Evidence for widespread hydrated minerals on asteroid (101955) Bennu

Early spectral data from the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission reveal evidence for abundant hydrated minerals on the surface of near-Earth asteroid (101955) Bennu in the form of a near-infrared absorption near 2.7 µm and thermal infrared spectral features that are most similar to those of aqueously altered CM-type carbonaceous chondrites. We observe these spectral features across the surface of Bennu, and there is no evidence of substantial rotational variability at the spatial scales of tens to hundreds of metres observed to date. In the visible and near-infrared (0.4 to 2.4 µm) Bennu’s spectrum appears featureless and with a blue (negative) slope, confirming previous ground-based observations. Bennu may represent a class of objects that could have brought volatiles and organic chemistry to Earth

The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements

The top-shaped morphology characteristic of asteroid (101955) Bennu, often found among fast-spinning asteroids and binary asteroid primaries, may have contributed substantially to binary asteroid formation. Yet a detailed geophysical analysis of this morphology for a fast-spinning asteroid has not been possible prior to the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission. Combining the measured Bennu mass and shape obtained during the Preliminary Survey phase of the OSIRIS-REx mission, we find a notable transition in Bennu’s surface slopes within its rotational Roche lobe, defined as the region where material is energetically trapped to the surface. As the intersection of the rotational Roche lobe with Bennu’s surface has been most recently migrating towards its equator (given Bennu’s increasing spin rate), we infer that Bennu’s surface slopes have been changing across its surface within the last million years. We also find evidence for substantial density heterogeneity within this body, suggesting that its interior is a mixture of voids and boulders. The presence of such heterogeneity and Bennu’s top shape are consistent with spin-induced failure at some point in its past, although the manner of its failure cannot yet be determined. Future measurements by the OSIRIS-REx spacecraft will provide insight into and may resolve questions regarding the formation and evolution of Bennu’s top-shape morphology and its link to the formation of binary asteroids

Flexural isostasy: Constraints from gravity and topography power spectra

We have used spherical harmonic coefficients that describe Earth's gravity anomaly and topography fields to quantify the role of isostasy in contributing to crustal and upper mantle structure. Power spectra reveal that the gravity effect of topography and its flexural compensation contributes significantly to the observed free-air gravity anomaly spectra for spherical harmonic degree 33 < n < 400, which corresponds to wavelength 100 < λ < 1200 km. The best fit is for an elastic plate (flexure) model with an elastic thickness, Te, of 34.0 ± 4.0 km. Smaller values underpredict the observed gravity spectra while higher values overpredict. The best fit Te is a global average and so there will be regions where Te is lower and higher. This is confirmed in studies of selected regions such as the Hawaiian-Emperor seamount chain and the Himalaya fold and thrust belt where we show that flexural isostatic anomalies are near zero in regions where Te ~34.0 km and of large amplitude in regions of lower and higher Te. Plate flexure may also contribute at higher (n > 400) and lower (n < 33) degrees, but topography appears either uncompensated or fully compensated at these degrees, irrespective of the actual Te. All isostatic models underpredict the spectra at 2 < n < 12 and so we interpret the low order Earth's gravity field as caused, at least in part, by non-isostatic processes due to dynamic motions such as those associated with convective upwellings and downwellings in Earth's mantle

Flexural isostasy: Constraints from gravity and topography power spectra

We have used spherical harmonic coefficients that describe Earth's gravity anomaly and topography fields to quantify the role of isostasy in contributing to crustal and upper mantle structure. Power spectra reveal that the gravity effect of topography and its flexural compensation contributes significantly to the observed free-air gravity anomaly spectra for spherical harmonic degree 33 400) and lower (n < 33) degrees, but topography appears either uncompensated or fully compensated at these degrees, irrespective of the actual Te. All isostatic models underpredict the spectra at 2 < n < 12 and so we interpret the low order Earth's gravity field as caused, at least in part, by non-isostatic processes due to dynamic motions such as those associated with convective upwellings and downwellings in Earth's mantle

Signaling or Not-Signaling: Variation in Vulnerability and Defense Tactics of Armored Ground Crickets (Acanthoplus Speiseri: Orthoptera, Tettigoniidae, Hetrodinae)

Male Orthoptera singing from exposed perches are at risk from acoustically- and visually-hunting predators. The defensive reactions of armored ground crickets (Acanthoplus speiseri) include falling silent, dropping from their perch, alarm stridulation and autohaemorrhaging. Male and female ground crickets show different reactivity (i.e. the number or intensity of defense tactics used) to predation, depending on level of exposure: calling males were more reactive when approached during daylight, compared with in the dark. During daylight, calling males were more reactive than silent, cryptic, males and females. The level of response presumably reflected the riskiness of the individual’s behavior and situation at that time. Plasticity of response to predation allows individuals to balance risky behavior (i.e. acoustic signaling from exposed perches) by being more reactive to potential threats