Colors and Shapes of the Irregular Planetary Satellites

TL;DR

This study analyzes the colors and shapes of 43 irregular satellites of outer planets to infer their origins and the processes shaping them, revealing differences in surface composition and similarities in physical structure.

Contribution

It provides the first comprehensive optical color survey of irregular satellites and compares their properties with other small bodies to understand their origins.

Findings

Ultrared matter is depleted in irregular satellites compared to Kuiper belt objects.

Irregular satellites and main-belt asteroids share similar shapes, suggesting collisional shaping.

Irregular satellites lack the ultrared surface material common in Kuiper belt objects.

Abstract

It is widely recognized that the irregular satellites of the giant planets were captured from initially heliocentric orbits. However, the mechanism of capture and the source region from which they were captured both remain unknown. We present an optical color survey of 43 irregular satellites of the outer planets conducted using the LRIS camera on the 10-meter telescope at the Keck Observatory in Hawaii. The measured colors are compared to other planetary bodies in search for similarities and differences that may reflect upon the origin of the satellites. We find that ultrared matter (with color index B-R $\ge$ 1.6), while abundant in the Kuiper belt and Centaur populations, is depleted from the irregular satellites. We also use repeated determinations of the absolute magnitudes to make a statistical estimate of the average shape of the irregular satellites. The data provide no evidence that the satellites and the main-belt asteroids are differently shaped, consistent with collisions as the major agent shaping both.