Crater Retention Ages Research Articles

The Mars Water Cycle at Other Epochs: Recent History of the Polar Caps and Layered Terrain

The Martian polar caps and layered terrain presumably evolve by the deposition and removal of small amounts of water and dust each year; the current attributes of the cap therefore represent the incremental transport during a single year as integrated over long periods of time. We have investigated the role of condensation and sublimation of water ice in this process by examining the seasonal water cycle during the past 10 7 years. In our model, axial obliquity, eccentricity, and L s of perihelion vary according to dynamic models. At each epoch, we calculate the seasonal variations in temperature at the two poles, keeping track of the seasonal CO 2 cap and the summertime sublimation of water ice into the atmosphere; net exchange of water between the two caps is calculated based on the difference in the summertime sublimation between the two caps (or on the sublimation from the other cap if one is covered with CO 2 frost all year). Despite the simple nature of our model and the tremendous complexity of the martian climate system, our results suggest two significant conclusions: (1) Only a relatively small amount of the available water in the polar deposits actually cycles between the poles on orbital—evolution time scales, such that it is to some extent the same water molecules moving back and forth between the two caps. (2) The difference in elevation between the two caps results in different seasonal behavior, such that there is a net transport of water from south to north averaged over long time scales; this transport would cause the north cap to grow at the expense of the south cap until the product of water sublimation rate and cap area was equal for the caps, minimizing any subsequent net transport. These results can help explain (1) the apparent inconsistency between the timescales inferred for layer formation and the much older crater retention age of the cap and (2) the difference in sizes of the two residual caps, with the south cap being smaller than the north.

Gravitational spreading of high terrain in Ishtar Terra, Venus

Magellan altimetry measurements indicate that the mountain belts and plateau margins of Ishtar Terra have topographic slopes in the range 2–30°. Magellan radar images reveal that numerous sets of narrow, closely spaced troughs and lineations, which we interpret as graben and normal faults, are located in many of these regions of high slope. The orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading. In several of the mountain belts, the extensional structures are parallel to the apparent direction of shortening, consistent with the interpretation that gravitational spreading occurred while mountain building was still active. To explore the implications of this spreading for the tectonic evolution of Ishtar Terra, we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range. Flow in the crust is assumed to be governed by a nonlinear, depth‐dependent rheology. We predict brittle failure and relaxation of topographic relief as functions of time, for ranges of crustal thickness (10–30 km) and thermal gradient (5–25 K/km) and for observed ranges in topographic elevation (1–6 km) and slope (1–30°). This parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief. For a crustal thickness greater than 10 km, a thermal gradient of 15 K/km or more, and average values of relief (3 km) and slope (3°), the topography relaxes to 25% of its original height in less than 10 m.y. Although initial failure, in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands, is predicted to occur much earlier, values of horizontal surface strain that are likely to be observable (∼1%) do not accumulate until significant relaxation of relief begins. Given the predicted rate of topographic relaxation, either the crust of Ishtar Terra must be anomalous in some way (either much stronger than predicted by standard flow laws or very thin or cold) or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for Venus of 500 Ma. The large apparent depth of compensation, the abundance of volcanism, and the uncertainty in the local age favor the view that mountain building in Ishtar Terra has occurred until times at least as recent as 10 Ma.

Resurfacing history of Tempe Terra and surroundings

The resurfacing history of the Tempe Terra region is determined using the Neukum and Hiller [1981] technique of breaking cumulative frequency curves into separate branches where the curves depart from a standard production curve. We find four surfaces recorded in the heavily cratered portions of Tempe Terra, with crater retention ages N(1) = [242,100], [95,700], [20,800], and [5500]. This is interpreted to indicate three major resurfacing events occurred in this region, ending at N(1) = [95,700], [20,800], and [5500]. The ridged plains on the Tempe Terra plateau have an oldest recorded surface age of [20,400], identical to that of the second resurfacing event recorded in the heavily cratered areas. A single resurfacing of the ridged plains (which may have been a second episode of ridged plains volcanism) occurred at N(1) = [7800]. The knobby plains to the north west and east of Tempe Terra also show a resurfacing at [20,000] and additional events at [5400] and perhaps [1600]. Mottled plains to the northeast record surfaces with crater retention age [7500], [5000], and [3900], where the first age is determined by a single surviving crater. It appears that the Lunae Planum Age (LPA) (at N(1) = [20,000]) resurfacing event seen elsewhere on Mars was widespread and effective in this region. A second widespread event appears to have ended at N(1) = [5000]. We estimate the thickness of the resurfacing materials corresponding to the LPA event to be less than 90 m in the heavily cratered areas but 300–500 m in the ridged plains themselves. Later resurfacing materials were generally thicker farther north in the mottled plains (300 m) than in the knobby plains (200 m).

On the nature and rate of resurfacing of Venus

Crater production and obliteration are modeled for the plains of Venus, using: (1 ) the observed distribution of Venus‐crossing asteroids and comets, (2) viscous relaxation of crater topography, and (3) erosion and burial by atmospheric, volcanic, and tectonic processes. Crater lifetimes are assumed to be proportional to crater depths for both classes of obliterative processes although the individual criteria vary. An average crater retention age between 0.4 to 2.0 Ga is estimated for plains under the assumption that craters are produced and not removed. The range is driven by uncertainty in identifying degraded impact as opposed to volcanic craters. On the other hand, crater retention ages greater than about 1.6 Ga are unlikely if viscous relaxation operates without loading of crater floors by burial. Our preferred model has plains subject to crater production and obliteration processes that vary over both space and time. In some areas, radar‐bright crater ejecta haloes are preserved for long periods of time because volcanism, tectonism, and weathering occurs at rates « 1 km/Ga. Viscous relaxation has probably generated numerous shallow craters in these relatively quiescent regions. In other areas, volcanism and tectonism have resurfaced the terrain at rates greater than several km/Ga. The global coverage and high resolution SAR and altimetry data expected from Magellan will allow testing of this model, based on detailed crater observations (diameter, depth distributions; morphologic criteria; surface scattering properties) and their association with volcanic and tectonic features.

Volcanism and tectonics of venus: Venera [formula omitted] results

Volcanism and tectonics are responsible for the majority of landforms observed on Venera 15 16 radar images covering the northern quarter of Venus' surface. Exogenetic resurfacing on Venus has been found negligible. The plains (about 75% of the area) are predominant with numerous features evidencing their formation as a result of basaltic volcanism. Tectonic deformations are mainly concentrated within uplands which occupy about 25% of the area. The most abundant are uplands bearing the crossecting systems of ridges and grooves (“parquet” or tesserae) which indicate deformations due to horizontally oriented stress or strain. Large dome-like uplifts with plains-like morphology complicated by apical systems of subparallel grooves represent another major type of tectonic landforms (Beta Regio etc.). The ridge belts bearing the controversional evidences of their compressional and/or extensional origin and observed on many venusian plains may be specific surficial manifestations of global spreading on Venus. Average crater retention age of the area surveyed is estimated as 0.5 to 1.0 b.y. Tectonic-volcanic resurfacing on Venus during the dated period of its history was an order of magnitude less than at Earth's oceanic floor.

Mars: Stratigraphy and gravimetry of Olympus Mons and its aureole

Olympus Mons and its surroundings have been investigated using impact crater statistics. Crater retention ages of all major geological units show that their emplacement occurred during three time intervals. The underlying fractured plains are part of the northern plains in general (N(1) > 4 × 10−3 (km−2)). This time is followed by the time interval represented by crater retention ages N(1) = 8 ×10−4 to 3 × 10−3 (km−2). Some lava flows in the fractured plains, the smooth plains, and the aureole materials were emplaced during this time. The aureole probably developed by some sort of collapse of the outer parts of an ancient volcano (‘Proto‐Olympus Mons’) underlying the present shield. Its remnants may show in the scarp and small parts of the shield not covered by younger lava flows. The third interval covers crater retention ages N(1) = 5 × 10−5 to 3 × 10−4 (km−2). This volcanic activity followed a period of dormancy. A series of lava flows forming caldera and present shield surface and flooding a depression around the shield are distinguishable. The regional stratigraphy is discussed as part of the global geological history. The question of the genesis of the aureole material is addressed by gravity models. The topographic mass of Olympus Mons contributes about 80% to the aureole gravity high northwest of the mountain. The gravity high of the entire aureole can be explained by a density model consisting of the topographic masses of Olympus Mons and the aureole materials. It does not require additional high‐density materials underneath the aureole. Thus aureole materials of an allochthonous origin by collapse are in agreement with the gravity data. However, the gravity data do not preclude an autochthonous and volcanic origin of the aureole materials. In the latter case, possible subsurface intrusions should have no density contrasts with the surroundings.

Martian ages

The subjects of this paper are a discussion of the methodology of relative age determination by impact crater statistics, a comparison of currently proposed Martian impact chronologies for the determination of absolute ages from crater frequencies, a report on our work of dating Martian volcanoes and erosional features by impact crater statistics, and an attempt to understand the main features of Martian history through a synthesis of our crater frequency data and those published by other authors. Two cratering chronology models are presented and used for inference of absolute ages from crater frequency data: model I, with nearly equal Martian and lunar cratering rates around (ca.) 4‐ to 10‐km crater sizes, and model II, equivalent to model I for ages > 3.5 · 109 years but with a factor of 2 higher Martian cratering rate at ages <3 · 109 years. Those model cratering chronologies are applied to the data. The interpretation of all crater frequency data available and tractable by our methodology leads to a global Martian geological history that is characterized essentially by two epochs of activity. The division between the two epochs is measured at a cumulative crater frequency value for 1‐km craters (crater retention age) of N(1) = 8 · 10−4 (km−2) corresponding to an absolute age of ca. 3 · 109 years (applying model I cratering chronology) and of ca. 1.5 · 109 years (applying model II cratering chronology). In the ancient epoch all major events like emplacement of the plains lavas, the piling up of most volcanic constructs, and large‐scale erosion of channels and mensae (highland/northern lowland boundary) have taken place. During the younger epoch, only the big Tharsis shield volcanoes were active, and some minor erosion took place. This means that Mars is not a youthful planet but an ancient one with respect to most of its surface features.

Planetary crater retention ages

Planetary crater retention ages

Relative crater production rates on planets

Dynamical histories of planetesimals in specified orbits, calculated by Wetherill (1975) and others, have estimates of relative numbers of impacts on different planets. These impact rates, F , are converted to crater production rates, F, by means of tables developed in this paper. Conversions are dependent on impact velocity and surface gravity. Crater retention ages can then be derived from (crater density)/(crater production rate). Such calculations of impact rates and their histories give the only basis, independent of sample dating, for establishing absolute geologic histories of the planets, contrary to published implications that this can be done by comparison of photos alone. A survey of the results, from orbits of interplanetary objects studied to date, indicates that the terrestial planets have crater production rates within a factor ten of each other, and that planet's crater retention ages can probably be determined with a factor of ±3. Further calculations of orbital histories of additional interplanetary bodies are suggested to put photogeologic analyses from spacecraft imagery on a firmer basis. Applications to Mars, as an example, using least-squares fits to crater-count data, suggest an average age of 0.3 to 3 b.y. for two types of channels. The Tharsis volcanics are found to be slightly younger than the channels (strongly confirmed by photomorphology since they are not cut by channels) and Olympus Mons is about 0.06 to 0.6 b.y. old, contrary to recent assertions that Olympus Mons is 2.5 b.y. old and most Martian volcanic provinces older than 3 b.y. Data strongly support the hypothesis that Martian channels formed in a fluvial climate that persisted on Mars until the Tharsis volcanism caused a change in the Martian obliquity state, as outlined by Toon, Ward, and Burns (1977).

Martian Cratering

New counts of Martian and lunar craters are used in a new analysis of the Mariner IV photographs. The difference in impact velocities on the two planets, neglected by other authors, is found to introduce an appreciable effect. The age of a Martian surface layer capable of retaining craters larger than 50-km diameter is found to be about 4 × 10 9 years, within a factor of about 2. The Martian craters have the diameter distribution expected from impacts of asteroidal fragments, within errors of measurement. Although the age quoted above apparently refers to the formation of the present surface layer (to a depth on the order of 3 km), the “crater retention ages” at smaller diameters are less, owing to an assumed erosive process. The crater retention time for a structure of about 1-km diameter is estimated to be on the order of 10 8 years, and it appears that an erosive process flattens large-scale relief at a rate of about 10 −4 cm/year. This indicates that fundamental conclusions about Mars' erosion history cannot be drawn from the ages of the large craters alone.