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A A Simon

Publications and source records attributed to A A Simon.

13 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2022

Cross‐Instrument Comparison of MapCam and OVIRS on OSIRIS‐REx

Two of the instruments onboard the OSIRIS-REx spacecraft, the MapCam color imager and the OVIRS visible and 20 infrared spectrometer, observed the surface of asteroid (101955) Bennu in partially overlapping wavelengths. 21 Significant scientific advances have been enabled by using data from these two instruments in tandem, but a robust 22 statistical understanding of their relationship is needed for future analyses to cross-compare their data as accurately 23 and sensitively as possible. Here we present a cross-instrument comparison of data acquired by MapCam and 24 OVIRS, including methods and results for all global and site-specific observation campaigns in which both 25 instruments were active. In our analysis, we consider both the absolute radiometric offset and the relative 26 (normalized) variation between the two instruments; we find that both depend strongly on the photometric and 27 instrumental conditions during the observation. The two instruments have a large absolute offset (>15%) due to their 28 independent radiometric calibrations. However, they are very consistent (relative offset as low as 1%) when each 29 instrument’s response is normalized at a single wavelength, particularly at low phase angles where shadows on 30 Bennu’s rough surface are minimized. We recommend using the global datasets acquired at 12:30 pm local solar 31 time for cross-comparisons; data acquired at higher phase angles have larger uncertainties.

D R Golish↗
NASA NTRS2020

Weak Spectral Features on (101995) Bennu from the OSIRIS-REx Visible and InfraRed Spectrometer

Context. The NASA Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission has obtained thousands of spectra of asteroid (101955) Bennu with the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS). Aims. We present a spectral search for minor absorption bands and determine compositional variations on the surface of Bennu. Methods. Reflectance spectra with low and high spatial resolutions were analyzed for evidence of weak absorption bands. Spectra were also divided by a global average spectrum to isolate unique spectral features, and variations in the strongest band depths were mapped on a surface shape model. The global visible to near-IR spectrum of Bennu shows evidence of several weak absorption bands with depths of a few percent. Results. Several observed bands are consistent with phyllosilicates, and their distribution correlates with the stronger 2.74-μm hydration band. A 0.55-μm band is consistent with iron and is deepest in the spectrally reddest areas on Bennu. The presence of hydrated phyllosilicates and iron oxides indicates substantial aqueous alteration in Bennu’s past. Conclusions. Bennu’s spectra are not identical to a limited set of carbonaceous chondrite spectra, possibly due to compositional properties and spatial scale differences; however, returned samples should contain a mixture of common chondrite materials.

OSIRIS-REx↗
NASA NTRS2020

Asteroid (101955) Bennu’s Weak Boulders and Thermally Anomalous Equator

Thermal inertia and surface roughness are proxies for the physical characteristics of planetary surfaces. Global maps of these two propertiesdistinguish the boulder population on near-Earth asteroid (NEA) (101955)Bennuinto two typesthat differ in strength,andboth havelower thermal inertiathan expectedfor boulders and meteorites. Neither has strongly temperature-dependent thermal properties. The weakerbouldertypeprobably would not survive atmospheric entry and thus may not berepresented in the meteorite collection. The maps also show ahigh-thermal-inertia band at Bennu’s equator, which might be explained by processes such as compaction or strength sorting during mass movement, but these explanations are not wholly consistent with other data. Our findings imply that other C-complex NEAs likely have boulderssimilar to those on Bennu,rather than finer-particulate regoliths.A tentative correlation between albedo and thermal inertia of C-complex NEAs may be due to relative abundances of boulder types.

B Rozitis↗
NASA NTRS2020

Exogenic Basalt on Asteroid (101955) Bennu

When rubble-pile asteroid 2008 TC3 impacted Earth on October 7, 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material. However, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption. Here we report the presence of meter-scale exogenic boulders on the surface of near-Earth asteroid (101955) Bennu - the 0.5-km, rubble-pile target of the OSIRIS-REx mission which has been spectroscopically linked to the CM carbonaceous chondrite meteorites. Hyperspectral data indicate that the exogenic boulders have the same distinctive pyroxene composition as the howardite-eucrite-diogenite (HED) meteorites that come from (4) Vesta, a 525-km diameter asteroid that has undergone differentiation and extensive igneous processing. Delivery scenarios include the infall of Vesta fragments directly onto Bennu or indirectly onto Bennu’s parent body, where the latter’s disruption created Bennu from a mixture of endogenous and exogenic debris. Our findings demonstrate that rubble-pile asteroids can preserve evidence of inter-asteroid mixing that took place at macroscopic scales well after planetesimal formation ended. Accordingly, the presence of HED-like material on the surface of Bennu provides previously unrecognized constraints on the collisional and dynamical evolution of the inner main belt.

Asteroids↗
NASA NTRS2019

A Complex Storm System in Saturn's North Polar Atmosphere in 2018

Saturn’s convective storms usually fall in two categories. One consists of mid-sized storms ~ 2,000 km wide, appearing as irregular bright cloud systems that evolve rapidly, on scales of a few days. The other includes the exceptional Great White Spots (GWS), planetary-scale giant storms that disturb a full latitude band, and which have been observed only seven times. Here we report a new intermediate type, observed in 2018 in the North Polar Region. Four large storms (the first one lasting longer than 200 days) formed sequentially in close latitudes, experiencing mutual encounters, and leading to zonal disturbances affecting a full latitude band ~ 8,000 km wide, during at least 8 months. Dynamical simulations indicate that each storm required energies ~ 100 times smaller than those necessary for a GWS. This event occurred at about the same latitude and season as the GWS in 1960, in close correspondence with the cycle of approximately 60 years hypothesized for equatorial GWSs.

A Sánchez-Lavega↗
NASA NTRS2019

Saturn Atmospheric Dynamics One Year After Cassini: Long-Lived Features and Time Variations in the Drift of the Hexagon

We examine Saturn's atmospheric dynamics with observations in the visible range from ground-based telescopes and Hubble Space Telescope (HST). We present a detailed analysis of observations acquired during 2018 obtaining drift rates of major meteorological systems from the equator to the North polar hexagon. A system of polar storms that appeared in the planet in March 2018 and remained active with a complex phenomenology at least until September is analyzed elsewhere [Sanchez-Lavega et al., 2019]. Many of the regular cloud features visible in 2018 are long-lived and can be identified in Saturn images in 2017, and in some cases, for up to a decade using also Cassini ISS images. Without considering the polar storms, the most interesting long-lived cloud systems are: i) A bright white spot in the Equatorial Zone that can be tracked continuously since 2014 with minimal changes in its zonal velocity, which was 444:3 +or- 3:1 ms(exp -1) in 2014 and 452:4 + or - 1:7 ms(exp -1) in 2018. This velocity is remarkably different from the zonal winds at the cloud level at its latitude during the Cassini mission, and is closer to zonal winds obtained at the time of the Voyagers flybys and to zonal winds from Cassini VIMS infrared images of the lower atmosphere. ii) A large long-lived Anticyclone Vortex, here AV, that formed after the Great White Spot of 2010-2011. This vortex has changed significantly in visual contrast, drift rate and latitude with minor changes in size over the last years. iii) A system of subpolar vortices at latitudes 60-65N present at least since 2011. These vortices and additional atmospheric features here studied follow drift rates consistent with zonal winds obtained by Cassini. We also present a study of the positions of the vertices of Saturn's North polar hexagon from 2015 to 2018. These measurements are compared with previous analyses during the Cassini mission (2007-2014), observations with HST in the 90s, and data from the Voyagers in 1980-1981 to explore the long term variability of the hexagon's drift rate. We find variations in the drift rate of the hexagon through these epochs that can not be fit by seasonal changes in the polar area. Instead, the different drift rates reinforce the role of the North Polar Spot that was present in the Voyager epoch and in the early 90s to cause a faster drift rate of the hexagon at that time compared with the current slower one.

R Hueso↗
NASA NTRS2019

The operational environment and rotational acceleration of asteroid (101955) Bennu from OSIRIS-REx observations

During its approach to asteroid (101955) Bennu, NASA's Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft surveyed Bennu's immediate environment, photometric properties, and rotation state. Discovery of a dusty environment, a natural satellite, or unexpected asteroid characteristics would have had consequences for the mission's safety and observation strategy. Here we show that spacecraft observations during this period were highly sensitive to satellites (sub-meter scale) but reveal none, although later navigational images indicate that further investigation is needed. We constrain average dust production in September 2018 from Bennu's surface to an upper limit of 150g s −1 averaged over 34 min. Bennu's disk-integrated photometric phase function validates measurements from the pre-encounter astronomical campaign. We demonstrate that Bennu's rotation rate is accelerating continuously at 3.63+- 0.52 x10 −6 degrees A day -2 , likely due to the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, with evolutionary implications.

Carl William Hergenrother↗
NASA NTRS2015

Stratospheric Benzene and Hydrocarbon Aerosols Detected in Saturn's Auroral Regions

Context. Saturn's polar upper atmosphere exhibits significant auroral activity; however, its impact on stratospheric chemistry (i.e. the production of benzene and heavier hydrocarbons) and thermal structure remains poorly documented. Aims. We aim to bring new constraints on the benzene distribution in Saturn's stratosphere, to characterize polar aerosols (their vertical distribution, composition, thermal infrared optical properties), and to quantify the aerosols' radiative impact on the thermal structure. Methods. Infrared spectra acquired by the Composite Infrared Spectrometer (CIRS) on board Cassini in limb viewing geometry are analysed to derive benzene column abundances and aerosol opacity profiles over the 3 to 0.1 mbar pressure range. The spectral dependency of the haze opacity is assessed in the ranges 680-900 and 1360-1440 cm(exp −1). Then, a radiative climate model is used to compute equilibrium temperature profiles, with and without haze, given the haze properties derived from CIRS measurements. Results. On Saturn's auroral region (80 deg S), benzene is found to be slightly enhanced compared to its equatorial and mid-latitude values. This contrasts with the Moses & Greathouse (2005, J. Geophys. Res., 110, 9007) photochemical model, which predicts a benzene abundance 50 times lower at 80◦S than at the equator. This advocates for the inclusion of ion-related reactions in Saturn's chemical models. The polar stratosphere is also enriched in aerosols, with spectral signatures consistent with vibration modes assigned to aromatic and aliphatic hydrocarbons, and presenting similarities with the signatures observed in Titan's stratosphere. The aerosol mass loading at 80 deg S is estimated to be 1−4 × 10(exp −5) g cm(exp −2), an order of magnitude less than on Jupiter, which is consistent with the order of magnitude weaker auroral power at Saturn.We estimate that this polar haze warms the middle stratosphere by 6 K in summer and cools the upper stratosphere by 5 K in winter. Hence, aerosols linked with auroral activity can partly account for the warm polar hood observed in Saturn's summer stratosphere.

satellites↗
NASA NTRSDate not supplied

Spectral Properties of Dust on Asteroid (101955) Bennu with the OSIRIS-REx Visible and Infrared Spectrometer (OVIRS)

The Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft successfully collected a sample of regolith from asteroid (101955) Bennu on October 20, 2020[1]. The Touch-and-Go (TAG) spacecraft maneuver used to collect the sample mobilized material, including very fine particles (dust),from up to ~1m in depth and ~10m in diameter from the point of contact with the asteroid [1,2].TAG created a dust plume that intercepted the spacecraft and accumulated on the instruments. As a result, all instruments show degradation in optical throughput, with the degree of degradation depending on their aperture size, orientation, and position on the spacecraft [1]. In this study, we analyzed OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) from before and after TAG to assess the dust on the instrument with implications for Bennu’s dust composition and scattering properties, and future visible–near-infrared (VNIR)spectral observations with the OSIRIS-REx spacecraft. OVIRS is a point spectrometer that measures reflected light at VNIR wavelengths from 0.4 to 4.3 μm with a 4 mrad circular field of view(FOV) [3]. Previous observations with OVIRS revealed widespread hydrated minerals and carbon-bearing materials on Bennu [4,5]. A ~3 μm absorption feature is observed globally in OVIRS spectra, with a band position and depth that is consistent with Mg/Fe phyllosilicates and suggests a similar degree of aqueous alteration to that experienced by some carbonaceous chondrite meteorites[4,6]. A series of weak (<5%) VNIR spectral features, potentially associated with hydrated phyllosilicates and iron oxides, are also found in OVIRS spectra (Fig. 1), though not typically observed in meteorite spectra [7]. OSIRIS-REx Thermal Emission Spectrometer (OTES) spectra are consistent with a very thin accumulation (a few to ~ten microns) of fine particles (<~65–100 microns in size)across the surface of Bennu, and particularly on the roughest boulders, an interpretation that is supported by the dust plume seen during TAG [1,8]. An analysis of TAG contamination on the OTES optics revealed a 15% decrease in throughput, and OTES observations of space with and without that contamination allow a recreation of the thermal infrared spectrum [1]. The spectrum of the contamination lacks volume scattering features and exhibits a stronger Mg-OH absorption (~16.5 μm) than average Bennu, suggesting a Mg-rich phyllosilicate composition [1,6]. Here we present the first compositional analysis of Bennu dust at VNIR wavelengths by analyzing the contaminant on OVIRS.

H H Kaplan↗
NASA NTRSDate not supplied

OSIRIS-APEX: an OSIRIS-REx Extended Mission to Apophis

Goal 1: Establish the processes that drive the evolution of small asteroids, including tidal effects from close encounters with terrestrial planets. Goal 2: Determine the characteristics of an S-complex object to establish the link with its parent-asteroid family and its dynamical evolution from main belt through multiple Earth encounters. Goal 3: Examine the properties of an S-complex, potentially hazardous asteroid (PHA) as an analog for other PHAs and to defend the planet.

Asteroids↗