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| Titel |
How old is Autolycus crater? |
| VerfasserIn |
Harald Hiesinger, Jan Henrik Pasckert, Carolyn H. van der Bogert, Mark S. Robinson |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250129528
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| Publikation (Nr.) |
 EGU/EGU2016-9655.pdf |
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| Zusammenfassung |
| Accurately determining the lunar cratering chronology is prerequisite for deriving absolute
model ages (AMAs) across the lunar surface and throughout the Solar System [e.g.,
1]. However, the lunar chronology is only constrained by a few data points over
the last 1 Ga and there are no calibration data available between 1 and 3 Ga and
beyond 3.9 Ga [2]. Rays from Autolycus and Aristillus cross the Apollo 15 landing
site and presumably transported material to this location [3]. [4] proposed that at
the Apollo 15 landing site about 32% of any exotic material would come from
Autolycus crater and 25% would come from Aristillus crater. [5,6] proposed that the
39Ar-40Ar age of 2.1 Ga derived from three petrologically distinct, shocked Apollo 15
KREEP basalt samples, date Autolycus crater. Grier et al. [7] reported that the optical
maturity (OMAT) characteristics of these craters are indistinguishable from the
background values despite the fact that both craters exhibit rays that were used to infer
relatively young, i.e., Copernican ages [8,9]. Thus, both OMAT characteristics and
radiometric ages of 2.1 Ga and 1.29 Ga for Autolycus and Aristillus, respectively,
suggest that these two craters are not Copernican in age. [10] interpreted newer U-Pb
ages of 1.4 and 1.9 Ga from sample 15405 as the formation ages of Aristillus and
Autolycus. If Autolycus is indeed the source of the dated exotic material collected at the
Apollo 15 landing site, than performing crater size frequency distribution (CSFD)
measurements for Autolycus offers the possibility to add a new calibration point to the lunar
chronology, particularly in an age range that was previously unconstrained. We used
calibrated and map-projected LRO NAC images to perform CSFD measurements within
ArcGIS, using CraterTools [11]. CSFDs were then plotted with CraterStats [12], using
the production and chronology functions of [13]. We determined ages of 3.72 and
3.85 Ga for the interior (Ai1) and ejecta area Ae3, which we reject because our
CSFDs show evidence of secondary craters. Areas Ae1 and Ae2 show very young
AMAs (<∼0.5-0.6 Ga), which are too young, considering the fact that Aristillus
superposes Autolycus and the results of OMAT studies [7]. Areas Ae4 and Ae5 yielded
ages of 3.20 and 3.45 Ga, respectively. Although these ages are least affected by
secondaries from Aristillus, they are much older than the 2.1 Ga sample ages that
were linked to the formation of Autolycus crater [5,6]. This either implies that the
dated samples are not related to Autolycus or that the CSFD measurements are so
heavily affected by resurfacing and secondary cratering from Aristillus that they
do not represent the formation age of Autolycus. In either case, because of these
uncertainties Autolycus can not currently be used as a calibration point for the lunar
chronology function. A dedicated mission to either sample terrains with ages of 1-3
Ga or in situ dating such surfaces is of high priority to further constrain the lunar
chronology.
[1] Hiesinger et al. (2012) JGR 117; [2] Stöffler and Ryder (2001) Chronology and
Evolution of Mars; [3] Wilhelms (1987) USGS Spec. Pub. 1348; [4] Schultz (1986) Tech.
Rep. 86-03; [5] Ryder et al. (1991) Geology 19; [6] Bogard et al. (1990) Geochim.
Cosmochim. Acta 54; [7] Grier et al. (1999) LPSC 30; [8] Hackman (1966) USGS I-463; [9]
Page (1970) USGS I-666; [10] Grange et al. (2013) JGR 118; [11] Kneissl et al. (2012) PSS
59; [12] Michael and Neukum, (2010) EPSL 294; [13] Neukum et al. (2001) SSR 96. |
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