Polarimetric signature of the oceans as detected by near-infrared Earthshine observations

TL;DR

This study demonstrates that near-infrared polarimetry of Earthshine can detect the polarimetric signature of Earth's oceans, providing a potential method for identifying oceanic exoplanets.

Contribution

First detection of Earth's ocean polarimetric signature in disk-integrated light using near-infrared polarimetry of Earthshine.

Findings

Positive correlation between polarization degree and ocean fraction

Hourly polarization variations match Earth's rotation and ocean distribution

Polarimetric signature consistent with ocean reflection detected

Abstract

Context. The discovery of an extrasolar planet with an ocean has crucial importance in the search for life beyond Earth. The polarimetric detection of specularly reflected light from a smooth liquid surface is anticipated theoretically, though the polarimetric signature of Earth's oceans has not yet been conclusively detected in disk-integrated planetary light. Aims. We aim to detect and measure the polarimetric signature of the Earth's oceans. Methods. We conducted near-infrared polarimetry for lunar Earthshine and collected data on 32 nights with a variety of ocean fractions in the Earthshine-contributing region. Results. A clear positive correlation was revealed between the polarization degree and ocean fraction. We found hourly variations in polarization in accordance with rotational transition of the ocean fraction. The ratios of the variation to the typical polarization degree were as large as ~0.2-1.4. Conclusions. Our observations provide plausible evidence of the polarimetric signature attributed to Earth's oceans. Near-infrared polarimetry may be considered a prospective technique in the search for exoplanetary oceans.