EPSC Abstracts Vol. 12, EPSC2018-238-1, 2018 European Planetary Congress 2018 EEuropeaPn PlanetarSy Science CCongress c Author(s) 2018

The Depth of Simple Craters and the Shadows they Cast: Evidence for on and the

Lior Rubanenko, Jaahnavee Venkatraman and A. Paige University of California, Los Angeles, USA ([email protected])

Abstract (MLA, 250 m/px) and gridded Lunar Orbiter Laser Altime- ter (LOLA, 120 m/px), as shown in Figure 1. On Mercury Mercury and the Moon are similar in their tiny obliquity and we measured craters between latitudes 75° 86°, where the − airless environment, but are vastly different in their observed most reliable MLA data is found. On the Moon, we measured ice quantities. Here we report craters on both planetary craters in latitudes 75° 90°, and plan to extend this range − bodies become shallower in latitudes in which ice is expected to include both lower and higher latitudes. Overall, we mea- to accumulate in the permanent shadows (PSRs) they cast. We sured 1003 craters on Mercury and 1237 craters on the Moon. measure the thickness of this infill and use it to constrain the Next, we model the distribution these craters would have had origin and age of this ice. if they were filled with ice up to the permanent shadow limit, and compare them with the measured distribution. 1. Introduction Topographic depressions near the poles of Mercury and the Moon may trap ice for billions of years inside PSRs [1,10,15]. On Mercury, evidence for water ice deposits that are at least a few meters thick was remotely sensed in [6]. Data obtained by the MErcury Surface, Space ENvironment, GEo- chemistry, and Ranging (MESSENGER) revealed bright and dark deposits in areas -enough to trap water ice according to a thermal model [3, 8, 11]. More recently, evidence for the presence of ice inside individual small craters ( 1 km) and ∼ micro cold-traps (1 10 m) was found using data obtained by − the Mercury Laser Altimeter (MLA) [5, 13]. N In contrast, lunar cold-traps were not observed to con- tain similar ice quantities. Evidence for a thin layer of ice -1.5 was found in several lunar craters such as Shackelton [16], but -based RADAR observations did not detect areas -2 > 1 km2 with high backscattering [14], indicating that any

Elevation (km) -2.5 existing ice must be thinner than a few decimeters or is in the 0 1 2 3 4 5 6 form of distributed grains [2]. Horizontal Distance (km) Here we constrain the thickness of the ice inside small crater using a statistical approach; we measure the depths Figure 1: An example showing our method of measurement. of small craters (diameters 3 15 km) on Mercury and the The blue line drawn on top of the crater in the top panel marks − Moon, and compare them to the depths these craters would the the elevation profile shown in blue in the lower panel. have had if they were filled with ice. If the cold-trap volatile outflux is greater than the volatile influx, the craters depth should remain constant with latitude. If the influx is greater than the outflux, with time ice should accumulate inside 3 Maximum Possible Infill craters, making them shallower. Due the exponential dependence of the sublimation rate on the 2. Methods ice can only persist inside the permanent shadow cast by the crater. During the , the crater casts transient We begin by identifying small (3 15 km), simple craters − shadows in different directions. We calculate the depth of this on the Mercury Dual Imaging System (MDIS) and the Lunar transient shadow ds assuming a hemispherical bowl-shaped Reconnaissance Orbiter Camera (LROC) global basemaps. crater, We measure the craters’ elevation along a south-north pro- d 1 s = 1 cot θ (1) file on the gridded Mercury Laser Altimeter polar map d − 2∆ where d is the crater’s depth, θ is the incidence angle and ∆ is the crater d/D ratio. The depth of the PSR constrains the depth of ice accumulated inside the crater. Therefore, if we subtract the modeled PSR depth from the measured d/D in lower latitudes (where only a small amount of ice is expected to persist [10, 12]) we should receive the d/D distribution of craters as if they were filled with ice up to the PSR limit. This assumes the d/D does not significantly change with latitude due to some other inherent geologic property.

4 Results and Discussion

Figure 2 shows the d/D distribution for Mercury (a) and the Moon (b). The mean d/D of crater on Mercury decreases from 0.96 0.0036 in latitudes 75° 78° to 0.075 0.0034 in ± − ± latitudes 83° 86°. On the Moon craters do not become shal- − lower until near-polar latitudes; the mean d/D decreases from 0.123 0.0038 in latitudes 75° 78° to 0.097 0.0033 in ± − ± latitudes 87° 90°. The values above are provided along with − the standard error of the mean. As explained above, we model the distribution of craters as if they were filled with ice to the permanent shadow limit (dashed line). We find that on Mer- cury craters are filled roughly to half of their capacity, while craters on the Moon far from being filled. Figure 2: (a) The d/D distribution on Mercury for two latitude Craters on both Mercury and the Moon become shallower rings. The mean crater depth decreases as deep (d/D>0.1) are at latitudes in which ice is expected to accumulate in the PSRs replaced by shallow (d/D<0.1) craters. (b) The d/D distri- they cast. If this infill is due to accumulation of ice, we es- bution on the Moon for three latitude rings. On the Moon, timate its thickness to be 10 100 m. This implies a net craters do not become significantly shallower until near-polar − delivery rate of a few meters per Ga, in accord with previous latitudes. theoretical [7, 9] and observational [4] estimates. Addition- ally, the difference between the mean measured and modeled [7] JI Moses et al. External sources of water for mercury’s filled crater distributions indicates the historic mean volatile putative ice deposits. Icarus, 1999. accumulation rate is greater - but probably of the same order [8] GA et al. Bright and dark polar deposits on - as the erosion rate. mercury: Evidence for surface volatiles. Science, 2013. [9] L Ong et al. Volatile retention from cometary impacts References on the moon. Icarus, 2010.

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