Spacecraft magnetometers recorded a magnetic field near Europa that varies in step with Jupiter's changing field at the moon's location — the signature of a response induced inside the moon rather than generated by it.
A changing external magnetic field drives electric currents in any electrically conducting layer it passes through. Those currents create a secondary field. Salty liquid water conducts well; cold solid ice does not.
Europa's surface adds a second, independent line of reasoning: it is young, cracked and largely free of impact craters, which points to resurfacing rather than a static shell.
- OBSERVATION
- Induced magnetic response synchronised with Jupiter's field
- MEASUREMENT
- Field strength and phase relative to the driving field
- MODEL
- Conducting shell within the moon
- INFERENCE
- Global salty liquid layer beneath the ice
The measured magnetic response is best explained by a conducting layer inside Europa, and salty liquid water is the most plausible candidate given everything else known about the moon.
Ice-shell thickness, ocean depth, salinity and chemistry are not settled. Whether the ocean contacts a rocky sea floor — a key question for habitability — is unresolved, and habitability itself is not evidence of life.
"Europa could host life" is a hypothesis about conditions. No biological measurement of Europa exists.
- NASA Science //
- NASA Science //