Robert E. Grimm

Impact craters and Venus resurfacing history

Venusian impact crater size-frequency distributions, locations, and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism. An atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km, the size-frequency distribution is close to the atmosphere-free case. With this result, and assuming that the surface records a crater production population (a catastrophic resurfacing model, CRM), an age of cessation of rapid resurfacing of ∼ 500 Ma is obtained. Crater locations are widely dispersed across Venus and the hypothesis that they are completely spatially random (CSR) cannot be rejected. However, craters that show embayment by plains materials or modification by throughgoing faults (i.e., tectonized) are preferentially found in areas with relatively few craters overall. The primary region where these modified craters are found is the Aphrodite volcanotectonic zone, extending from Ovda Regio on the west to the region east of Atla Regio. These results, together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy, imply that a range of surface ages are recorded by the impact crater population; e.g., the Aphrodite zone is relatively young. An end-member model (equilibrium resurfacing model, ERM) was developed to quantify resurfacing scenarios. In the ERM, Venus has been resurfacing at an average rate of approximately 1 km2 yr−1. However, the CRM and ERM are idealized end-member representations of possible resurfacing histories. For both models, the resurfacing rate can be expressed as the product of resurfacing patch area a (normalized by planetary surface area) and the frequency ω of resurfacing events. Numerical simulations of resurfacing showed that there are two solution branches that satisfy the CSR constraint: a 0.1 (74° diameter circle). The former range corresponds to resurfacing diameters smaller than the average intercrater distance, whereas the latter is associated with large, infrequent events, resurfacing 10% of the planet every 50 Ma to 100% every 500 Ma. The observed fraction of embayed and tectonized craters further constrains values of a and only values near 0.0003 are admissible. The resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs, in any given geological episode, in only a limited number of regions on the planet.

Venus tectonics: An overview of Magellan observations

The nearly global radar imaging and altimetry measurements of the surface of Venus obtained by the Magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet. Many areas of Venus record a superposition of different episodes of deformation and volcanism. This deformation is manifested both in areally distributed strain of modest magnitude, such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions, as well as in zones of concentrated lithospheric extension and shortening. The common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes. Ridge belts and mountain belts, which have characteristic widths and spacings of hundreds of kilometers, represent successive degrees of lithospheric shortening and crustal thickening. The mountain belts of Venus, as on Earth, show widespread evidence for lateral extension both during and following active crustal compression. Venus displays two principal geometrical variations on lithospheric extension: the quasi-circular coronae (75–2600 km diameter) and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers. Both are sites of significant volcanic flux, but horizontal displacements may be limited to only a few tens of kilometers. Few large-offset strike slip faults have been observed, but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening. Several large-scale tectonic features have extremely steep topographic slopes (in excess of 20°–30°) over a 10-km horizontal scale; because of the tendency for such slopes to relax by ductile flow in the middle to lower crust, such regions are likely to be tectonically active. In general, the preserved record of global tectonics of Venus does not resemble oceanic plate tectonics on Earth, wherein large, rigid plates are separated by narrow zones of deformation along plate boundaries. Rather tectonic strain on Venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers. These characteristics are shared with actively deforming continental regions on Earth. The styles and scales of tectonic deformation on Venus may be consequences of three differences from the Earth: (1) The absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well-preserved. (2) A high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller-scale deformational features. (3) A strong coupling of mantle convection to the upper mantle portion of the lithosphere, probably because Venus lacks a mantle low-viscosity zone, leads to crustal stress fields that are coherent over large distances. The lack of a global system of tectonic plates on Venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on Earth.

Water and the thermal evolution of carbonaceous chondrite parent bodies

Many carbonaceous chondrites have been aqueously altered within their parent bodies. From chemical and textural data on these meteorites and from studies of collision mechanics, we pose two hypotheses for the aqueous alteration environment. In the first model, alteration occurs throughout the parent body interior; in the second, alteration occurs in a postaccretional surface regolith. Both models are based on the assumptions of an initially homogeneous mixture of ice and rock and heating by decay of 26Al. Under the interior-alteration model, linked bounds on the initial ice-to-rock ratio and 26Al abundance are found that satisfy alteration temperatures derived from oxygen isotope studies. We find that water may play a strong role in controlling chondrite evolution by acting as a thermal buffer that allows substitution of low-temperature aqueous alteration instead of high-temperature recrystallization. Additional constraints imposed by the inferred water volume consumed by the alteration reaction and the total water volume that exchanged oxygen isotopes with host rocks are best explained if alteration occured ina regolith. We show quantitatively how liquid water may be introduced there by hydrothermal circulation, by diffusion of vapor from below, or by venting due to fracture when interior pore pressures exceed the parent body strength. A sealed permafrost zone is not required to ensure insulation of water long enough for aqueous alteration. Retention of primordial ice is probably not limited by sublimation or by collisional comminution, but by shock vaporization. If large, C-type asteroids are representative of carbonaceous chondrite parent bodies, they may still contain significant quantities of ice.

Iron meteorites as remnants of planetesimals formed in the terrestrial planet region

Iron meteorites are core fragments from differentiated and subsequently disrupted planetesimals. The parent bodies are usually assumed to have formed in the main asteroid belt, which is the source of most meteorites. Observational evidence, however, does not indicate that differentiated bodies or their fragments were ever common there. This view is also difficult to reconcile with the fact that the parent bodies of iron meteorites were as small as 20 km in diameter and that they formed 1–2 Myr earlier than the parent bodies of the ordinary chondrites. Here we show that the iron-meteorite parent bodies most probably formed in the terrestrial planet region. Fast accretion times there allowed small planetesimals to melt early in Solar System history by the decay of short-lived radionuclides (such as 26Al, 60Fe). The protoplanets emerging from this population not only induced collisional evolution among the remaining planetesimals but also scattered some of the survivors into the main belt, where they stayed for billions of years before escaping via a combination of collisions, Yarkovsky thermal forces, and resonances. We predict that some asteroids are main-belt interlopers (such as (4) Vesta). A select few may even be remnants of the long-lost precursor material that formed the Earth.

Hot-spot evolution and the global tectonics of Venus

The global tectonics of Venus may be dominated by plumes rising from the mantle and impinging on the lithosphere, giving rise to hot spots. Global sea-floor spreading does not take place, but direct convective coupling of mantle flow fields to the lithosphere leads to regional-scale deformation and may allow lithospheric transport on a limited scale. A hot-spot evolutionary sequence comprises (i) a broad domal uplift resulting from a rising mantle plume, (ii) massive partial melting in the plume head and generation of a thickened crust or crustal plateau, (iii) collapse of dynamic topography, and (iv) creep spreading of the crustal plateau. Crust on Venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea-floor spreading.

Anatomy of a Venusian hot spot: Geology, gravity, and mantle dynamics of Eistla Regio

Eistla Regio is a series of broad swells, each up to a few thousand kilometers in diameter, in the equatorial highlands of Venus. It is characterized by strong positive free-air gravity anomalies, shield volcanoes, and rift systems. We present a two-part study of the western and central portions of this Venusian hot spot. First, Magellan radar images were mapped in order to understand the general geologic history of the region. Radial fracture systems both on the rises and volcanoes indicate uplift and associated faulting accompanied volcanic construction. Prominent fracture zones strike WNW to NW, parallel to the long axis of the highlands. The largest of these, Guor Linea, displays a progressive deformation history that may include minor clockwise rotation in addition to bulk NNE-SSW extension. Other regional structural patterns also suggest this sense of rotation. The parts of Eistla studied here appear to be structurally distinct from surrounding regions, although volcanic radar-bright plains and ridge-and-groove deformation to the west indicate an earlier, more diffuse thermal uplift. In the second part of our analysis, Pioneer Venus line-of-sight accelerations were inverted for vertical gravity which, when combined with topography, were used to solve for mass anomalies on the crust-mantle boundary and in the upper levels of the mantle convective system. Both western and central Eistla appear to be sites of vigorous mantle upwelling. A linear upwelling appears beneath Guor Linea, indicating that rifting is actively driven by mantle flow. Minor crustal thinning may be present beneath western Eistla and the northern plains, whereas some thickening occurs beneath central Eistla and the southern plains. Heng-O corona, in particular, may be the site of a small shallow mass anomaly of either compositional (thickened crust) or thermal origin. Mantle return flow is asymmetrically distributed about Eistla, being much stronger to the south than the north, and little flow is inferred at all to the NE in Bereghinya Planitia. These differences introduce a clockwise rotation to finite strain, in agreement with observations.

Groundwater‐controlled valley networks and the decline of surface runoff on early Mars

[1] Fluvial erosion on early Mars was dominated by valley networks created through a combination of groundwater processes and surface runoff. A reduced greenhouse effect due to CO 2 loss, together with a declining geothermal heat flux, promoted the growth of a cryosphere and a Hesperian hydrologic regime dominated by outflow channel formation. We test the hypothesis that the transition from valley network to outflow channel formation was preceded by a more subtle evolution characterized by a weakening of surface runoff, leaving groundwater processes as the dominant, final source of valley network erosion. Our hypothesis, supported by a terrestrial analog in the Atacama desert of Chile, is related to the groundwater sapping reactivation hypothesis for densely dissecting highland valley networks on Mars suggested by Baker and Partridge in 1986 and focuses on the age analysis of large, sparsely dissecting valley networks such as Nanedi Valles, Nirgal Vallis, valleys in fretted terrain, and tributaries of outflow channels and Valles Marineris chasmata. We find that these features are consistently late Noachian to Hesperian in age, younger than Noachian densely dissecting dendritic valley networks in the southern highlands. In the Tharsis region the observation of dense and sparse valley network morphologies on Hesperian terrain suggests that while surface runoff gave way to groundwater processes consistent with our hypothesis, the transition may have occurred later than elsewhere on the planet. The volcanic nature of Tharsis suggests that geothermal heat and volatile production led to episodically higher volumes of surface runoff in this region during the Hesperian.

Recent deformation rates on Venus

Constraints on the recent geological evolution of Venus may be provided by quantitative estimates of the rates of the principal resurfacing processes, volcanism and tectonism. This paper focuses on the latter, using impact craters as strain indicators. The total postimpact tectonic strain lies in the range 0.5–6.5%, which defines a recent mean strain rate of 10−18–10−17 s−1 when divided by the mean surface age. Interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 Ma (catastrophic resurfacing model). If distributed tectonic resurfacing contributed strongly before that time, as suggested by the widespread occurrence of tessera as inliers, the mean global strain rate must have been at least ∼10−155 S−1, which is also typical of terrestrial active margins. Numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years. Parameterized convection models predict that the mean global thermal gradient decreases by only about 5 K/km over this time; even with the exponential dependence of viscosity upon temperature, the surface strain rate drops by little more than one order of magnitude. Strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model. An alternative, uniformitarian resurfacing hypothesis holds that Venus is resurfaced in quasi-random “patches” several hundred kilometers in size that occur in response to changing mantle convection patterns. Under such a model, the observed crater strain distribution indicates that about 1% of the planets surface is tectonically active at any time. However, this model requires a very weak crustal rheology to achieve surface velocities ∼100 mm/yr appropriate to the required “patch” size. Without well-developed lateral weak zones, Venus is essentially a one-plate planet, but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limited horizontal movement. The net rate of tectonic activity is logarithmically intermediate between Earth and Mars: about 100 times slower than plate tectonics, but up to 100 times faster than planets where tectonic stress arises largely from lithospheric cooling and contraction.

Gravity anomalies, compensation mechanisms, and the geodynamics of western Ishtar Terra, Venus

Pioneer Venus line-of-sight orbital accelerations have been used to calculate the geoid and vertical gravity anomalies for western Ishtar Terra on various planes of altitude Z0. The apparent depth of isostatic compensation at Z0 = 1400 km is 180 ± 20 km based on the usual method of minimum variance in the isostatic anomaly. We seek to explain this observation, as well as the regional elevation, peripheral mountain belts, and inferred age of western Ishtar Terra in terms of one of three broad geodynamic models. Under local mantle upwelling, western Ishtar is the surface expression of a hotspot which produces topography both dynamically and by volcanic construction. This model holds that the mountain belts form as a result of incipient downward mantle return flow. An upwelling with its centroid at 160-km depth and buoyancy stress 100 MPa can satisfy the observed long-wavelength geoid and topography provided that the crust-mantle density contrast does not control the response of the crust (so that late-time subsidence does not occur) and that the mantle viscosity does not increase markedly with depth. Mantle upwelling produces a combination of strike-slip motion and azimuthal extension on the uplift itself; radial compression is restricted to the surrounding plains. Under local mantle downwelling, cold sinking mantle beneath western Ishtar induces shear tractions on the crust which thicken and elevate it; volcanism is potentially the result of basal crustal remelting. The formation of mountain belts is assumed to be a by-product of crustal thickening. A deeply seated downwelling (300–700 km, buoyancy stress 200–300 MPa) can satisfy the geoid and central topography provided that the crust can respond separately to mantle flow and that a more viscous lower mantle exists. The predicted surface strains during intermediate stages of the uplift are in qualitative agreement with observations (azimuthally oriented thrusts at the uplift margin). Incorporation of the effective viscosities of diabase and olivine suggests that uplift above a plume will be rapid, but the thermal gradient over downgoing flow must be at least as high as the expected mean value for Venus (∼20 K/km) in order to attain substantial uplift within several hundred million years. A third scenario, regional compression, may be distinguished from the second in that western Ishtar is simply a locus of strain accumulation and crustal thickening from regional stress fields; neither broad downward mantle flow nor lithospheric subduction need occur. However, both the source of stress and geoid anomaly are problematic. If the topography of western Ishtar is assumed to be Airy compensated, then the geoid signature must arise from unrelated mantle density anomalies. In this case, the lithosphere must be decoupled from mantle flow or else the mantle must be effectively rigid in order to suppress dynamic topography. A more complex model than those analyzed in detail here is needed to produce both the correct surface strains and apparent compensation depth. We favor upwelling models in which a mantle plume has flattened out and the surface has been subsequently deformed. Western Ishtar Terra must be distinguished from other paradigm hotspots on Venus by different environmental conditions or by being in a unique stage of hotspot evolution. Further quantitative investigations of the interaction of mantle shear tractions with a laterally heterogeneous lithosphere are required.

Controls on Martian Hydrothermal Systems: Application to Valley Network and Magnetic Anomaly Formation

magnetic anomalies. For host rock permeabilities as low as 10 � 17 m 2 and intrusion volumes as low as 50 km 3 , the total discharge due to intrusions building that part of the southern highlands crust associated with magnetic anomalies spans a comparable range as the inferred discharge from the overlying valley networks. INDEX TERMS: 1832 Hydrology: Groundwater transport; 1860 Hydrology: Runoff and streamflow; 1545 Geomagnetism and Paleomagnetism: Spatial variations (all harmonics and anomalies); 5440 Planetology: Solid Surface Planets: Magnetic fields and magnetism; 5114 Physical Properties of Rocks: Permeability and porosity; KEYWORDS: Hydrothermal, groundwater, runoff, crustal magnetism, intrusions