The trail of sulphur: from molecular clouds to life

Project Information

SUL4LIFE

Grant agreement ID: 101096293

DOI

10.3030/101096293

EC signature date 19 June 2023

Start date 1 September 2023

End date 31 August 2028

Funded under

• European Research Council (ERC)

Total cost

€ 2 191 875,00

EU contribution

€ 2 191 875,00

Coordinated by

AGENCIA ESTATAL CONSEJO SUPERIOR DE INVESTIGACIONES CIENTIFICAS
Spain

Periodic Reporting for period 1 - SUL4LIFE (The trail of sulphur: from molecular clouds to life)

Reporting period: 2023-09-01 to 2026-02-28

Summary of the context and overall objectives of the project

Sulfur is the tenth most abundant element in the Universe and plays a crucial role in biological systems. Some sulfur compounds have even been proposed as essential catalysts for the formation of amino acids in space. However, while the cosmic budgets of carbon and oxygen are relatively well constrained, the gas-phase abundance of sulfur remains uncertain by several orders of magnitude. This prevents us from using sulfur as a reliable probe of star and planet formation.
In the Solar System, sulfur is widespread across planets and moons. It is found in solid form within Mercury’s polar regions, as sulfur oxides in the atmosphere of Venus, and as sulfate minerals across the surface of Mars. On Earth, it is present in nearly every environmental reservoir, atmosphere, oceans, rocks, and living organisms.
In the interstellar medium, however, many questions remain open about where sulfur is stored and in which molecular species. Even the relative amounts of sulfur in the gas and solid phases are poorly constrained. This is largely because the main reservoirs of sulfur are difficult to observe with current ground-based and space facilities. The SUL4LIFE project takes a decisive step toward solving the long-standing mystery of sulfur chemistry in space through an innovative approach built on three main pillars.
(i) We are building an unprecedented database of high-quality observations of sulfur-bearing molecules. This will allow us to trace how sulfur is distributed and transformed, from the earliest molecular clouds to the protoplanetary disks where planets take shape.
(ii) We are carrying out ab initio calculations and laboratory experiments to determine the key reaction rates that control sulfur chemistry. These results are crucial to refine chemical models and make them truly reliable.
(iii) We now know that the chemical composition of planet-forming disks is largely inherited from the very first stages of star formation. Advanced 3D bi-fluid (gas + dust) magneto-hydrodynamic simulations coupled with chemistry (chemo-MHD) tare used o follow the journey of matter from natal clouds all the way to planet-forming regions, enabling us to make accurate chemical predictions.
By combining these three approaches, SUL4LIFE will reveal how sulfur is incorporated into protoplanetary disks, a key step in understanding planet formation and, ultimately, the chemical origins of life.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The SUL4LIFE team has achieved a major breakthrough with the first imaging of the sulfur-bearing molecule S2H in the Horsehead Nebula. To date, S2H is the only interstellar species identified with a disulfide bond (S–S), a key chemical bond in terrestrial biochemistry. In living systems, disulfide bonds are fundamental for protein folding, stability, and solubility. The discovery of S2H in space therefore provides crucial insight into how complex and potentially prebiotic molecules may originate under interstellar conditions.
The identification of S-S molecules is an exciting examaple of the synergy between astronomers and experimentalists. Laboratory irradiation of H2S-containing ices had previously demonstrated the formation of sulfur chains (Sₓ, H2Sₓ), suggesting that such compounds could exist in space. Yet their detection in space is technically demanding, and S2H remains the only confirmed case. Complementary experiments and theoretical studies within SUL4LIFE and by collaborators are now advancing our understanding of the chemistry of these elusive species.
Observations with the James Webb Space Telescope further revealed that SO2 is a major sulfur reservoir in interstellar ices. Since SO2 is also abundant in the gas phase—detected in protoplanetary disks and exoplanetary atmospheres—it represents a key link between solid- and gas-phase chemistry in star- and planet-forming regions. Laboratory experiments within SUL4LIFE have demonstrated that SO and SO2 can be efficiently photodesorbed by UV photons, and for the first time their desorption yields have been quantified. These measurements are critical for establishing reliable connections between ice and gas chemistries, where the precursors of complex organic molecules are expected to emerge.
On the modeling side, a central effort of the project’s first two years has been the development of chemo-MHD simulations that couple state-of-the-art magnetohydrodynamical collapse models with detailed chemical post-processing, explicitly including grain growth. This pioneering simulation enabled us to determine at which stages of star formation large sulfur-bearing molecules can form.
Taken together, these results highlight the unique strength of SUL4LIFE: the integration of laboratory experiments, astronomical observations, and advanced numerical simulations. This synergy is beginning to unravel the long-standing puzzle of sulfur chemistry in space and its potential connection with the origin of life in Earth.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The SUL4LIFE team is pushing the frontiers of astrochemistry by combining innovative methodologies with the optimized use of world-class observatories. Using the JWST, we detected for the first time the [SI] line at 25 μm in a photodissociation region, enabling the most precise determination of sulfur abundance in the Orion Bar to date. In parallel, we carried out the first survey of hydrogen sulfide (H2S) in a large sample of young stars with the Northern European Millimeter, Array (NOEMA) showing that each star-forming environment has its own unique sulfur chemistry.
One of our most exciting discoveries has been the first images of the sulfur molecule S2H in the Horsehead Nebula, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Our team had previously detected this molecule, but how it formed in space was still uncertain. Thanks to the new SUL4LIFE images, we now know that its origin is closely tied to chemical reactions taking place on the surfaces of tiny interstellar dust grains, where ices and starlight interact to create new molecules.
Observations alone cannot answer all our questions, because many molecules are invisible to our telescopes and because planet-forming regions are too small and distant for detailed study. To overcome these limitations, we developed state-of-the-art simulations that combine physics and chemistry, following the evolution of gas and dust from interstellar clouds to young stars. For the first time, these simulations included the growth of dust grains, which is essential to understanding planet formation.
Finally, our laboratory program is measuring how efficiently sulfur molecules are released from ices under starlight — data that had never been available before. These measurements are essential for building accurate models of interstellar chemistry.
Together, these breakthroughs — from the discovery of new molecules in space to cutting-edge simulations and laboratory experiments — are reshaping the way we think about sulfur chemistry in the cosmos. Step by step, we are uncovering the hidden pathways that transform simple atoms into the complex ingredients of life.

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