Chirality Amplifies at the Wet-Dry Interface

01The evidence

• Evaporative crystallisation of sodium chlorate produces conglomerates, the classic demonstration that a solution can resolve into handed crystals through a physical process rather than a chiral reagent.Chemical Society Reviews
• Mechanisms of chirality amplification have been classified by what they do to the excess, which makes the amplification step a measurable operation rather than a narrative one.Chirality amplification taxonomy

02The tension

Prebiotic chemistry can generate small excesses through circularly polarised light or magnetite surfaces, but biology uses essentially complete homochirality. The gap between a few per cent and near-complete is the hard part, and asymmetric synthesis does not close it.

03The theory

The amplification happens at a phase boundary. In a saturated solution, crystals of one handedness grow at the expense of the other, and repeated cycles of dissolution and re-crystallisation drive the system toward a single handedness. Each cycle compounds the bias.

If the process is a random walk in the excess with a drift term proportional to the current excess, the excess grows exponentially in the number of cycles. If it is diffusion-limited at the crystal surface, it grows as the square root. The two predictions differ, which is what makes the mechanism testable.

The geological consequence is a search rule. Settings with many wet-dry cycles, such as tidal flats and evaporitic basins, should show the highest excess, and settings that stayed wet should show none regardless of how much organic material they contain.

This also explains why the search for chiral excess in meteorites has been confusing: the parent body's alteration history determines how many cycles occurred, and that is not the same as how much water was present.

04What this is not

05Predictions

• P1Enantiomeric excess should scale with the square root of the number of wet-dry cycles, in controlled laboratory cycling.
• P2Wet-only and dry-only controls should show no amplification, while alternating cycles should.
• P3Samples from evaporitic or tidal settings should show higher excess than samples from permanently wet settings, at matched organic content.

06What would refute it

If excess is independent of cycle number, amplification is happening somewhere else and the phase-boundary account is wrong.

07What it would change

It would give the search for prebiotic chirality a geological predictor, and reframe the origin of homochirality as a geochemical processing problem.