Published in the Publications of the Astronomical Society of Japan on June 27, 2025, this international study marks the first direct detection of both solid and gaseous sulfur in the interstellar medium, using data from the Japan-led XRISM (X-ray Imaging and Spectroscopy Mission) satellite.
A Unique X-ray View Of Interstellar Sulfur
The recent findings from the XRISM spacecraft—launched by JAXA in collaboration with NASA and the European Space Agency (ESA)—highlight the mission’s ability to detect and distinguish between different physical states of elements across the galaxy. Using its Resolve instrument, XRISM performed high-resolution X-ray spectroscopy, analyzing emissions from two distant binary star systems. This novel approach allowed scientists to directly observe sulfur in both gas and solid forms, a feat not previously accomplished with this level of detail.
The sulfur was detected in regions of the interstellar medium (ISM)—a mixture of gas and dust that fills the space between stars—by analyzing how the X-rays from bright background sources interacted with intervening material.
Like a cosmic X-ray scan, the process works similarly to how a medical X-ray highlights differences between tissues and bones by varying absorption levels. By positioning one of these stellar X-ray sources, GX 340+0, behind a well-chosen region of space, the team observed how sulfur absorbed and interacted with high-energy photons.
How Sulfur Moves Through The Galaxy
In previous studies using ultraviolet light, astronomers had only been able to detect sulfur in its gaseous state in diffuse regions of the ISM. However, sulfur tends to vanish in denser molecular clouds, prompting hypotheses that it condenses into solid compounds—perhaps by freezing onto dust grains or chemically binding with other elements. The new XRISM data confirms this assumption: sulfur not only exists as a gas but also appears in solid forms, likely mixed with iron.
The research team suggests the detected solids may include compounds like pyrrhotite, troilite, or pyrite—minerals commonly found in meteorites and referred to in Earth-based geology. These iron-sulfur compounds are critical clues for understanding how material condenses and travels through the galaxy, eventually contributing to the birth of new stars and planetary systems.
Multiple Targets Confirm Sulfur’s Hidden Presence
To validate their detection, the scientists extended their observations to a second X-ray binary, 4U 1630-472, which lies in a different region of the Milky Way. This comparative analysis confirmed that sulfur exists in both gaseous and solid forms across a wider section of the galaxy than previously known.
The result not only enhances confidence in XRISM’s data but also showcases the Resolve instrument’s ability to investigate the chemical composition of cosmic environments. According to Brian Williams, XRISM project scientist at NASA’s Goddard Space Flight Center, these observations open a new window into large-scale chemical mapping of the galaxy: “XRISM’s X-ray observations are a unique probe of sulfur in a large section of the Milky Way. There’s still so much to learn about the galaxy we call home.”
Why Sulfur Matters Beyond Earth
Sulfur plays a pivotal role not only in astrophysics but also in planetary science and biology. On Earth, sulfur is essential to life—it is a component of amino acids, proteins, and enzymes. In space, understanding where sulfur resides helps scientists refine models of planet formation, habitability, and astrochemistry.
Tracking sulfur through its different cosmic environments may also help explain the delivery of volatile elements to young planets via meteorites and comets. The fact that sulfur can switch states so easily—from gas to solid and back again—makes it a highly dynamic tracer of chemical processes across varying densities and temperatures in the galaxy.
XRISM’s Mission And What Comes Next
Launched in 2023, XRISM is the latest major observatory dedicated to X-ray astrophysics, following in the footsteps of NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton. The Resolve instrument, jointly developed by NASA and JAXA, operates as a microcalorimeter—a device that measures the slight temperature rise caused by absorbing individual X-ray photons, providing ultra-precise energy measurements.
The sulfur study led by Lía Corrales of the University of Michigan is the first of many XRISM investigations to come. Corrales emphasized that lab modeling is underway to simulate more compounds and better match future observations.
This landmark sulfur mapping effort highlights how cutting-edge space instrumentation can illuminate the chemical complexity of our galaxy. By pinpointing where critical elements are hiding between stars, scientists are piecing together the cosmic pathways that feed star birth, planet formation, and potentially, the ingredients for life itself.
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