Crater Age Research Articles

Geology of central Chryse Planitia and the Viking 1 landing site: Implications for the Mars Pathfinder mission

1:500,000‐scale geologic mapping in the central Chryse Planitia region of Mars was correlated with “ground‐truth” data gathered by the Viking 1 lander. Materials within the Chryse basin can be subdivided into plains and channel units that are typically separated from one another by gradational contacts. Hesperian Ridged plains materials, unit 1 (Hr1) are the oldest materials mapped. Typically, these materials contain numerous fresh impact craters and have sharply defined, mare‐like wrinkle ridges similar to those appearing on the lunar maria. These materials grade into Hesperian Ridged plains materials, unit 2 (Hr2), which are characterized by buried and eroded impact craters and subdued wrinkle ridges. From analyses of crater age dates and their associated geologic contacts, channel materials appear to have formed at the same time as Hr2 materials, and it is likely both units represent fluvial sediments. Measurements of buried craters contained in Hr2 materials suggest that in places this unit may be ∼50 m thick, but crater size‐frequency distribution curves suggest that the areal average may be closer to ∼170 m. Based on these observations, our interpretation is that Hr2 materials were deposited into a standing body of water during channel formation. This interpretation implies that many of the rocks visible in the Viking 1 lander images were emplaced by fluvial processes. Possibly, finer‐grained sediments remained in suspension and were subsequently transported out of Chryse Planitia and into the northern plains during draining of the ponded water. East‐west trending surface undulations, visible in lander views toward the south, may represent aeolian dunes, lava flow fronts, or sediment waves formed at the bottom of the standing body of water. Broad physiographic units seen at the surface are not clearly visible in Viking orbiter images; however, they can be projected at the resolution of the orbiter images. These units show that concentrations of drift materials are oriented in a northwesterly direction, contrary to the strongest prevailing wind direction which is toward the northeast. These materials were probably deposited on Ridged plains materials, unit 2, during a period of time when aeolian processes were more active in the region. Both Earth‐based radar and Viking thermal data suggest that the Mars Pathfinder landing site will be similar geologically to the Viking 1 site. If this is true, then the Mars Pathfinder mission provides the opportunity for building directly on results of the Viking program. Some of the outstanding questions that Mars Pathfinder may be able to address include determining the aeolian modification history of the Chryse Planitia region, the degree and possibly the relative rate of sediment induration, the fraction of rocks and boulders emplaced by impact processes, the possibility that some materials are the result of in situ weathering, and whether materials were emplaced by fluvial processes and the associated depositional environment.

A STATISTICAL TEST OF CORRELATIONS AND PERIODICITIES IN THE GEOLOGICAL RECORDS

Matsumoto & Kubotani argued that there is a positive and statistically significant correlation between cratering and mass extinction. This argument is critically examined by adopting a method of Ertel used by Matsumoto & Kubotani but by applying it more directly to the extinction and cratering records. It is shown that on the null-hypothesis of random distribution of crater ages, the observed correlation has a probability of occurrence of 13%. However, when large craters are excluded whose ages agree with the times of peaks of extinction rate of marine fauna, one obtains a negative correlation. This result strongly indicates that mass extinction are not due to accumulation of impacts but due to isolated gigantic impacts.

Arabia Terra, Mars: Tectonic and Palaeoclimatic Evolution of a Remarkable Sector of Martian Lithosphere

A regional geologic study of Arabia Terra, a densely cratered area of Mars northern hemisphere, has revealed the individuality of this province. This is best expressed by an equatorial belt with a crater age distinctly younger as compared to the northern part of Arabia Terra and to Noachis Terra to the south. We interpret this as an incipient back-arc system provoked by the subduction of Mars lowlands under Arabia Terra during Noachian times. The regional fracture patterns are also best explained in this manner, making it unnecessary to appeal to a rotational instability of the planet, which is not supported by the palaeoclimatic indicators in the area. This model could be the first regional-scale confirmation of Sleep's (1994) hypothesis of a limited plate consumption as an explanation of the martian dichotomy.

Terrestrial impact craters: Their spatial and temporal distribution and impacting bodies

The terrestrial impact record contains currently ~145 structures and includes the morphological crater types observed on the other terrestrial planets. It has, however, been severely modified by terrestrial geologic processes and is biased towards young (≤200 Ma) and large (≥2 0 km) impact structures on relatively well-studied cratonic areas. Nevertheless, the ground-truth data available from terrestrial impact structures have provided important constraints for the current understanding of cratering processes. If the known sample of impact structures is restricted to a subsample in which it is believed that all structures ≥20 km in diameter (D) have been discovered, the estimated terrestrial cratering rate is 5.5±2.7 x 10-15km-2a-1 for D ≥20 km. This rate estimate is equivalent to that based on astronomical observations of Earth-crossing bodies. These rates are a factor of two higher, however, than the estimated post-mare cratering rate on the moon but the large uncertainties preclude definitive conclusions as to the significance of this observation. Statements regarding a periodicity in the terrestrial cratering record based on time-series analyses of crater ages are considered unjustified, based on statistical arguments and the large uncertainties attached to many crater age estimates. Trace element and isotopic analyses of generally siderophile group elements in impact lithologies, particularly impact melt rocks, have provided the basis for the identification of impacting body compositions at a number of structures. These range from meteoritic class, e.g., C-l chondrite, to tentative identifications, e.g., stone?, depending on the quality and quantity of analytical data. The majority of the identifications indicate chondritic impacting bodies, particularly with respect to the larger impact structures. This may indicate an increasing role for cometary impacts at larger diameters; although, the data base is limited and some identifications are equivocal. To realize the full potential of the terrestrial impact record to constrain the character of the impact flux, it will be necessary to undertake additional and systematic isotopic and trace element analyses of impact lithologies at well-characterized terrestrial impact structures.

Dating volcanism and rifting on Venus using impact crater densities

Although the distribution of impact craters on Venus is indistinguishable from a completely spatially random population on the basis of crater data alone, the addition of geologic information indicates that the craters are not random with respect to geology. Areas of low crater density correlate with concentrations of tectonic and volcanic features, as well as high proportions of faulted and embayed craters. High‐resolution mapping of large volcanoes, flood‐type lava flow fields, rifts, and coronae shows that these features have low crater densities that are unlikely to have occurred merely by chance. Because these features also appear young based on stratigraphic data, difference in age is the most likely reason for differences in crater density between geologically defined terrains. Crater densities may be used to determine reliable relative ages for sufficiently large geologic terrains containing at least eight to nine craters, as long as the mapping criteria are independent of impact crater locations. This result provides a tool to begin identifying and dating major geologic provinces, and to begin developing a geologic history of Venus. Using a simple data model, and calculating crater ages relative to an assumed global mean age of 300 Ma, we show that the ages of large volcanoes (72 ± 45 Ma), flow fields (128 ± 91 Ma), rifts (130 ± 145 Ma), and coronae (120 ± 115 Ma) are all substantially younger than the mean plains age, and probably represent ongoing volcanic and tectonic activity, rather than the end of a brief global resurfacing event with a mean age of 300 Ma.

Impact crater degradation on venusian plains

In venusian plains, impact craters without extensive low backscatter ejecta deposits are more likely to have low backscatter floors, be embayed by volcanic deposits, and exhibit fractures as compared to craters with extensive low backscatter ejecta. We interpret these trends as evidence of ongoing degradation of low backscatter ejecta by aeolian activity, weathering, and volcanism. Using a crater age sequence based on extent of preservation of low backscatter ejecta, together with Monte Carlo simulations, we find that tectonic activity has extended over a longer time period than volcanism.

First Images of Asteroid 243 Ida

The first images of the asteroid 243 Ida from Galileo show an irregular object measuring 56-kilometers by 24 kilometers by 21 kilometers. Its surface is rich in geologic features, including systems of grooves, blocks, chutes, albedo features, crater chains, and a full range of crater morphologies. The largest blocks may be distributed nonuniformly across the surface; lineaments and dark-floored craters also have preferential locations. Ida is interpreted to have a substantial regolith. The high crater density and size-frequency distribution (-3 differential power-law index) indicate a surface in equilibrium with saturated cratering. A minimum model crater age for Ida-and therefore for the Koronis family to which Ida belongs-is estimated at 1 billion years, older than expected.

The age of the Roter Kamm impact crater, Namibia: Constraints from 40Ar‐39Ar, K‐Ar, Rb‐Sr, fission track, and 10Be‐26Al studies

Abstract The well‐preserved 2.5 km diameter Roter Kamm impact crater is located in the Namib desert in Namibia. The impact has occurred in Precambrian granitic and granodioritic orthogneisses of the 1200–900 Ma old Namaqualand Metamorphic Complex which were partly covered by Gariep metasediments; the granites are invaded by quartz veins and quartz‐feldspar‐pegmatites. Previous geological field evidence suggested a crater age of about 5–10 Ma. In order to constrain this age, we selected a set of basement rocks (granites, granodiorites) exposed at the crater rim and studied the Rb‐Sr, K‐Ar, 40Ar‐39Ar, and 10Be‐26Al isotopic systems as well as apatite fission track ages of these samples. The Rb‐Sr isotopic systematics confirm the derivation of these samples from the Namaqualand basement (age about 1.29 Ga), which underwent Damaran orogenesis at about 650 Ma. No basement rocks with Rb‐Sr ages younger than about 410 Ma were identified. The K‐Ar ages of pseudotachylite and melt breccia samples show that these samples are dominated by incompletely degassed fragments of basement rocks, with some retaining their original metamorphic ages of about 470 Ma. The apatite fission track ages range from 20–28 Ma, which may be interpreted as an extension of the 25 Ma Burdigalian peneplanation event, or as incomplete resetting of the apatite fission tracks during the impact event. The 10Be and 26Al exposure age of a quartz sample isolated from a quartz‐pegmatite was found to be 150 ka; it is likely that the exposure of the sample began after material covering it had been removed by erosion 150 ka ago. Two glassy fractions extracted from a rim granite were dated by 40Ar‐39Ar analysis. One sample gives practically a plateau age of 3.7 ± 0.3 Ma, while the other gives a minimum age of 3.6 Ma. The best available age estimate for the Roter Kamm crater is therefore 3.7 ± 0.3 Ma.

Major episodes of geologic change: correlations, time structure and possible causes

Published data sets of major geologic events of the past ∼ 250Myr (extinction events, sea-level lows, continental flood-basalt eruptions, mountain-building events, abrupt changes in sea-floor spreading, ocean-anoxic and blackshale events and the largest evaporite deposits) have been synthesized (with estimated errors). These events show evidence for a statistically significant periodic component with an underlying periodicity, formally equal to 26.6 Myr, and a recent maximum, close to the present time. The cycle may not be strictly periodic, but a periodicity of ∼ 30Myr is robust to probable errors in dating of the geologic events. The intervals of geologic change seem to involve jumps in sea-floor spreading associated with episodic continental rifting, volcanism, enhanced orogeny, global sea-level changes and fluctuations in climate. The period may represent a purely internal earth-pulsation, but evidence of planetesimal impacts at several extinction boundaries, and a possible underlying cycle of 28–36 Myr in crater ages, suggests that highly energetic impacts may be affecting global tectonics. A cyclic increase in the flux of planetesimals might result from the passage of the Solar System through the central plane of the Milky Way Galaxy—an event with a periodicity and mean phasing similar to that detected in the geologic changes.

On the significance of crater ages: New ages for Dellen (Sweden) and Araguainha (Brazil)

Samples of a few grams in weight from the impact melt at the Dellen crater (Sweden) and the Araguainha dome (Brazil) were analyzed by Rb-Sr and Sm-Nd techniques. For Dellen, pure labradorite, orthopyroxene, and glass separates from a clast-free impact melt rock with holohyaline ground mass yield an internal Rb-Sr isochron with 89.0 ± 2.7 Ma ( I sr = 0.83571 ±0.00007; 2σ), compared to Ar-Ar plateau ages of 102 ± 3.2 Ma and 109.6 ±2.0 Ma published for total melt rocks. Various whole-rock values not plotting on this Rb-Sr isochron indicate primary variations in 87Sr 86 Sr over the melt sheet. The age of the Araguainha dome (Brazil) is bracketed by the Rb-Sr model age of 243 ± 19 Ma ( I Sr = 0.7226 ± 0.0005), for altered cordierite in the impact melt, and by the internal Rb-Sr isochron age of 449 ± 9 Ma ( I Sr = 0.7140 ± 0.0007), for an uplifted granite (shock stage Ib). In melt rocks from both impact structures 143Nd 144Nd was not homogenized during total rock melting: Sm-Nd data either scatter around the primary isochron (Araguainha) or point to a partial resetting (Dellen). This data array may be caused by tiny refractory REE-rich inclusions in the main mineral phases which dominate the Sm-Nd system of the impact melt rocks. The significant difference between the new isochron age for Dellen and Ar-Ar “ages” show that the data base for analysis of periodicity in cratering record most probably lack precision and accuracy.

Periodicity and decay of craters over the past 600 myr

The data set of Grieve which provides diameters and ages of craters is analyzed to obtain periodicity of the formation rate and decay constant of craters. It is confirmed that large craters (D ⩾ 10 km) do not exhibit any periodicity while small ones appear to satisfy the Broadbent criterion for quantum (periodicity) hypothesis at P ≃ 29.5 myr. The result is consistent with a recent study of Bailey who showed that large craters are largely due to asteroids. Allowing for the decay of craters, an excess of 4 small craters within the nearest past is detected. In this sense, one may argue that the solar system is now in a moderate comet shower.

Isotope systematics and shock-wave metamorphism: III. K-Ar in experimentally and naturally shocked rocks; the Haughton impact structure, Canada

This is a report on the first comparative study of the effects of experimental and natural shock on the K-Ar system. First, mineral separates from a 451 ± 1 Ma old gneiss, experimentally shocked up to 60 GPa, were investigated by stepwise heating 40Ar- 39Ar technique. Compared with an unshocked sample, the shocked samples have lower 40Ar- 39Ar ages, and the diffusion properties have changed depending on peak shock pressure and mineralogic composition. Total resetting of the K-Ar clock did not occur. Secondly, to compare these results with the effects of natural shock, samples from the ~23 Ma old Haughton impact crater, Devon Island, Canada, with different shock stages were investigated. Up to shock stage III, no total resetting of the K-Ar clock was observed, but severe argon losses, depending on shock pressure, result in 40Ar- 39Ar plateau ages which are incorrect in terms of the true crater age, yet they provide an upper limit to the time of the cratering event.

Structural development of Coprates Chasma and Western Ophir Planum, Valles Marineris Rift, Mars

A portion of Valles Marineris was mapped in detail in order to clarify the dominant processes responsible for the formation of Coprates Chasma. New crater counts indicate that the caprock on western Ophir Planum plateau has a Late Hesperian crater age, whereas trough floor preserved in western Coprates Chasma has an Early to Late Hesperian crater age. Caprock on western Ophir Planum correlates in relative age with the Syria Planum Formation, and the caprock may overlie Lower Hesperian ridged plains material. The conspicuous absence of wrinkle ridges on Ophir Planum may reflect burial by this later material. Trough floor material in Coprates Chasma correlates in relative age with ridged plains material on the adjacent Lunae Planum and Coprates plateaus and represents a structurally coherent block displaced downward by normal faulting. The crater counts and detailed structural relationships demonstrate the commonly accepted view that Coprates Chasma occupies a graben. The floor of this graben is only thinly mantled by intratrough deposits and is relatively intact; the trough shows no evidence for a chaotically broken and jumbled floor, as required by genesis models of collapse and subsurface drainage. Formation of Coprates Chasma by keystone collapse of locally elevated topography is not supported by available topographic data, but a general association between the trough and volcanotectonic activity in the Tharsis region is considered to be a likely explanation for the trough forming stresses. Subtle asymmetries in topography and boundary fault development across Coprates Chasma may imply a gentle, down‐to‐the‐north half‐graben geometry for the trough. Normal faults that define the northern margin of western Coprates Chasma and those that deform the adjacent Ophir Planum plateau change trend systematically with position, implying a 30° rotation of the local horizontal principal stresses in this region during trough growth. Faulting on Ophir Planum appears related to the faulting that created the Coprates Chasma trough. The geology and structure of Coprates Chasma are comparable to those of other troughs such as Melas, Ius, and perhaps Candor chasmata, suggesting that these troughs may also have formed as grabens. The revised crater age for western Ophir Planum indicates that the initiation of trough faulting and subsidence in the Valles Marineris system is early Late Hesperian and may have been synchronous across the system relative to the prevailing cratering rate.

Terrestrial impact: The record in the rocks*

Abstract— Approximately 130 terrestrial craters are currently known. They range up to 140 km, and perhaps as much as 200 km, in diameter and from Recent to ∼2 billion years in age. The known sample, however, is highly biased to geologically young craters on the better known cratonic areas. The sample is also deficient in small (D < 20 km) craters compared to other planetary bodies. These biases are largely the result of active terrestrial geologic processes and their effects have to be considered when interpreting the record. The strength of the terrestrial cratering record lies in the availability of ground truth data, particularly on the structural and lithological nature of craters, which can be interpreted to understand and constrain large‐scale impact processes. Some contributions include the definition of the concept of transient cavity formation and structural uplift during cratering events. Depths of excavation are poorly constrained, as very few terrestrial craters have preserved ejecta.Unlike their planetary counterparts, terrestrial impact craters are mostly recognized not by morphology but by the occurrence of characteristic shock metamorphic effects. Their study has led to models of shock wave attenuation and an understanding of the character and formation of various impact‐lithologies, including impact melt rocks. They, in turn, aid in interpreting the nature of extraterrestrial samples, particularly samples from the lunar highlands. The recognition of diagnostic shock metamorphic effects and the signature of projectile contamination through geochemical anomalies in impact lithologies provide the basis for recognizing the impact signature in K/T boundary samples. The record also provides a basis for testing hypotheses of periodic cometary showers. Although inherently not suitable to define short wavelength periods in time due to relatively large uncertainties associated with crater ages, the current record shows no evidence of periodicity. Future directions in terrestrial impact studies will likely continue to focus on the K/T and related problems, including the recognition of other impact signatures in the stratigraphic record. Some emphasis will likely be given to the economic potential of craters and individual large structures, such as Sudbury, will provide an increasingly better understood context for interpreting planetary impact craters. To live up to the full potential of the record to constrain impact processes, however, more basic characterization studies are required, in addition to emphasis on topical areas of study.

In situ10Be-26Al exposure ages at Meteor Crater, Arizona

A new method of dating the surface exposure of rocks from in situ production of 10Be and 26Al has been applied to determine the age of Meteor Crater, Arizona. A lower bound on the crater age of 49,200 ± 1,700 years has been obtained by this method.

The significance of pollen‐rich inorganic lake sediments in the Cratére du Nouveau‐Quèbec area, Ungava, Canada

Unexpectedly high pollen concentrations characterize the basal, silty part of the postglacial sediments accumulated in two lakes from the Cratère du Nouveau‐Quebéc area, Ungava. These lacustrine silts and their pollen content result from early postglacial washing of a pollen bearing till. The till must therefore ahve incorporated pollen that relates to events prior to the last glacial event. The matrix of the till deposits surrounding the lakes shows outstandingly high pollen concentrations. It is hypothesized that because of the proximity of the ice divide during the last (and earlier) ice advance(s) in Ungava, the previously depoisited till and the pollen that haad accumulated in its matrix during the interglacial interval(s) were preserved in relict till plains or recycled into the till of teh last glaciation. The crater's age has been established at 1.4 Ma and holds a minimum thickness of 95 m of sediments. It is very likely filled with successive tills or related glacigenic deposits perhaps representing the whole length of time since the crater was formed. Alike the most recent till, these deposits should pollen. there is thus the prospect ofr a 1.4 Ma old pollen record for Ungava.

Tectonic and volcanic evolution of dark terrain and its implications for the internal structure and evolution of Ganymede

Furrows in ancient dark terrain on Ganymede form three systems that are each hemispheric or greater in scale. The oldest of the systems, designated system III, is dominated by approximately concentric troughs centered on about 60°N, 50°W. System I, in the anti‐Jovian hemisphere, contains concentric and subradial furrows arrayed around a large, degraded palimpsest centered at 15°S, 165°W. Furrows in each system formed on and locally are buried by dark volcanic materials that embay and infill preexisting topographic features; they crosscut extremely few well preserved older craters; and they occur on surfaces having significantly different relative crater ages. System II, also in the anti‐Jovian hemisphere, contains widely spaced, radially arrayed furrows commonly 500–2000 km in length, which are organized around a large area of dark smooth resurfacing material, intense dark terrain fracturing, and some of the globally oldest light material. The total thickness of dark terrain resurfacing, estimated using the stratigraphy of different dark material deposits and the rim heights of the largest crater size class whose density was depleted by each deposit, is probably in the range of 3–8 km. Multiple models of the origin of each furrow system were tested using observed geologic features and patterns. Systems I and III were found to be most consistent with reactivation of impact‐generated, multiringed structures by endogenic global extension, during a period of widespread dark material volcanism that obliterated a dense, ancient crater population. System II was found to be most consistent with fracturing of a single, circular, isostatic uplift covering an entire hemisphere. On the basis of geologic observations and interpretations and theoretical models of convection in spheres, it is hypothesized that the uplift developed by long‐term warming of the upwelling current of a single axisymmetric convection cell in an initially cooler, undifferentiated interior. Such wanning would also have created global expansion and supplied the tensional stress inferred to have formed systems I and III. This hypothesis is supported by the concentration around the center of system II of intense fracturing and relatively young dark volcanic deposits, suggestive of high lithospheric heat flow, lithospheric thinning, and stress concentration. An observed long‐term decrease in the width of extensional tectonic features interpreted to be of endogenic origin is also consistent with lithospheric thinning due to warming of a cool interior.

Crater densities and crater ages of different terrain types on Ganymede

Crater densities and size-frequency distributions on Ganymede were measured for separate areas of dark furrowed and dark smooth terrains, reticulate terrain, complex grooved terrain, and groups of groved polygons and groove lanes having a variety of relative ages. These measurements were used first, together with results of previous studies, to test alternate models of heliocentric and planetocentric crater-forming bombardments of Ganymede and Callisto. The spatial distribution of crater production functions on Ganymede is consistent with a single impactor population having dominated cratering of that satellite's observed surface. In addition, relative crater ages of different Ganymedean material deposits are better reconciled with the deposits' stratigraphic relations if a heliocentric impactor population is assumed. These results are consistent with results of previous studies, interpreted here to indicate that the spatial distributions of most classes of craters on Ganymede and younger craters on Callisto are more consistent with heliocentric than with planetocentric bombardment. The measurements were then used to infer the sequence and style of surface modification of Ganymede; bounds on absolute ages depend on the assumption of heliocentric bombardment. Dark terrain was emplaced during a prolonged period of volcanic resurfacing of older, heavily cratered terrain, probably at least 10 8 yrs in duration and at least 3.8–4.0 Gyr ago. Furrows generally formed on blanket-like deposits of dark material, typically hundreds of meters or more in thickness, soon after their emplacement; furrow formation continued globally throughout the period of dark-material deposition. Next, dark terrain was deformed locally by reticulate terrain formation. Light terrain emplacement subsequently began, at least regionally within “complex grooved terrain.” It continued for an extended period (perhaps several times 10 8 yrs) in grooved lanes, grooved polygons, andsmooth terrain, where a light-material layer averaging several hundred meters or more in thickness was emplaced.

Dome craters on Ganymede

Voyager observations of Ganymede show broad, high‐albedo, topographic domes situated within the central pits of some impact craters, referred to in this study as “dome craters”. Of 56 dome craters identified on Ganymede, all but two can be placed into one of two classes, based on the ratio of dome diameter to crater rim diameter. Two new hypotheses for the origin of the domes involving diapirism as an agent of dome formation are offered. Implicit in both hypotheses are possible regional heat flux variations. Under these scenarios, “relaxation” of crater relief may not be homogeneous in space and/or time, and crater morphology may not be a consistent indicator of crater age. Plutonic intrusions within the upper lithospheres of Ganymede (and Callisto) may have played a far more important role in heat transport on these satellites than previously noted.

Numerical simulations and the problem of periodicity in the cratering record

Ages of craters in the record of impacts on earth may be uniformly period, totally random, or a mixture of the two. These alternatives are studied through numerical simulation wherein time-series analysis is performed on real and simulated sequences to which random noise has been added to represent age-dating uncertainty. We conclude that the real record is most likely to have been generated by a mixture of random and periodic impacts, with the random events constituting the majority.