Detection of Silica-Rich Deposits on Mars Squyres, S.W; Arvidson, R.E; Ruff, S ...
Science (American Association for the Advancement of Science),
05/2008, Volume:
320, Issue:
5879
Journal Article
Peer reviewed
Mineral deposits on the martian surface can elucidate ancient environmental conditions on the planet. Opaline silica deposits (as much as 91 weight percent SiO₂) have been found in association with ...volcanic materials by the Mars rover Spirit. The deposits are present both as light-toned soils and as bedrock. We interpret these materials to have formed under hydrothermal conditions and therefore to be strong indicators of a former aqueous environment. This discovery is important for understanding the past habitability of Mars because hydrothermal environments on Earth support thriving microbial ecosystems.
Asteroid crater retention ages have unknown accuracy because projectile-crater scaling laws are difficult to verify. At the same time, our knowledge of asteroid and crater size-frequency ...distributions has increased substantially over the past few decades. These advances make it possible to empirically derive asteroid crater scaling laws by fitting model asteroid size distributions to crater size distributions from asteroids observed by spacecraft. For D > 10 km diameter asteroids like Ceres, Vesta, Lutetia, Mathilde, Ida, Eros, and Gaspra, the best matches occur when the ratio of crater to projectile sizes is f ∼ 10. The same scaling law applied to 0.3 < D < 2.5 km near-Earth asteroids such as Bennu, Ryugu, Itokawa, and Toutatis yield intriguing yet perplexing results. When applied to the largest craters on these asteroids, we obtain crater retention ages of ∼1 billion years for Bennu, Ryugu, and Itokawa and ∼2.5 billion years for Toutatis. These ages agree with the estimated formation ages of their source families and could suggest that the near-Earth asteroid population is dominated by bodies that avoided disruption during their traverse across the main asteroid belt. An alternative interpretation is that f > 10, which would make their crater retention ages much younger. If true, crater scaling laws need to change in a substantial way between D > 10 km asteroids, where f ∼ 10, and 0.3 < D < 2.5 km asteroids, where f > 10.
In May of 2011, NASA selected the
O
rigins,
S
pectral
I
nterpretation,
R
esource
I
dentification, and
S
ecurity–
R
egolith
Ex
plorer (OSIRIS-REx) asteroid sample return mission as the third mission ...in the New Frontiers program. The other two New Frontiers missions are
New Horizons
, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and
Juno
, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.
Context.
Asteroid (101955) Bennu is the target of NASA’s Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission. The spacecraft’s instruments ...have characterized Bennu at global and local scales to select a sampling site and provide context for the sample that will be returned to Earth. These observations include thermal infrared spectral characterization by the OSIRIS-REx Thermal Emission Spectrometer (OTES).
Aims.
To understand the degree of compositional and particle size variation on Bennu, and thereby predict the nature of the returned sample, we studied OTES spectra, which are diagnostic of these properties.
Methods.
We created and mapped spectral indices and compared them with the distribution of geomorphic features. Comparison to laboratory spectra of aqueously altered carbonaceous chondrites constrains the amount of compositional variability that is observable.
Results.
The OTES spectra exhibit two end-member shapes (or types), and compositional variability appears limited at the spatial resolution of the observations. The global distribution of these spectral types corresponds with the locations of regions composed of (i) large, dark, relatively rough boulders and (ii) relatively smooth regions lacking large boulders.
Conclusions.
The two spectral types appear to be diagnostic primarily of particle size variations, with contributions from other properties. The spectra resemble experimental data of solid substrates with very thin accumulations (a few to tens of microns) of fine particles (<~65–100 μm). The dustier surfaces commonly correspond with rougher rocks that may produce and/or act as traps for the particles. Anhydrous silicates are limited in abundance, and the bulk mineralogy is consistent with the most aqueously altered carbonaceous chondrites. We expect the returned samples to include these physical and mineralogical characteristics.
We investigated the origin of unusual pitted terrain on asteroid Vesta, revealed in images from the Dawn spacecraft. Pitted terrain is characterized by irregular rimless depressions found in and ...around several impact craters, with a distinct morphology not observed on other airless bodies. Similar terrain is associated with numerous marþian craters, where pits are thought to form through degassing of volatile-bearing material heated by the impact. Pitted terrain on Vesta may have formed in a similar manner, which indicates that portions of the surface contain a relatively large volatile component. Exogenic materials, such as water-rich carbonaceous chondrites, may be the source of volatiles, suggesting that impactor materials are preserved locally in relatively high abundance on Vesta and that impactor composition has played an important role in shaping the asteroid's geology.
Dawn at Vesta: Testing the Protoplanetary Paradigm Russell, C. T.; Raymond, C. A.; Coradini, A. ...
Science (American Association for the Advancement of Science),
05/2012, Volume:
336, Issue:
6082
Journal Article
Peer reviewed
The Dawn spacecraft targeted 4 Vesta, believed to be a remnant intact protoplanet from the earliest epoch of solar system formation, based on analyses of howardite-eucrite-diogenite (HED) meteorites ...that indicate a differentiated parent body. Dawn observations reveal a giant basin at Vesta's south pole, whose excavation was sufficient to produce Vesta-family asteroids (Vestoids) and HED meteorites. The spatially resolved mineralogy of the surface reflects the composition of the HED meteorites, confirming the formation of Vesta's crust by melting of a chondritic parent body. Vesta's mass, volume, and gravitational field are consistent with a core having an average radius of 107 to 113 kilometers, indicating sufficient internal melting to segregate iron. Dawn's results confirm predictions that Vesta differentiated and support its identification as the parent body of the HEDs.
The exploration of near‐Earth asteroids has revealed dynamic surfaces characterized by mobile, unconsolidated material that responds to local geophysical gradients, resulting in distinct morphologies ...and boulder distributions. The OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer) mission confirmed that asteroid (101955) Bennu is a rubble pile with an unconsolidated surface dominated by boulders. In this work, we documented morphologies indicative of mass movement on Bennu and assessed the relationship to slope and other geologic features on the surface. We found globally distributed morphologic evidence of mass movement on Bennu up to ~70° latitude and on spatial scales ranging from individual boulders (meter scale) to a single debris flow ~100 m long and several meters thick. The apparent direction of mass movement is consistent with the local downslope direction and dominantly moves from the midlatitudes toward the equator. Mass movement appears to have altered the surface expression of large (≥30m diameter) boulders, excavating them in the midlatitudes and burying them in the equatorial region. Up to a 10 ± 1 m depth of material may have been transported away from the midlatitudes, which would have deposited a layer ~5 ± 1 m thick in the equatorial region assuming a stagnated flow model. This mass movement could explain the observed paucity of small (<50‐m diameter) craters and may have contributed material to Bennu's equatorial ridge. Models of changes in slope suggest that the midlatitude mass movement occurred in the past several hundred thousand years in regions that became steeper by several degrees.
Plain Language Summary
Mass movement is the flow of loose material such as rock fragments across the surface of a planetary body (for instance, a landslide). We searched images of the surface of asteroid (101955) Bennu for evidence of mass movement. We found that rocks of various sizes have moved downslope, and evidence of this movement is apparent at most locations on the asteroid. By measuring the distribution of, and surface elevation around, the largest boulders on the surface of Bennu, we also found that the downslope movement of material appears to have excavated large boulders from the subsurface in the midlatitudes and buried large boulders near the equator. Our observation that material on Bennu has moved in what is currently the local downslope direction is not necessarily expected, because the downslope direction can change based on how quickly the asteroid is rotating, which varies over time. Thus, we can infer that this movement happened in the geologically recent past—probably within the past several hundred thousand years. These results can help us understand how geologic features like craters are erased, how the equatorial ridge formed, and how Bennu (and potentially other asteroids) change shape over time.
Key Points
Signatures of mass movement on Bennu are globally distributed at multiple spatial scales
Mass movement may have removed a ~10‐m‐thick layer of material from the midlatitudes and deposited a ~5‐m‐thick layer near the equator
Mass movement that left visible evidence on Bennu occurred within the past several hundred thousand years
Abstract DNA damage can cause (and result from) oxidative stress and mitochondrial impairment, both of which are implicated in the pathogenesis of Parkinson's disease (PD). We therefore examined the ...role of mitochondrial DNA (mtDNA) damage in human postmortem brain tissue and in in vivo and in vitro models of PD, using a newly adapted histochemical assay for abasic sites and a quantitative polymerase chain reaction (QPCR)-based assay. We identified the molecular identity of mtDNA damage to be apurinic/apyrimidinic (abasic) sites in substantia nigra dopamine neurons, but not in cortical neurons from postmortem PD specimens. To model the systemic mitochondrial impairment of PD, rats were exposed to the pesticide rotenone. After rotenone treatment that does not cause neurodegeneration, abasic sites were visualized in nigral neurons, but not in cortex. Using a QPCR-based assay, a single rotenone dose induced mtDNA damage in midbrain neurons, but not in cortical neurons; similar results were obtained in vitro in cultured neurons. Importantly, these results indicate that mtDNA damage is detectable prior to any signs of degeneration — and is produced selectively in midbrain neurons under conditions of mitochondrial impairment. The selective vulnerability of midbrain neurons to mtDNA damage was not due to differential effects of rotenone on complex I since rotenone suppressed respiration equally in midbrain and cortical neurons. However, in response to complex I inhibition, midbrain neurons produced more mitochondrial H2 O2 than cortical neurons. We report selective mtDNA damage as a molecular marker of vulnerable nigral neurons in PD and suggest that this may result from intrinsic differences in how these neurons respond to complex I defects. Further, the persistence of abasic sites suggests an ineffective base excision repair response in PD.
Simple mass‐balance and thermodynamic constraints are used to illustrate the potential geochemical and geophysical diversity of a fully differentiated Vesta‐sized parent body with a eucrite crust ...(e.g., core size and density, crustal thickness). The results of this analysis are then combined with data from the howardite–eucrite–diogenite (HED) meteorites and the Dawn mission to constrain Vesta's bulk composition. Twelve chondritic compositions are considered, comprising seven carbonaceous, three ordinary, and two enstatite chondrite groups. Our analysis excludes CI and LL compositions as plausible Vesta analogs, as these are predicted to have a negative metal fraction. Second, the MELTS thermodynamic calculator is used to show that the enstatite chondrites, the CV, CK and L‐groups cannot produce Juvinas‐like liquids, and that even for the other groups, depletion in sodium is necessary to produce liquids of appropriate silica content. This conclusion is consistent with the documented volatile‐poor nature of eucrites. Furthermore, carbonaceous chondrites are predicted to have a mantle too rich in olivine to produce typical howardites and to have Fe/Mn ratios generally well in excess of those of the HEDs. On the other hand, an Na‐depleted H‐chondrite bulk composition is capable of producing Juvinas‐like liquids, has a mantle rich enough in pyroxene to produce abundant howardite/diogenite, and has a Fe/Mn ratio compatible with eucrites. In addition, its predicted bulk‐silicate density is within 100 kg m−3 of solutions constrained by data of the Dawn mission. However, oxidation state and oxygen isotopes are not perfectly reproduced and it is deduced that bulk Vesta may contain approximately 25% of a CM‐like component. Values for the bulk‐silicate composition of Vesta and a preliminary phase diagram are proposed.
Abstract
Mesosiderites are an amalgamation of crustal silicates and molten metal, and their formational history is not well understood. It is widely believed that redox reactions occurred in the ...mesosiderites during metal–silicate mixing. Previous studies evaluated redox reactions by studying the silicates within mesosiderites, but little attention has been given to the metal for complementary evidence of such processes. Here, the evidence for redox within the metal portion of five mesosiderites is documented, most notably lower P content in the matrix metal relative to clast metal (nodule). These observations, together with the noted FeO reduction in silicates, provide further support for redox reactions occurring during metal–silicate mixing. Samples with differing Ir concentrations, such as Chaunskij and RKP A70015, have been previously classified as anomalous. However, the marked variation in highly siderophile element concentrations in all of these mesosiderites is consistent with fractional crystallization. These compositional trends could be explained by isolated metallic masses that underwent fractional crystallization before mixing or by hit‐and‐run collisions that produced metallic masses that ranged in size.
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