Marian Horstmann and Addi Bischoff

Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

On October 7, 2008, a small asteroid named 2008 TC₃ was detected in space about 19 h prior to its impact on Earth. Numerous world-wide observations of the object while still in space allowed a very precise determination of its impact area: the Nubian Desert of northern Sudan, Africa. The asteroid had a pre-atmospheric diameter of ~4 m; its weight is reported with values between ~8 and 83 t, and the bulk density with ~2–3 g/cm³, translating into a bulk porosity in the range of ~20–50%. Several dedicated field campaigns in the predicted strewn field resulted in the recovery of more than 700 (monolithological) meteorite fragments with a total weight of ~0.5 kg. These meteorites were collectively named “Almahata Sitta”, after the nearby train station 6, and initially classified as an anomalous polymict ureilite. Further work, however, showed that Almahata Sitta is not only a ureilite but a complex polymict breccia containing chemically and texturally highly variable meteorite fragments, including different ureilites, a ureilite-related andesite, metal-sulfide assemblages related to ureilites, and various chondrite classes (enstatite, ordinary, carbonaceous, Rumuruti-like). It was shown that that chondrites and ureilites derive from one parent body, i.e., asteroid 2008 TC₃, making this object, in combination with the remotely sensed physical parameters, a loosely aggregated, rubble-pile-like object. Detailed examinations have been conducted and mineral-chemical data for 110 samples have been collected, but more work on the remaining samples is mandatory.
Detailed study of Almahata Sitta allows insights into the formation and evolution of ureilites and their parent body. These results support the catastrophic impact disruption of the ureilite parent body and re-accretion of the dispersed ureilitic material into second generation ureilite asteroids. Almahata Sitta shows that different chondritic materials were present in the region of re-accretion and mixed into the newly formed rubble-pile-like asteroid. Asteroid 2008 TC₃ was part of a late-formed ureilitic second generation body in the main belt and was liberated ∼20 Ma ago, finally moving into Earth-crossing orbits that ultimately led to its impact on Earth. The abundant samples of Almahata Sitta, fragments of Asteroid 2008 TC₃, allow study of not only different types of meteorites, but offer the unique opportunity to gain further insights into processes in the asteroid belt of our Solar System such as migration, collision, mixing, and (re-)accretion of asteroidal bodies. Beyond that, this event has the potential to further the understanding of the meteorite–asteroid links, which is a major goal of meteorite science.

Reference
Horstmann M and Bischoff A (in press) The Almahata Sitta polymict breccia and the late accretion of asteroid 2008 TC3. Chemie der Erde
[doi:10.1016/j.chemer.2014.01.004]
Copyright Elsevier

Link to Article

Heather B. Franz^1,2 et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Center for Research and Exploration in Space Science and Technology, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
²Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland 20742, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here.

Reference
Franz et al. (in press) Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars. Nature
[doi:10.1038/nature13175]

Link to Article

Robert V. Wagner and Mark S. Robinson

School of Earth and Space Exploration, Arizona State University, 1100 S. Cady Mall, Tempe, AZ 85287-3603

Lunar Reconnaissance Orbiter Camera images reveal the presence of steep-walled pits in mare basalt (n=8), impact melt deposits (n=221), and highland terrain (n=2). Pits represent evidence of subsurface voids of unknown extents. By analogy with terrestrial counterparts, the voids associated with mare pits may extend for hundreds of meters to kilometers in length, thereby providing extensive potential habitats and access to subsurface geology. Because of their small sizes relative to the local equilibrium crater diameters, the mare pits are likely to be post-flow features rather than volcanic skylights. The impact melt pits are indirect evidence both of extensive subsurface movement of impact melt and of exploitable sublunarean voids. Due to the small sizes of pits (mare, highland, and impact melt) and the absolute ages of their host materials, it is likely that most pits formed as secondary features.

Reference
Wagner RV and Robinson MS (in press) Distribution, Formation Mechanisms, and Significance of Lunar Pits. Icarus
[doi:10.1016/j.icarus.2014.04.002]
Copyright Elsevier

Link to Article

Elisa V. Quintana^1,2 and Jack J. Lissauer²

¹SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043, USA
²Space Science and Astrobiology Division 245-3, NASA Ames Research Center, Moffett Field, CA 94035, USA

Models of planet formation have shown that giant planets have a large impact on the number, masses, and orbits of terrestrial planets that form. In addition, they play an important role in delivering volatiles from material that formed exterior to the snow line (the region in the disk beyond which water ice can condense) to the inner region of the disk where terrestrial planets can maintain liquid water on their surfaces. We present simulations of the late stages of terrestrial planet formation from a disk of protoplanets around a solar-type star and we include a massive planet (from 1 M_⊕ to 1 M_J) in Jupiter’s orbit at ~5.2 AU in all but one set of simulations. Two initial disk models are examined with the same mass distribution and total initial water content, but with different distributions of water content. We compare the accretion rates and final water mass fraction of the planets that form. Remarkably, all of the planets that formed in our simulations without giant planets were water-rich, showing that giant planet companions are not required to deliver volatiles to terrestrial planets in the habitable zone. In contrast, an outer planet at least several times the mass of Earth may be needed to clear distant regions of debris truncating the epoch of frequent large impacts. Observations of exoplanets from radial velocity surveys suggest that outer Jupiter-like planets may be scarce, therefore, the results presented here suggest that there may be more habitable planets residing in our galaxy than previously thought.

Reference
Quintana EV and Lissauer JJ (2014) The Effect of Planets Beyond the Ice Line on the Accretion of Volatiles by Habitable-zone Rocky Planets. The Astrophysical Journal 786:33.
[doi:10.1088/0004-637X/786/1/33]

Link to Article