How Does Water go From Interstellar Clouds to Habitable Worlds?

Water moves.  On Earth, it moves in the form of rivers, rain, or ocean swells.  In space, its movements are more subtle but no less more important, and so far we understand very little about that process.  Luckily, we had a tool to help us try to understand it better – the Hershel Space Observatory.  Though it has been out of commission for over 8 years, a team of scientists have now compiled all a review of all of the papers using Hershel data to track water from its birth in interstellar clouds to its eventual resting place on planets. There are still some gaps, but it’s a worthy step towards a better understanding.

Hershel, which was launched in 2009, had a specific instrument called the Heterodyne Instrument for Far-Infrared (HIFI).  Designed by Dutch scientists, one of HIFI’s primary objectives was to look for water in space.

Artist depiction of the HIFI instrument that was used as a basis of the review paper.
Artist depiction of the HIFI instrument that was used as a basis of the review paper.
Credit: Caltech / NASA / ESA

Tracing one of the most common molecules in the universe isn’t easy, partially because it can be found almost everywhere.   From shockwaves caused by star formation to plumes off Saturn’s moon Enceladus, water is abundant throughout the cosmos.  But figuring out how it moves about in those different stages of cosmic evolution is trickier.

Data from Hershel was a good place to start trying to understand, and there was a lot of it.  Published over dozens of papers in the last few years, a team led by Dr. Ewine van Dishoeck at Leiden University collected as much of that data as they could and discovered several new tidbits along the way.

ESA video describing some of Herschel’s accomplishments.
Credit: ESA

The first of those is how quickly water ice in early solar systems tied up with dust particles.  Those combinations of water and dust then go on to form the building blocks of planets surrounding young stars.  There’s much more water available in these gaseous discs than would be expected. Enough water surrounds a typical young star to fill thousands of oceans.  It doesn’t look like oceans when starting out though – the formation of those ice / dust slivers was another finding from this research. The team found that ice forms in layers on dust particles, collecting the type of water that is most readily available (e.g. regular H2O or “heavy” water).

Finalizing this research effort likely means that scientists have gleaned about as much as possible about water migration out of the data from Hershel.  Unfortunately, Herschel’s successor, with similar capabilities, won’t launch until at least 2040.  The paper’s authors put forward this review paper as a way of keeping collective memory during the drought of new data coming in over the next 20 years.

Artist's impression of the Herschel Space Telescope. Credit: ESA/AOES Medialab/NASA/ESA/STScI
Artist’s impression of the Herschel Space Telescope.
Credit: ESA/AOES Medialab/NASA/ESA/STScI

All hope for new data is not lost though – there are a few instruments that could help push along our understanding of the pathway of water.  First is the James Webb Space Telescope, which has an instrument called the Mid-Infrared Instrument (MIRI). MIRI could potentially collect some additional data, but will largely be focused on JWST’s larger mission objectives outside of just understanding water.  Additionally, ALMA has the ability to peer through the water in Earth’s atmosphere to try to track it out in space, but interpreting its data is still in the early stages, though an understanding of Hershel’s data will help contribute there as well.

Sometimes review papers truly are one of the best tools of science, as it helps us maintain knowledge throughout the scientific generations. The one surmising the years of data that Herschel managed to collect is a fitting tribute to that space observatory.

Astronomie NL – Long-awaited review reveals journey of water from interstellar clouds to habitable worlds
arXiv – Water in star-forming regions (WISH): Physics and chemistry from clouds to disks as probed by Herschel spectroscopy
NASA – Herschel Finds Oceans of Water in Disk of Nearby Star
UT – Speedy Science: Here’s Four Years Of Herschel Telescope Work In A Short Video

Lead Image:
Artist depiction of Herschel and the water it was searching for.
Credit: ESA / ALMA / NASA / L.E. Kristensen