Paper Title: Pristine ices in a planet-forming disk revealed by heavy water
Authors: M. Leemker, J. J. Tobin, S. Facchini, P. Curone, A. Booth, K. Furuya, M. L. R. van ‘t Hoff
First-author institution: Dipartimento di Fisica, Universit`a degli Studi di Milano, Via Celoria 16, Milano, 20133, Italy
Status: Nature Astronomy, Volume 9, 1486–1494. (Open Access)
The story of water on Earth is pretty complicated. We still don’t understand where it came from (was it from comets? From the protoplanetary disc in which Earth formed? Or did it form on the Earth itself?) so a lot of scientists are working hard to answer this question. But, there lies a fundamental question beneath this one: where was this water originally created? Did it come from the interstellar medium (called “inheritance”), or was it destroyed and reformed in the Sun’s protoplanetary disc (called “reset” or “processing”)?
Using observations from the Atacama Large Millimetre Array (ALMA), the authors of today’s article have found evidence supporting inheritance, meaning Earth’s water – if it comes from comets or the protoplanetary disc – must originate from the interstellar medium!The object of interest is V883 Ori, a Class II protoplanetary disc that is a few million years old; this is the stage protoplanetary discs spend most of their life in. It’s currently undergoing a “stellar accretion outburst” – that is, it’s really, really bright, and the disc gets extraordinarily hot due to a bunch of matter suddenly being dumped onto the star. This turns a whole bunch of ice to vapour, which we can then observe using ALMA, letting us probe the ice inventory of these discs.
A quick (but necessary) chemistry lesson
Let’s run through a quick physics and chemistry lesson: deuterated hydrogen, or deuterium, (D) is hydrogen that has an additional neutron, which is typically added to the interstellar medium (ISM) by supernovae. Deuterated water comes in two forms: singly deuterated water HDO (also called semi-heavy water) and doubly-deuterated water D2O (also called heavy water). When water (H2O) ice forms in the interstellar medium at temperatures of ~25K, you simultaneously form D2O due to the deuterium hanging around in the ISM. If this water and D2O are processed above temperatures of 500K (the “reset/processing” scenario), then H2O can later reform, but D2O cannot. Therefore, D2O is a fantastic tracer of the history of water in protoplanetary discs. If you have D2O levels consistent with the ISM, then water must have been inherited from the ISM.
So where does the water come from?
Whew, okay. So, what’s the story the authors are telling? Based on their inferences of the D2O to H2O ratio from observations of V883 Ori (D2O/H2O), the authors argue that the history of water is consistent with the “inheritance” story – that is, water must come from the ISM.
They worked this out by comparing their inferences with outputs from chemical processing simulations that simulate the collapse of a molecular cloud. These simulations provide different D2O/H2O levels depending on whether the “reset” or “inheritance” scenario is true; you can see the observations versus simulations in Figure 1, indicating that V883 Ori is consistent with the inheritance story.
What’s more is that V883 Ori has D2O/H2O levels consistent with extremely young protoplanetary discs that are still accreting material from the ISM (called Class 0 objects, which are a few million years younger than V883 Ori). These Class 0 discs are unlikely to have had time to process their water and D2O, so the fact that V883 Ori has similar D2O/H2O levels is strong evidence of a lack of processing – again supporting the inheritance scenario!
Finally, the authors compared the D2O/H2O abundances against values inferred from the comet 67P/Churyumov-Gerasimenko and found that they were, again, similar. This provides evidence of a link between the very early Class 0 stage (i.e. when the protoplanetary disc was first forming), the main protoplanetary disc stage V883 Ori is in, and the modern day solar system. The water inheritance scenario has a very exciting corollary: the water on Earth may be older than the Sun itself!
The authors also checked if their results were robust: some dust in the inner disc of V883 Ori could be hiding further D2O that we can’t observe, but this means that the observed D2O/H2O levels are a minimum possible value. Increasing this value (due to the hidden D2O) only strengthens the inheritance argument.If water comes from the interstellar medium, then, what else does? Does the content of comets also hail from the cold reaches of space between stars? Complex organic molecules may also come from the ISM, so is Earth’s water – and maybe the original organic molecules – more alien than we thought?
Astrobite edited by Maggie Verizon
