What does deuterium tell us about life in space?

[Ann]

Over at the Breaking Science News forum, bystander recently posted a link to an interesting article about the Earth's water and its origin in view of its deuterium content. The question under discussion was whether the Earth's water originates in the interstellar ices that became part of the protoplanetery disk that gave rise to the solar system and survived unchanged there as the solar system was born, or if the original ices were destroyed and then recreated and modified inside the protoplanetary disk of the solar system.

https://carnegiescience.edu/news/earth% ... _older_sun wrote:
“Why this is important? If water in the early Solar System was primarily inherited as ice from interstellar space, then it is likely that similar ices, along with the prebiotic organic matter that they contain, are abundant in most or all protoplanetary disks around forming stars,” Alexander explained. “But if the early Solar System’s water was largely the result of local chemical processing during the Sun’s birth, then it is possible that the abundance of water varies considerably in forming planetary systems, which would obviously have implications for the potential for the emergence of life elsewhere.”

The scientists used deuterium as a diagnostic to whether the ices of the solar system were chemically modified inside the protoplanetary disk of the solar system or whether the deuterium of these ices stayed the same after they were born in interstellar space:

https://carnegiescience.edu/news/earth% ... _older_sun wrote:
For example, interstellar water-ice has a high ratio of deuterium to hydrogen because of the very low temperatures at which it forms.

I'm interested in how much we know about the deuterium content of interstellar ices. But I would also like to know if there might be alternative explanations to the deuterium content of the Earth's water.

This summer I visited a science fair in London, where I had the opportunity to talk to scientists working on the Rosetta mission. I asked them what their mission at the comet actually was, and I was told that one of the most important tasks was to measure the deuterium content of the water ices or water vapor on or right next to the surface of 67P/Churyumov–Gerasimenko. The scientists wanted to know if the deuterium content of the water on or right next to the surface of the comet matched the deuterium content found on the Earth.

The scientist explained a hypothesis about the deuterium content of the Earth's water, namely, that the deuterium content has been rising over time. That is because water can escape from the Earth as chemical reactions separates the hydrogen from the oxygen, and then the lightweight hydrogen can disappear into space. But it is more likely that "heavy hydrogen", deuterium, will remain on the Earth while the more lightweight "normal hydrogen" escapes. The deuterium can then react with oxygen to form water again, but now it will be "heavy water". When these processes go on for billions of years, the deuterium content in the Earth's water will rise.

So it could be that the deuterium content of the Earth's water today doesn't match the deuterium content of the Earth's water three or four billion years ago.

I'm writing this because the scientists in the Carnegie Science article seemed to be comparing the Earth's water and its deuterium content with interstellar ices (and what exactly do we know about the deuterium content in them?) and possible processes going on four and a half to five billion years ago inside the original solar nebula. But isn't it possible that the deuterium content of the Earth's water is partly a result of processes that have been going on on the Earth for billions of years?

Ann

[geckzilla]

I understand the study of the deuterium is something like looking at the fingerprint of water here and there to try to mount evidence that it was all originated at the same time but I really don't see how it relates to life since the processes of life started independently and later on. Could you further elaborate on what you were asking about it in regards to life?

[Ann]

Good question, Geck. I was really mostly asking about the deuterium. But the article in Carnegie Science focused on the life aspect of deuterium in space:

Washington, D.C.—Water was crucial to the rise of life on Earth and is also important to evaluating the possibility of life on other planets. Identifying the original source of Earth’s water is key to understanding how life-fostering environments come into being and how likely they are to be found elsewhere.

I was really asking two questions. One was how sure we are why the deuterium content in the water of the Earth is what it is. And that was really my main question.

My second question was what the whole life in space thing has to do with the deuterium in the Earth's water. But when it comes to that question, I was really just sort of grumbling.

The question of the deuterium in the water of the Earth is more interesting, I think.

Ann

[geckzilla]

Well, two ways I can think of to introduce doubt into the deuterium idea would be to discover a way for it to become naturally synthesized other than by the Big Bang or a way for it to dissociate other than by exposure to gamma radiation. Either of those could potentially destroy the validity of any measurements made with that method. Otherwise, it sounds pretty solid to me!

[Chris Peterson]

So it could be that the deuterium content of the Earth's water today doesn't match the deuterium content of the Earth's water three or four billion years ago.

It definitely hasn't stayed the same. Indeed, the change is measurable by looking at so-called fossil water sources (water that has been protected from chemical interactions for millions of years or more).

The deuterium/protium ratio is much higher on all the terrestrial planets than it is in the Universe as a whole (or in the gas giants), precisely because of chemical processes that concentrate deuterium. It is also higher in periodic comets, whose surfaces have undergone similar processing in repeated passes near the Sun.