Stuart J. Robbins^a and Brian M. Hynek^b

^aLaboratory for Atmospheric and Space Physics, University of Colorado at Boulder, 3665 Discovery Dr., Boulder, CO 80309, United States
^bLaboratory for Atmospheric and Space Physics and Department of Geological Sciences, University of Colorado at Boulder, 3665 Discovery Dr., Boulder, CO 80309, United States

Impact craters (“craters”) are ubiquitous across most solid surfaces in the Solar System. The most common use of crater populations (populations as defined by diameter- or “size-” frequency) is to estimate relative and absolute model surface ages based on two assumptions: Craters will form spatially randomly across the planetary body, and craters will form following a random distribution around a known or assumed temporal flux. Secondary craters – craters that form from the ejecta of a crater formed by an extraplanetary-sourced impactor – belie both of these assumptions and so will affect crater-based ages if not removed from crater counts. A question unanswered with observational data to this point has been, what is the population of primary versus secondary craters on a given planet? We have answered this question for Mars for craters larger than 1 km in diameter by using a recently published global crater database, classifying craters as primary or secondary, and creating maps of the population statistics. Our approach was to err on the side of a crater being primary by default and hence our work is a conservative measurement. We show that, globally, secondary craters are at least 24% as numerous as primary craters (comprising 19% of the total population) for diameters D≥1 km. However, there are many “hot spots” across the globe where secondary craters are more numerous than primary craters for diameters as large as 9 km. This is the first time such a study has been conducted globally for any body and it shows that, not only are secondary craters numerous, but they can significantly affect crater populations in a non-uniform way across a planetary surface

Reference
Robbins SJ and Hynek BM (2014) The secondary crater population of Mars. Earth and Planetary Science Letters 400:66.
[doi:10.1016/j.epsl.2014.05.005]
Copyright Elsevier

Link to Article

Arpita Roy^1,2, Jason T. Wright^1,2 and Steinn Sigurðsson^1,2

¹Department of Astronomy and Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
²Center for Exoplanets and Habitable Worlds, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA

The lunar farside highlands problem refers to the curious and unexplained fact that the farside lunar crust is thicker, on average, than the nearside crust. Here we recognize the crucial influence of Earthshine, and propose that it naturally explains this hemispheric dichotomy. Since the accreting Moon rapidly achieved synchronous rotation, a surface and atmospheric thermal gradient was imposed by the proximity of the hot, post-giant impact Earth. This gradient guided condensation of atmospheric and accreting material, preferentially depositing crust-forming refractories on the cooler farside, resulting in a primordial bulk chemical inhomogeneity that seeded the crustal asymmetry. Our model provides a causal solution to the lunar highlands problem: the thermal gradient created by Earthshine produced the chemical gradient responsible for the crust thickness dichotomy that defines the lunar highlands.

Reference
Roy A, Wright JT, and Sigurðsson S (2014) Earthshine on a Young Moon: Explaining the Lunar Farside Highlands. The Astrophysical Journal – Letters 788:L42.
[doi:10.1088/2041-8205/788/2/L42]

Link to Article

F. Jourdan^a, G. Benedix^b, E. Eroglu^c, P.A. Bland^b and A. Bouvier^d

^aWestern Australian Argon Isotope Facility, Department of Applied Geology and JdL-CMS, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
^bDepartment of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845
^cSchool of Chemistry and Biochemistry, The University of Western Australia, Crawley, WA 6009, Australia
^dDepartment of Earth Sciences, University of Western Ontario, London, ON, N6A 3K7, Canada

The Bunburra Rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse-, medium- and fine-grained lithologies. Petrographic observations constrain the limited shock pressure to between ca. 10 GPa and 20 GPa. In this study, we carried out nine ⁴⁰Ar/³⁹Ar step-heating experiments on distinct single-grain fragments extracted from the coarse and fine lithologies. We obtained six plateau ages and three mini-plateau ages. These ages fall into two internally concordant populations with mean ages of 3640 ± 21 Ma (n=7; P=0.53) and 3544 ± 26 Ma (n=2; P=0.54), respectively. Based on these results, additional⁴⁰Ar/³⁹Ar data of fusion crust fragments, argon diffusion modeling, and petrographic observations, we conclude that the principal components of the Bunburra Rockhole basaltic achondrite are from a melt rock formed at ~3.64 Ga by a medium to large impact event. The data imply this impact generated high enough energy to completely melt the basaltic target rock and reset the Ar systematics, but only partially reset the Pb-Pb age. We also conclude that a complete ⁴⁰Ar∗ resetting of pyroxene and plagioclase at this time could not have been achieved at solid-state conditions. Comparison with a terrestrial analogue (Lonar crater) shows that the time-temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve. Ar data also suggest that a second medium-size impact event occurred on a neighboring part of the same target rock at ~3.54 Ga. Concordant low-temperature step ages of the nine aliquots suggest that, at ~3.42 Ga, a third smaller impact excavated parts of the ~3.64 Ga and ~3.54 Ga melt rocks and brought the fragments together. The lack of significant impact activity after 3.5 Ga, as recorded by the Bunburra Rockhole suggest that (1) either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts, or (2) it was covered by a porous heat-absorbing regolith blanket which, when combined with the diminishing frequency of large impacts in the solar system, protected Bunburra from subsequent major heating events. Finally we note that the total (K/Ar) resetting impact event history recorded by some of the brecciated eucrites (peak at 3.8-3.5 Ga) is similar to the large impact history recorded by the Bunburra Rockhole parent body (ca. 3.64-3.54 Ga; this study) and could indicate a similar position in the asteroid belt at that time.

Reference
Jourdan F, Benedix G, Eroglu E, Bland PA and Bouvier A (in press) 40Ar/39Ar impact ages and time-temperature argon diffusion history of the Bunburra Rockhole anomalous basaltic achondrite. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.039]
Copyright Elsevier

Link to Article

E.L. Walton^a,b, T.G. Sharp^c, J. Hu^c and J. Filiberto^d

^aMacEwan University, Department of Physical Sciences, 10700-104 Ave, City Centre Campus, Edmonton, AB, T5J 4S2, Canada
^bUniversity of Alberta, Department of Earth & Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB, T6G 2E3, Canada
^cArizona State University, School of Earth and Space Exploration, Tempe, AZ, 85287-1404, U.S.A
^dSouthern Illinois University, Department of Geology, Carbondale, IL

The microtexture and mineralogy of shock melts in the Tissint martian meteorite were investigated using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and synchrotron micro X-ray diffraction to understand shock conditions and duration. Distinct mineral assemblages occur within and adjacent to the shock melts as a function of the thickness and hence cooling history. The matrix of thin veins and pockets of shock melt consists of clinopyroxene + ringwoodite ± stishovite embedded in glass with minor Fe-sulfide. The margins of host rock olivine in contact with the melt, as well as entrained olivine fragments, are now amorphosed silicate perovskite + magnesiowüstite or clinopyroxene + magnesiowüstite. The pressure stabilities of these mineral assemblages are ~15 GPa and >19 GPa, respectively. The ~200-μm-wide margin of thicker, mm-size (up to 1.4 mm) shock melt vein contains clinopyroxene + olivine, with central regions comprising glass + vesicles + Fe-sulfide spheres. Fragments of host rock within the melt are polycrystalline olivine (after olivine) and tissintite + glass (after plagioclase). From these mineral assemblages the crystallization pressure at the vein edge was as high as 14 GPa. The interior crystallized at ambient pressure. The shock melts in Tissint quench-crystallized during and after release from the peak shock pressure; crystallization pressures and those determined from olivine dissociation therefore represent the minimum shock loading. Shock deformation in host rock minerals and complete transformation of plagioclase to maskelynite suggest the peak shock pressure experienced by Tissint ⩾29−30 GPa. These pressure estimates support our assessment that the peak shock pressure in Tissint was significantly higher than the minimum 19 GPa required to transform olivine to silicate perovskite plus magnesiowüstite
Small volumes of shock melt (<100 μm) quench rapidly (0.01 s), whereas thermal equilibration will occur within 1.2 seconds in larger volumes of melt (1 mm²). The apparent variation in shock pressure recorded by variable mineral assemblages within and around shock melts in Tissint is consistent with a shock pulse on the order of 10−20 ms combined with a longer duration of post-shock cooling and complex thermal history. This implies that the impact on Mars that shocked and ejected Tissint at ~1 Ma was not exceptionally large

Reference
Walton EL, Sharp TG, Hu J and Filiberto J (in press) Heterogeneous mineral assemblages in Martian meteorite Tissint as a result of a recent small impact event on Mars. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.023]
Copyright Elsevier

Link to Article

Hurwitz D. M. and D. A. Kring

Center for Lunar Science and Exploration, Lunar and Planetary Institute, Houston, Texas, USA

We modeled the differentiation of the South Pole–Aitken (SPA) impact melt sheet to determine whether noritic lithologies observed within SPA formed as a result of the impact. Results indicate differentiation of SPA impact melt can produce noritic layers that may accommodate observed surface compositions but only in specific scenarios. One of nine modeled impact melt compositions yielded layers of noritic materials that account for observations of noritic lithologies at depths of ~6 km. In this scenario, impact occurred before a hypothesized lunar magma ocean cumulate overturn. The 50 km deep melt sheet would have formed an insulating quenched layer at the surface before differentiating. The uppermost differentiated layers in this scenario have FeO and TiO₂ contents consistent with orbital observations if they were subsequently mixed with the uppermost quenched melt layer and with less FeO- and TiO₂-enriched materials such as ejecta emplaced during younger impacts. These results verify that noritic lithologies observed within SPA could have formed as a direct result of the impact. Therefore, locations within SPA that contain noritic materials represent potential destinations for collecting samples that can be analyzed to determine the age of the SPA impact. Potential destinations include central peaks of Bhabha, Bose, Finsen, and Antoniadi craters, as well as walls of Leibnitz and Schrödinger basins. Additionally, potential remnants of the uppermost quenched melt may be preserved in gabbroic material exposed in “Mafic Mound.” Exploring and sampling these locations can constrain the absolute age of SPA, a task that ranks among the highest priorities in lunar science.

Reference
Hurwitz DM and Kring DA (in press) Differentiation of the South Pole–Aitken basin impact melt sheet: Implications for lunar exploration. Journal of Geophysical Research: Planets 
[doi:10.1002/2013JE004530]
Published by arrangement with John Wiley & Sons

Link to Article

Vasavada¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

The Mars Science Laboratory mission reached Bradbury Landing in August 2012. In its first 500 sols, the rover Curiosity was commissioned and began its investigation of the habitability of past and present environments within Gale Crater. Curiosity traversed eastward toward Glenelg, investigating a boulder with a highly alkaline basaltic composition, encountering numerous exposures of outcropping pebble conglomerate, and sampling aeolian sediment at Rocknest and lacustrine mudstones at Yellowknife Bay. On sol 324, the mission turned its focus southwest, beginning a year-long journey to the lower reaches of Mt. Sharp, with brief stops at the Darwin and Cooperstown waypoints. The unprecedented complexity of the rover and payload systems posed challenges to science operations, as did a number of anomalies. Operational processes were revised to include additional opportunities for advance planning by the science and engineering teams.

Reference
Vasavada et al. (in press) Overview of the Mars Science Laboratory mission: Bradbury Landing to Yellowknife Bay and beyond. Journal of Geophysical Research: Planets
[doi:10.1002/2014JE004622]
Published by arrangement with John Wiley & Sons

Link to Article

Birger Schmitz^a,b et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aDepartment of Physics, Lund University, Lund, Sweden
^bHawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, HI, USA

About a quarter of all meteorites falling on Earth today originate from the breakup of the L-chondrite parent body ~470 Ma ago, the largest documented breakup in the asteroid belt in the past ~3 Ga. A window into the flux of meteorites to Earth shortly after this event comes from the recovery of about 100 fossil L chondrites (1–21 cm in diameter) in a quarry of mid-Ordovician limestone in southern Sweden. Here we report on the first non-L-chondritic meteorite from the quarry, an 8 cm large winonaite-related meteorite of a type not known among present-day meteorite falls and finds. The noble gas data for relict spinels recovered from the meteorite show that it may be a remnant of the body that hit and broke up the L-chondrite parent body, creating one of the major asteroid families in the asteroid belt. After two decades of systematic recovery of fossil meteorites and relict extraterrestrial spinel grains from marine limestone, it appears that the meteorite flux to Earth in the mid-Ordovician was very different from that of today.

Reference
Schmitz et al. (in press) A fossil winonaite-like meteorite in Ordovician limestone: A piece of the impactor that broke up the L-chondrite parent body? Earth and Planetary Science Letters 400:145.
[doi:10.1016/j.epsl.2014.05.034]
Copyright Elsevier

Link to Article

Derek W. G. Sears

Space Science and Astrobiology Division, NASA Ames Research Center/BAER Institute, Mountain View, California, USA

In this interview, William Hartmann (Bill, Fig. 1) describes how he was inspired as a teenager by a map of the Moon in an encyclopedia and by the paintings by Chesley Bonestell. Through the amateur journal “Strolling Astronomer,” he shared his interests with other teenagers who became lifelong colleagues. At college, he participated in Project Moonwatch, observing early artificial satellites. In graduate school, under Gerard Kuiper, Bill discovered Mare Orientale and other large concentric lunar basin structures. In the 1960s and 1970s, he used crater densities to study surface ages and erosive/depositional effects, predicted the approximately 3.6 Gyr ages of the lunar maria before the Apollo samples, discovered the intense pre-mare lunar bombardment, deduced the youthful Martian volcanism as part of the Mariner 9 team, and proposed (with Don Davis) the giant impact model for lunar origin. In 1972, he helped found (what is now) the Planetary Science Institute. From the late 1970s to early 1990s, Bill worked mostly with Dale Cruikshank and Dave Tholen at Mauna Kea Observatory, helping to break down the Victorian paradigm that separated comets and asteroids, and determining the approximately 4% albedo of comet nuclei. Most recently, Bill has worked with the imaging teams for several additional Mars missions. He has written three college textbooks and, since the 1970s, after painting illustrations for his textbooks, has devoted part of his time to painting, having had several exhibitions. He has also published two novels. Bill Hartmann won the 2010 Barringer Award for impact studies and the first Carl Sagan Award for outreach in 1997.

Reference
Sears DWG (in press) Oral histories in meteoritics and planetary science—XXIV: William K. Hartmann. Meteoritics & Planetary Science
[doi:10.1111/maps.12298]
Published by arrangement with John Wiley & Sons

Link to Article

Norbert Schorghofer¹ and Oded Aharonson²

¹Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii, Honolulu, HI 96822, USA
²Helen Kimmel Center for Planetary Science, Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, 76100, Israel

It has long been suggested that water ice can exist in extremely cold regions near the lunar poles, where sublimation loss is negligible. The geographic distribution of H-bearing regolith shows only a partial or ambiguous correlation with permanently shadowed areas, thus suggesting that another mechanism may contribute to locally enhancing water concentrations. We show that under suitable conditions, water molecules can be pumped down into the regolith by day-night temperature cycles, leading to an enrichment of H₂O in excess of the surface concentration. Ideal conditions for pumping are estimated and found to occur where the mean surface temperature is below 105 K and the peak surface temperature is above 120 K. These conditions complement those of the classical cold traps that are roughly defined by peak temperatures lower than 120 K. On the present-day Moon, an estimated 0.8% of the global surface area experiences such temperature variations. Typically, pumping occurs on pole-facing slopes in small areas, but within a few degrees of each pole the equator-facing slopes are preferred. Although pumping of water molecules is expected over cumulatively large areas, the absolute yield of this pump is low; at best, a few percent of the H₂O delivered to the surface could have accumulated in the near-surface layer in this way. The amount of ice increases with vapor diffusivity and is thus higher in the regolith with large pore spaces.

Reference
Schorghofer N and Aharonson O (2014) The Lunar Thermal Ice Pump. The Astrophysical Journal 788:169.
[doi:10.1088/0004-637X/788/2/169]

Link to Article

M. Rubin¹, N. Fougere², K. Altwegg^1,3, M. R. Combi², L. Le Roy³, V. M. Tenishev² and N. Thomas^1,3

¹Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
²Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109, USA
³Center for Space and Habitability, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

Comets are surrounded by a thin expanding atmosphere, and although the nucleus’ gravity is small, some molecules and grains, possibly with the inclusion of ices, can get transported around the nucleus through scattering (atoms/molecules) and gravitational pull (grains). Based on the obliquity of the comet, it is also possible that volatile material and icy grains get trapped in regions, which are in shadow until the comet passes its equinox. When the Sun rises above the horizon and the surface starts to heat up, this condensed material starts to desorb and icy grains will sublimate off the surface, possibly increasing the comet’s neutral gas production rate on the outbound path. In this paper we investigate the mass transport around the nucleus, and based on a simplified model, we derive the possible contribution to the asymmetry in the seasonal gas production rate that could arise from trapped material released from cold areas once they come into sunlight. We conclude that the total amount of volatiles retained by this effect can only contribute up to a few percent of the asymmetry observed in some comets.

Reference
Rubin M, Fougere N, Altwegg K, Combi MR, Le Roy L, Tenishev VM and Thomas N (in press) Mass Transport around Comets and its Impact on the Seasonal Differences in Water Production Rates. The Astrophysical Journal 788:168.
[doi:10.1088/0004-637X/788/2/168]

Link to Article