Determining the shapes and surface properties of low-activity Jupiter-family comets from their rotational lightcurves

In situ missions to Jupiter-family comets (JFCs) have revealed that the population exhibits a diverse range of physical characteristics, from the shapes of their nuclei to their levels of activity. The nucleus properties of such comets may be dictated by the specific processes that they experience throughout their lifetimes as they evolve from the Kuiper Belt onto confined, short period orbits. Recent studies have linked the erosion of global surface topography with thermal processing close to the Sun [1, 2, 3], and provided evidence that the extent of surface evolution may impact the photometric properties of the nucleus [4]. In addition, the prevalence of bi-lobed shapes among the spacecraft-visited comets has resulted in much speculation on whether or not this configuration can be attributed to formation processes [e.g. 5,6]. However, these findings are all based on measurements for only a handful of JFCs. Our present goal is to extend the number of comets with well-constrained nucleus properties via ground-based observations, in order to test the validity of these predictions.

In this work, we present an analysis of rotational lightcurves acquired for five of the largest JFCs: 137P/Shoemaker-Levy 2, 143P/Kowal-Mrkos, 162P/Siding Spring, 169P/NEAT and 172P/Yeung. The datasets are composed of newly acquired observations in combination with archival lightcurves, providing a long temporal baseline for each comet. For three of the comets in the sample (137P, 143P and 162P) the collected data provided sufficient geometry coverage to derive uniquely-defined models of the shapes and spin states of their nuclei using convex lightcurve inversion: to date, the only other JFC to have been modelled in this way was Rosetta mission target 67P prior to the spacecraft encounter [7]. The new JFC shape models indicate an elongated nucleus for 162P, and more rounded shapes for 137P and 143P. Artificial lightcurves generated using these models suggest lower limits of nucleus elongation (a/b) of 1.82, 1.20, and 1.54 respectively. If the convex shape model properties are representative of the true nucleus elongations, the observed bilobed fraction for all short period comets with well-constrained shapes is reduced from ~71% to 60%, which remains significantly higher than is estimated for other small body populations. 

Using the shape models to correct the lightcurves for rotational modulation, linear phase functions (β) were measured as 0.048 ± 0.01 mag/deg for 137P, 0.048 ± 0.002 mag/deg for 143P, and 0.051 ± 0.002 mag/deg for 162P. Three of the JFCs in our sample were observed at phase angles ɑ < 1° but interestingly none exhibited any evidence for an opposition surge. Using the effective radii of the targets as measured by SEPPCoN [8] and the absolute magnitude extrapolated from the linear phase function fits, we derived geometric albedos (pV) for each comet. These ranged from 0.022 for 162P to 0.046 for 143P, consistent with the dark surfaces typical of JFCs. We will discuss the implications of these findings at the meeting, including anticipated advances resulting from the vast quantities of sparse-in-time photometry that the upcoming Legacy Survey of Space and Time (LSST) at Vera Rubin Observatory will provide [9].

[1] Vincent et al. (2017) MNRAS 469 pp.S329–S338

[2] Benseguane et al. (2022) A&A 668 p.A132

[3] Guilbert-Lepoutre et al. (2023) PSJ 4 p.220

[4] Kokotanekova et al. (2018) MNRAS 479 4 pp.4665-4680

[5] Davidsson et al. (2016) A&A 592 p.A63

[6] Hirabayashi et al. (2016) Nature 534 7607 pp.352-355

[7] Lowry et al. (2012) A&A 548 p.A12

[8] Fernández et al. (2013) Icarus 226 1 pp.1138-1170

[9] Donaldson et al. (2024) PSJ 5 7 p.162