Timing of ancient extensional tectonic features on Mars

Although numerous studies have delineated the Tharsis and post-Tharsis volcanic/tectonic history on Mars, only a few attempts have examined the earlier epochs. This is not an easy task since unambiguous crater ages for pre-Tharsis and early Tharsis units are difficult to determine owing to a variety of active surface processes. Ancient tectonic features, however, have a sufficiently large superposed crater population that should permit relative dating. A technique for crater counting along linear features analagous to areal crater density is proposed. A modification of this approach has been tested and applied to a variety of ancient tectonic features

Floor-fractured crater models of the Sudbury structure, Canada

The Sudbury structure in Ontario, Canada, is one of the oldest and largest impact structures recognized in the geological record. It is also one of the most extensively deformed and volcanically modified impact structures on Earth. Although few other terrestrial craters are recognized as volcanically modified, numerous impact craters on the Moon have been volcanically and tectonically modified and provide possible analogs for the observed pattern of modification at Sudbury. We correlate the pattern of early deformation at Sudbury to fracture patterns in two alternative lunar analogs and then use these analogs both to estimate the initial size of the Sudbury structure and to model the nature of early crater modification at Sudbury

Variation in multiring basic structures as a function of impact angle

Previous studies have demonstrated that the impact process in the laboratory varies as a function of impact angle. This variation is attributed to changes in energy partitioning and projectile failure during the impact and, in simple craters, produces a sequence of progressively smaller and more asymmetric crater forms as impact angle decreases from approximately 20 degrees. Variations in impact angle can produce differences in the appearance of multiring impact basins. Comparisons of Orientale to the more oblique impact structure at Crisium also suggests that these differences primarily reflect the degree of cavity collapse. The relative changes in massif ring topography, basin scarp relief, and the distribution of peripheral mare units are consistent with a reduction in degree of cavity collapse with decreasing impact angle. The prominent uprange basin scarps and the restriction of tectonically derived peripheral mare units along uprange ring structures also may indicate an uprange enhancement of failure during cavity collapse. Finally, although basin ring faults appear to be preferred pathways for mare volcanism, fault-controlled peripheral mare volcanism occurs most readily uprange of an oblique impact; elsewhere such volcanism apparently requires superposition of an impact structure on the ring fault

Floor-fractured crater models for igneous crater modification on Venus

Although crater modification on the Earth, Moon, and Mars results from surface erosion and crater infilling, a significant number of craters on the Moon also exhibit distinctive patterns of crater-centered fracturing and volcanism that can be modeled as the result of igneous crater modification. Here, we consider the possible effects of Venus surface conditions on this model, describe two examples of such crater modification, and then briefly discuss the constraints these craters place on conditions at depth

Igneous intrusion models for floor fracturing in lunar craters

Lunar floor-fractured craters are primarily located near the maria and frequently contain ponded mare units and dark mantling deposits. Fracturing is confined to the crater interior, often producing a moat-like feature near the floor edge, and crater depth is commonly reduced by uplift of the crater floor. Although viscous relaxation of crater topography can produce such uplift, the close association of modification with surface volcanism supports a model linking floor fracture to crater-centered igneous intrusions. The consequences of two intrusion models for the lunar interior are quantitatively explored. The first model is based on terrestrial laccoliths and describes a shallow intrusion beneath the crater. The second model is based on cone sheet complexes where surface deformation results from a deeper magma chamber. Both models, their fit to observed crater modifications and possible implications for local volcanism are described

Grape canopy structure, light microclimate and photosynthesis. 1. A two-dimensional model of the spatial distribution of surface area densities and leaf ages in two canopy systems

The development of a model of the vertical and horizontal distribution of leaf, shoot, and fruit surface area densities, SAD (surface area per unit of canopy volume, m2 · m-3), as well as leaf types (main and lateral leaves) and leaf ages is described for two canopy systems (Espalier-type, S-System, and 3-wire Cordon, C-System). Leaf, shoot and fruit SAD increased towards the canopy interior for both systems and reached maximum values of 12, 0.8, and 1.2 m2 · m-3, respectively, for the S-System, and 10, 0.4, and 1.0 m2 · m-3 for the C-System. The fruit zone was always located near canopy areas of highest density. The distribution of surfaces of leaves of different ages from primary and secondary shoots was distinctly different for the two canopy types tested

Photosynthesis of sun and shade leaves of field-grown grapevine (Vitis vinifera L.) in relation to leaf age. Suitability of the plastochron concept for the expression of physiological age

The relationship of photosynthesis to leaf age for sun and shade leaves was investigated in a field experiment over two years. The objective was to test the suitability of the plastochron index concept, proposed by ERICKSON and MICHELINI (1957), for the description of age-related changes in leaf photosynthetic activity in the field. The duration of a plastochron decreased strongly and similarly for sun and shade shoots with increasing temperature, whether temperature was expressed as a daily heat sum (degree days > 10-degrees-C.dd-1) or as the mean of daily minima and maxima. The responsiveness to temperature decreased during the season and the lower threshold for development increased. Plastochron duration was insensitive to the amount of light received in the canopy. Expressing time elapsed between leaf unfolding and attaining maximum rates of photosynthesis in degree days or days yielded changing relationships depending on when during the season leaves unfolded. In contrast, this relationship was constant if time was expressed in plastochrons. The results show that the plastochron is an adequate indicator of time as the plant experiences it, and is superior to the thermal time (degree day) and chronological time expressions commonly used to relate photosynthesis to development

Discovery of meteorites on a blue-ice field near the Frontier Mountains, North Victoria Land, Antarctica

A high concentration of meteorites were discovered on a blue ice field northeast of the Frontier Mountains. As a result of a systematic search, a total of 42 meteorites were recovered. The current glacial situation has evolved through various stages, which are discussed in relationship to the concentration of meteorites. Ice flow patterns are summarized. The chemical composition and terrestrial ages of the meteorites are discussed

The effect of cave illumination on bats

Artificial light at night has large impacts on nocturnal wildlife such as bats, yet its effect varies with wavelength of light, context, and across species involved. Here, we studied in two experiments how wild bats of cave-roosting species (Rhinolophus mehelyi, R. euryale, Myotis capaccinii and Miniopterus schreibersii) respond to LED lights of different colours. In dual choice experiments, we measured the acoustic activity of bats in response to neutral-white, red or amber LED at a cave entrance and in a flight room – mimicking a cave interior. In the flight room, M. capaccinii and M. schreibersii preferred red to white light, but showed no preference for red over amber, or amber over white light. In the cave entrance experiment, all light colours reduced the activity of all emerging species, yet red LED had the least negative effect. Rhinolophus species reacted most strongly, matching their refusal to fly at all under any light treatment in the flight room. We conclude that the placement and light colour of LED light should be considered carefully in lighting concepts for caves both in the interior and at the entrance. In a cave interior, red LED light could be chosen – if needed at all – for careful temporary illumination of areas, yet areas important for bats should be avoided based on the precautionary principle. At cave entrances, the high sensitivity of most bat species, particularly of Rhinolophus spp., towards light sources almost irrespective of colour, calls for utmost caution when illuminating cave entrances