  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.
