51st Lunar and Planetary Science Conference (2020) 2589.pdf

ASTEROIDS, COMETS, AND KUIPER BELT OBJECTS: SOURCES OF INNER AND OUTER SOLAR SYSTEM CRATER POPULATIONS. S.J. Robbins*,a, C.M. Lisseb, Y.R. Fernándezc, J.M. Bauerd, A.F. Chengb, H.A. Weaverb, W.B. McKinnone, J.J. Kavelaarsf, S.A. Sterna, V.J. Brayg, C.B. Beddingfieldh,i, J.R. Spencera, C.B. Olkina, J.W. Parkera, J.M. Moorei, O.M. Umurhanh,i, W. Grundyj, L.A. Younga, A. Verbiscerk, M. Kinczykl and the New Horizons GGI Science Team. *[email protected], 1Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302 aSouthwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, United States; bThe Johns Hopkins University, Baltimore, MD 21218, United States; cUniversity of Central Florida, Orlando, FL 32816, United States; dUniversity of Maryland, College Park, MD 20742, United States; eWashington University in St. Louis, St. Louis, MO 63130, United States; fUniversity of Victoria, Victoria, BC V8P 5C2, Canada; gLunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, United States; hSagan Center at the SETI Institute and NASA Ames Research Center, United States; iNASA Ames Research Center, Moffett Field, CA 94043, United States; jLowell Observatory, Flagstaff, AZ 86001, United States; kUniversity of Virginia, PO Box 400325, Charlottesville, VA 22904, United States; lNorth Carolina State Univ.

Introduction: The size-frequency distribution are thought to dominate. Therefore, we have measured (SFD) of Solar System objects is diagnostic of the for- impact craters in a few young regions of Mercury and mational and collisional history of those objects. As similarly attempted to exclude secondaries. smaller objects impact larger ones, craters form which Moon: Maria unit craters were extracted from [5]; a can be used to infer the impactor population through the general, spoken (unciteable) rule-of-thumb for lunar im- use of various scaling laws. The lunar crater population pacts is that secondaries tend to dominate near D ≈ 1–3 is well studied and forms the basis and model for inner km, so craters D < 3 km were removed. solar system bodies, while near-Earth asteroids (NEAs) Mars: The largest Mars crater database is outdated provide a present-day comparison population. Com- relative to modern imagery, so we used new, un- bined, these can be used to model the main asteroid published data in young, Amazonian terrains, assem- belt’s (MBA’s) SFD (e.g., [1]). bled from CTX imagery that excluded secondary im- The outer solar system (OSS) likely underwent a pacts. This database should be complete to D ≥ 0.75 km. distinct collision and dynamic history, which should be Vesta: Data from both [6] and [7] do not match other reflected in differences in the SFD of impact craters and crater data (Fig. 1) and we are investigating this. small bodies that formed them. However, the OSS is Ceres: Unpublished data from [8] was used that had much more difficult to observe: Many fewer spacecraft a stated completeness limit of 1 km; to be conservative, have imaged solid OSS surfaces, and ground- or near- we clipped this dataset at D ≥ 2 km. Earth-based telescopic surveys of comet nuclei are NEA: Data from [9] was used. much more difficult than asteroid surveys reaching MBA: The latest models based on [1] were used. down to the same size. ISS Results: Preliminary analysis in Fig. 1 shows an Recent advances from telescopic surveys (e.g., [2- RSFD normalized such that the different crater and im- 3]) have offered at least two independent estimates of pactor populations are visible in a narrow vertical range. the SFD of Jupiter-family comet (JFC) and short- and From this preliminary work, most of the different bodies long-period comet (SPC, LPC) nuclei from the Kuiper show the same basic trend, indicating a similar source- Belt. Independently, NASA’s New Horizons spacecraft impact population. The primary outlier is Vesta, which in the past five years has provided unprecedented imag- we are further investigating. ing of Pluto, Charon, their satellite Nix, and Kuiper Belt Outer Solar System Craters: For this work, we Object (KBO) Arrokoth that spatially resolved impact chose to focus on well-imaged craters in the Saturn and craters. The Pluto-Charon system provides an im- Pluto-Charon systems, in addition to Arrokoth. portant crater population because the giant planets’ sat- Mimas, Iapetus, Rhea: Craters by [10] were used, ellite systems likely exchange material, such that their which are complete for D ≳ 3 km, 3 km, and 1 km in crater populations are likely contaminated by planeto- regions they cover, respectively. The best coverage centric impactors. around Inktomi crater on Rhea was removed to avoid This work seeks to answer two very broad questions: secondary craters, and there is no good evidence of re- Given these recent surveys and spacecraft data, (1) is solved secondary craters elsewhere on these bodies. there convincing evidence for distinct inner and outer Tethys, Dione: Craters by [11-12] were used, which solar system impactor populations, and (2) is there con- should be complete for ≳5 km. vincing evidence that the cometary SFD is the source of Charon: To avoid resurfacing effects on Pluto, OSS impact craters? Charon’s craters on Vulcan Planum were used based on Inner Solar System and Asteroids: Numerous da- new counts which closely match those by [13]. The tasets need to be assembled. counts should be complete for D ≳ 3 km. Mercury: The most complete Mercury crater data- Nix: Craters from [14] were used which should be base [4] was designed to eliminate secondary impact complete for D ≳ 1–2 km. craters. However, examination shows that that effort Arrokoth: Craters from [15] were used which, based might have eliminated too many craters, for the SFD on the highest spatial scale imaging, should be complete shows a substantial decrease – instead of maintaining – down to ~100s of meters. the slope at crater diameters where secondary impacts Completeness estimates are important to understand. 51st Lunar and Planetary Science Conference (2020) 2589.pdf

Some researchers have based completeness upon a SFD [5] Robbins (2019) doi:10.1029/2018JE005592. [6] Marchi et al. (2012) doi:10.1126/science.1218757. [7] Liu et al. (2018) transition to shallower power-laws based on the ISS, doi:10.1016/j.icarus.2018.04.006. [8] Zeilnhofer & Barlow (2019, where the SFD is well known to continue with a ≈–3 pers. comm.) Abs #1259. [9] Harris & D’Abramo (2015) power-law slope (differential SFD) from kilometers doi:10.1016/j.icarus.2015.05004. [10] Robbins et al. (2015) Abs #1654. [11] Kirchoff & Schenk (2010) doi:10.1016/j.icarus. through meters of size. When using this assumption for 2009.12.007. [12] Kirchoff & Schenk (2015) doi:10.1016/j.icarus. Saturnian system craters, completeness limits have been 2015.04.010. [13] Singer et al. (2019) doi:10.1126/science.aap8628. [14] Robbins et al. (2017) doi:10.1016/j.icarus.2016.09.027. [15] placed near D ≈ 5–10 km. However, using a limit of Spencer et al. (in rev.). ≈8–10 pixels, completeness would be closer to ≈1–3 km. Comets— JFCs, SPCs, LPCs: Today’s JFCs Funding: This work was funded by NASA’s New Horizons likely originated from scattered KBOs that were once in mission. the trans-Neptunian region. There have been many re- 10 9 cent attempts to measure the JFC SFD based on tele- 8 Mercury 7 Moon scopic surveys. Most attempts have derived sizes using 6 Mars 5 Vesta optical photometry of sunlight scattered by the nucleus 4 Ceres and an assumed albedo, but newer work using Spitzer Near-Earth Asteroids (Dx25) 3 Main Belt Asteroids (Dx25) and WISE infrared photometry can directly measure =10km) sizes, for thermal emissivities vary little. The IR studies D 2 demonstrate cometary SFDs are different from inner so- lar system asteroid and crater SFDs: Asteroid SFDs fol- 1 9 low a reasonably constant power-law at diameters from 8 7 ~10 km to at least ~10 m. By contrast, the JFC SFD ap- 6 pears to have an inflection point to a shallower SFD 5 slope starting ~1–4 km. However, de-biasing these sur- 4 3

veys based on observational limitations is difficult, and (normalized to RSFDValue past authors have tended to propose ad hoc mechanisms 2 for removing small nuclei to reconcile the SFDs. How-

ever, is this necessary? 0.1 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 For our comparison to the OSS crater population, we 1 10 100 used the JFC survey from [2] coupled with the SPC and Diameter (km) LPC survey from [3] – two independent surveys that ar- rived at similar observed SFDs. However, like OSS cra- Figure 1: Inner solar system crater populations with NEA and MBA populations overlaid. Extremely simple ters, some authors have assumed that some form of SFD scaling to the asteroid populations has been applied, de-biasing is needed in order to match the inner solar shifting their diameters by a factor of 25. We are work- system or asteroid SFDs. ing to make this more physically correct.

Outer Solar System Synthesis: Figure 2 shows a ` preliminary synthesis of the OSS comet and moon SFDs. 10 Mimas Tethys Regarding the general SFD shapes, the Saturnian satel- 6 Dione Iapetus 5 lites follow similar SFDs, though the shallower slopes 4 Rhea Charon Nix Arrokoth (that some interpret as completeness issues) are differ- 3 Jupiter-Family Comets* 2 Short-Period Comets* ent. However, Charon, Nix, and Arrokoth have ex- Long-Period Comets* *10x Diameter scaling applied tremely similar – and shallow – slopes that closely 1 match Mimas and Iapetus. Additionally, the comet =10 km, cometsarbitrary)=10 km, 6 slopes are similar to the shallow SFD components of D 5 4 these three bodies. The mismatch at ~10s of km crater 3 diameters vs comets that would produce ~10s of km cra- 2 ters is still a subject we are investigating. However, this clearly demonstrates not only that the 0.1 6 shallow slopes on Saturnian satellite crater populations 5 is likely real, but by extension, the Charon, Nix, and Ar- 4 3

rokoth data indicate that comet surveys likely are sam- 2 pling the true SFD of ~km-scale nuclei. 0.01 Ongoing Work: At the LPSC2020 conference, we (cratersnormalized to RSFDValue 2 3 4 5 6 2 3 4 5 6 2 3 4 5 6 2 expect to have more accurate scaling between craters 1 10 100 Diameter (km) and impactors, and to have more quantitative compari- sons between the different populations. Figure 2: Outer solar system crater populations with References: [1] Bottke et al. (2005, & pers. comm.) doi:10.1016/j.ic- comet populations overlaid. Extremely simple scaling arus.2004.10.026. [2] Fernández et al. (2013) doi: 10.1016/j.ica- to the comet populations has been applied, shifting their rus.2013.07.021. [3] Bauer et al. (2017) doi:10.3847/ 1538- diameters by a factor of 10. We are working to make 388/aa72df. [4] Herrick et al. (2018) doi:10.1029/ 2017JE005516. this more physically correct.