Detecting surface color heterogeneity of small Solar System bodies with LSST

Surface color heterogeneity on small Solar System objects (SSOs) can be attributed to a wide variety of geological processes, including internal differentiation [1]; volatile condensation, evaporation, and transportation [2]; deposition of exogenous materials [3,4]; and spectral changes due to space weathering across surfaces of varying ages [5]. In-depth characterization of SSOs displaying surface heterogeneity, whether via ground based observing campaigns or spacecraft based remote sensing and in-situ exploration promises to shed light on these geological processes. Efficiently and accurately identifying those SSOs that display surface color heterogeneity from a growing total population of millions is therefore a priority to focus future research directions towards those objects most likely to provide unique insights into SSO geology.

 

Large scale, multi-filter time-domain sky surveys, including the LSST, provide a wealth of full-disk photometric measurements of SSOs over time. Taking advantage of the relatively short rotational and orbital timescales of SSOs, given knowledge of an object’s rotational state and illumination conditions, the temporal resolution provided by time-domain surveys can be transformed into a probe of spatial variability. Previous studies have used time-domain photometry to infer the shapes [6] and the color and albedo distributions of possible surface features of SSOs [7]. Unfortunately, most time-domain surveys do not measure the colors of objects as a function of time; photometric measurements taken in different filters are typically widely separated in time in comparison to rotational timescales. Straightforward interpretations of color measurements are therefore complicated by a variety of confounding factors affecting the relative brightness of an object, including its changing effective cross section, motion relative to the observer and Sun, and wavelength dependent phase function [8].

 

Despite these limitations, we argue that SSOs displaying surface color variability can indeed be detected in non-contemporaneous, multi-filter time-domain survey data [9]. We describe a statistical test to identify such objects and, using simulated photometry of a set of synthetic SSOs with predetermined physical properties, explore the test’s performance. To implement the test, a photometric model of each SSO under consideration must be developed. We assess the detection and false-positive rates as a function of model accuracy to evaluate the performance of the test and consider the feasibility of its application to present day datasets. Finally, we report recent progress, results, and lessons learned from applying the test to real photometric datasets.

 

[1] Johnson, B.C., Sori, M.M., & Evans, A.J. (2019) Nature Astronomy, 4 41-44

[2] Olkin, C.B. et al. (2015) The Astronomical Journal, 154, 258

[3] McCord, T.B. et al. (2012) Nature 491, 83-86

[4] Reddy, V. et al. (2012) Icarus 221, 544-559

[5] Schröder, S. E. et al. (2015) Planetary and Space Science, 117, 236-245

[6] Kaasalainen, M. & Torppa, J. (2001) Icarus, 153, 24

[7] Lacerda, P. (2009) AJ, 137, 3404

[8] Carry, B., et al., (2024) Astronomy & Astrophysics, 687 A38

[9] Humes, O.A. & Agarwal, J. (2026) Astronomy & Astrophysics, in press