Nebular ingassing as the primary source of water and light noble gases to Earth

Proposed mechanisms for volatile addition to the Terrestrial Planets include: 1) primary accretion of volatile-rich material (e.g., pebble accretion); 2) late accretion of carbonaceous chondrites; and 3) nebular ingassing. Primary accretion results in similar water concentrations for both Earth and Mars, a result at odds with most martian water estimates. Neither primary accretion nor late addition effectively ingasses helium. Nebular ingassing is an effective mechanism for light rare gas addition, but it does not deliver enough heavy rare gases to Earth. It is likely that all three mechanisms contribute partly to the Earth's volatile budget.

Ingassing occurs when a planet reaches ~0.3 Earth Mass (M_E) in the presence of a solar nebula. Smaller bodies, such as Mars, would never reach the basalt solidus, so that ingassing would have been negligible. The amount of volatiles ingassed to Earth is critically dependent on the duration of the nebula and growth rate of the planet. Assuming a nebular age of 4 My (based on the youngest chondrules) and a 10 My accretion time, 1.1-1.4×10²⁴ g (8-10 ocean-equivalents) H₂O would be ingassed to Earth [1]. Ingassing leads to an excess of He, Ne and Ar, and a deficit of Kr and Xe. The (ingassed/present-day) rare gas ratios decrease from ~700 for He to less than 10 for Ar, varying linearly on a semilogarithmic plot vs. atomic mass. Such a linear fit is expected based on models of hydrodynamic escape [2] and gives an m_c value of ~48, representing the mass above which hydrodynamic escape is negligible. The deficit of Kr and Xe can be satisfied by late addition of ~1.3 wt% CI chondrites. The late addition would not add appreciably to the lighter rare gases, as they would be lost by hydrodynamic escape. Simple mixing of these three proposed volatile sources does not satisfy the isotopic constraints from the heavy rare gases unless additional isotope fractionation mechanisms are invoked. Olson, P.L. and Z.D. Sharp (2019) Phys. Earth Planet. Int. 106294. Hunten, D.M., et al. (1987) Icarus, 69: p. 532-549.