Audrey Hughes Rager^a,b, Eugene I. Smith^b,*, Bettina Scheu^c and Donald B. Dingwell^c

^aMaterials Engineering and Research Lab (MERL), U.S. Bureau of Reclamation, Denver Federal Center, Bldg. 56, Room 1400, Denver, CO 80225, United States
^bDepartment of Geoscience, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010, United States
^cGeo- und Umweltwissenschaften, Ludwig-Maximilians-Universität München, Theresienstrasse 41/III, 80333 München, Germany

Crater and ejecta morphology provide insight into the composition and structure of the target material. Fluidized ejecta surrounding Martian rampart craters are thought to result from the addition of water to the ejecta during impact into a water-rich (ice or liquid) regolith. Here we test experimentally an alternate hypothesis. We propose that the decompression of a rock–water mixture across the water vaporization curve during the excavation stage of impact cratering results in an increased proportion of fines in the ejecta. This enables the ejecta to flow with little or no liquid water present. To test this hypothesis, fragmentation experiments were conducted on sandstone (28 vol% open porosity) from the northern Eldorado Mountains, Nevada, using a shock-tube apparatus at the LMU Munich, Germany. Rock samples with 0–92% of their open pore space filled with water were pressurized to 15 MPa at 177 °C or 300 °C and rapidly decompressed. As the water vaporization curve is crossed, the water in the pore space rapidly flashes to steam causing, together with the expanding gas in the water-free pore space, the sample to fragment. The presence of water has a significant effect on the grain size distribution and grain shape of the fragmented rock samples. In comparison with (dry) control samples, samples with water with 15–50% open pore space exhibit much smaller grain sizes. The predominant grain shape of dry as well as partially water-saturated samples is bladed, reflecting fracturing parallel to the decompression front. Samples with >80% water in open pore space had an increase in fines and larger particles but less intermediate sized particles. Fragments from experiments with>80% water in open pore space also displayed a more equant grain shape, indicating that the decompression of water caused fracturing independent of the orientation of the decompression front. These results may provide insight into the morphology of Martian rampart craters. We propose here that even relatively low water contents in the target (∼16%) may be sufficient to produce a significant increased proportion of fines allowing the ejecta to flow with little or no water present.

Reference
Rager AH, Smith EI, Scheu B and Dingwell DB (2014) The effects of water vaporization on rock fragmentation during rapid decompression: Implications for the formation of fluidized ejecta on Mars. Earth and Planetary Science Letters 385:68–78.
[doi:10.1016/j.epsl.2013.10.029]
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David A. Rothery^a,*, Rebecca J. Thomas^a, Laura Kerber^b

^aDepartment of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
^bLaboratoire de Météorologie Dynamique du CNRS, Université Paris 6, Paris, France

A 27×13 km ‘rimless depression’ 100 km inside the southwest rim of the Caloris basin is revealed by high resolution orbital imaging under a variety of illuminations to consist of at least nine overlapping volcanic vents, each individually up to 8 km in diameter. It is thus a ‘compound’ volcano, indicative of localised migration of the site of the active vent. The vent floors are at a least 1 km below their brinks, but lack the flat shape characteristically produced by piston-like subsidence of a caldera floor or by flooding of a crater bottom by a lava lake. They bear a closer resemblance to volcanic craters sculpted by explosive eruptions and/or modified by collapse into void spaces created by magma withdrawal back down into a conduit. This complex of overlapping vents is at the summit of a subtle edifice at least 100 km across, with flank slopes of about only 0.2 degrees, after correction for the regional slope. This is consistent with previous interpretation as a locus of pyroclastic eruptions. Construction of the edifice could have been contributed to by effusion of very low viscosity lava, but high resolution images show that the vent-facing rim of a nearby impact crater is not heavily embayed as previously supposed on the basis of lower resolution flyby imaging. Contrasts in morphology (sharpness versus blurredness of the texture) and different densities of superposed sub-km impact craters inside each vent are consistent with (but do not prove) substantial differences in the age of the most recent activity at each vent. This suggests a long duration of episodic magmagenesis at a restricted locus. The age range cannot be quantified, but could be of the order of a billion years. If each vent was fed from the same point source, geometric considerations suggest a source depth of at least 50 km. However, the migration of the active vent may be partly controlled by a deep-seated fault that is radial to the Caloris basin. Other rimless depressions in this part of the Caloris basin fall on or close to radial lines, suggesting that elements of the Pantheon Fossae radial fracture system that dominates the surface of the central portion of the Caloris basin may continue at depth almost as far as the basin rim.

Reference
Rothery DA, Thomas RJ and Kerber L (2014) Prolonged eruptive history of a compound volcano on Mercury: Volcanic and tectonic implications. Earth and Planetary Science Letters 385:59–67.
[doi:10.1016/j.epsl.2013.10.023]
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R. Sakai^a,*, H. Nagahara^a, K. Ozawa^a, S. Tachibana^b

^aDepartment of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
^bDepartment of Natural History Sciences, Hokkaido University, N10 W8, Sapporo 060-0810, Japan

The present study aims to constrain the composition of the initial lunar magma ocean (LMO) with fluid dynamic and thermodynamic consideration. A plausible range of the initial LMO composition is investigated by developing an incremental polybaric fractional crystallization model with variable fractionation efficiency to satisfy three conditions for the anorthosite crust formation: (1) the amount of anorthite crystallized from the LMO is abundant enough to form the crust with the observed thickness, (2) the Mg# (= Mg/(Mg+Fe)) of orthopyroxene crystallized with anorthite in the cooling LMO is consistent with that observed in the lunar highland rocks, ferroan anorthosite, and (3) crystallized anorthite separated to float in the turbulent LMO. A plausible range of FeO and Al₂O₃ contents of the bulk LMO is successfully constrained as a crescent region tight for FeO and loose for Al₂O₃. The FeO content must be higher than 1.3 times the bulk silicate Earth (BSE) and lower than 1.8 xBSE unless the Al₂O₃ content of the Moon is extremely higher than the Earth. These upper and lower limits for FeO are positively correlated with the initial Al₂O₃ content and fractionation efficiency. The FeO-rich LMO composition may suggest that the circum-Earth disk just after the giant impact of the Earth-Moon system formation was more oxidizing or the impactor was richer in FeO than the Earth’s mantle.

Reference
Sakai R, Nagahara H, Ozawa K and Tachibana S (in press) Composition of the lunar magma ocean constrained by the conditions for the crust formation. Icarus
[doi:10.1016/j.icarus.2013.10.031]
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A. Belloche¹, H. S. P. Müller^1,2, K. M. Menten¹, P. Schilke^1,2 and C. Comito¹

¹Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Context. The discovery of amino acids in meteorites fallen to Earth and the detection of glycine, the simplest of them, in samples returned from a comet to Earth strongly suggest that the chemistry of the interstellar medium is capable of producing such complex organic molecules and that they may be widespread in our Galaxy.
Aims. Our goal is to investigate the degree of chemical complexity that can be reached in the interstellar medium, in particular in dense star-forming regions.
Methods. We performed an unbiased, spectral line survey toward Sgr B2(N) and (M), two regions where high-mass stars are formed, with the IRAM 30 m telescope in the 3 mm atmospheric transmission window. Partial surveys at 2 and 1.3 mm were performed in parallel. The spectra were analyzed with a simple radiative transfer model that assumes local thermodynamic equilibrium but takes optical depth effects into account.
Results. About 3675 and 945 spectral lines with a peak signal-to-noise ratio higher than 4 are detected at 3 mm toward Sgr B2(N) and (M), i.e. about 102 and 26 lines per GHz, respectively. This represents an increase by about a factor of two over previous surveys of Sgr B2. About 70% and 47% of the lines detected toward Sgr B2(N) and (M) are identified and assigned to 56 and 46 distinct molecules as well as to 66 and 54 less abundant isotopologues of these molecules, respectively. In addition, we report the detection of transitions from 59 and 24 catalog entries corresponding to vibrationally or torsionally excited states of some of these molecules, respectively, up to a vibration energy of 1400 cm^-1 (2000 K). Excitation temperatures and column densities were derived for each species but should be used with caution. The rotation temperatures of the detected complex molecules typically range from ~50 to 200 K. Among the detected molecules, aminoacetonitrile, n-propyl cyanide, and ethyl formate were reported for the first time in space based on this survey, as were five rare isotopologues of vinyl cyanide, cyanoacetylene, and hydrogen cyanide. We also report the detection of transitions from within twelve new vibrationally or torsionally excited states of known molecules. Absorption features produced by diffuse clouds along the line of sight are detected in transitions with low rotation quantum numbers of many simple molecules and are modeled with ~30–40 velocity components with typical linewidths of ~3–5 km s^-1.
Conclusions. Although the large number of unidentified lines may still allow future identification of new molecules, we expect most of these lines to belong to vibrationally or torsionally excited states or to rare isotopologues of known molecules for which spectroscopic predictions are currently missing. Significant progress in extending the inventory of complex organic molecules in Sgr B2(N) and deriving tighter constraints on their location, origin, and abundance is expected in the near future thanks to an ongoing spectral line survey at 3 mm with ALMA in its cycles 0 and 1. The present single-dish survey will serve as a solid basis for the line identification and analysis of such an interferometric survey.

Reference
Belloche A, Müller HSP, Menten KM, Schilke P and Comito C (in press) Complex organic molecules in the interstellar medium: IRAM 30 m line survey of Sagittarius B2(N) and (M). Astronomy & Astrophysics 559:A47.
[doi:10.1051/0004-6361/201321096]
Reproduced with permission © ESO

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