Thursday, 20 March 2014

Larry Nittler
Sulfur-Depleted Composition of Mercury's Largest Pyroclastic Deposit: Implications for Explosive Volcanism and Surface Reflectance on the Innermost Planet

"Multispectral reflectance data from Mercury’s surface returned by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during its three flybys and orbital mission have revealed 51 sites interpreted to be pyroclastic deposits (PDs) formed through explosive volcanism. These deposits have relatively high albedo, steep or “red” slopes of spectral reflectance versus wavelength, and diffuse borders that surround irregularly shaped rimless pits, thought to be the source vents. The presence of the PDs indicates that the source magmas had substantial volatile contents, although the specific volatile phases are unknown. From a spatially resolved, targeted X-Ray Spectrometer (XRS) measurement, we report the chemical composition of the largest PD on Mercury, located northeast of Rachmaninoff basin. We have determined that this deposit has a similar major-element composition to surrounding materials with the notable exception of sulfur, which is strongly depleted. This result suggests that S is a dominant volatile species driving explosive volcanism on the planet, and that sulfides may play a role in darkening much of Mercury’s surface."

Alan Boss
Short-Lived Radioisotope Injection into the Solar Nebula: First 3D Calculations of Shock Interactions with Rotating Presolar Cloud Models

"The presence of live 60Fe during the formation of chondrites has long been seen as the strongest argument in favor of the injection of 60Fe and other short-lived radioisotopes (SLRIs) into the presolar cloud (or the solar nebula) by a shock wave emanating from a Type II supernova (SNe) that synthesized the SLRIs. However, recent work has lowered the inferred initial abundances of 60Fe to values that are more consistent with the galactic background abundance rather than a nearby supernova. This explanation might require the high levels of initial 26Al found in chondrites to be derived from a Wolf-Rayet (WR) star wind, which is expected to be rich in 26Al and poor in 60Fe. Scenarios have also been advanced for accounting for the inferred levels of both 60Fe and 26Al through supernova injection into giant molecular clouds, though abundance problems remain.

The presence of live 26Al in refractory inclusions (e.g., CAIs) was the original motivation for the SNe trigger hypothesis. The fact that the FUN refractory inclusions show no evidence for live 26Al, coupled with the significant 26Al depletions found in some CAIs and refractory grains, implies that these refractory objects may have formed prior to the injection, mixing, and transport of 26Al into the refractories-forming region of the solar nebula. The 26Al data alone, therefore, seem to require the late arrival of SLRIs derived from a SNe into the inner region of the solar nebula, as opposed to injection into a giant molecular cloud.

Detailed hydrodynamical modeling has shown that SNe shock waves are the preferred means for simultaneously achieving triggered collapse of the presolar cloud and injection of SLRIs carried by the shock wave. WR star winds are likely to shred cloud cores rather than induce collapse. These studies also showed that injection into a rotating cloud can increase injection efficiencies by as much as a factor of 10. However, the models were restricted to axisymmetry (2D). Other previous models have studied shock wave interactions with fully 3D cloud cores. However, these 3D clouds were not assumed to be rotating. Here we extend the modeling effort to consider rotating 3D target clouds.

The rotating presolar clouds are similar to those previously studied in 2D: 2.2 M⊙ cloud cores with radii of 0.053 pc, rotating with angular veloc- ities of either Ω = 1014 or 1013 rad s1. The shock waves propagate along the rotation axis at speeds of vs = 20 or 40 km s1, shock widths of ws = 3×103 or 3×104 pc, and shock densi- ties ranging from ρs = 3.6 × 1019 to 2.1 × 1017 g cm3. Most of the clouds were triggered into collapse. Several of the clouds with Ω = 1013 collapsed and fragmented into multiple protostars.

The figures show the results for two models with vs = 40, ws = 3×10−4, and ρs = 7.2×10−18 in the previous units. Model H, with Ω = 10−13, formed a large (∼ 500 AU radius) protostellar disk with spiral arms that might undergo fragmentation, while model L, with Ω = 10−14, formed a smaller diameter (∼ 150 AU radius) protostellar disk with a single protostar. The injection efficiency fi, defined as the fraction of the incident shock wave material that is injected into the collapsing cloud core, was fi ∼ 0.03 for both models H and L, identical to fi for the non-rotating-cloud version of this same model. This differs from the expectations of the 2D models, where it was found that rotation could increase fi by a factor as large as 10. However, the rotating 2D models studied only the standard shock (ρs = 3.6 × 10−20, ws = 3 × 10−3), whereas models H and L considered denser, thinner shocks that are much more conducive to injection than the standard shock, so that the addition of rotation did not increase the injection efficiency further. Other shock assumptions can result in rotation leading to increased fi, provided the shock is aligned with the rotation axis."

Christian Klimczak
Limits on the Brittle Strength of Planetary Lithospheres Undergoing Global Contraction

"An assessment of the near-surface brittle strength of the lithospheres of Mercury, the Moon, and Mars reveals that all of these bodies could have accommodated substantial amounts of global contraction prior to the onset of thrust faulting. In particular, their lithospheres were sufficiently strong so as not to be markedly deformed until they experienced changes in radius of as high as 1.6 km (Mercury), 1.1 km (the Moon), and 2.2 km (Mars). As a consequence, these figures should be added to estimates to radius changes derived from thrust fault analyses, increasing the total estimates of global contraction of Mercury to as much as ΔR = 7.9 km, of the Moon to at least 1.2 km, and of Mars to 6.0 km." 

Jemma Davidson
Petrography of Very Primitive CO3 Chondrites: Dominion Range 08006, Miller Range 07687, and Four Others

"After interplanetary dust particles, carbonaceous chondrites provide some of the most pristine extraterrestrial samples available for study. The least altered chondrites typically contain abundant presolar grains and other isotopically anomalous matter, which can provide valuable insight into the early Solar System and parent body processing. The CO3 chondrites exhibit the complete metamorphic sequence from type 3.0 to 3.9. For the least altered CO3s, it is possible to further subdivide them, from type 3.00 to 3.2, and identify the most pristine samples for additional study by determining the Cr content of ferroan olivine. The same technique has been ap- plied to the ordinary chondrites (OC), but they do not follow the same (poorly defined) trend as the CO3 chondrites. It was suggested that Dominion Range (DOM) 03238 represents a missing link in the CO3 chondrite metamorphic sequence. However, here we report the results of a petrographic study of six other Antarctic CO3 chondrites that appear to define the CO trend and suggest that DOM 03238 is, in fact, an outlier. We also present detailed petrography for DOM 08006 that indicates it is the most primitive CO3." 

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