SPHEREx will scan the galaxy to look for life's ingredients

Where is all the water that might fill oceans on distant planets and moons? According to NASA, the forthcoming SPHEREx astrophysics mission will take stock of water’s cosmic origins by scanning the galaxy for hidden reservoirs. 

Experts say this ambitious endeavor may help uncover how water, the key molecule for life, accumulates on interstellar dust grains in vast clouds of gas and dust.

Since every living organism on Earth needs water to survive, scientists searching for life outside of our solar system are frequently guided by the phrase “follow the water.”

The SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) mission is set to launch on Thursday, February 27 from the Vandenberg Space Force Base in California. The explorer will be launched on a SpaceX Falcon 9 rocket.

Then, SPHEREx will begin its hunt for water, carbon dioxide, carbon monoxide, and other basic ingredients for life.

Mapping ice within molecular clouds

While there are no free-floating oceans in space, scientists have long suspected that frozen water – formed by bonding oxygen and hydrogen atoms on tiny dust grains – accounts for much of the universe’s water content. 

This ice is believed to accumulate in molecular clouds, the giant swaths of gas and dust from which stars and planetary systems eventually emerge.

SPHEREx will systematically survey these molecular clouds, where newly forming stars and the disks of matter surrounding them often mask hidden ice.

By analyzing key infrared wavelengths, the telescope can detect how water and other molecules absorb passing starlight, leaving a signature in the spectral data.

Rather than producing standard 2D images, SPHEREx will gather 3D spectral information along each line of sight, enabling researchers to see how much ice is present at different depths in the cloud. 

Over the course of more than 9 million observations, the mission will deliver the largest-ever inventory of interstellar ice, clarifying how environments in these clouds influence the abundance of water and other frozen compounds.

Where is the missing water?

Scientists’ interest in interstellar water has been longstanding. Yet a puzzle emerged in 1998, when NASA launched the Submillimeter Wave Astronomy Satellite (SWAS) to look for gaseous water in molecular clouds. 

SWAS detected far less water in gas form than predicted, raising the question: where was it hiding?

“We eventually realized that SWAS had detected gaseous water in thin layers near the surface of molecular clouds, suggesting that there might be a lot more water inside the clouds, locked up as ice,” said Gary Melnick, a senior astronomer at the Center for Astrophysics | Harvard & Smithsonian and a member of the SPHEREx science team.

SWAS also found less oxygen gas than expected, leading researchers to conclude that oxygen atoms had adhered to dust grains and combined with hydrogen to form water. 

The clouds’ interiors shield these ices from harsh cosmic radiation, preventing them from breaking apart. SPHEREx’s highly sensitive survey will now reveal more precisely where ice begins to form and how these ices evolve when stars are born.

Exposing the secrets of cosmic water

SPHEREx excels at broad, large-scale mapping, while telescopes like NASA’s James Webb Space Telescope (JWST) specialize in zooming in on a much smaller area with greater sensitivity to certain wavelengths. 

These complementary designs will allow SPHEREx to flag regions of high interest, which Webb can then examine in detail.

“If SPHEREx discovers a particularly intriguing location, Webb can study that target with higher spectral resolving power and in wavelengths that SPHEREx cannot detect,” Melnick said. “These two telescopes could form a highly effective partnership.”

By coupling SPHEREx’s panoramic survey with Webb’s close-up observations, scientists hope to piece together a comprehensive picture of how cosmic water emerges, accumulates, and ultimately finds its way onto forming planets and moons.

Tracing water’s cosmic life cycle

The logic behind “follow the water” rests on the knowledge that all known life depends on liquid H₂O. If large amounts of water ice are identified in molecular clouds, that material likely feeds into protoplanetary disks, eventually supplying the water for oceans and lakes. 

Thus, SPHEREx’s broad inventory may help confirm or refine theories on how water-rich worlds like Earth were assembled.

Although space telescopes such as NASA’s Spitzer and the retired Herschel have cataloged water and other compounds in select regions, SPHEREx will be the first mission to look for these ices at a truly galactic scale. 

Melnick and his colleagues plan to incorporate SPHEREx’s findings into existing catalogs of stars and dust, clarifying how and where water is being replenished – or destroyed – in cosmic neighborhoods.

Ultimately, the results will speak not only to the matter of where water resides but also to how it journeys from the frozen interiors of molecular clouds to become part of a rocky planet. 

By following water’s cosmic life cycle, SPHEREx could reveal the processes that made Earth’s oceans possible. It may also shed light on how these processes shaped the conditions for life on our planet, and perhaps on countless other worlds.

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