How reliable are Titan's sea depths? An IHO S-44 classification of Cassini bathymetry and the volume of Titan's surface liquids

Titan's lakes and seas are the only stable surface liquids known beyond Earth, yet their published depths 8 come from methods of very different reliability: radar soundings with and without a detected floor, 9 calibrated radar backscatter, microwave radiometry and geometric models. We classify every published 10 Titan depth with the International Hydrographic Organization standard S-44 (Edition 6.2.0), whose 11 Specification Matrix accommodates non-navigational data, and assign each result an evidence class and 12 provenance and completeness flags. No Cassini sounding reaches the coarsest navigational order of the 13 standard. Of 99 inverted radar bursts, 54% fall in the 25 m Matrix cell and 40% in the 50–100 m cells, 14 and liquid-permittivity uncertainty places every sounded body in the same depth-proportional cell. 15 Footprint radii of 3–10 km exceed the coarsest horizontal cell 6- to 20-fold, so we propose extension 16 cells. Along the Ligeia Mare track, depth correlates poorly with distance from shore, so geometric depths 17 carry a root-mean-square misfit of about 33 m. Sounded depths imply shore gradients 2–6 times those of 18 the uniform-slope volume model. A concave profile fitted to the soundings gives 58,600–68,600 km3 for 19 the three seas, which with the lakes overlaps the ~70,000 km3 lower limit from backscatter and an earlier 20 geometric model, which a single shared uniform slope cannot reach. Kraken Mare reaches 300 m only 21 under an extremely convex shore-distance profile or with area-based scaling. Most of Titan's surface-22 liquid inventory rests on assumptions bounded only by radar non-detections; we recommend that future 23 Titan bathymetry report uncertainties in S-44 form. 24