485 research outputs found
The Complex Geomorphology of Neukum Crater on Mars
No full textNeukum Crater, located at about 28°E/45°S at Noachis Terra on Mars, contains various geomorphological features that indicate a diverse geological history forming the crater during the past up to 4 Ga years. Most prominent features of this 102 km-crater are the large dark dune field and the two pits on its floor. The basaltic composition of the dunes sands, enriched in high-calcium pyroxenes, does not differ from the overall composition of the dark dunes found in various places elsewhere on Mars. Avalanches of the dune slip faces evidence recent seasonal mass movement processes within the dune field. Aeolian processes also left its traces in the form of countless dust devil tracks widespread on the crater floor as well as in the form of transverse aeolian ridges within the pits. These pits, also found in some neighbouring craters, act as geological windows to the subsurface and might have contributed as sources of the dark dune materials of this region. Moraine-like features and mass movements in smoothed terrain along the crater wall whiteness an era of active glacial and periglacial processes at Neukum crater. Superposed ejecta onto these landforms allow determining the minimum age of the features and constraining the timing of geological processes
Neukum Crater in Noachis Terra, Mars: Absolute Model Ages and Stratigraphy
No full textWe geologically mapped the Neukum crater in Noachis Terra, Mars, using a variety of data sets (e.g., HRSC, CTX,
MOC, HiRISE, THEMIS, CRISM, and MOLA), and identified 21 geomorphologic units. Several plains units on
the crater floor show distinct morphologies (smooth, rough, hummocky, furrowed), albedo, surface roughness, and
thermal inertia, and are large enough to be dated with crater size-frequency distribution (CSFD) measurements.
Like other craters in the vicinity, Neukum crater hosts pits on its floor, showing fine-scale layering, likely related to
the deposition of a regional unit that filled these craters. The wide distribution of pits indicates that the degradational
process that created them must have been active at regional scale. Thus, Neukum crater can serve as a case study for
processes that shaped significant portions of Noachis Terra. We used CTX images to perform CSFD measurements
of seven mapped geologic units to study their stratigraphy and absolute model ages (AMAs). In some cases it was
challenging to obtain robust CSFDs, particularly for the hummocky units, when surface roughness and crater sizes
were similar. These crater sizes, however, were not used for the fitting of the production function, and thus, do
not affect the derived AMAs. Our CSFD measurements suggest that Neukum crater is at least 3.5-3.7 Ga old, as
indicated by our age for unit Nr, which represents the crater rim and the older age of unit Nfh2, which is exposed
on the crater floor. Neukum crater might even be older and certainly is a Noachian crater. While the crater itself is
old, most of our map units exhibit significantly younger ages, indicating modification of the crater. At least three
units show evidence for resurfacing, i.e. units Hfh2, Hfs, and Nhpu2. The youngest AMA of 66 Ma was derived for
the floor of the northeastern pit within Neukum crater. This map unit Nhpu2 also exhibits additional AMAs of 450
Ma and 2.2 Ga. Similar ages (within the error bars) to the 450 Ma AMA have been found for units Hfr1 (460 Ma)
and Hfs (570 Ma). Within their error bars, units Hfh1 and Hfr2 also show overlapping ages of 340 Ma and 300 Ma,
respectively. Taking into account the error bars, the oldest ages of units Nhpu2 (2.2 Ga) and Hfs (1.8 Ga) are also
similar to each other. Thus, several geologic units associated with Neukum crater might share similar modification
histories, implying that the responsible processes affected large areas within the crater and possibly even outside
the crater. In fact, our geologic study indicates that aeolian processes might have substantially modified the crater
and its floor until the recent past. Thus, our AMAs are consistent with our morphologic observations of deflation
of many of our map units. Evidence for accumulation of material only exists in a few places in form of transverse
and longitudinal dunes
Gaspra's Cratering Record.
No full textThe images of the asteroid 951 Gaspra obtained by Galileo show impact craters down to the resolution limit of 50m/pixel. The observed crater population has characteristics which very much resemble those seen on the inner solar system planets. The remarkable finding is that the distribution is steep. The cumulative size-frequency distribution can be approximated by a power law with an exponent (population index) of between -3.3 and -3.7 in the size range 200m to 800m. This is very close to the distribution characteristics found on the Earth's moon, on Mars, and on Mercury with population indices (cumulative) around -3.5 in that size range. At larger sizes (Dɣkm) there appears to be a slight flattening similar to what is found for distributions on the inner solar system objects. The lunarcurve(standard distribution, cf. Neukum et al., 1975) can be approximates very well to the Gaspra distribution taking into account the differences in the impact velocity and surface gravity conditions. A steep curve was expected for the asteroid belt cratering record (cf. Neukum 1983) if the cratering record in the inner solar system is directly linked to this region in terms of a source region. The findings on Gaspra support the view long held by the first author that (1) the small craters(D<1km) on the inner solar system bodies are essentially primary impact craters and not caused by "background secondaries", and (2) the inner solar system cratering is due to the bombardement by one and the same family of bodies derived from the asteroid belt. The time dependence of the cratering ratein the asteroid belt and in the inner solar system may have been quite different. If Gaspra is between 200 and 500 m.y. old as estimated in combining the crater frequency measurement with impact probability data derives from astronomical observations of large-size asteroids, then it appears impossible that the time
Hazard analysis and mapping. In: Zwahlen, F. (Ed.), Vulnerability and Risk Mapping for the Protection of Carbonate (karst) Aquifers
No full text
The Impact of Soil Tension on Isotope Fractionation, Transport, and Interpretations of the Root Water Uptake Origin
No full textThe new isotope module in HYDRUS-1D can be used to infer the origin of root water uptake
(RWU), a suitable dynamic indicator for agriculture and forest water management. However, evidence shows
that the equilibrium fractionation between liquid water and water vapor within the soil is affected not only by
soil temperature but also by soil tension. How soil tension affects isotope transport modeling and interpretations
of the RWU origin is still unknown. In this study, we evaluated three fractionation scenarios on model
performance for a field data set from Langeoog Island: (a) no fractionation (Non_Frac), (b) the soil temperature
control on equilibrium fractionation as described by the standard Craig-Gordon equation (CG_Frac), and
(c) CG_Frac plus the soil tension control on equilibrium fractionation (CGT_Frac). The model simulations
showed that CGT_Frac led to more depleted isotopic compositions of surface soil water than CG_Frac. The
vertical origin of RWU was estimated using the water balance (WB) calculations and the Bayesian mixing
model (SIAR). While the former directly used water flow outputs, the latter used as input simulated isotopic
compositions (using different fractionation scenarios) of RWU and soil water. Both methods provided similar
variation trends with time and depth in different soil layers' contributions to RWU. The contributions of all soil
layers interpreted by the CGT_Frac scenario were always between Non_Frac and CG_Frac. The temporal origin
of RWU was deduced from particle tracking (PT, releasing one hypothetical particle for individual precipitation
event and tracking its movement based on the water balance between particles) and a virtual tracer experiment
(VTE, assigning a known isotope composition to individual precipitation event and tracking its movement
based on the cumulative isotope flux). Both methods revealed similar variation trends with time in drainage
and root zone (RZ) travel times. The interpreted drainage and RZ travel times were generally ranked as Non_
Frac > CGT_Frac > CG_Frac. Overall, the factors considered in the standard CG equation dominated isotope
fractionation, transport, and interpretations of the RWU origin. Isotope transport-based methods (SIAR, VTE)
were more computationally demanding than water flow-based methods (WB, PT)
De marasmo senili sive marcore naturali
No full textquam ... pro summis in medicina honoribus & privilegiis doctoralibus rite obtinendis, publico eruditorum examini subiicit Georg. Michael Neukum Scaphusio-Helvetus. Ad diem 10. Iulii, M D C C XLIII. ...Enthält 25 ThesenDiss. med. Basel, 174
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