Paradigms and Paradoxes: Dawn at Vesta

While confirming the popular paradigm of Vesta as the parent body of the HED meteorites, Dawn measurements have discovered many unexpected aspects of the vestan surface. First, an olivine layer was not found in the bottom of the large basin near the south pole of Vesta. In fact, while patches of olivine have been found in the north, it is rare on the surface. Secondly, while Vesta has little gravity and appears to have completely differentiated, it is not completely dry evidence for transient flows and pits resulting from devolatization have been found, implying a substantial amount of accessible water. Thirdly, transport of material to the surface of Vesta from elsewhere in the asteroid belt appears as dark material buried near the top of the crust to Vesta. This may have arrived in a single large impact and been spread around the surface and buried, later to be re-excavated. However, it is not certain that this is the only scenario possible for the source of this material. In short, Dawn's observations of Vesta have been both reassuring but unsettling at the same time

Characteristics of ion flow in the quiet inner plasma sheet

Abstract We use AMPTE/IRM and ISEE 2 data to study the properties of the high beta (βi \u3e 0.5) plasma sheet, the inner plasma sheet (IPS). Bursty bulk flows (BBFs) are excised from the two databases, and the average flow pattern in the non-BBF (quiet) IPS is constructed. At local midnight this ensemble-average flow is predominantly duskward; closer to the flanks it is mostly earthward. The flow pattern agrees qualitatively with calculations based on the Tsyganenko [1987] model (T87), where the earthward flow is due to the ensemble-average cross tail electric field and the duskward flow is the diamagnetic drift due to an inward pressure gradient. The IPS is on the average in pressure equilibrium with the lobes. Because of its large variance the average flow does not represent the instantaneous flow field. Case studies also show that the non-BBF flow is highly irregular and inherently unsteady, a reason why earthward convection can avoid a pressure balance inconsistency with the lobes. The ensemble distribution of velocities is a fundamental observable of the quiet plasma sheet flow field

Small crater populations on Vesta

The NASA Dawn mission has extensively examined the surface of asteroid Vesta, the second most massive body in the main belt. The high quality of the gathered data provides us with an unique opportunity to determine the surface and internal properties of one of the most important and intriguing main belt asteroids (MBAs). In this paper, we focus on the size frequency distributions (SFDs) of sub-kilometer impact craters observed at high spatial resolution on several selected young terrains on Vesta. These small crater populations offer an excellent opportunity to determine the nature of their asteroidal precursors (namely MBAs) at sizes that are not directly observable from ground-based telescopes (i.e., below ~100 m diameter). Moreover, unlike many other MBA surfaces observed by spacecraft thus far, the young terrains examined had crater spatial densities that were far from empirical saturation. Overall, we find that the cumulative power-law index (slope) of small crater SFDs on Vesta is fairly consistent with predictions derived from current collisional and dynamical models down to a projectile size of ~10 m diameter (Bottke et al., 2005a,b). The shape of the impactor SFD for small projectile sizes does not appear to have changed over the last several billions of years, and an argument can be made that the absolute number of small MBAs has remained roughly constant (within a factor of 2) over the same time period. The apparent steady state nature of the main belt population potentially provides us with a set of intriguing constraints that can be used to glean insights into the physical evolution of individual MBAs as well as the main belt as an ensemble.Comment: Accepted by PSS, to appear on Vesta cratering special issu

Physical science research needed to evaluate the viability and risks of marine cloud brightening

Marine cloud brightening (MCB) is the deliberate injection of aerosol particles into shallow marine clouds to increase their reflection of solar radiation and reduce the amount of energy absorbed by the climate system. From the physical science perspective, the consensus of a broad international group of scientists is that the viability of MCB will ultimately depend on whether observations and models can robustly assess the scale-up of local-to-global brightening in today\u27s climate and identify strategies that will ensure an equitable geographical distribution of the benefits and risks associated with projected regional changes in temperature and precipitation. To address the physical science knowledge gaps required to assess the societal implications of MCB, we propose a substantial and targeted program of research-field and laboratory experiments, monitoring, and numerical modeling across a range of scales

Dawn and the Vesta-HED Connection

Although it is difficult to explain exactly how eucrites and diogenites are related through simple magmatic processes, their shared oxygen isotopic compositions and the common occurrence of clasts of both lithologies in howardite breccias support derivation from a common parent body. For decades, HED meteorites have been linked to asteroid 4 Vesta, based on spectral similarities [1] and the discovery of a dynamical family (Vestoids) that provides a bridge between Vesta and nearby resonance escape hatches [2]. Although recently derived constraints on the rapidity of HED parent body differentiation, based on measurements of Al-26 in diogenites, have been used to argue against the Vesta-HED connection [3], new thermal evolution models [e.g., 4] appear to be heated and melted fast enough to account for this constraint. Data from the Dawn orbiter strengthen the Vesta - HED linkage and provide new insights into petrogenetic interpretations of these meteorites

Olivine or Impact Melt: Nature of the "Orange" Material on Vesta from Dawn

NASA's Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types, a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), b) lobate patches with well-defined edges, and c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed "Leslie feature" first identified by Gaffey (1997) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt

Global Chemical Transport on Hot Jupiters: Insights from 2D VULCAN photochemical model

The atmospheric dynamics of tidally-locked hot Jupiters is dominated by the equatorial winds. Understanding the interaction between global circulation and chemistry is crucial in atmospheric studies and interpreting observations. Two-dimensional (2D) photochemical transport models shed light on how the atmospheric composition depends on circulation. In this paper, we introduce the 2D photochemical transport model, VULCAN 2D, which improves on the pseudo-2D approaches by allowing for non-uniform zonal winds. We extensively validate our VULCAN 2D with analytical solutions and benchmark comparisons. Applications to HD 189733 b and HD 209458 b reveal distinct characteristics in horizontal transport-dominated and vertical mixing-dominated regimes. Motivated by the inferred carbon-rich atmosphere by Giacobbe et al. (2021), we find that HD 209458 b with super-solar carbon-to-oxygen ratio (C/O) exhibits pronounced C2H4 absorption on the morning limb but not on the evening limb, owing to horizontal transport from the nightside. We discuss when a pseudo-2D approach is a valid assumption and its inherent limitations. Finally, we demonstrate the effect of horizontal transport in transmission observations and its impact on the morning-evening limb asymmetry with synthetic spectra, highlighting the need to consider global transport when interpreting exoplanet atmospheres.Comment: 18 pages, 20 figures, submitted to Ap

Regulation of Exogenous and Endogenous Glucose Metabolism by Insulin and Acetoacetate in the Isolated Working Rat Heart A Three Tracer Study of Glycolysis, Glycogen Metabolism, and Glucose Oxidation

Abstract Myocardial glucose use is regulated by competing substrates and hormonal influences. However, the interactions of these effectors on the metabolism of exogenous glucose and glucose derived from endogenous glycogen are not completely understood. In order to determine changes in exogenous glucose uptake, glucose oxidation, and glycogen enrichment, hearts were perfused with glucose (5 mM) either alone, or glucose plus insulin (40 U/ml), glucose plus acetoacetate (5 mM), or glucose plus insulin and acetoacetate, using a three tracer ( 3 H, 14 C, and 13 C) technique. Insulinstimulated glucose uptake and lactate production in the absence of acetoacetate, while acetoacetate inhibited the uptake of glucose and the oxidation of both exogenous glucose and endogenous carbohydrate. Depending on the metabolic conditions, the contribution of glycogen to carbohydrate metabolism varied from 20-60%. The addition of acetoacetate or insulin increased the incorporation of exogenous glucose into glycogen twofold, and the combination of the two had additive effects on the incorporation of glucose into glycogen. In contrast, the glycogen content was similar for the three groups. The increased incorporation of glucose in glycogen without a significant change in the glycogen content in hearts perfused with glucose, acetoacetate, and insulin suggests increased glycogen turnover. We conclude that insulin and acetoacetate regulate the incorporation of glucose into glycogen as well as the relative contributions of exogenous glucose and endogenous carbohydrate to myocardial energy metabolism by different mechanisms. ( J. Clin. Invest. 1997. 100:2892-2899.) Key words: citric acid cycle • NMR • isotopomer analysi