Crater Retention Ages Research Articles

Ages of very large impact basins on Mars: Implications for the late heavy bombardment in the inner solar system

A sharp peak in both relative crater retention ages and inferred model absolute ages for the largest impact basins on Mars may be a Martian equivalent of a late heavy bombardment (LHB) terminal lunar cataclysm (TLC). If so, it supports a NICE‐type LHB, provides an important temporal connection between lunar and Martian chronologies, and suggests the current Martian chronology overestimates the age of large basins and perhaps other events in early Martian history.

Rapid decrease in Martian crustal magnetization in the Noachian era: Implications for the dynamo and climate of early Mars

The magnetic signatures and crater retention ages of the 19 largest (>1000 km diameter) impact basins on Mars are examined to constrain the history of the acquisition of crustal magnetization during the Noachian era. The 5 most clearly impact‐demagnetized basins are younger than the 14 basins within which lies substantially re‐magnetized crust. Poisson analysis shows that the most likely time of this magnetization cessation was 4.115–4.13 Ga (model age) and that it occurred quickly, taking less than 20 Ma. A global decrease in effective crustal magnetic susceptibility due, e.g., to a decrease in the rate of hydrothermal alteration, is one possible explanation. Alternatively, the cessation of post‐impact magnetization reflects the rapid death of the Martian dynamo.

Buried impact basin distribution on Mars: Contributions from crustal thickness data

Crustal thickness data (derived from Mars Global Surveyor (MGS) gravity field and topographic data) exposes a number of circular thin areas (CTAs) that may represent deeply buried impact basins, which are often not visible in topography alone. A data set which combines quasi‐circular depressions (QCDs) revealed by the Mars Orbiter Laser Altimeter (MOLA) on the MGS spacecraft with a population of non‐QCD CTAs is a better estimate of the true crater retention ages of the buried surfaces of Mars. This study finds that all regions have older crater retention ages than previously thought based on QCDs alone. The highlands and lowlands appear to have the same basement crater retention age, but Tharsis is younger.

An improved crustal magnetic field map of Mars from electron reflectometry: Highland volcano magmatic history and the end of the martian dynamo

We apply improved kinetic modeling of electron transport in the martian thermosphere to fit pitch angle distributions measured by the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER), together with appropriate filtering, binning, averaging and error correction techniques, to create the most reliable ER global map to date of crustal magnetic field magnitude at 185 km altitude, with twice the spatial resolution and considerably higher sensitivity to crustal fields than global maps of magnetic field components produced with MAG data alone. This map compares favorably to sparsely sampled dayside MAG data taken at similar altitudes, insofar as a direct comparison is meaningful. Using this map, we present two case studies. The first compares the magnetic signatures of two highland volcanoes, concluding that the comparatively greater thermal demagnetization at Syrtis Major compared with Tyrrhena Patera is likely due to a higher ratio of intruded to extruded magmas. The second uses the map along with topographic data to compare the magnetic signatures and crater retention ages of the demagnetized Hellas impact basin and magnetized Ladon impact basin. From this comparison, we determine that the martian global dynamo magnetic field went from substantial to very weak or nonexistent in the absolute model age time interval 4.15 ± 0.05 to 4.07 ± 0.05 Ga ago.

Titan's young surface: Initial impact crater survey by Cassini RADAR and model comparison

Cassini's Radar instrument has mapped over 10% of Titan's surface through the T18 flyby in August 2006 and has detected remarkably few impact craters. Three impact craters have been conclusively identified and named, and a handful of unconfirmed candidate impact structures are documented. These results indicate that the overall crater density is a factor of ∼5 higher than Earth, and has a size‐distribution slope very similar to Earth's indicating rapid obliteration of smaller structures by erosion and burial. The paucity of small craters was expected from atmospheric shielding considerations, but the rarity of medium (20–100 km diameter) craters – some 30–300 times less abundant than on nearby Saturnian satellites – implies rapid resurfacing. Erosion and burial appear to be the dominant means of crater obliteration, with viscous relaxation playing a lesser role. Models of the 20–100 km impact crater production rate suggest an overall crater retention age of 100 Myr–1 Gyr, the same as the corresponding age for the Earth.

Venus: Divergent outcomes of terrestrial planet formation

Although Venus is often referred to as the most Earth-like of the terrestrial planets, its runaway greenhouse has resulted in a dry, hot, uninhabitable surface. Its surface geology is complex, with volcanoes ranging from 500 km across, lava flows fields >800km across, mountain belts, rift zones, and terrains unique to Venus such as tesserae and coronae. It surface has an average crater retention age similar to Earth's continents, but the random nature of the impact crater population renders it useless in providing time constraints for the geologic history of Venus. At some point in the past, Venus lost an ocean's worth of water. If this water persisted on the surface for long periods of time in Venus's early history, life may have evolved. Untangling, the complex history of Venus, and what it reveals for the evolution of habitable planets, will require future missions to the surface.

Impact constraints on, and a chronology for, major events in early Mars history

Large‐diameter visible and buried impact basins, seen as “quasi‐circular depressions” (QCDs) in MOLA gridded data, provide a self‐consistent chronology for major events on early Mars in terms of N(200) crater retention ages. On the basis of a conversion to model absolute ages, this chronology extends back hundreds of millions of years into a previously unknown “pre‐Noachian” epoch during which a now buried highlands surface was established, in which several very large impact basins formed while the global magnetic field was still present. A cluster of very large “lowland‐making” basins occurred at a model age of about 4.13 GY (or earlier), forming the fundamental topography of the Mars crustal dichotomy at a time prior to the age of the oldest visible highland crust. This early event in Martian history marks the transition from the “pre‐Noachian” to the Early Noachian when the well‐preserved Hellas, Argyre, and Isidis basins formed, all after the global magnetic field died.

Recent high‐latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement

Martian high‐latitude zones are covered with a smooth, layered ice‐rich mantle containing characteristic polygonal patterns. The mantle is especially uniform and homogeneous in the northern plains. We performed a survey of decameter‐scale mantle textures and circular features, including impact craters, in the northern lowlands down to 50°N latitude. Strongly altered and mantled craters of impact and non‐impact origin form a population of subtle circular features below the mantle. Pits of non‐impact origin are numerous in some regions at lower latitudes. Impact craters superposed on the mantle are small and very sparse. The inferred mean crater retention age of the mantle is ∼0.1 Ma. The spatial distribution of young craters suggests an age difference between the highest latitudes (younger) and some lower‐latitude regions (older). Latitudinal trends in polygon textures agree with impact crater evidence for a latitudinal age progression of mantle properties.

Martian cratering 8: Isochron refinement and the chronology of Mars

This paper reviews and refines the technique of dating martian surfaces by using impact-crater isochrons (defined as size distributions of impact craters on undisturbed martian surfaces of specified ages). In the 1970s, this system identified not only abundant ancient martian volcanic surfaces, but also extensive lava plains with ages of a few 10 8 y-old; this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites. The present update utilizes updated estimates of the Mars/Moon cratering ratio (the most important calibration factor), improves treatment of gravity and impact velocity scaling effects, combines aspects of the crater size distribution data from earlier work by both Neukum and Hartmann, and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from Popova et al. (2003, Meteorit. Planet. Sci. 38, 905–925). The updated isochrons are not radically different from the previous “2002 iteration” but fit observed data better and give somewhat older model ages for features dated from small craters (diameter D < 100 m ). Crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in D, confirming the general isochron shape and giving crater retention ages in the range of some 10 6 to some 10 8 y, interpreted as lava flow ages. More complex, older units are also discussed. Uncertainties are greatest if only small craters ( D ≲ 100 m ) are used. Suggestions by other workers of gross uncertainties, due to local secondary craters and deposition/exhumation, are discussed; they do not refute our conclusions of significant volcanic, fluvial, and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation. Indeed, crater count data suggest certain very recent episodes of deposition, exhumation, and ice flow, possibly associated with obliquity cycles of ∼ 10 7 y timescale. Evidence from ancient surfaces suggests higher rates of volcanism, fluvial activity, glaciation, and other processes in Noachian/Hesperian time than in Amazonian time.

North polar cap of Mars: Polar layered deposit characterization and identification of a fundamental climate signal

The record of recent climate change on Mars is encoded in the polar layered deposits within the north polar cap. Individual Mars Orbiter Camera (MOC) images of exposed layer sequences in cliffs and troughs provide the equivalent of high resolution “cores” through many sections in the upper part of the north polar layered terrain. In order to decode this record it is necessary 1) to quantitatively characterize the layers in individual “cores” and 2) to assess possible correlations between “cores” in vertical layered deposit sequences across the cap. We use two techniques commonly employed in paleoceanography for the study of deep‐sea sediment cores on Earth to establish the characteristics of layers in individual cores (Fourier analysis) and to determine the correlation between cores (curve‐shape matching algorithms). Application to “cores” (vertical sections) of the north polar layered terrain on Mars reveals several fundamental properties of north polar cap stratigraphy: 1) Fourier analysis of the layer vertical sequences reveals a characteristic and repetitive wavelength of ∼30 m thickness throughout the upper part (Zone 1) of all sequences analyzed. 2) Application of curve‐shape matching algorithms demonstrates that layers correlate across at least three quarters of the cap (∼6 × 105 km2) in the 13 images analyzed to date. 3) Assessment of geometric relationships shows that layers are not horizontal, but rather have an apparent dip of approximately 0.5 degrees. We interpret these results as follows: 1) The fundamental ∼30 m signal is interpreted as a climate signal that may correspond to a 51 kyr insolation cycle. 2) The lateral correlation and broad distribution of these layer sequences strongly imply that layer accumulation processes are widespread across the cap, rather than confined within a single trough or region. 3) Local to regional variability in individual layer thicknesses (and thus accumulation and sublimation rates) is typically less than a factor of 2.5, providing the ability to study regional trends, but often making it difficult to correlate visually the vertical sequences in individual cores. Finally, initial examination of layers located deeper in the stratigraphic sequence within the north polar cap than the ∼300 m thick Zone 1 provides evidence for a unit less than 100 m thick (Zone 2) in which the fundamental ∼30 m sequence is not detected. We interpret this as a deposit having formed during a recent high‐obliquity phase of Mars, during which time polar volatiles underwent mobilization and were transport equatorward, leaving a polar lag of dust‐rich material. The most recent “ice age” (∼0.5–2 Ma) offers a plausible candidate for this period of ice cap removal and lag deposit formation. An underlying Zone 3 (∼200 m) contains a dominant 35 m signal, and a lowermost Zone 4 (∼200 m) contains multiple signals but no dominant one. Together these four zones represent ∼800 m of vertical stratigraphic section, about one‐fourth of the total thickness of the cap. These findings support earlier interpretations that orbital parameter variations could cause significant erosion and possibly complete removal of the polar caps. The interpreted crater retention ages of the layered terrain are consistent with the correlations and vertical sequences described here, suggesting that the polar caps wax and wane throughout geological history, depending on the evolution of orbital parameters. Definition of the ∼30 m unit signal holds promise for determining 1) the detailed origin of individual layer types, 2) the nature of deposition and sublimation processes and their relation to insolation geometry across the polar cap, and 3) correlation with and comparison to the south polar layered terrain record.

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.

Comparison of the North and South Polar Caps of Mars: New Observations from MOLA Data and Discussion of Some Outstanding Questions

New high-resolution data from the Mars Orbiter Laser Altimeter (MOLA) have provided detailed topographic maps for the north and south polar regions. These new data allow one to compare the overall topography and geologic histories of the two polar regions and to highlight some specific outstanding questions in Mars polar studies, following earlier comparisons using Viking and Mars Global Surveyor data. The new data show that the centers of symmetry of the polar cap deposits in map view (which include both the layered terrain and residual ice) are offset from the current rotational pole in antipodal directions. Offset in the north appears to be due to retreat of the polar materials from predominantly one direction (180° W). Lines of evidence for movement and melting in different forms (e.g., lobes in young craters, kettle-like depressions, candidate residual mantles overlying polar layered terrain, and possible eskers) have been seen at both poles. The exact timing and causes of movement and melting is yet unknown. Differences in underlying topography (large, low, flat depression in the north; broad, cratered high and edge of a large impact basin in the south) may influence the accumulation, flow, and movement of polar material and the storage and movement of meltwater. The small number of superposed craters has been interpreted to indicate a Late Amazonian age for both caps, with the southern cap being somewhat older (7–15×10 6 years) than the northern cap (<100×10 3 years). The Late Amazonian-aged caps are surrounded and underlain by Hesperian-aged material, indicating an apparent hiatus almost 3-byr in duration. This apparent hiatus in the geologic record from the Late Hesperian to Late Amazonian at both poles may be accounted for in one of three ways. (1) Polar caps are recent events in the history of Mars: This scenario requires that conditions in the Late Amazonian changed to produce environments favorable for cap formation late in the history of Mars, or that polar wander brought the caps to their present position very recently. (2) The polar caps are oscillating: In this scenario, caps are periodically deposited and then retreat and disappear, possibly due to extremes in the obliquity cycle. In this model, the present caps are the latest example of cap deposition and represent the type of deposit that has come and gone numerous times in the Amazonian. (3) The present caps are old, but have been renewed: In this scenario, the present caps have been in their current position for much of the Amazonian, but some process (e.g., melting, flow) is periodically destroying preexisting craters to produce cap surfaces that appear very young in terms of crater retention ages. Documenting the nature of the emplacement history of the present polar deposits and distinguishing among these disparate scenarios for their history are among major challenges facing Mars polar studies in the coming years.

Stratigraphy and Isotope Ages of Lunar Geologic Units: Chronological Standard for the Inner Solar System

The absolute ages of cratered surfaces in the inner solar system, including Mars, are derived by extrapolation from the impact flux curve for the Moon which has been calibrated on the basis of absolute ages of lunar samples. We reevaluate the lunar flux curve using isotope ages of lunar samples and the latest views on the lunar stratigraphy and the principles of relative and absolute age dating of geologic surface units of the Moon. The geological setting of the Apollo and Luna landing areas are described as far as they are relevant for this reevaluation. We derive the following best estimates for the ages of the multi-ring basins and their related ejecta blankets and present alternative ages for the basin events (in parentheses): 3.92 ± 0.03 Gyr (or 3.85 ± 0.05 Gyr) for Nectaris, 3.89 ± 0.02 Gyr (or 3.84 ± 0.04 Gyr) for Crisium, 3.89 ± 0.01 Gyr (or 3.87 ± 0.03 Gyr) for Serenitatis, and 3.85 ± 0.02 Gyr (or 3.77 ± 0.02 Gyr) for Imbrium. Our best estimates for the ages of the mare landing areas are: 3.80 ± 0.02 Gyr for Apollo 11 (old surface), 3.75 ± 0.01 Gyr for Apollo 17, 3.58 ± 0.01 Gyr for Apollo 11 (young surface), 3.41 ± 0.04 Gyr for Luna 16, 3.30 ± 0.02 Gyr for Apollo 15, 3.22 ± 0.02 Gyr for Luna 24, and 3.15 ± 0.04 Gyr for Apollo 12. The ages of Eratosthenian and Copernican craters remain: ~ 2.1 (?) Gyr (Autolycus), 800 ± 15 Myr (Copernicus), 109 ± 4 Myr (Tycho), 50.3 ± 0.8 (North Ray crater, Apollo 16), and 25.1 ± 1.2 (Cone crater, Apollo 14). When plotted against the crater densities of the relevant lunar surface units, these data result in a revised lunar impact flux curve which differs from the previously used flux curve in the following respects: (1) The ages of the stratigraphically most critical impact basins are notably younger, (2) the uncertainty of the calibration curve is decreased, especially in the age range from about 4.0 to 3.0 Gyr, (3) any curve for ages older than 3.95 Gyr (upper age limit of the Nectaris ejecta blanket) is abandoned because crater frequencies measured on such surface formations cannot be correlated with absolute ages obtained on lunar samples. Therefore, the impact flux curve for this pre-Nectarian time remains unknown. The new calibration curve for lunar crater retention ages less than about 3.9 Gyr provides an updated standard reference for the inner solar system bodies including Mars.

Martian cratering VI: Crater count isochrons and evidence for recent volcanism from Mars Global Surveyor

Abstract— This paper develops a methodology to establish absolute Martian ages by deriving isochrons on a plot of Martian impact crater density vs. crater diameter, calibrated by lunar crater/age data. The isochrons illustrated here are based on a Mars/Moon cratering ratio of 1.6 at constant size, but there is a factor of 2 to 4 uncertainty in this ratio and the consequent model ages. Martian crater diameter distributions are determined in several areas down to diameter D = 16–45 m; the shapes of the curves in young areas are found to be close to that of the predicted isochrons and close to the standard production function found by Neukum. The youngest areas studied here display the lunar‐like production function down to D ∼30 m, where saturation equilibrium sets in. Model crater retention ages of several volcanic units are found to be a few hundred million years or less, with estimated uncertainties ranging from a factor of 2 lower to a factor of 4 higher. The results are consistent with Martian meteorite ages. Volcanism on Mars has probably persisted into the last 10 to 15% of the planet's history and is likely ongoing. Because surfaces as young as a few hundred million years have reached crater saturation equilibrium at D &lt; ∼60 to 100 m, Mars is likely to have widespread impact‐produced regoliths at least a few meters deep, and this may contribute to the widespread mobile dust and boulder fields of Mars.

The geologic history of Venus: A stratigraphic view

On the basis of regional and global stratigraphic analyses, we outline the major events in the geologic history of Venus determined by photogeological study of surface features. Because the morphological signatures of terrain emplaced prior to the time of tessera formation are not preserved, the stratigraphic record presented comprises only the last 10–20% of the total history of Venus. The estimated range of the mean crater retention age of the surface (from ∼200 to 1600 million years) leads us to describe the timing and duration of different events in terms of fractions of the mean surface age T. The beginning of the observed history of Venus was characterized by intensive tectonic deformation of global or semi‐global scale which formed the tessera terrain. Termination of the compressional stage is estimated to have occurred at about 1.4T while the tensional stage lasted for another 0.1–0.2T. After tessera formation, several stages of extensive volcanism occurred, burying vast areas of tessera and forming what are now observed as regional plains. The combined duration of the emplacement of these plains is estimated to be about 0.2–0.3T, with an implied average global rate of volcanism of a few cubic kilometers per year. Regional plains‐forming materials can be subdivided and are separated from each other, and from underlying and overlying units, by unconformities. These unconformities are formed, from oldest to youngest, by tessera‐forming deformation, dense fracturing, broad ridging, and, finally, wrinkle ridging. These tectonic episodes are interpreted to be generally globally synchronous and to represent successive episodes characterized by the dominance of compression, then tension, then again compression, and, finally, tension. The last global‐scale tectonic episode, extensive wrinkle ridging, happened at about time T, which was very close in time to the emplacement of the most areally abundant plains unit. This marked the transition to the present stage of the history of Venus, which is characterized by a predominance of regional rifting and related volcanism. This stage appears to have lasted from about time T to the present, making it the longest time duration among the stratigraphic units considered, although the resulting tectonic and volcanic features and deposits cover only 10–20% of the surface of Venus. These observations mean that the general intensity of tectonics and the flux of volcanism (a few tenths of a cubic kilometer per year) in this latest period were much lower than those in earlier times. In summary, the morphologically observable part of the history of Venus was characterized by two key characteristics that stand in contrast to the comparable period of Earth history (approximately the Phanerozoic) when global geodynamic processes were dominated by plate tectonics: (1) Venus shows no signature of plate tectonics; instead, its global tectonic environment passed from an initial dominance of compression, through tension, then again compression, and finally tension, with the density of deformational structures and the strain rate declining with time. (2) In the beginning of this period of time on Venus, plains‐forming volcanism occurred at a rate comparable to volcanism at mid‐ocean ridges but was emplaced in an entirely different style. The predominant component of volcanism on Earth during this time was the extrusive volcanism at mid‐oceanic ridges. For the last few hundred million years, Venus has been dominated primarily by rift‐associated volcanism emplaced at a production rate comparable to or even lower than present intraplate volcanism production rates on Earth.

Duration of tessera deformation on Venus

The density and distribution of impact craters superposed on the highly deformed tessera terrain on Venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains. Eighty percent of tesserae craters are undeformed. No existing craters exhibit evidence of contractional deformation, suggesting that the early compressional stage of tessera deformation ended abruptly. The small number of craters fractured by late‐stage tessera extension constrains the duration of this phase to less than 20% of the average crater retention age of the tesserae, or approximately 30–60 Ma. These results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains.

Regional and global stratigraphy of Venus: a preliminary assessment and implications for the geological history of Venus

Photogeologic analysis of Magellan images for 36 widespread sites and several larger areas permits the definition and characterization of a sequence of mappable stratigraphic units and tectonic structures deforming them. Seven rock-stratigraphic units, three related time-stratigraphic units (Systems), and three geologic time subdivisions (Periods) are proposed to describe the vast majority of the areas under study. The first widespread geologic unit preserved on Venus is the highly deformed tessera terrain formed during the Fortunian Period, apparently the result of an event that destroyed the morphology of preexisting terrain and any superposed craters. Global image data reveal no evidence for extensive terrain dating from the pre-Fortunian time, comprising the first 80–90% of the history of Venus, although rocks dating from this period are almost certainly contained within the tessera. Immediately following the Fortunian an extensive period of plains volcanism began, the Guineverian Period, during which the majority of Venus was volcanically resurfaced. During the Guineverian early widespread plains of the Sigrun Group were deformed by extensive and closely-spaced graben systems. Continued widespread plains emplacement occurred and units of the Lavinia Group were deformed, some into extensive ridge belts, recording a change from distributed extensional deformation to often-focused compressional deformation. Plains of the Rusalka Group are the most widespread currently exposed, and are characterized by extensive development of wrinkle ridges of compressional origin. These Guineverian Period plains must have been emplaced and deformed over a relatively short period of time (probably less than about a hundred million years) because the vast majority of impact craters are superposed on the plains, and the crater retention age of this surface is of the order of 300–500 Ma. This extensive plains volcanism then gave way to materials of the Atla Group, local volcanic edifices and flow units with sources associated with coronae and rifts that were emplaced in the late Guineverian and Aurelian Periods. The Aurelian Period, defined by impact craters with dark parabolas, is interpreted to extend from the present back to about 30–50 Ma ago. During this period extensive rifting occurred in several areas of Venus and volcanism has continued at a reduced level relative to the earlier parts of the Guineverian period. Our observations favor a model in which the observed part of the geologic history of Venus (which is the last 20% or less) started with catastrophic tectonic deformation and volcanic resurfacing, followed by the period of declining surface activity of endogenic origin which lasts until now. The lack of surface units representing the first 80–90% of the history of Venus is remarkable from the standpoint of its present low level of activity. This contrast, together with the emerging geologic history of the last 10% of the lifetime of Venus, suggest that catastrophic and/or episodic global processes may have characterized Venus in its earlier history. This factor may also provide an insight into earliest Earth history.

Evidence for Active Hotspots on Venus from Analysis of Magellan Gravity Data

The 500-Myr average crater retention age for Venus has raised questions about the present-day level of tectonic activity. In this study we examine the relationship between the gravity and topography of four large volcanic swells, Beta, Atla, Bell, and Western Eistla Regiones, for clues about their stage of evolution. The Magellan line-of-sight gravity data are inverted using a point mass model of the anomalous mass to solve for the local vertical gravity field. Spectral admittance calculated from both the local gravity inversions and a spherical harmonic model is compared to three models of compensation: local compensation, a "flexural" model with local and regional compensation of surface and subsurface loads, and a "hotspot" model of compensation that includes top loading by volcanoes and subsurface loading due to a deep, low density mass anomaly. The coherence is also calculated in each region, but yields an elastic thickness estimate only at Bell Regio. In all models, the long wavelengths are compensated locally. The long-wavelength estimates of the deep compensation depth for Bell, Atla, Western Eistla, and Beta Regiones are 125, 175, 200, and 225 km, respectively, with an error of approximately ± 35 km. The flexural model is rejected primarily because it gives values of effective elastic thickness of approximately 100 km, which is at least a factor of two larger than estimates of effective elastic thickness obtained from the coherence spectra at Bell Regio and from other studies that model the topographic expression of flexure at Bell Regio and in other tectonic settings globally. Assuming a crustal thickness of 30 km, the fit to the hotspot model at Atla Regio gives an effective elastic thickness of 30 ± 5 km. Atla Regio is interpreted as an active hotspot because of the deep compensation depth and a strong subsurface loading signature. At Bell Regio, effective elastic thickness is 30 ± 5 km at short wavelengths and 50 ± 5 km at long wavelengths, using a crustal thickness of 10 km. The 30-km value is interpreted as the effective elastic thickness at the time when the volcanoes are emplaced; the 50-km value is believed to reflect the present-day effective elastic thickness. Bell Regio has a relatively shallow compensation depth, 125 km, and a weak bottom-to-top-loading ratio determined from the coherence spectra. These results may indicate a relatively old, possibly inactive plume. The data at Beta and Western Eistla Regiones are of relatively poor quality and do not permit detailed interpretations. These areas are interpreted as active hotspots because of their large compensation depths, greater than typical of the rest of the planet.

The global resurfacing of Venus

The impact cratering record on Venus is unique among the terrestrial planets. Fully 84% of the craters are in pristine condition, and only 12% are fractured. Remarkably, only 2.5% of the craters and crater‐related features are embayed by lava, although intense volcanism and tectonism have affected the entire planet. Furthermore, the spatial and hypsometric distribution of the craters is consistent with a completely random one, including stochastic variations. Monte Carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed, or unobserved nonrandom crater distributions for resurfacing areas between 0.03% and 100% of the planet's surface. These models also are not consistent with the number and nonrandom distribution of volcanoes, and the nonrandom distribution of embayed and heavily fractured craters. The constraints imposed by the cratering record strongly indicate that Venus experienced a global resurfacing event about 300 m.y. ago followed by a dramatic reduction of volcanism and tectonism. This global resurfacing event ended abruptly (&lt;10 m.y.). The present crater population has accumulated since then and remains largely intact. Thermal history models suggest that similar global resurfacing events probably occurred episodically in the past. The tesserae statistically have the same average surface age (crater retention age) as the rest of the planet, but they probably represent older rock units deformed by earlier episodes of global resurfacing. Although they largely survived the latest global resurfacing event, their surfaces were severely deformed by it. Monte Carlo simulations indicate that only about 4%–6% of the planet has been volcanically resurfaced since the global event, and that the lava production rate has been no more than 0.01–0.15 km3/yr during this time. This rate is significantly less than the current rate of intraplate volcanism on Earth (0.33–0.5 km3/yr). Most of Venus' recent volcanism occurs in the Beta‐Atla‐Themis region, and most of the recent tectonism is associated with the major global‐scale tectonic disruption zones that lie within and connect the equatorial highlands. The approximately 33% of the planet's surface bounded by latitudes 30°N and 30°S, longitudes 60° and 300°E contains twice as many heavily fractured craters and 1.4 times more lava‐embayed craters as the planetary average. This region includes most of the major tectonic belts in the equatorial region. Because the craters are indistinguishable from a statistically random distribution, both spatially and hypsometrically, this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m.y. However, these craters are simply fractured and/or embayed, and very few have been subjected to complete tectonic disruption, complete burial, and subsequent removal from the surface, as was the case during the global resurfacing event. We show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record. The effects of recent geologic activity are much less than those of the earlier global resurfacing event, when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity. Episodic regional resurfacing events that had global effects also occurred on Earth (e.g., the mid‐Cretaceous superplume) and probably on Mars. On Mars they may have triggered the catastrophic releases of water that formed the outflow channels.

Density and morphology of impact craters on Tessera Terrain, Venus

Densities of impact craters on tessera, which is complex ridged terrain of tectonic origin, and on the remainder of the planet, which is mostly volcanic plains, were studied using Magellan images for about 96% of the surface of Venus. The density of large (D&gt;16 km) impact craters on tessera is higher by a factor of about 1.4 than on the remainder of the planet. This means that the tessera crater retention age is larger than the age of the plains. This is in agreement with the well known fact that tessera is embayed by the surrounding volcanic plains. The density of small (D&lt;16 km) impact craters on tessera is lower than on the remainder of the planet, evidently an observational bias caused by a difficulty in recognizing the small craters on rough tessera terrain. The absence of recognizable tectonic deformation in most of the large on‐tessera craters means that during the period of crater emplacement most of the studied tesserae were tectonically stable and did not undergo noticeable degree of deformation.