The discovery of water vapour and ice particles erupting from Saturn's moon Enceladus fuelled speculation that an internal ocean was the source1 2 3. Alternatively, the source might be ice warmed, melted or crushed by tectonic motions4. Sodium chloride (that is, salt) is expected to be present in a long-lived ocean in contact with a rocky core. Here we report a ground-based spectroscopic search for atomic sodium near Enceladus that places an upper limit on the mixing ratio in the vapour plumes orders of magnitude below the expected ocean salinity5. The low sodium content of escaping vapour, together with the small fraction of salt-bearing particles6, argues against a situation in which a near-surface geyser is fuelled by a salty ocean through cracks in the crust1. The lack of observable sodium in the vapour is consistent with a wide variety of alternative eruption sources, including a deep ocean6, a freshwater reservoir, or ice. The existing data may be insufficient to distinguish between these hypotheses.