47th Lunar and Planetary Science Conference (2016) 1616.pdf

REVISITING THE CSFDS OF THE TAURUS LITTROW DARK MANTLE DEPOSIT: IMPLICATIONS FOR AGE DETERMINATIONS OF PYROCLASTIC DEPOSITS. C. H. van der Bogert1, L. Gaddis2, H. Hie- singer1, M. Ivanov1,3, B. Jolliff4, P. Mahanti5, and J. H. Pasckert1, 1Institut für Planetologie, Westfälische Wilhelms- Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany ([email protected]); 2Astrogeology Science Center, U.S. Geological Survey, Flagstaff, AZ; 3Vernadsky Insitute, Moscow, Russia; 4Washington Univer- sity, St. Louis, MO, USA; 5Arizona State University, Tempe, AZ, USA.

Introduction: The crater size-frequency distribu- basalts, to serve as ground truth for our investigation. tions (CSFDs) of small craters on dark deposits on the The same area was investigated before and after the Moon were first studied in preparation for the Apollo Apollo 17 mission [3,16]; however, that work was missions. The subdued appearance of the dark man- tling material in Taurus Littrow was interpreted to in- dicate that it was young [1,2]. This observation was one of the motivations for the selection of Taurus Littrow as the Apollo 17 landing site. However, the glasses collected during the mission and interpreted to originate from the dark mantle deposits (DMDs) have ages similar to fragments of the underlying and adja- cent mare basalts (ca. 3.7 Ga)[3]. Thus, [3] proposed that the paucity of craters with diameters <500 m, par- ticularly those with diameters of 100-200 m, must be caused by an abnormally high rate of degradation due

to the thick, unconsolidated nature of the DMDs. In- deed, DMDs have decreased radar returns at both 3.8 and 12.6 cm wavelengths compared to adjacent high- land and mare units, suggesting that they contain no large rocks (~1-50 cm diameter) for at least 1-5 meters in depth [4-6]. The challenges encountered during the interpreta- tion of the Taurus Littrow (TL) DMD are relevant to CSFD measurements on other possible pyroclastic deposits [7-10]. For example, pyroclastics may be pre- sent at both the Compton-Belkovich Volcanic Com- plex and Lassell Massif based on their morphologic, spectral, and radar characteristics [5,9,11]. However, it is problematic to date small occurrences of these de- posits with CSFDs owing to their apparent sensitivity to either degradation and/or target property effects [8,9,12]. At both locations, CSFDs of small craters are subparallel to the nominal equilibrium condition. We interpreted these CSFDs to indicate the influence of a continuous resurfacing process such as mass-wasting, for which pyroclastic deposits would be particularly vulnerable. Because of these challenges, such deposits are often dated indirectly by bracketing the age via CSFD measurement of adjacent stratigraphic units [10,13]. A bracketing approach is also necessary in cases where the DMD may be too thin to have effec- tively reset the crater chronometer. Approach: We revisit the CSFDs of the TL DMD, Figure 1. (top) Locations of count areas on mare in using Kaguya Terrain Camera (TC) [14] and Lunar and near Taurus Littrow shown on the TC evening mo- Reconnaissance Orbiter Wide Angle (WAC) and Nar- saic. S5 contains thick DMDs, while S11 and S15 are row Angle Camera (NAC) [15] data, because radio- unmantled mare. (bottom) Our CSFD measurements metric ages are available for it and its related mare are similar to those derived by [3]. 47th Lunar and Planetary Science Conference (2016) 1616.pdf

done prior to the establishment of a calibrated lunar chronology [17], we are able to fit ages and provide a chronology [17,18]. We used the mare basalt map of refined interpretation of the origins of the differently [19] to select new count areas within the old mare (unit sloped portions of the CSFDs. Our analyses suggest S5, 3.70±0.04 Ga [19]) surrounding the Apollo 17 that the apparent deficit of small craters on the DMD is landing site, as well as in two adjacent younger mare caused by two effects. First, the DMD is older than the deposits (units S11, 3.55+0.05-0.09 Ga and S15, surrounding mare, and seems to overlie an even older 3.44+0.03-0.06 Ga [19]). CSFDs were measured on mare unit with an age of 3.83 Ga (Fig. 2). Because the WAC and TC data sets, with counts on NAC imagery DMD is older, equilibrium is achieved at larger crater to follow. Radiometric ages of mare basalts at the diameters (~250 m) than for the younger mare. The Apollo 17 landing site are consistent with the absolute youngest adjacent mare (S11), which exhibits a 3.17 model ages of [19], including 3.7-3.8 Ga [20], 3.56- Ga age in our work, has a production population down 3.84 Ga [21], and 3.59-3.79 Ga [summarized by 22]. to ~140 m diameters. Thus, the CSFDs of the units Orange glasses associated with the DMDs [23,24] have different slopes over different diameter ranges, so were determined to be 3.48 to 3.66 Ga [25,26]. that they cross at crater diameters of ~150 m. Second, Results and Discussion: Qualitatively, we ob- the level of saturation reached in the equilibrated served the same morphologies as [3] for the small cra- branch of each area is different likely due to the faster ters in the DMD: subdued appearance and shallower degradation of craters in the DMD with respect to the depths [see also 27]. The dark albedo and subdued adjacent mare surfaces, as suggested by [3]. crater rims make it challenging to measure the crater An interesting new result of applying the calibrated diameters on the TC mosaic. Some craters also appear lunar chronology is that a resurfacing event at 3.63 Ga to be partially filled with DMD material and/or basalt can be fit in the S5 NW count area (Fig. 2). This resur- flows. Our CSFDs (Fig. 1), are also similar to those facing age is consistent with the radiometric age of the presented in Figure 2 of [3]. However, using the lunar orange glass sampled by Apollo 17. Using the method of [28], we estimate the thickness of the DMD and its overlying regolith at the S5 NW count area to be ~26- 40 m. This is consistent with an estimate of 20 m by [3], and the inference of [6] that the western part of the mantle deposit is thicker than at the landing site (10-14 m for DMD plus regolith [29-31]). Implications: Our study indicates that it is possi- ble to derive reliable absolute model ages from thick DMD deposits that are consistent with radiometric ages of nearby samples. Thin DMDs are more prob- lematic and may require bracketing to constrain their ages [10,13]. As part of our ongoing study, we will measure CSFDs on thinner areas of the TL DMD to investigate the effects of deposit thickness on the in- terpretation of CSFDs and absolute model ages.

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