Adam R. Sarafian

Early accretion of water in the inner solar system from a carbonaceous chondrite–like source

History recorded in asteroids water Astronomers know that interstellar water is abundantly available to young planetary systems—our blue planet collected (or accreted) plenty of it. Still, the details of waters movement in the inner solar system are elusive. Sarafian et al. measured water isotopes in meteorite samples from the asteroid Vesta for clues to the timing of water accretion. Their samples have the same isotopic fingerprint of volatiles as both Earth and carbonaceous chondrites, some of the most primitive meteorites. The findings suggest that Earth received most of its water relatively early from chondrite-like bodies. Science, this issue p. 623 Similar volatile isotopes of Earth and ancient meteorites point to an early accumulation of water for terrestrial bodies. Determining the origin of water and the timing of its accretion within the inner solar system is important for understanding the dynamics of planet formation. The timing of water accretion to the inner solar system also has implications for how and when life emerged on Earth. We report in situ measurements of the hydrogen isotopic composition of the mineral apatite in eucrite meteorites, whose parent body is the main-belt asteroid 4 Vesta. These measurements sample one of the oldest hydrogen reservoirs in the solar system and show that Vesta contains the same hydrogen isotopic composition as that of carbonaceous chondrites. Taking into account the old ages of eucrite meteorites and their similarity to Earth’s isotopic ratios of hydrogen, carbon, and nitrogen, we demonstrate that these volatiles could have been added early to Earth, rather than gained during a late accretion event.

Experimental constraints on the damp peridotite solidus and oceanic mantle potential temperature

Turning up the mantle temperature The temperature at which Earths mantle begins to melt is a long-standing question in geology. Sarafian et al. present a clever set of experiments to determine the impact of small amounts of water on the melting temperature of mantle rock (see the Perspective by Asimow). This allowed them to reinterpret geophysical observations of melting in the mantle and revise estimates of mantle temperature upward. A hotter mantle has a multitude of implications for mantle melting and geodynamic processes. Science, this issue p. 942; see also p. 908 Experiments on mantle rock with small amounts of water provide constraints on the temperature of Earth’s mantle. Decompression of hot mantle rock upwelling beneath oceanic spreading centers causes it to exceed the melting point (solidus), producing magmas that ascend to form basaltic crust ~6 to 7 kilometers thick. The oceanic upper mantle contains ~50 to 200 micrograms per gram of water (H2O) dissolved in nominally anhydrous minerals, which—relative to its low concentration—has a disproportionate effect on the solidus that has not been quantified experimentally. Here, we present results from an experimental determination of the peridotite solidus containing known amounts of dissolved hydrogen. Our data reveal that the H2O-undersaturated peridotite solidus is hotter than previously thought. Reconciling geophysical observations of the melting regime beneath the East Pacific Rise with our experimental results requires that existing estimates for the oceanic upper mantle potential temperature be adjusted upward by about 60°C.