Collisional Excavation of Asteroid (596) Scheila

TL;DR

This study observed asteroid (596) Scheila's ejecta cloud, measuring its spectrum and dust ejection, concluding it was likely caused by an impact rather than volatile activity, with no detected gases or ice.

Contribution

First spectral measurement of Scheila's ejecta revealing reddening and absorption features, supporting impact origin over cometary activity.

Findings

No gases associated with volatile activity detected

Estimated dust mass ejected was 6 x 10^8 kg

Impact likely caused the outburst, not sublimation

Abstract

We observed asteroid (596) Scheila and its ejecta cloud using the Swift UV-optical telescope. We obtained photometry of the nucleus and the ejecta, and for the first time measured the asteroid's reflection spectrum between 290 - 500 nm. Our measurements indicate significant reddening at UV wavelengths (13% per 1000 {\AA}) and a possible broad, unidentified absorption feature around 380 nm. Our measurements indicate that the outburst has not permanently increased the asteroid's brightness. We did not detect any of the gases that are typically associated with either hypervolatile activity thought responsible for cometary outbursts (CO+, CO2+), or for any volatiles excavated with the dust (OH, NH, CN, C2, C3). We estimate that 6 x 10^8 kg of dust was released with a high ejection velocity of 57 m/s (assuming 1 {\mu}m sized particles). While the asteroid is red in color and the ejecta have the same color as the Sun, we suggest that the dust does not contain any ice. Based on our observations, we conclude that (596) Scheila was most likely impacted by another main belt asteroid less than 100 meters in diameter.