NASA’s SPHEREx Launches Soon and Will Search For Water in Molecular Clouds

As far as we can tell, life needs water. Cells can’t perform their functions without it. Some have suggested that other exotic liquids, like liquid methane, could do the job on worlds like Saturn’s moon Titan. That idea is highly speculative, though.

So, it makes sense that NASA is launching a spacecraft dedicated to the search for water.

SPHEREx stands for Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer. It’s scheduled to launch on February 27th. It has a single instrument and one observing mode. Part of its mission is to map the sky in near-infrared and measure the spectra of 450 million galaxies. The results will help scientists understand the expansion of the Universe and the origin and evolution of galaxies.

Its other scientific goal is to probe molecular clouds for water ice and other frozen pre-biotic molecules. These ices are frozen onto the surface of dust grains in molecular clouds, and somehow, through a long journey, they become part of planets, where they can form oceans and potentially foster the appearance of life.

Infrared observations show that in cold, dense regions of space in molecular clouds, chemicals critical to life are locked into dust grains. Water is the primary one, of course, but there are other pre-biotic molecules as well: carbon dioxide (CO2), carbon monoxide (CO), methanol (CH3OH), the nitrogen-bearing molecule ammonia (NH3) plus various carbon-nitrogen stretch molecules (XCN), and the important sulphur-bearing molecule, carbonyl sulphide (COS). Carbon-nitrogen stretch molecules are everywhere in organic and biological molecules and play critical roles in biological processes. Carbonyl-sulphide plays a role in the formation of peptides, which are the building blocks of proteins.

There’s a vast amount of water frozen in dust grains in molecular clouds, and scientists think this is where the bulk of the water in the galaxy and even in the Universe resides. These grains are the source of water for Earth’s oceans and for any exoplanets or moons that might harbour oceans.

SPHEREx will examine molecular clouds and try to understand how much water they contain. It will also examine stars in those clouds and the rings of material that form around them, out of which planets form.

Put succinctly, SPHEREx is trying to answer this question: How does ice content evolve from diffuse clouds to dense clouds to planetary disks and then to planets?

There’s little doubt that ices play an important role in the formation of planetesimals in disks around young stars. Likewise, there’s little doubt that these ices are sources of water and organic molecules, too. But how does it all happen? Ice’s journey from translucent to dense molecular clouds and then to protoplanetary disks is not well understood. Scientists want to know if the ices in the disks are simply inherited from the interstellar medium or if they’re altered in the disks somehow.

The SPHEREx mission hopes to answer this question and others with its infrared absorption spectroscopy.

SPHEREx will generate spectra for between 8 and 9 million sources and should transform our understanding of ices in molecular clouds, young stellar objects, and protoplanetary disks.

In infrared wavelengths, ices have unique spectral signatures. Prior to the JWST, scientists had only about 200 ice absorption spectra available. The JWST is changing that, but it has lots of other important work to do.

The JWST is already advancing our understanding of these ices. Like other infrared observatories, it can see through dust, but it is far more powerful and sensitive. A key to SPHEREx’s design and performance is its ability to be as accurate as the JWST.

There is no shortage of targets for SPHEREx. Some research shows that there are over 8,000 molecular clouds in the Milky Way. Not all of them are great targets for SPHEREx, but many are.

SPHEREx has a catalogue of targets that includes molecular clouds in the Large and Small Magellanic Clouds and several constellations, including Monoceros, home of the Monoceros R2 Molecular Cloud.

It’s axiomatic that stars and planets have the same compositions as the molecular clouds that fostered them. But the specifics of planet formation are mysterious and the study of the processes has produced some surprises.

In 1998, NASA launched the Submillimeter Wave Astronomy Satellite (SWAS). Similar to SPHEREx, it studied the chemical composition of interstellar clouds and surveyed the galaxy to determine how much water vapour was present in molecular clouds. Surprisingly, it found far less than expected.

“This puzzled us for a while,” said Gary Melnick, a senior astronomer at the Center for Astrophysics | Harvard & Smithsonian and a member of the SPHEREx science team. “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.”

The SWAS team figured out that hydrogen and oxygen atoms were being frozen onto the surfaces of ice grains where they formed water ice. Subsequent research confirmed their suspicions. On the unprotected surfaces of molecular clouds, cosmic radiation can break the H2O molecules apart, but protected inside molecular clouds, the molecules persisted.

The water ice and other ices create spectroscopic signatures separate from their liquid counterparts, and SPHEREx is designed to detect them.

It will do more than detect them, though. The spacecraft will also determine how deep inside the clouds the ices form, how their abundance changes with cloud density, and how the abundance changes when a star forms.

SPHEREx will also cooperate with other telescopes, including the JWST, which will perform more powerful follow-up observations when merited.

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

SPHEREx will launch on February 27th in a Falcon Heavy rocket from Vandenberg Air Base. It will follow a Sun-Synchronous orbit at about 700 km altitude. In its nominal 25-month mission, SPHEREx will map the entire sky four times.

• Press Release: NASA’s SPHEREx Space Telescope Will Seek Life’s Ingredients
• CalTech SPHEREx Science
• An All-Sky Spectral Survey
• The Astrophysical Journal: The SPHEREx Target List of Ice Sources (SPLICES)
• JPL: The Origin of Water – and Other Pre-biotic Molecules – in Planetary Systems

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