The universe's most common ice isn't fully glassy after all

The most common form of ice in the Universe is low-density amorphous ice (LDA), a glassy solid whose water molecules are frozen in a jumble rather than the neat hexagonal lattice of freezer ice. A 2025 study in Physical Review B found that this “glass” is not fully disordered after all: it hides tiny crystals.

LDA makes up the bulk of comets, coats icy moons and pervades the cold dust clouds where stars and planets form. It arises when water vapour freezes onto extremely cold surfaces such as dust grains. It was discovered nearly 90 years ago, yet its structure has been debated ever since.

How do we know it contains crystals? Researchers at UCL and the University of Cambridge, led by Michael Davies, froze virtual boxes of water molecules to −120 °C at different speeds. Simulated ice that was up to about 20% crystalline best matched X-ray diffraction measurements of real LDA, and a second method gave a similar fit at 25%. The hidden crystals are about 3 nanometres wide, slightly wider than a strand of DNA.

The team then made real LDA in several ways, including depositing vapour onto a very cold surface and warming crushed high-density amorphous ice, and gently heated the samples until they crystallised. The final crystals differed depending on how each ice had formed. A truly disordered material would keep no such memory of its origin.

“Ice on Earth is a cosmological curiosity due to our warm temperatures.” — Christoph Salzmann, UCL

The result bears on panspermia, the speculative idea that comets delivered life’s building blocks to Earth inside amorphous ice. Davies said a partly crystalline ice has less space in which molecules such as amino acids could be embedded, although its amorphous regions could still carry some.