Nereid from space: Rotation, size and shape analysis from Kepler/K2, Herschel and Spitzer observations

TL;DR

This study analyzes Nereid's rotation, size, and shape using data from Kepler/K2, Herschel, and Spitzer, revealing a stable low-amplitude rotation state, a near-spherical shape, and a highly cratered surface.

Contribution

It provides the first detailed rotation period and shape constraints of Nereid based on space-based observations, challenging previous assumptions of a highly elongated shape.

Findings

Nereid's rotation period is approximately 11.6 hours.

The shape is constrained to an axis ratio of at most 1.3:1.

Nereid has a highly cratered surface.

Abstract

In this paper we present an analysis of Kepler K2 mission Campaign 3 observations of the irregular Neptune satellite, Nereid. We determined a rotation period of P=11.594(+/-)0.017 h and amplitude of dm=0.0328(+/-)00018, confirming previous short rotation periods obtained in ground based observations. The similarities of light curve amplitudes between 2001 and 2015 show that Nereid is in a low-amplitude rotation state nowadays and it could have been in a high-amplitude rotation state in the mid 1960's. Another high-amplitude period is expected in about 30 years. Based on the light curve amplitudes observed in the last 15 years we could constrain the shape of Nereid and obtained a maximum a:c axis ratio of 1.3:1. This excludes the previously suggested very elongated shape of a:c=1.9:1 and clearly shows that Nereid's spin axis cannot be in forced precession due to tidal forces. Thermal emission data from the Spitzer Space Telescope and the Herschel Space Observatory indicate that Nereid's shape is actually close to the a:c axis ratio limit of 1.3:1 we obtained, and it has a very rough, highly cratered surface