Titan’s Strong Tidal Dissipation Precludes a Subsurface Ocean

The Cassini mission provided unprecedented insights into Saturn’s largest moon, Titan, from its atmosphere to the deep interior [1]. The moon’s large measured response to the tides exerted by Saturn was interpreted as evidence of the existence of a subsurface ocean [2, 3]. This response, twice the value predicted in pre-Cassini studies, has escaped complete explanation. Here we report the detection of the signature of tidal dissipation in Titan’s gravity field enabled by a reanalysis of the the radiometric data acquired by Cassini with improved techniques. We found that substantial energy is being dissipated in the interior (∼ 3-4 TW, corresponding to a tidal quality factor Q ∼ 5), consistent with recent studies of Titan’s rotational state [4]. A model in which dissipation is concentrated in a high-pressure ice layer close to its melting point robustly explains Titan’s observed large tidal response, dissipation and rotational state in a self-consistent manner, reconciling all available geophysical measurements. Because the presence of a liquid layer reduces the tidal dissipation generated below it [5], these new measurements are not consistent with the presence of a subsurface ocean on Titan. Efficient ice shell convection can prevent widespread melting and ocean formation, but a slushy high-pressure ice layer is consistent with expectations [6], indicating that it likely hosts liquid water pockets. The forthcoming Dragonfly mission to Titan will provide a further test of whether a subsurface ocean exists.