The Planetary Geochemistry Group
CosmoGeo Lab
Research
Unravelling the origin of life-essential volatiles like nitrogen (N), carbon (C), and water (H2O) in the rocky planets of our Solar System and beyond is key to answering a fundamental question in modern science: what is the recipe to form a habitable planet? N-C-H2O inventories of rocky planets are the result of a complex series of processes starting from primitive dust, organics, and ices in a protoplanetary disk, moving through planetesimals and planetary embryos, and ending with oligarchic growth and giant impact stages of planetary accretion. CosmoGeo Lab's goal is to comprehensively understand how these processes affect the inventories of life-essential volatiles in rocky planets. To accomplish this, we explore the geochemistry of protoplanetary and planetary scale differentiation processes like core formation, degassing of magma oceans, and magma ocean crystallization using high pressure-temperature experiments, isotope geochemistry, and thermo-chemical modelling supported by meteoritic data. We constrain the role of hitherto poorly understood physical and chemical processes within protoplanetary and planetary interiors that modulate the final N-C-H2O inventories of rocky planets in our solar system and around other stars. Understanding these processes will not only lead to the development of a general framework for the origin of N-C-H2O in rocky exoplanets but also provide important insights into the processes that eventually lead to the formation of habitable planets and, therefore, life itself. Some of the topics explored by members of the CosmoGeo Lab are listed below:
Elemental fractionation of volatiles during core-mantle differentiation
The bulk silicate reservoirs of rocky protoplanets and planets are depleted in volatiles in comparison to chondrites. The segregation of these volatiles into the metallic cores could be one of the primary causes behind their depletion. Our research group is dedicated to elucidating the intricate elemental partitioning mechanisms governing the distribution of these volatiles between metallic and silicate melts. By conducting high-pressure and high-temperature experiments, we aim to gain deeper insights into the multifaceted geochemical processes that transpired during the early stages of planetary formation. Specifically, our investigations encompass a wide range of accretion scenarios and differentiation conditions, shedding light on the potential incorporation of volatiles into the cores as well as their retention in the mantles and atmospheres of protoplanets and planets.
Isotopic fractionation of volatiles during core-mantle differentiation
The isotopic compositions of volatiles in the mantles and cores of differentiated protoplanets and planets deviate from those observed in primitive chondrites. Our research group employs high-pressure and high-temperature experiments to precisely quantify the extent of isotopic fractionation that occurs during core-mantle differentiation in planetesimals and planets. For instance, our investigations have revealed that nitrogen isotopes exhibit only minor fractionation during this process, implying that the nitrogen isotopic ratios found in iron meteorites accurately represent the compositions of their parent bodies. These discoveries hold significant implications for the application of the isotopic signatures of volatiles in differentiated bodies as tracers to decipher the origins of volatiles in rocky planets. For more details, see - Grewal et al (2022) GCA.
Speciation of volatiles in magma oceans
The dissolution mechanism of volatiles in silicate melts during early magma oceans and subsequent partial melting in planets remains poorly understood. Our research on this topic aims to enhance our comprehension of the speciation of volatiles within magma oceans and partial melts and assesses its profound impact on the distribution of volatiles among the principal reservoirs within rocky bodies. Utilizing a combination of high-pressure and high-temperature experiments and spectroscopy methods like Raman and FTIR, we determine the dissolution mechanisms of these volatiles in silicate melts under extreme conditions relevant to protoplanetary and planetary interiors. Understanding the speciation of volatiles in magma oceans and partial melts is of paramount importance because it also dictates the chemical compositions of planetary atmospheres formed by magma ocean and volcanic degassing.
This facet of our research plays a critical role in advancing our knowledge of the chemical compositions of atmospheres in rocky planets and exoplanets throughout their entire geological histories. For more details, see - Grewal 2020 et al (2020) GCA.
Volatile reservoirs in the solar protoplanetary disk
The timing of the origin of volatiles in the rocky planets is a subject of major debate. The discussion revolves around whether these planets accreted volatiles from their inception or had an exclusively exogenous volatile delivery from the outer solar system. Our approach based on meteorites involves examining the isotopic and chemical characteristics of the earliest-formed planetesimals within the inner Solar System. Our research strongly indicates that these early planetesimals were indeed volatile-bearing, and this legacy of early-accreted volatiles is also retained in the present-day volatile inventory of rocky planets. These findings hold great importance for comprehending the chemical makeup of volatile carriers, the distribution of volatile reservoirs in the disk as a function of space and time, and the thermal structure of the disk. For more details, see - Grewal et al (2021) Nature Astronomy, Grewal (2022) APJ, and Grewal et al (2024) Nature Astronomy.
Volatile inventories in the parent bodies of iron meteorites
Magmatic iron meteorites are the remnants of the metallic cores of the earliest formed planetesimals both in the inner and outer solar system. Our research leverages the elemental and isotopic compositions of these meteorites to reconstruct the volatile abundances and isotopic compositions of their parent bodies. These constraints are pivotal for understanding the volatile contents of the building blocks of rocky planets. To achieve this, we employ high-pressure and high-temperature experiments simulating the fractional crystallization processes within the parent cores of iron
meteorites and core-mantle differentiation within their parent bodies in conjunction with thermo-chemical models. This facet of our research is of critical importance in the endeavor to trace the volatile inventories in the earliest planetesimals and how they were affected by high-temperature processes like thermal metamorphism, melting, and core-mantle differentiation. For more details, see - Grewal et al (2021) Nature Astronomy, Grewal et al (2022) EPSL, Grewal et al 2022 GCA, and Grewal et al (2023) GCA.