The mystery of where Earth's water comes from deepens with a new study

In a conclusion that has relevance to our search for life on other planets, the researchers understood that melted or “achondrite” meteorites cannot be the main source of water on Earth.

The study involved looking at samples from 7 meteorites, originating from either the inner and outer reaches of the solar system, that eventually slammed into Earth. This likely happened billions of years after they split off from at least five so-called planetesimals — objects which collided with others to form our solar system’s planets. These planetesimals underwent melting due to the heat from the decay of radioactive elements, splitting into layers containing a crust, mantle, and core, as explained in the press release from the University of Maryland. 

What’s remarkable about the study is that while it is often assumed water ended up on Earth from the outer solar system, the researchers showed that not all outer solar system objects have much water. In fact, we don’t yet conclusively know what objects carried water because as soon as the meteorites the scientists studied melted, no water remained. Overall, water accounted for less than two-millionths of the mass of the achondrite meteorite samples they looked at. This likely means water came to our planet on unmelted meteorites. 

To compare, meteorites with the greatest amount of water content, so-called carbonaceous chondrites, are comprised of up to 20% of water by weight. That’s about 100,000 times more water than in the samples analyzed by Newcombe and her co-authors.

The scientists believe the ramifications of their work extend past geology as water is so crucial to our search for life. Exoplanets, in particular, have been looked at as potential sources of water and hosts for extraterrestrial organisms. Figuring out which space bodies may have the greatest water content and how it gets there is paramount to figuring out the mystery of both life’s origins and its spread throughout our solar system and beyond.  

Interesting Engineering (IE) reached out to Professor Newcombe for more insight on the paper.  

The following exchange has been lightly edited for clarity and flow.

Interesting Engineering: If water came to Earth on unmelted meteorites, how did they avoid the fate of the achondrite meteorites and not melt upon impact?

Professor Newcombe: Great question. Achondrite meteorites are thought to have melted in response to heating by radioactive decay during the first few million years of solar system history. Radioactive heating was very intense during the first couple of million years of solar system formation, because there was a lot of 26Al, which is a short-lived radionuclide. Planetesimals that accreted during those first few million years likely experienced extensive melting, unless they were very tiny (less than around 20 km across). Unmelted meteorites (which we call chondrites) could have escaped this melting if their parent bodies formed after most of the [Aluminum-26] had already decayed. Alternatively, their parent bodies may have been very small so that they didn't contain enough Al-26 to drive melting.

Meteorites found on Earth normally have a fusion crust - a thin layer of melt on their outer surface that forms as the meteorite burns up on entry into our atmosphere. Chondrites don't escape this melting on their outer surfaces, but their cores remain unmelted.

IE: What are the implications of your study on the search for the origin of life on Earth?

Water is considered to be a necessary ingredient for life to flourish. The abundance of water on Earth's surface is not easy to explain, given that we are relatively close to the sun, and therefore it might be expected that the material that accreted onto the proto-Earth was likely quite dry. Our study provides insight into the origin of Earth's water, by ruling out delivery of water by melted material. 

IE: What were the challenges of analyzing the water content in the different samples you studied?

The main challenge is removing terrestrial contamination. We had to clean our samples very carefully, we baked them in an oven at ~100 degrees C to remove terrestrial water on the surfaces of the samples, and we left the samples under vacuum for a month prior to analyzing them. Our analyses are done under vacuum. We use an ion beam to ablate the surface of the sample in order to remove any remaining surface contamination before each analysis.

IE: What is next for your research team?

We are all obsessed with water! We are analyzing water in other meteorites. We also make our own magma in the lab and we explore how water behaves in magma. Some of us study volcanic eruptions -- we want to understand what controls the magnitude (or explosivity) of volcanic eruptions.

Check out the new study “Degassing of early-formed planetesimals restricted water delivery to Earth,” published in the journal Nature.

Study abstract

The timing of delivery and the types of body that contributed volatiles to the terrestrial planets remain highly debated1,2. For example, it is unknown if differentiated bodies, such as that responsible for the Moon-forming giant impact, could have delivered substantial volatiles3,4 or if smaller, undifferentiated objects were more probable vehicles of water delivery5,6,7. Here we show that the water contents of minerals in achondrite meteorites (mantles or crusts of differentiated planetesimals) from both the inner and outer portions of the early Solar System are ≤2 μg g−1 H2O. These are among the lowest values ever reported for extraterrestrial minerals. Our results demonstrate that differentiated planetesimals efficiently degassed before or during melting. This finding implies that substantial amounts of water could only have been delivered to Earth by means of unmelted material.