F. E. DeMeo^1,2 and B. Carry^3,4

¹Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-16, Cambridge, Massachusetts 02138, USA
²Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology (MIT), 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
³Institut de Mécanique Céleste et de Calcul des Éphémérides, Observatoire de Paris, UMR8028 CNRS, 77 avenue Denfert-Rochereau, 75014 Paris, France
⁴European Space Astronomy (ESA) Centre, PO Box 78, Villanueva de la Cañada 28691, Madrid, Spain

We are still seeking a copyright agreement with Nature to display their abstracts.

Reference
DeMeo FE and Carry B (2014) Solar System evolution from compositional mapping of the asteroid belt. Nature 505:629–634.
[doi:10.1038/nature12908]

Link to Article

Emmanuel Jacquet

Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St Georges Street, Toronto, ON M5S 3H8, Canada

Primitive meteorites are dominated by millimeter-size silicate spherules called chondrules. The nature of the high-temperature events that produced them in the early solar system remains enigmatic. Beside their thermal history, one important clue is provided by their size which shows remarkably little variation (less than a factor of 6 for the mean chondrule radius of most chondrites) despite the extensive range of ages and heliocentric distances sampled. It is however unclear whether chondrule size is due to the chondrule melting process itself, or has been simply inherited from the precursor material, or yet results from some sorting process. I examine these different possibilities in terms of their analytical size predictions. Unless the chondrule-forming “window” was very narrow, radial sorting can be excluded as size-determining processes because of the large variations it would predict. Molten planetesimal collision or impact melting models, which derive chondrules from the fragmentation of larger melt bodies, would likewise predict too much size variability by themselves; more generally any size modification during chondrule formation is limited in extent by evidence from compound chondrules and the considerable compositional variability of chondrules. Turbulent concentration would predict a low size variability but lack of evidence of any accretion bias in carbonaceous chondrites may be difficult to reconcile with any form of local sorting upon agglomeration. Growth by sticking (especially if bouncing-limited) of aggregates as chondrule precursors would yield limited variations of their final radius in space and time, and would be consistent with the relatively similar size of other chondrite components such as refractory inclusions. This suggests that the chondrule-melting process(es) simply melted such nebular aggregates with little modification of mass.

Reference
Jacquet E (in press) The quasi-universality of chondrule size as a constraint for chondrule formation models. Icarus
[doi:10.1016/j.icarus.2014.01.012]
Copyright Elsevier

Link to Article

Colin M. Dundas¹, Shane Byrne², Alfred S. McEwen², Michael T. Mellon³, Megan R. Kennedy⁴, Ingrid J. Daubar², Lee Saper⁴

¹Astrogeology Science Center, U. S. Geological Survey, Flagstaff, Arizona, USA
²Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
³Southwest Research Institute, Boulder, Colorado, USA
⁴Malin Space Science Systems, San Diego, California, USA

Twenty small new impact craters or clusters have been observed to excavate bright material inferred to be ice at mid-latitudes and high latitudes on Mars. In the northern hemisphere, the craters are widely distributed geographically and occur at latitudes as low as 39°N. Stability modeling suggests that this ice distribution requires a long-term average atmospheric water vapor content around 25 precipitable micrometers, more than double the present value, which is consistent with the expected effect of recent orbital variations. Alternatively, near-surface humidity could be higher than expected for current column abundances if water vapor is not well mixed with atmospheric CO₂, or the vapor pressure at the ice table could be lower due to salts. Ice in and around the craters remains visibly bright for months to years, indicating that it is clean ice rather than ice-cemented regolith. Although some clean ice may be produced by the impact process, it is likely that the original ground ice was excess ice (exceeding dry soil pore space) in many cases. Observations of the craters suggest small-scale heterogeneities in this excess ice. The origin of such ice is uncertain. Ice lens formation by migration of thin films of liquid is most consistent with local heterogeneity in ice content and common surface boulders, but in some cases, nearby thermokarst landforms suggest large amounts of excess ice that may be best explained by a degraded ice sheet.

Reference
Dundas CM, Byrne S, McEwen AS, Mellon MT, Kennedy MR, Daubar IJ and Saper L (in press) HiRISE observations of new impact craters exposing Martian ground ice. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004482]
Published by arrangement with John Wiley & Sons

Link to Article

Kirsten L. Siebach and John P. Grotzinger

Geological and Planetary Sciences Division, California Institute of Technology, Pasadena, California, USA

While the presence of water on the surface of early Mars is now well known, the volume, distribution, duration, and timing of the liquid water have proven difficult to determine. This study makes use of a distinctive boxwork-rich sedimentary layer on Mount Sharp to map fluid-based cementation from orbital imagery and estimate the minimum volume of water present when this sedimentary interval was formed. The boxwork structures on Mount Sharp are decameter-scale light-toned polygonal ridges that are unique compared to previous observations of Martian fractured terrain because they are parallel-sided ridges with dark central linear depressions. This texture and the sedimentary setting strongly imply that the ridges are early diagenetic features formed in the subsurface phreatic groundwater zone. High-resolution orbital imagery was used to map the volume of light-toned cemented ridges. Based on the cemented volume, a minimum of 5.25 × 105 m³ of cement was deposited within the fractures. Using a brine composition based on observations of other Martian cements and modeling the degree of evaporation, each volume of cement requires 800–6700 pore volumes of water, so the mapped boxwork ridge cements require a minimum of 0.43 km³ of water. This is a significant amount of groundwater that must have been present at the −3620 m level, 1050 m above the current floor of Gale Crater, providing both a new constraint on the possible origins of Mount Sharp and a possible future science target for the Curiosity rover where large volumes of water were present, and early mineralization could have preserved a once-habitable environment.

Reference
Siebach KL and Grotzinger JP (in press) Volumetric estimates of ancient water on Mount Sharp based on boxwork deposits, Gale Crater, Mars. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004508]
Published by arrangement with John Wiley & Sons

Link to Article