Richard A. Kerr

Recent results from the Curiosity Mars rover have helped scientists formulate a plan for the next phase of its mission: looking for possible “molecular fossils” left by ancient martian microbes. Analyses of rocks show that Curiosity landed near a former lake that at least intermittently held enough water to have supported life. Now, papers published online in Science show that rocks that once formed the lakebed and bottom mud layer are high in organic carbon molecules. Researchers can’t tell yet whether the molecules came from ancient life or rained down from space. But future analyses—especially of recently eroded rocks that spent most of their history shielded from the cosmic rays thought to sterilize the top meter or so nearest the martian surface—should help researchers determine whether Mars ever harbored life.

Reference
Kerr RA (2013) New Results Send Mars Rover on a Quest for Ancient Life. Science 342:1300-1301.
[doi:10.1126/science.342.6164.1300]
Reprinted with permission from AAAS

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Stuart J. Mills^1,*, Fabrizio Nestola², Volker Kahlenberg³, Andrew G. Christy⁴, Clivia Hejny³ and Günther J. Redhammer⁵

¹Geosciences, Museum Victoria, GPO Box 666, Melbourne 3001, Victoria, Australia
²Dipartimento di Geoscienze, Università di Padova, Via Gradenigo 6, Padova I-35131, Italy
³Institut für Mineralogie und Petrographie der Universität Innsbruck, Innrain 52, 6020 Innsbruck, Austria
⁴Centre for Advanced Microscopy, Australian Natioanl University, Canberra, ACT 0200, Australia
⁵Department of Materials Engineering and Physics, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria

Single-crystal diffraction of jarosite, KFe₃³⁺(SO₄)₂(OH)₆, has been undertaken at low temperatures that proxy for martian surface conditions. Room-temperature data are consistent with literature data [a = 7.2913(5), c = 17.1744(17), and V = 790.72(11) in R3̄m], while the first low-temperature data for the mineral is presented (at 253, 213, 173, and 133 K). Data collections between 297 and 133 K show strongly anisotropic thermal expansion, with the c axis much more expandable than the a axis. Much of the anisotropy is due to strong distortion of the KO₁₂ polyhedron, which increases by 8% between 297 and 133 K. The data sets can aid in the identification of jarosite by X-ray diffraction of martian soils using the Curiosity Rover’s CheMin instrument.

Reference
Mills SJ, Nestola F, Kahlenberg V, Christy AG, Hejny C and Redhammer GJ (2013) Looking for jarosite on Mars: The low-temperature crystal structure of jarosite. American Mineralogist 98:1966-1971.
[doi:10.2138/am.2013.4587]
Copyright: The Mineralogical Society of America

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Sourav Chatterjee¹ and Jonathan C. Tan²

¹Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
²Departments of Astronomy & Physics, University of Florida, Gainesville, FL 32611, USA

The compact multi-transiting planet systems discovered by Kepler challenge planet formation theories. Formation in situ from disks with radial mass surface density, Σ, profiles similar to the minimum mass solar nebula but boosted in normalization by factors  10 has been suggested. We propose that a more natural way to create these planets in the inner disk is formation sequentially from the inside-out via creation of successive gravitationally unstable rings fed from a continuous stream of small (~cm-m size) “pebbles,” drifting inward via gas drag. Pebbles collect at the pressure maximum associated with the transition from a magnetorotational instability (MRI)-inactive (“dead zone”) region to an inner MRI-active zone. A pebble ring builds up until it either becomes gravitationally unstable to form an ~1 M ⊕ planet directly or induces gradual planet formation via core accretion. The planet may undergo Type I migration into the active region, allowing a new pebble ring and planet to form behind it. Alternatively, if migration is inefficient, the planet may continue to accrete from the disk until it becomes massive enough to isolate itself from the accretion flow. A variety of densities may result depending on the relative importance of residual gas accretion as the planet approaches its isolation mass. The process can repeat with a new pebble ring gathering at the new pressure maximum associated with the retreating dead-zone boundary. Our simple analytical model for this scenario of inside-out planet formation yields planetary masses, relative mass scalings with orbital radius, and minimum orbital separations consistent with those seen by Kepler. It provides an explanation of how massive planets can form with tightly packed and well-aligned system architectures, starting from typical protoplanetary disk properties.

Reference
Chatterjee S and Tan JC (2014) Inside-out Planet Formation. The Astrophysical Journal 780:53.
[doi:10.1088/0004-637X/780/1/53]

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E. Sellentin, J. P. Ramsey, F. Windmark and C. P. Dullemond

Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Überle-Str. 2, Heidelberg 69120, Germany

Context. The growth process of dust particles in protoplanetary disks can be modeled via numerical dust coagulation codes. In this approach, physical effects that dominate the dust growth process often must be implemented in a parameterized form. Due to a lack of these parameterizations, existing studies of dust coagulation have ignored the effects a hydrodynamical gas flow can have on grain growth, even though it is often argued that the flow could significantly contribute either positively or negatively to the growth process.
Aims. We intend to qualitatively describe the factors affecting small particle sweep-up under hydrodynamical effects, followed by a quantification of these effects on the growth of dust particles, such that they can be parameterized and implemented in a dust coagulation code.
Methods. Using a simple model for the flow, we numerically integrate the trajectories of small dust particles in disk gas around a proto-planetesimal, sampling a large parameter space in proto-planetesimal radii, headwind velocities, and dust stopping times.
Results. The gas flow deflects most particles away from the proto-planetesimal, such that its effective collisional cross section, and therefore the mass accretion rate, is reduced. The gas flow however also reduces the impact velocity of small dust particles onto a proto-planetesimal. This can be beneficial for its growth, since large impact velocities are known to lead to erosion. We also demonstrate why such a gas flow does not return collisional debris to the surface of a proto-planetesimal.
Conclusions. We predict that a laminar hydrodynamical flow around a proto-planetesimal will have a significant effect on its growth. However, we cannot easily predict which result, the reduction of the impact velocity or the sweep-up cross section, will be more important. Therefore, we provide parameterizations ready for implementation into a dust coagulation code.

Reference
Sellentin E, Ramsey JP, Windmark F and Dullemond CP (2013) A quantification of hydrodynamical effects on protoplanetary dust growth. Astronomy & Astrophysics 560:A96.
[doi:10.1051/0004-6361/201321587]
Reproduced with permission © ESO

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