Astronomers plan for interstellar water search

One of the James Webb Space Telescope’s preliminary missions will be to search for water in a nearby star-forming region


IC 2631, a reflection nebula, is part of the Chamaeleon Complex, and the JWST will reveal its complex chemistry. Image credit: ESO

Water makes up roughly 60 per cent of our bodies, but where did it come from? How did it form? These are the questions scientists hope to answer when NASA’s James Webb Space Telescope (JWST) is launched and operational. The highly anticipated space telescope will use its infrared capabilities to look into frigid molecular clouds, regions which contain the special conditions necessary to create water. By peering into these cosmic reservoirs astronomers will be able to understand the origin and evolution of water and other key compounds for habitable planets.

Deep within molecular clouds, where dust is struck by intense ultraviolet radiation from newly born stars, complex molecules are formed as a result of chemical reactions. These clouds of gas and dust have a complexity ranging from molecular hydrogen (H2) to carbon-based organics. Molecular clouds are known for holding the highest concentration of water in the universe, as well as being the birthing place for stars and planets.

On the surfaces of tiny dust grains hydrogen atoms combine with oxygen to form water molecules. This happens in the same way that carbon combines with hydrogen to make methane, or nitrogen combines with hydrogen to produce ammonia. These molecules are formed on the surfaces of dust grains and frozen over by a layer of ice, preserving them for over millions of years. When planets are being formed they can collect these cosmic snowflakes, thus providing the materials needed for life. “If we can understand the chemical complexity of these ices in the molecular cloud, and how they evolve during the formation of a star and its planets, then we can assess whether the building blocks of life should exist in every star system,” says Melissa McClure of the Universiteit van Amsterdam, the principal investigator on a research project to investigate cosmic ices.

This investigation will be one of the JWST’s Director’s Discretionary Early Release Science projects, locating what ices are present within a nearby star-forming region. “We plan to use a variety of Webb’s instrument modes and capabilities, not only to investigate this one region, but also to learn how best to study cosmic ices with Webb,” says Klaus Pontoppidan of the Space Telescope Science Institute (STScI). By utilising JWST’s high-resolution spectrographs, NIRSpec and MIRI, astronomers can get observations with five-times better accuracy that any prior space telescope at near- and mid-infrared wavelengths.

The team of astronomers, led by McClure, plan to target the Chamaeleon Complex. This is a star-forming region visible in the southern hemisphere, sitting around 500-light-years away from Earth and containing hundreds of protostars. Protostars are not quite stars, but are in the process of being formed from constituent materials; in fact, the oldest of these protostars are just 1 million years old. “This region has a bit of everything we’re looking for,” said Pontoppidan.

The simulated spectrum highlights what molecules JWST may detect using its spectrographs. Image credit: NASA/ESA/Hubble Heritage Team/M. McClure (Universiteit van Amsterdam) & A. Boogert (University of Hawaii)

The JWST will observe the light coming from stars behind the molecular cloud. The light from these stars will pierce through the cloud and, while it’s on its travels, the ices in the cloud will absorb some of the light at a specific wavelength. By observing many background stars across the sky, astronomers can map the ices within the clouds and locate where the different ones are formed. The astronomers will also target individual protostars in the cloud, uncovering how their ultraviolet light leads to the creation of complex molecules.

Astronomers will have a look at the birthplaces of planets as well, which are the rotating disks of gas and dust that surround protostars, known as protoplanetary disks. JWST’s optics will be able to measure the amounts and relative abundances of ices as close as 8 billion kilometres (5 billion miles), which is the rough equivalent of Pluto’s distance away from the Sun.

“Comets have been described as dusty snowballs. At least some of the water in Earth’s oceans likely was delivered by the impacts of comets early in our Solar System’s history. We’ll be looking at the places where comets form around other stars,” explains Pontoppidan.

Experiments must be done in the laboratory in order to understand the data collected by JWST. The spectrographs will split the infrared light into its constituent colours and wavelengths, seeing which molecules have been absorbing the background stars’ light. The tests conducted in laboratories will provide a database of molecular ‘fingerprints’, which scientists can refer to when identifying the molecules within the star-forming region.

“Laboratory studies will help address two key questions. The first is what molecules are present but, just as important, we’ll look at how the ices got there. How did they form? What we find with Webb will help inform our models and allow us to understand the mechanisms for ice formation at very low temperatures,” explains Karin Öberg of the Harvard-Smithsonian Center for Astrophysics. “It will take years to fully mine the data that comes out of Webb.”

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