► Valleys feeding the Eberswalde fan crosscut Holden impact crater ejecta blanket. ► Holden crater formation likely postdates the Early Hesperian. ► Modeling shows Eberswalde fan can form at a ...geologically rapid rate. ► The role of Holden crater in Eberswalde fluvial activity is highlighted.
The fan deposit in Eberswalde crater has been interpreted as strong evidence for sustained liquid water on early Mars with a paleolake formed during the Noachian period (>3.7Gy). This location became a key region for understanding the Mars paleo-environment. Eberswalde crater is located 50km north of the rim of the 150km diameter crater Holden. Stratigraphic relationships and chronology obtained using recent Mars Express High Resolution Stereo Camera and Mars Reconnaissance Orbiter Context Camera images show that Eberswalde fluvial activity crosscuts Holden ejecta and thus postdates Holden crater, whose formation age is estimated from crater counts as Late Hesperian (∼3.5Gy, depending on models). Fluvial modeling shows that short term activity (over several years to hundreds of years) involving dense flows (with sediment:water ratio between 0.01 and 0.3) may be as good an explanation of the fluvial landforms as dilute flow over longer durations. Modeling of the thermal effect of the Holden impact in the Eberswalde watershed is used to evaluate its potential role in aqueous activity. The relative timing of the Holden impact and Eberswalde’s fan is a constraint for future studies about the origin of these landforms. Holden ejecta form a weak and porous substrate, which may be easy to erode by fluvial incision. In a cold climate scenario, impact heating could have produced runoff by melting snow or ground ice. Any attempt to model fluvial activity at Eberswalde should take into account that it may have formed as late as in the Late Hesperian, after the great majority of valley network formation and aqueous mineralization on Mars. This suggests that hypotheses for fan formation at Eberswalde by transient and/or localized processes (i.e. impact, volcanism, unusual orbital forcing) should be considered on a par with globally warmer climate.
We report on the discovery of stellar occultations, observed with Kepler, which recur periodically at 15.685 hr intervals, but which vary in depth from a maximum of 1.3% to a minimum that can be less ...than 0.2%. The star that is apparently being occulted is KIC 12557548, a V = 16 mag K dwarf with T sub(eff, s) Asymptotically = to 4400 K. The out-of-occultation behavior shows no evidence for ellipsoidal light variations, indicating that the mass of the orbiting object is less than ~3 M sub(J) (for an orbital period of 15.7 hr). Because the eclipse depths are highly variable, they cannot be due solely to transits of a single planet with a fixed size. We discuss but dismiss a scenario involving a binary giant planet whose mutual orbit plane precesses, bringing one of the planets into and out of a grazing transit. This scenario seems ruled out by the dynamical instability that would result from such a configuration. We also briefly consider an eclipsing binary, possibly containing an accretion disk, that either orbits KIC 12557548 in a hierarchical triple configuration or is nearby on the sky, but we find such a scenario inadequate to reproduce the observations. The much more likely explanation-but one which still requires more quantitative development-involves macroscopic particles escaping the atmosphere of a slowly disintegrating planet not much larger than Mercury in size. The particles could take the form of micron-sized pyroxene or aluminum oxide dust grains. The planetary surface is hot enough to sublimate and create a high-Z atmosphere; this atmosphere may be loaded with dust via cloud condensation or explosive volcanism. Atmospheric gas escapes the planet via a Parker-type thermal wind, dragging dust grains with it. We infer a mass-loss rate from the observations of order 1 M sub(+ in circle) Gyr super(-1), with a dust-to-gas ratio possibly of order unity. For our fiducial 0.1 M sub(+ in circle) planet (twice the mass of Mercury), the evaporation timescale may be ~0.2 Gyr. Smaller mass planets are disfavored because they evaporate still more quickly, as are larger mass planets because they have surface gravities too strong to sustain outflows with the requisite mass-loss rates. The occultation profile evinces an ingress-egress asymmetry that could reflect a comet-like dust tail trailing the planet; we present simulations of such a tail.
The presence of perennially wet surface environments on early Mars is well documented
, but little is known about short-term episodicity in the early hydroclimate
. Post-depositional processes driven ...by such short-term fluctuations may produce distinct structures, yet these are rarely preserved in the sedimentary record
. Incomplete geological constraints have led global models of the early Mars water cycle and climate to produce diverging results
. Here we report observations by the Curiosity rover at Gale Crater indicating that high-frequency wet-dry cycling occurred in early Martian surface environments. We observe exhumed centimetric polygonal ridges with sulfate enrichments, joined at Y-junctions, that record cracks formed in fresh mud owing to repeated wet-dry cycles of regular intensity. Instead of sporadic hydrological activity induced by impacts or volcanoes
, our findings point to a sustained, cyclic, possibly seasonal, climate on early Mars. Furthermore, as wet-dry cycling can promote prebiotic polymerization
, the Gale evaporitic basin may have been particularly conducive to these processes. The observed polygonal patterns are physically and temporally associated with the transition from smectite clays to sulfate-bearing strata, a globally distributed mineral transition
. This indicates that the Noachian-Hesperian transition (3.8-3.6 billion years ago) may have sustained an Earth-like climate regime and surface environments favourable to prebiotic evolution.
Mars' climate history depends in part on its atmospheric pressure evolution, but most existing constraints on atmospheric pressure are indirect. Thin atmospheres allow small objects to reach the ...surface and form impact craters; therefore, ancient impact craters can constrain past atmospheric pressure. To identify ancient craters preserved in sedimentary rocks and exhumed by wind erosion, we use HiRISE orthoimages, anaglyphs, and digital terrain models (DTMs). We compare measured crater populations from two sites to predictions from an atmosphere‐impactor interaction model for atmospheres of different pressures. Our upper limits on continuous atmospheric pressure are 1.9±0.1 bar around 4 Ga and 1.5±0.1 bar at 3.8±0.2 Ga. We demonstrate that atmospheric pressure cannot have been continuously above these upper limits. During the interval 3.8±0.2 Ga, our crater counts require that atmospheric pressure was less than 5% of Earth's modern pressure for at least 104 yrs, or at higher pressure for a correspondingly longer duration of time (at least 105−106 years at 1.5 bar for our Mawrth phyllosilicates and Meridiani Planum data, respectively). Therefore, atmospheric pressure around 4 Ga was either continuously 1.9±0.1 bar or varied between higher (>1.9 bar) and lower (<1.9 bar) pressures. Similarly, atmospheric pressure at 3.8±0.2 Ga was either continuously 1.5±0.1 bar, or varied between higher (>1.5 bar) and lower (<1.5 bar) pressures. Finally, we synthesize all available paleopressure estimates for early Mars to constrain a 2‐component model of Mars' long‐term atmospheric pressure evolution. In our model, atmospheric pressures <1 bar early in Mars' history best fit existing paleopressure constraints.
Plain Language Summary
Atmospheric pressure helps control Mars' climate over time. Ancient impact craters give us upper limits on past atmospheric pressure. If atmospheric pressure is high, objects travel through a thicker, denser atmosphere. This stops more small objects from reaching the ground and forming craters. A group of ancient craters with lots of small craters indicates low atmospheric pressure during crater accumulation. We compare the proportion of small, ancient craters at 2 sites on Mars to models of craters for different atmospheres. This gives an upper limit of 1.9 bar around 4 billion years ago and 1.5 bar at 3.8±0.2 billion years ago. It is possible that our observed craters formed while atmospheric pressure was changing. To form the groups of craters at our study sites, Mars' atmospheric pressure could either have been less than 5% of Earth's modern pressure for at least 10,000 years or could have been at higher pressure for a longer duration of time. We combine our new pressure results with other limits on Mars' past atmospheric pressure to model how Mars' atmospheric pressure has evolved over the last 4.4 billion years. Models where Mars' atmospheric pressure starts low fit existing data best.
Key Points
Constraints on continuous and time‐varying atmospheric pressure are derived from preserved, ancient crater populations
Crater chronology functions are used to constrain accumulation time for crater populations at different atmospheric pressures
Existing paleopressure data are most consistent with low atmospheric pressures early in Mars' history
Chloride‐bearing deposits on Mars record high‐elevation lakes during the waning stages of Mars' wet era (mid‐Noachian to late Hesperian). The water source pathways, seasonality, salinity, depth, ...lifetime, and paleoclimatic drivers of these widespread lakes are all unknown. Here we combine reaction‐transport modeling, orbital spectroscopy, and new volume estimates from high‐resolution digital terrain models, in order to constrain the hydrologic boundary conditions for forming the chlorides. Considering a T = 0°C system, we find that (1) individual lakes were >100 m deep and lasted decades or longer; (2) if volcanic degassing was the source of chlorine, then the water‐to‐rock ratio or the total water volume were probably low, consistent with brief excursions above the melting point and/or arid climate; (3) if the chlorine source was igneous chlorapatite, then Cl‐leaching events would require a (cumulative) time of >10 years at the melting point; and (4) Cl masses, divided by catchment area, give column densities 0.1–50 kg Cl/m2, and these column densities bracket the expected chlorapatite‐Cl content for a seasonally warm active layer. Deep groundwater was not required. Taken together, our results are consistent with Mars having a usually cold, horizontally segregated hydrosphere by the time chlorides formed.
Key Points
Halite lake deposits suggest very limited water‐rock interaction on early Mars, different to the extensive alteration evident at Gale Crater
Lakes were >100 m deep and lasted >(101–103) years
Punctuated high rates of volcanism raising temperatures above freezing could have supplied the chlorine for the salt deposits
Crater Mound Formation by Wind Erosion on Mars Steele, L. J.; Kite, E. S.; Michaels, T. I.
Journal of geophysical research. Planets,
January 2018, 2018-01-00, 20180101, Volume:
123, Issue:
1
Journal Article
Peer reviewed
Open access
Most of Mars' ancient sedimentary rocks by volume are in wind‐eroded sedimentary mounds within impact craters and canyons, but the connections between mound form and wind erosion are unclear. We ...perform mesoscale simulations of different crater and mound morphologies to understand the formation of sedimentary mounds. As crater depth increases, slope winds produce increased erosion near the base of the crater wall, forming mounds. Peak erosion rates occur when the crater depth is ∼2 km. Mound evolution depends on the size of the host crater. In smaller craters mounds preferentially erode at the top, becoming more squat, while in larger craters mounds become steeper sided. This agrees with observations where smaller craters tend to have proportionally shorter mounds and larger craters have mounds encircled by moats. If a large‐scale sedimentary layer blankets a crater, then as the layer recedes across the crater it will erode more toward the edges of the crater, resulting in a crescent‐shaped moat. When a 160 km diameter mound‐hosting crater is subject to a prevailing wind, the surface wind stress is stronger on the leeward side than on the windward side. This results in the center of the mound appearing to “march upwind” over time and forming a “bat‐wing” shape, as is observed for Mount Sharp in Gale crater.
Key Points
Slope wind erosion makes sedimentary mounds shorter in small craters and thinner in larger craters, in agreement with observations
Crescent‐shaped moats are formed by slope winds in large craters covered with a kilometer thick sedimentary layer
Results provide an explanation for Mount Sharp's bat‐wing shape and its offset from the crater center
Understanding when, where, and how frequently liquid water was stable on Mars since the Late Noachian/Early Hesperian (3.2-3.9 Ga) is important for understanding the evolution of Mars' climate and ...hydrology. Some relatively young features on Mars require multiple wetting events to form, whereas others are consistent with single wetting events. Small and rare exit breach craters or “pollywogs” are craters between 0.5 and 15 km in diameter with valleys leading away from the lowest point on their rims but no visible inlet valleys. These craters must have been filled with water to the point of overspill to form the observed valleys. The two possible water sources are precipitation and groundwater. In this paper we use measurements from Digital Elevation Models (DEMs) of 18 pollywog craters (21 outlet valleys) and a fixed channel width 0-D breach erosion model to determine whether pollywog exit breach valleys are consistent with a single crater overspill event, or if their formation requires multiple overspill events. Our model, which we compare to a selection of dam breaching events on Earth, predicts runaway erosion for two pollywog exit breaches. No runaway erosion is observed. We discuss potential explanations for this mismatch between the data and our model. We show that the majority of pollywog craters on Mars are consistent with formation during a single crater overspill event, incorporating a work around for the long-standing problem of unknown grainsize into our approach. Three pollywog craters require either multiple events or sustained water supply to drive erosion. We discuss potential source mechanisms for crater-filling water and conclude that pollywogs either formed in a single erosion event, driven by groundwater discharge, or through many small erosion events, driven by draining of small meltwater lakes formed on crater-filling bodies of ice.
•Small exit breach craters record wet events on Hesperian/Amazonian Mars.•We use a fixed channel width 0-D breach erosion model to predict valley incision in one overspill event.•Pollywogs either formed during a single, groundwater-driven crater overspill event, or smaller repeated climate-driven draining events.
Essential oil compositions of fresh and freeze-dried leaves were determined for 16 accessions of
Ocimum basilicum belonging to different varieties to see whether they could be used as infraspecific ...taxonomic characters. One accession of
O. x citriodorum was also studied. Some 30 monoterpenoids, sesquiterpenoids and phenylpropanoids were identified, the major components (more than 20% of the total essential oil composition in one or more accessions) being geranial and neral in
O. x citriodorum, and linalool, methyl chavicol, eugenol, methyl eugenol and geraniol in
O. basilicum. Based on a combination of the latter compounds, five major essential oil profiles could be distinguished in the accessions studied for
O. basilicum. These profiles were largely the same for fresh and freeze-dried material of the same plant, although in dried leaves, methyl chavicol and eugenol concentrations had generally declined in comparison to those of linalool. There appeared to be little correlation between essential oil patterns and varietal classification within
O. basilicum. In view of the chemical heterogeneity of
O. basilicum and its use as an essential oil-producing crop, culinary herb, medicinal plant and insect-controlling agent, in all of which chemicals play an important role, the infraspecific classification of this taxon should take chemical characters into consideration. A system for the classification of essential oil chemotypes in
O. basilicum is proposed.
Severe acute pancreatitis is associated with an early increase in intestinal permeability and endotoxemia. Endotoxin is a potent stimulator for the production and release of procalcitonin and its ...components (calcitonin precursors; CTpr). The aim of this study is to evaluate the role of plasma CTpr as an early marker for gut barrier dysfunction in patients with acute pancreatitis.
Intestinal permeability to macromolecules (polyethylene glycol 3350), serum endotoxin and antiendotoxin core antibodies, plasma CTpr, and serum C-reactive protein (CRP) were measured on admission in 60 patients with acute pancreatitis. Attacks were classified as mild (n = 48) or severe (n = 12) according to the Atlanta criteria.
Compared with mild attacks of acute pancreatitis, severe attacks were significantly associated with an increase in intestinal permeability index (median: 0.02 vs. 0.006, P < 0.001), the frequency of endotoxemia (73% vs. 41%, P = 0.04), and the extent of depletion of serum IgM antiendotoxin antibodies (median: 43 MMU vs. 100 MMU, P = 0.004). Plasma CTpr levels were significantly elevated in patients with severe attacks compared with mild attacks on both the day of admission and on day 3 (median: 64 vs. 22 fmol/mL, P = 0.03; and 90 vs. 29 fmol/mL, P = 0.003 respectively). A positive and significant correlation was observed between the admission serum endotoxin and plasma CTpr levels on admission (r = 0.7, P < 0.0001) and on day 3 (r = 0.96, P < 0.0001), and between plasma CTpr on day 7 and the intestinal permeability index (r = 0.85, P = 0.0001). In contrast, only a weak positive correlation was observed between peak serum levels of CRP and plasma CTpr on admission (r = 0.3, P = 0.017) and on day 7 (r = 0.471, P = 0.049), as well as between CRP and each of the admission serum endotoxin (r = 0.3, P = 0.03) and the intestinal permeability index (r = 0.375, P = 0.007).
In patients with acute pancreatitis, plasma concentrations of CTpr appear to reflect more closely the derangement in gut barrier function rather than the extent of systemic inflammation.
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