Title: Origin, Cycle, and Escape of Volatiles on Terrestrial Planets

Speaker: Dr. Hiroyuki Kurokawa (JSPS Research Fellow, ELSI)

Abstract:

The supply of volatile elements (H, C, N, O, S, and noble gases) to terrestrial planets is necessary for the emergence of habitable environments on their surfaces and, consequently, for the emergence of life. The cycle and loss processes of these volatiles control the sustainability of the habitability. In this talk, I present our previous and ongoing theoretical studies combined with geochemistry and geology to understand the origin, cycle, and escape of volatiles on terrestrial planets.

First, I introduce our studies on the escape of atmospheres and water of Earth and Mars. Exploration missions has suggested that Mars once sustained a habitable condition on its surface--oceans covered by an atmosphere denser than that of present-day. We utilized isotope (D/H, 15N/14N, and 38Ar/36Ar) analysis of Martian meteorites whose absolute ages have been reported to constrain when volatiles were lost and to understand what caused the loss. Our study show that Mars had a dense atmosphere at 4 billion years ago, whereas more than half of surface water had lost at the time. Because Mars had the core dynamo about 4 billion years ago, the result suggests that the stripping of the atmosphere by the solar wind following the cessation of the dynamo is partially responsible for transforming Mars from a warm wet world into a cold desert world.

Contrary to Mars, geologic evidence has shown that Earth has maintained oceans throughout its history except for the duration of snowball Earth events. Our model of Earth's deep water cycle combined with D/H data of Archean seawater show that the initial volume of oceans might be twice larger than that of present-day. The major process to remove Earth's seawater is the subduction into the mantle and the volume removed by atmospheric escape is small. These studies suggest that the terrestrial planets in the solar system are likely to have acquired similar amounts of volatiles and their fates diverged because of the cycle and loss processes.

Next, I show our ongoing studies to constrain the origin of volatile on terrestrial planets. Classical planet formation theory regards these volatiles as delivered by late accretion of planetesimals (asteroids and comets) derived from the outer solar system. We modeled the evolution of atmospheric volumes and compositions during the late accretion taking the erosion and replenishment of volatiles by numerous impacts into account. We found that, to satisfy the present-day C/H and N/H ratios of Earth's atmosphere and hydrosphere, the impactors of late accretion should be volatile poor and that volatiles are likely to have been delivered during planet formation.

A new theory of planet formation--pebble accretion regards cm-sized dusts (pebbles) drifting from the outer to inner protoplanetary disks as building blocks of planets. One possible mechanism to deliver volatiles to terrestrial planets in the pebble accretion paradigm is the accretion of icy pebbles derived from the outer solar system during planet formation. However, previous studies have shown that the icy pebble accretion causes "water-excess"--Earth would acquired a huge amount of water. We performed hydrodynamical simulations of disk gas past a terrestrial planet. Our simulations show that the planet induces a gas flow which leaves the planet in the mid plane regions. Because the speed of outflow is comparable with that of the pebble drift, the interaction of the planet-induced flow with icy pebbles may reduce the accretion rate onto the planet and overcome the water-excess problem.

Finally, I introduce my research plan to tackle the origin of volatiles on terrestrial planets by participating exploration missions: Dawn, Hayabusa-2, and MMX. Analysis of near infrared spectra of dwarf planet Ceres, asteroid Ryugu, and Martian moons will provide constraints on their volatile abundances and on the contribution of asteroids to the volatiles on the terrestrial planets. Because the accretion rate of pebbles has been estimated to differ significantly between Ceres and small bodies, combining the data analysis with theoretical modeling will unveil the process to deliver the volatile elements to the terrestrial planets.