^1,2Juan Luis Rizos,^1,2Julia de León,^1,2Javier Licandro,³Humberto Campins,^1,2,5Marcel Popescu,³Noemí Pinilla-Alonso,⁴Dathon Golish,³Mario de Prá,⁴Dante Lauretta
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.03.007]
¹Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain
²Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
³Physics Department, University of Central Florida, P.O. Box 162385, Orlando, FL 32816-2385, USA
⁴Lunar and Planetary Laboratory, University of Arizona, 1415 N. Sixth Ave., Tucson, AZ 85705-0500, USA
⁵Astronomical Institute of the Romanian Academy, 5 Cuţitul de Argint, 040557 Bucharest, Romania
Copyright Elsevier

The OSIRIS-REx asteroid sample-return mission is investigating primitive near-Earth asteroid (101955) Bennu. Thousands of images will be acquired by the MapCam instrument onboard the spacecraft, an imager with four color filters based on the Eight-Color Asteroid Survey (ECAS): b′ (473 nm), v (550 nm), w (698 nm), and x (847 nm). This set of filters will allow identification and characterization of the absorption band centered at 700 nm and associated with hydrated silicates. In this work, we present and validate a spectral clustering methodology for application to the upcoming MapCam images of the surface of Bennu. Our procedure starts with the projection, calibration, and photometric correction of the images. In a second step, we apply a K-means algorithm and we use the Elbow criterion to identify natural clusters. This methodology allows us to find distinct areas with spectral similarities, which are characterized by parameters such as the spectral slope S′ and the center and depth of the 700-nm absorption band, if present. We validate this methodology using images of (1) Ceres from NASA’s Dawn mission. In particular, we analyze the Occator crater and Ahuna Mons. We identify one spectral cluster–located in the outer parts of the Occator crater interior–showing the 700-nm hydration band centered at 698 ± 7 nm and with a depth of 3.4 ± 1.0%. We interpret this finding in the context of the crater’s near-surface geology.

^1,2Haiyang S.Wang,^1,2,3Charles H.Lineweaver,^2,3Trevor R.Ireland
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.03.018]
¹Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
²Planetary Science Institute, The Australian National University, Canberra, ACT 2611, Australia
³Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia
Copyright Elsevier

We present new estimates of protosolar elemental abundances based on an improved combination of solar photospheric abundances and CI chondritic abundances. These new estimates indicate CI chondrites and solar abundances are consistent for 60 elements. Our estimate of the protosolar “metallicity” (i.e. mass fraction of metals, Z) is 1.40%, which is consistent with a value of Z that has been decreasing steadily over the past three decades from ∼1.9%. We compare our new protosolar abundances with our recent estimates of bulk Earth composition (normalized to aluminium), thereby quantifying the devolatilization in going from the solar nebula to the formation of the Earth. The quantification yields a linear trend log (f) = α log (TC) + β, where f is the Earth-to-Sun abundance ratio and TC is the 50% condensation temperature of elements. The best fit coefficients are: α = 3.676 ± 0.142 and β =  − 11.556 ± 0.436. The quantification of these parameters constrains models of devolatilization processes. For example, the coefficients α and β determine a critical devolatilization temperature for the Earth TD(E) = 1391 ± 15 K. The terrestrial abundances of elements with TC < TD(E) are depleted compared with solar abundances, whereas the terrestrial abundances of elements with TC > TD(E) are indistinguishable from solar abundances. The abundances of noble gases and hydrogen are depleted more than a prediction based on the extrapolation of the best-fit volatility trend. The terrestrial abundance of Hg (TC = 252 K) appears anomalously high under the assumption that solar and CI chondrite Hg abundances are identical. To resolve this anomaly, we propose that CI chondrites have been depleted in Hg relative to the Sun by a factor of 13 ± 7. We use the best-fit volatility trend to derive the fractional distribution of carbon and oxygen between volatile and refractory components (fvol, fref). For carbon we find (0.91 ± 0.08, 0.09 ± 0.08); for oxygen we find (0.80 ± 0.04, 0.20 ± 0.04). Our preliminary estimate gives CI chondrites a critical devolatilization temperature TD(CI) = 550−100+20 K.

¹Shigeru Wakita,¹Hidenori Genda
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.03.008]
¹Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Copyright Elsevier

Hydrous minerals are found on the surfaces of asteroids, but their origin is not clear. If their origin is endogenic, the hydrous minerals that were formed in the inner part of a planetesimal (or parent body) should come out on to the surface without dehydration. If their origin is exogenic, the source of hydrous minerals accreting onto asteroids is needed. Collisions in the asteroid belt would be related to both origins because collisions excavate the surface and eject the materials. However, the fate of hydrous minerals in large planetesimals during the collisional process has not been well investigated. Here, we explore planetesimal collisions by using the iSALE-2D code, and investigate the effect of an impact for the target planetesimal containing hydrous minerals. Our numerical results for the fiducial case (5 km/s of the impact velocity) show that hydrous minerals are slightly heated during the collisions. This moderate heating indicates that they can avoid the dehydration reaction and keep their original composition. Some hydrous minerals have larger velocity than the escape velocity of the collision system. This means that hydrous minerals can escape from the planetesimal and support the theory of exogenic origin for the hydrous minerals on asteroids. Meanwhile, the velocity of other hydrous minerals is smaller than the escape velocity of the system. This also indicates the possibility of an endogenic origin for the hydrous minerals on asteroids. Our results suggest that hydrous minerals on asteroids can be provided by planetesimal collisions.

J. ORM€O1, P. MINDE², A. T. NIELSEN³, and C. ALWMARK⁴
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13275]
¹Centro de Astrobiologıa (INTA-CSIC), ES-28850 Torrejon de Ardoz, Spain
²Bj€orkv€agen 28, SE-98336, Malmberget
³Department of Geosciences and Natural Resource Management, University of Copenhagen, DK-1350 Copenhagen, Denmark
⁴Department of Geology, Lund University, SE-22362 Lund, Sweden
Published by arrangement with John Wiley & Sons

The lower Cambrian Vakkejokk Breccia is a proximal ejecta layer from a shallow marine impact. It is exposed for ~7 km along a steep mountainside in Lapland, northernmost Sweden. In its central parts, the layer is up to ~27 m thick. Here the breccia shows a vertical differentiation into (1) a lower subunit consisting of strongly deformed target sediments mixed with up to decameter size, mainly crystalline basement clasts (i.e., lower polymict breccia [LPB]); (2) a middle subunit consisting of a polymict, blocky to gravelly breccia, commonly graded (i.e., graded polymict breccia [GPB]), that, in turn, is sporadically overlain by (3) a few dm thick, sandy bed (i.e., top sandstone [TS]). Previous work interpreted the graded beds as deposited by resurging water during early crater modification. We made three short (<1.35 m) core drillings through the graded beds. The line‐logging technique previously used on cores from other marine‐target craters was complemented by logging of equal‐sized cells in photos made along the cores. Granulometry and clast lithology determinations provide further evidence for the top beds of the breccia being resurge deposits. However, the magnitude of this resurge can only be assessed by future deep core drilling of the infill of the crater hidden below the mountain.

Tetsuya Komabayashi^a, Giacomo Pesce^a, Ryosuke Sinmyo^b, Takaaki Kawazoe^c, Helene Breton^a, Yuta Shimoyama^d, Konstantin Glazyrin^e, Zuzana Konôpková^e, Mohamed Mezouar^f
Earth and Planetary Science Letters 511, 12-24 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.056]
^aSchool of Geo Sciences and Centre for Science at Extreme Conditions, University of Edinburgh,EH93FE,UK
^bBayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
^cDepartment of Earth and Planetary Systems Science, Hiroshima University, Hiroshima, Japan
^dDepartment of Earth and Space Science, Osaka University, Osaka, Japan
^eDeutsches Elektronen-Synchrotron(DESY), Photon Science, Notekstrasse 85, 22607 Hamburg, Germany
^fEuropean SynchrotronRadiation Facility, BP220, F-38043 Grenoble Cedex, France
Copyright Elsevier

Phase relations in Fe–5 wt%Ni–4 wt%Si alloy was examined in an internally resistive heated diamond anvil cell under high pressure (P) and temperature (T) conditions to about 200 GPa and 3900 K by in-situ synchrotron X-ray diffraction. The hexagonal close-packed (hcp) structure was observed to the highest P–T condition, supporting the idea that the stable iron alloy structure in Earth’s inner core is hcp. The P–Tlocations of the phase transition between the face-centred cubic (fcc) and hcp structures were also constrained to 106 GPa. The transition occurs at 15 GPa and 1000 K similar to for pure Fe. The Clausius–Clapeyron slope is however, 0.0480 GPa/K which is larger than reported slopes for Fe (0.0394 GPa/K), Fe–9.7 wt%Ni (0.0426 GPa/K), and Fe–4 wt%Si (0.0394 GPa/K), stabilising the fcc structure towards high pressure. Thus the simultaneous addition of Ni and Si to Fe increases the dP/dT slope of the fcc–hcp transition. This is associated with a small volume change upon transition in Fe–Ni–Si. The triple point, where the fcc, hcp, and liquid phases coexist in Fe–5 wt%Ni–4 wt%Si is placed at 145 GPa and 3750 K. The resulting melting temperature of the hcp phase at the inner core-outer core boundary lies at 550 K lower than in pure Fe.

Zh. Wang (王兆鹏) and M. Cuntz
Astrophysical Journal 873, 113 Link to Article [DOI: 10.3847/1538-4357/ab0377 ]
Department of Physics University of Texas at Arlington, Arlington, TX 76019-0059, USA

In Papers I and II, a comprehensive approach was utilized for the calculation of S-type and P-type habitable regions in stellar binary systems for both circular and elliptical orbits of the binary components. This approach considered a joint constraint, including orbital stability for possible system planets and a habitable region, determined by the stellar radiative energy fluxes (“radiative habitable zone”; RHZ). Specifically, the stellar S-type and P-type RHZs are calculated based on the solution of a fourth-order polynomial. However, in concurrent developments, mostly during 2013 and 2014, important improvements have been made in the computation of stellar habitable zones for single stars based on updated climate models given by R. K. Kopparapu and collaborators. These models entail considerable changes for the inner and outer limits of the stellar habitable zones. Moreover, regarding the habitability limit given by the runaway greenhouse effect, notable disparities were identified between Earth, Mars, and super-Earth planets due to differences in their atmospheric models, thus affecting their potential for habitability. It is the aim of this study to compute S-type and P-type habitable regions of binaries in response to the updated planetary models. Moreover, our study will also consider improved relationships between effective temperatures, radii, and masses for low-luminosity stars.