Marc D. Norman^a,b and Alexander A. Nemchin^c

^aLunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA
^bResearch School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT 0200, Australia
^cDepartment of Applied Geology, Curtin University of Technology, Perth, WA 6845, Australia

A sharp rise in the flux of asteroid-size bodies traversing the inner Solar System at 3.9 Ga has become a central tenet of recent models describing planetary dynamics and the potential habitability of early terrestrial environments. The prevalence of ~3.9 Ga crystallization ages for lunar impact-melt breccias and U–Pb isotopic compositions of lunar crustal rocks provide the primary evidence for a short-lived, cataclysmic episode of late heavy bombardment at that time. Here we report U–Pb isotopic compositions of zirconolite and apatite in coarse-grained lunar melt rock 67955, measured by ion microprobe, that date a basin-scale impact melting event on the Moon at 4.22±0.01 Ga followed by entrainment within lower grade ejecta from a younger basin approximately 300 million yr later. Significant impacts prior to 3.9 Ga are also recorded by lunar zircons although the magnitudes of those events are difficult to establish. Other isotopic evidence such as ⁴⁰Ar–³⁹Ar ages of granulitic lunar breccias, regolith fragments, and clasts extracted from fragmental breccias, and Re–Os isotopic compositions of lunar metal is also suggestive of impact-related thermal events in the lunar crust during the period 4.1–4.3 Ga. We conclude that numerous large impactors hit the Moon prior to the canonical 3.9 Ga cataclysm, that some of those pre-cataclysm impacts were similar in size to the younger lunar basins, and that the oldest preserved lunar basins are likely to be significantly older than 3.9 Ga. This provides sample-based support for dynamical models capable of producing older basins on the Moon and discrete populations of impactors. An extended period of basin formation implies a less intense cataclysm at 3.9 Ga, and therefore a better opportunity for preservation of early habitable niches and Hadean crust on the Earth. A diminished cataclysm at 3.9 Ga suggests that the similarity in the age of the oldest terrestrial continental crust with the canonical lunar cataclysm is likely to be coincidental with no genetic significance.

Reference
Norman MD and Alexander A. Nemchin AA (in press) A 4.2 billion year old impact basin on the Moon: U–Pb dating of zirconolite and apatite in lunar melt rock 67955. Earth and Planetary Science Letters
[doi:10.1016/j.epsl.2013.11.040]
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Romain Tartèse^a and Mahesh Anand^b

^aPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
^bDepartment of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom

Refers To
Romain Tartèse, Mahesh Anand
Late delivery of chondritic hydrogen into the lunar mantle: Insights from mare basalts
Earth and Planetary Science Letters, Volume 361, 1 January 2013, Pages 480-486

The authors regret for an error which was introduced in an intermediate step involved in calculating the amount of chondritic material added to the lunar interior to account for the estimated H content of the mare basalt source regions (Section 6, lines 12–25). The revised text is as follows:
Assuming a 400 km deep solidified mantle with 25 ppm H and a density of 3300 kg m⁻³ implies that ~1×10¹⁸ kg of H has been added by chondrite-type impactors and efficiently mixed in the upper lunar mantle. Taking the measured H content of ~5000–15 000 ppm in CI chondrites (Alexander et al., 2012 and Kerridge, 1985), this represents a mass of about 6.2×10¹⁹ to 1.8×10²⁰ kg of CI-type material accreted to the lunar upper mantle. This corresponds to 0.2–0.5 wt.% of the 400 km deep upper mantle considered here. By comparison, HSE abundances in lunar basalts require an amount of ~1.6×10¹⁹ kgof chondritic material to have been accreted and mixed into the lunar upper mantle (Bottke et al., 2010 and Day et al., 2007), around 8±4 times less than that required for a lunar mantle with 25 ppm H.

Reference
Tartèse R and Mahesh Anand M (in press) Corrigendum to “Late delivery of chondritic hydrogen into the lunar mantle: Insights from mare basalts” [Earth Planet. Sci. Lett. 361 (2013) 480–486]. Earth and Planetary Science Letters
[doi:10.1016/j.epsl.2014.01.002]
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Pavlo P. Korsun^a, Philippe Rousselot^b, Irina V. Kulyk^a, Viktor L. Afanasiev^c, Oleksandra V. Ivanova^a

^aMain Astronomical Observatory of NAS of Ukraine, Akademika Zabolotnoho 27, 03680 Kyiv, Ukraine
^bUniversity of Franche-Comté, Observatoire des Sciences de l’Univers THETA, Institut UTINAM – UMR CNRS 6213, BP 1615, 25010 Besançon Cedex, France
^cSpecial Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz, 369167, Russia

Spectrophotometric monitoring of distant comet C/2002 VQ94 (LINEAR) was performed with the 6-m telescope of SAO RAS (Special Astrophysical Observatory of Russian Academy of Sciences) and with the 2.5-m Nordic Optical Telescope (Observatory del Roque de los Muchachos, Canarias, Spain) between 2008 and 2013. During this period the comet was on the outbound segment of its orbit, between heliocentric distances of 8.36 au and 16.84 au. Analysis of the spectra revealed the presence of the CO⁺ and N₂⁺emissions in the cometary coma at a distance of 8.36 au from the Sun. This distance is larger than ionic emissions have been detected in any previous objects. Only continuum, with no traces of emissions, was detected in the spectrum obtained in 2009 when the comet was at a distance of 9.86 au. From the spectra obtained in 2008, average column densities of 2.04×10⁹ mol cm^-2 for N₂⁺ and 3.26×10¹⁰ mol cm^-2 for CO⁺were measured in the cometary coma. The derived values correspond to N₂⁺/CO⁺=0.06 within the projected slit. Images obtained through a red continuum filter in 2008 showed a bright, dust coma, indicating a high level of physical activity. A considerably lower level of activity was observed in 2009 and 2011 at distances of 9.86 au and 13.40 au respectively. No noticeable activity was detected in 2013 at a heliocentric distance of 16.84 au.The Afρ parameter, which is used as an indicator of cometary activity, was measured as 2000 cm in 2008, and 800 cm in 2009 and 2011. The Afρ values correspond to dust production rates between 10-20 kg s^-1, 4-6 kg s^-1 and 3-5 kg s^-1 at 8.36, 9.86, and 13.40 au respectively. There is an obvious correlation between the decrease of the dust production rate of the nucleus and the disappearance of the emissions in the spectrum of C/2002 VQ94 (LINEAR) at heliocentric distances greater than 9 au. The colors and size of the nucleus of C/2002 VQ94 (LINEAR) were estimated from the images obtained during the late stage at a heliocentric distance of 16.84 au, when the activity had probable ceased. The B-V and V-R colors were estimated to be 1.07±0.05 and 0.54±0.03 respectively. The effective nucleus radius of 482 km is in agreement with the previously published results, obtained from the observations of the comet during its early inactive stage ( and ).

Reference
Korsun PP, Philippe Rousselot P, Kulyk IV, Afanasiev VL and Ivanova OV (in press) Distant activity of comet C/2002 VQ94 (LINEAR): optical spectrophotometric monitoring between 8.4 and 16.8 au from the Sun. Icarus
[doi:10.1016/j.icarus.2014.01.006]
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