Extended Data Fig. 2: Roughness caused by ice-trapped ebullition bubbles.

The main roughness phenomenon is caused by insulating bubbles leading to slower ice growth directly beneath the bubble column. The resulting upward cavities in the bottom of ice are filled by water (feature a), partly filled by gas (feature b), or completely filled by gas (feature c). Rough surfaces are shown by black arrows; arrow width depicts expected roughness intensity. Feature d shows a pause in ebullition (a weaker seep) that allows ice growth to ‘catch up’ with surrounding ice sheet. Feature e shows stacked bubbles from a strong seep in a ‘wedding cake’ formation. In this example, the feature has vented the gas to the atmosphere, resulting in multiple ridges and a near-vertical ice–water interface with strong SAR backscatter. Wedding cake cavities typically re-fill with gas and vent again in an alternating cycle. Real portions of dielectric constants for different media are represented by ɛ′.