One hundred forty parsecs, a distance light traverses in four and a half centuries, separate us from the Taurus molecular cloud. Within its depths hide dark condensations: prestellar cores, where gas and dust slowly contract under their own weight. Each dust grain here is a microscopic condensation nucleus, cloaked in a mantle of water ice. This ice is the main reservoir of oxygen and the future building material for planets. But seeing it directly is nearly impossible: the clouds are too dense and cold. Infrared spectroscopy comes to the rescue: water molecules absorb radiation at a wavelength of 3 micrometers, leaving a characteristic dip in the spectrum.
Until now, astronomers caught the ice signal by shining cores with beams of randomly positioned background stars. This method yields only individual probes through the cloud. The SPHEREx team took a different route: they used scattered galactic emission — coreshine. In the interstellar medium it is like fog lit by distant lamps. Ice absorbs part of this light, and from the depth of the shadow the amount of ice can be reconstructed. A measurement grid with a step of 7.2 arcseconds, about 1000 astronomical units at the distance of these cores, allowed continuous maps to be built. 1000 astronomical units is 30 times farther from the Sun than Neptune, and on such scales the outlines of future planetary systems emerge.
Radiative transfer modeling, including three-dimensional Monte Carlo simulations with RADMC-3D, could not explain the dip. Neither changing the illumination geometry, nor grain growth to 10 microns, nor the ice condensation threshold produces a central minimum. The only qualitative scenario is a sharp decrease in the fraction of water ice in the densest regions. It seems that at depths where the column density reaches AV>25–30 magnitudes (starlight is attenuated by billions of times), ice is actively processed into other molecules. Possibly it becomes part of complex organics or exchanges oxygen with other ices such as CO and CO₂.
Coreshine maps open a new era in astrochemistry. Previously, ice was studied pointwise; now, entire fields can be examined, independent of rare background stars. SPHEREx has already shown that about half of known prestellar cores scatter enough light for such analysis. The next step is to compare the distributions of different ice components: water, carbon monoxide, and carbon dioxide. This will allow us to trace where oxygen disappears in pre-collapse conditions and understand with what chemical baggage future planets begin their journey.
Dark cores are dark because they hide light. But in their scattered glow, shadows appear that tell of hidden ice. And the fact that in the most opaque heart the ice signature suddenly vanishes is a persistent reminder that no cloud is static. Even in the deepest cosmic cold, at 10 degrees above absolute zero, matter prepares for transformation.
SPHEREx is a true cosmic 'ice cartographer': every six months it builds a complete map of the sky, and its data already contains millions of spectra available for searching for traces of frozen water in the cradles of future planets.
Although scattered glow of prestellar cores does not directly appear in science fiction plots, the idea of 'ice that stores history' echoes the image of a giant ice library in Kim Stanley Robinson's novel '2312' — only here the chemical evolution of the cloud is recorded.