Differentiated Meteorites Unlikely to Be a Significant Source for Earth’s Water

Credit: NASA.

The timing and mechanism of water delivery to early Earth is a subject of active research. The central question is what type(s) of planetary bodies carried the water. Water could have been delivered to Earth from cometary ice, various types of chondritic (undifferentiated) meteorites, or various achondritic (melted and differentiated) meteorites. All known meteorites appear to have originated from one of two distinct isotopic reservoirs: non-carbonaceous (NC) and carbonaceous (CC). NC meteorites likely formed in the inner solar system, close to the Sun, and are thought to have accreted little to no water. In contrast, CC meteorites likely formed in the outer solar system, beyond Jupiter’s orbit and the water snowline, and therefore accreted abundant water. Indeed, carbonaceous chondrite meteorites from the CC reservoir contain up to 10-20 wt% water. Notably, differentiated meteorites occur in both the NC and the CC groups.

In a new study, Megan Newcombe (University of Maryland) and co-authors report secondary ion mass spectrometry measurements of water concentrations in the main silicate minerals used to estimate the bulk water contents of planets in differentiated meteorites, sampling both the NC and CC reservoirs. Results show that these meteorites are among the driest extraterrestrial materials ever studied (100,000 times drier than water-rich carbonaceous chondrites), most likely because they experienced efficient degassing during heating and melting on their parent asteroids. This rules them out as primary sources of Earth’s water. The results for the differentiated CC meteorites, in particular, contradict the prevalent view that all outer solar system objects are rich in water. This raises new questions about the exact link between the timing of NC/CC accretion and volatile addition to Earth because at least some CC meteorites would be depleted in water. Ruling out differentiated meteorites as a significant source of Earth’s water helps us understand water delivery on Earth and exoplanets. READ MORE