Katarina Miljković^1,*, Mark A. Wieczorek¹, Gareth S. Collins², Matthieu Laneuville¹, Gregory A. Neumann³, H. Jay Melosh⁴, Sean C. Solomon^5,6, Roger J. Phillips⁷, David E. Smith⁸, Maria T. Zuber⁸

¹Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, Case 7011, Lamarck A, 5, 35 rue Hélène Brion, 75205 Paris cedex 13, France.
²Department of Earth Sciences and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.
³Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
⁴Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907, USA.
⁵Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
⁶Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA.
⁷Planetary Science Directorate, Southwest Research Institute, Boulder, CO 80302, USA.
⁸Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.

Maps of crustal thickness derived from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission revealed more large impact basins on the nearside hemisphere of the Moon than on its farside. The enrichment in heat-producing elements and prolonged volcanic activity on the lunar nearside hemisphere indicate that the temperature of the nearside crust and upper mantle was hotter than that of the farside at the time of basin formation. Using the iSALE-2D hydrocode to model impact basin formation, we found that impacts on the hotter nearside would have formed basins with up to twice the diameter of similar impacts on the cooler farside hemisphere. The size distribution of lunar impact basins is thus not representative of the earliest inner solar system impact bombardment.

Reference
Miljković K, Wieczorek MA, Collins GS, Laneuville M, Neumann GA, Melosh HJ, Solomon SC, Phillips RJ, Smith DE and Zuber MT (2013) Asymmetric Distribution of Lunar Impact Basins Caused by Variations in Target Properties. Science 342:724-726.
[doi:10.1126/science.1243224]
Reprinted with permission from AAAS

Link to Article

Yudhijit Bhattacharjee

The Mars Atmosphere and Volatile EvolutioN (MAVEN), a NASA spacecraft to be launched to Mars later this month, will try to decipher billions of years of planetary history from careful study of the martian atmosphere. Eons ago, planetary scientists believe, Mars had a thick atmosphere that sheltered a surface awash with water—conditions in which life could have emerged and thrived. Today, that atmosphere is thin and depleted, and Mars is a cold, barren desert. What caused this remarkable transformation? Until now, planetary scientists have attempted to answer such questions mainly from the planet’s surface. MAVEN will take a new course: flying through the outer fringes of Mars’s atmosphere, measuring gases and monitoring conditions with eight instruments. The measurements should help researchers figure out how the solar wind, asteroid impacts, and chemical reactions gradually depleted the Red Planet’s atmosphere.

Reference
Bhattacharjee Y (2013) Orbiting MAVEN Mission Set to Trace a Planet’s History in Thin Martian Air. Science 342:681.
[doi:10.1126/science.342.6159.681]
Reprinted with permission from AAAS

Link to Article

Mariek E. Schmidt^a, Christian M. Schrader^b,* and Timothy J. McCoy^c

^aDepartment of Earth Sciences, Brock University, Saint Catharines, ON L2S 3A1, Canada
^bNASA, Marshall Space Flight Center, Huntsville, AL 35813, Unites States
^cDepartment of Mineral Sciences, Smithsonian Institution, MRC-0119, PO Box 37012, Washington, DC 20013-7012, United States

The primary oxygen fugacity (fO₂) of basaltic melts reflects the mantle source oxidation state, dictates the crystallizing assemblage, and determines how the magma will evolve. Basalts examined by the Spirit Mars Exploration Rover in Gusev Crater range from the K-poor Adirondack class (0.02 wt% K₂O) to K-rich Backstay class (up to 1.2 wt% K₂O) and exhibit substantially more variation than observed in martian basaltic meteorites. The ratios of ferric to total iron (Fe³⁺/Fe_T) measured by the Mössbauer spectrometer are high (equivalent to −0.76 to +2.98 ΔQFM; quartz-fayalite-magnetite buffer as defined by Wones and Gilbert, 1969), reflecting secondary Fe³⁺ phases. By combining the Fe³⁺/Fe_T of the igneous minerals (olivine, pyroxene, and magnetite) determined by Mössbauer spectrometer, we estimate primary fO₂ for the Gusev basalts to be −3.6 to 0.5 ΔQFM. Estimating the fO₂ as a function of the dependence of the CIPW normative fayalite/magnetite ratios on Fe³⁺/Fe_T yields a slightly smaller range of −2.58 to +0.57 ΔQFM. General similarity between the fO₂ estimated for the Gusev basalts and ranges in fO₂ for the shergottitic meteorites (−3.8 to 0.2 ΔQFM; Herd, 2003 and Goodrich et al., 2003) suggests that the overall range of fO₂ for the martian igneous rocks and mantle is relatively restricted. Like the shergottites (Herd, 2003), estimated fO₂ of three Gusev classes (Adirondack, Barnhill and Irvine) correlates with a proxy for LREE enrichment (K₂O/TiO₂). This suggests mixing between melts or fluids derived from reservoirs with contrasting fO₂ and REE characteristics. Oxygen fugacity estimates for the martian interior suggest that tectonic processes have not led to sufficient recycling of oxidized surface material into the martian interior to entirely affect the overall oxidation state of the mantle.

Reference
Schmidt ME, Schrader CM and McCoy TJ (2013) The primary fO2 of basalts examined by the Spirit rover in Gusev Crater, Mars: Evidence for multiple redox states in the martian interior. Earth and Planetary Science Letters 384:198–208.
[doi:10.1016/j.epsl.2013.10.005]
Copyright Elsevier

Link to Article

Yan-Fei Jiang (姜燕飞)¹, James M. Stone¹ and Shane W. Davis²

¹Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
²Canadian Institute for Theoretical Astrophysics, Toronto, ON M5S3H4, Canada

We study the long-term thermal stability of radiation-dominated disks in which the vertical structure is determined self-consistently by the balance of heating due to the dissipation of MHD turbulence driven by magneto-rotational instability (MRI) and cooling due to radiation emitted at the photosphere. The calculations adopt the local shearing box approximation and utilize the recently developed radiation transfer module in the Athena MHD code based on a variable Eddington tensor rather than an assumed local closure. After saturation of the MRI, in many cases the disk maintains a steady vertical structure for many thermal times. However, in every case in which the box size in the horizontal directions are at least one pressure scale height, fluctuations associated with MRI turbulence and dynamo action in the disk eventually trigger a thermal runaway that causes the disk to either expand or contract until the calculation must be terminated. During runaway, the dependence of the heating and cooling rates on total pressure satisfy the simplest criterion for classical thermal instability. We identify several physical reasons why the thermal runaway observed in our simulations differ from the standard α disk model; for example, the advection of radiation contributes a non-negligible fraction to the vertical energy flux at the largest radiation pressure, most of the dissipation does not happen in the disk mid-plane, and the change of dissipation scale height with mid-plane pressure is slower than the change of density scale height. We discuss how and why our results differ from those published previously. Such thermal runaway behavior might have important implications for interpreting temporal variability in observed systems, but fully global simulations are required to study the saturated state before detailed predictions can be made.

Reference
Jiang Y-F, Stone JM and Davis SW (2013) On the Thermal Stability of Radiation-dominated Accretion Disks. The Astrophysical Journal 778:65.
[doi:10.1088/0004-637X/778/1/65]

Link to Article

Philip J. Armitage^1,2, Jacob B. Simon¹ and Rebecca G. Martin^1,3

¹JILA, University of Colorado and NIST, 440 UCB, Boulder, CO 80309-0440, USA
²Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0391, USA
³Sagan Fellow.

Protoplanetary disks are likely to be threaded by a weak net flux of vertical magnetic field that is a remnant of the much larger fluxes present in molecular cloud cores. If this flux is approximately conserved its dynamical importance will increase as mass is accreted, initially by stimulating magnetorotational disk turbulence and subsequently by enabling wind angular momentum loss. We use fits to numerical simulations of ambipolar dominated disk turbulence to construct simplified one-dimensional evolution models for weakly magnetized protoplanetary disks. We show that the late onset of significant angular momentum loss in a wind can give rise to “two timescale” disk evolution in which a long phase of viscous evolution precedes rapid dispersal as the wind becomes dominant. The wide dispersion in disk lifetimes could therefore be due to varying initial levels of net flux. Magnetohydrodynamic (MHD) wind triggered dispersal differs from photoevaporative dispersal in predicting mass loss from small (<1 AU) scales, where thermal winds are suppressed. Our specific models are based on a limited set of simulations that remain uncertain, but qualitatively similar evolution appears likely if mass is lost from disks more quickly than flux, and if MHD winds become important as the plasma β decreases.

Reference
Armitage PJ, Simon JB and Martin RG (2013) Two Timescale Dispersal of Magnetized Protoplanetary Disks. The Astrophysical Journal – Letters 778:L14.
[doi:10.1088/2041-8205/778/1/L14]

Link to Article