Keeping one face toward the planet does not freeze the tide
Imagine standing on a small frozen moon. The planet stays near one part of the sky, but along an eccentric orbit its distance changes. Its apparent direction also rocks slightly because orbital speed changes while uniform synchronous spin does not. Both effects alter the stretching force. A perfectly circular, exactly synchronous orbit removes this particular changing tide.
The force that deforms the moon is a difference: the planet pulls on its near side, centre and far side by different amounts. Much of the common pull merely carries the entire moon around its orbit. Removing that common acceleration leaves the tide. In the Lab the arrows show the changing part of this differential pull, while the steady tidal bulge has been removed.
The shell window makes the response visible: an ice layer surrounds liquid, and both boundaries move according to the same displacement solution. The world is deliberately synthetic, with a radius of five hundred kilometres and equal density in both layers. Real ice and water differ in density, and real moons can contain rock, several ice phases and oceans with their own dynamics.
Reading: Beuthe, Tides on Europa: the membrane paradigm, arXiv:1410.4735v1
A late response leaves work behind
An ideal spring stores energy when stretched and gives it back when released. Over a closed cycle it keeps none. A material with viscosity also resists motion. Its deformation does not exactly follow the changing load, so the force and displacement trace a loop. That loop represents a net energy transfer per cycle.
The first data plot divides force and displacement by their separate peaks so that their timing can be compared. Their similar heights therefore do not mean equal physical units or equal energy. The second plot follows one radial surface component. Total heating requires the whole surface, all the changing tidal components and their physical scales; the power readout performs that calculation.
Try ideal elasticity. The body still bends, but the phase lag and heating vanish. Then choose the circular preset: the displayed changing deformation vanishes because its driver has gone. These are two different routes to zero power. One removes dissipation from the material; the other removes this time-dependent forcing.
Reading: Beuthe, Tides on Europa: the membrane paradigm, arXiv:1410.4735v1
A large tide is not a photograph of an ocean
A subsurface liquid can make an outer shell easier to deform. But a warm, soft solid can also bend strongly. The gravity response measures a mass redistribution, not the state of every layer. Within this Lab’s equal-density model, changing the shell thickness while rescaling stiffness and viscosity can preserve the entire external response. The solid preset demonstrates that exact ambiguity.
This matters when reading actual spacecraft results. A 2025 reanalysis of Cassini tracking estimated Titan’s complex gravity response and interpreted it with layered interior models. Its preferred warm high-pressure ice structure follows from those observations together with material, thermal and structural assumptions. The coloured interior diagram in that paper is an inference, not a radar image through the moon.
The next useful measurement is one that responds differently to competing interiors. Surface displacement, rotation and gravity can complement each other in a realistic stratified moon. A 2026 Dragonfly study explores such radio measurements through simulations. Its simulations estimate the precision of a proposed surface-response measurement.
Reading: Petricca et al., Titan’s strong tidal dissipation precludes a subsurface ocean, Nature 648 (2025) · Petricca & Buccino, A Radio Science Experiment for Dragonfly to Investigate Titan’s Rotation and Interior Structure, GRL 53 (2026)
Heat has to reach the right place
The default synthetic world produces about one hundred gigawatts in its viscoelastic ice. That number is a model prediction, not a measured moon luminosity. The thickness plot places it beside a separate conductive reference. That reference assumes fixed temperatures at the two boundaries and a constant thermal conductivity; it is not a coupled thermal evolution calculation.
Where the curves cross, the assumed heating balances the assumed conductive loss. Warming can change viscosity, convection can carry more heat, melting can move the boundary, and eccentricity itself can evolve. Those feedbacks decide whether liquid survives. The small experiment helps identify the terms that a more complete calculation must connect.
The apparent shape changes are magnified, and the screen states the gain. For each thickness and material the gain is referenced to the same small eccentricity, so increasing eccentricity visibly increases the changing bulge. Read the metre-valued displacement when comparing absolute sizes across different materials. No colour in the cutaway is a calculated temperature map.
Reading: Beuthe, Tides on Europa: the membrane paradigm, arXiv:1410.4735v1 · Petricca et al., Titan’s strong tidal dissipation precludes a subsurface ocean, Nature 648 (2025)