian rampart ejecta craters are quantified through measurement of the ejecta flow front perimeter and ejecta area. This quantity, called lobateness, was computed for each complete lobe of the 1582 single lobe (SL), 251 double lobe (DL), and 380 multiple lobe (ML) craters included in this study. A lobateness value of 1 indicates a circular ejecta blanket, whereas more sinuous ejecta perimeters have lobateness values >1. Although resolution does have an effect on the absolute values of lobateness, the general relationships between lobateness and morphology exist regardless of resolution. Evaluation of the lobateness values reveals that the outer lobes of DL and ML craters have higher median lobateness values (i.e., are more sinuous) than the inner lobes. The outermost lobe of ML craters displays higher lobateness values than the outer lobe of DL craters or the single lobe of SL craters. Previous reports of lobateness-diameter, lobateness-latitude, and lobateness-terrain relationships for rampart craters are not supported by this study. Many of the differences between the results of this study and the previous lobateness analyses can be attributed to the inclusion of resolution effects and the distinction between different ejecta morphologies in this study. The results of this study taken together with a previous analysis of the distribution and diameter dependence of different ejecta morphologies are most consistent with the theory that Martian lobate ejecta morphologies form from impact into subsurface volatiles