Stephanie C. Werner
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13263]
Centre for Earth Evolution and Dynamics, Department of Geosciences, University of Oslo, , 0315 Oslo, Norway
Published by arrangement with John Wiley & Sons

Crater densities on planetary surfaces allow assessing relative ages but so far firm calibration of so‐called cratering‐chronology models is available only for the Moon and limited to the past 4.1 billion years. Most planetary geological time scales are still model‐dependent, and essentially constrained by meteorite ages or by comparison to (dynamical) solar system evolution models. Here we describe in situ calibration of the Martian cratering chronology using cosmogenic and radiogenic isotope ages obtained by the NASA Curiosity rover. We determined the cratering‐rate ratio between Moon and Mars for recent times, and extended the calibration of cratering rates to earlier times than those based exclusively on lunar data. Our preferred interpretation supports monotonic flux decay since at least 4.24 Ga and likely since about 4.45 Ga, implying orbital migration of the giant planets, and its direct, transient, dynamical effect on the planetesimal populations was initiated early. But only Martian Sample Return will provide strongly needed capability for distinction of the different models currently available.

Andreas T. Hertwig^a, Kimura Makoto^b, Takayuki Ushikubo^c, Céline Defouilloy, ^aNoriko T.Kita^a
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.020]
^aWiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
^bNational Institute of Polar Research, Meteorite Research Center, Midoricho 10-3, Tachikawa, Tokyo 190-8518, Japan
^cKochi Institute for Core Sample Research, JAMSTEC, 200 Monobe-otsu, Nankoku, Kochi 783-8502 Japan
Copyright Elsevier

The ²⁶Al-²⁶Mg ages of FeO-rich (type II) chondrules from Acfer 094, one of the least thermally metamorphosed carbonaceous chondrites, were determined by SIMS analysis of plagioclase and olivine/pyroxene using a radio frequency (RF) plasma oxygen ion source. In combination with preexisting ²⁶Al-²⁶Mg ages of FeO-poor (type I) chondrules, the maximum range of formation ages recorded in chondrules from a single meteorite is determined to help provide constraints on models of material transport in the proto-planetary disk. We also report new SIMS oxygen three-isotope analyses of type II chondrules in Acfer 094. All but one of the plagioclase analyses show resolvable excesses in ²⁶Mg and isochron regressions yield initial ²⁶Al/²⁷Al ratios of type II chondrules that range from (3.62 ± 0.86) × 10^–6 to (9.3 ± 1.1) × 10^–6_, which translates to formation ages between 2.71 ^–0.22/_+0.28 Ma and 1.75 ^–0.11/_+0.12 Ma after CAI. This overall range is indistinguishable from that determined for type I chondrules in Acfer 094. The initial ²⁶Al/²⁷Al ratio of the oldest type II chondrule is resolved from that of all other type II chondrules in Acfer 094. Importantly, the oldest type I chondrule and the oldest type II chondrule in Acfer 094 possess within analytical error indistinguishable initial ²⁶Al/²⁷Al ratios and Δ¹⁷O values of ∼0‰. Ages and oxygen isotope ratios clearly set these two chondrules apart from all other chondrules in Acfer 094. It is therefore conceivable that the formation region of these two chondrules differs from that of other chondrules and in turn suggests that Acfer 094 contains two distinct chondrule generations.