[Paper Review] A new perspective on interiors of ice-rich planets: Ice-rock mixture rather than a layered structure.

Allona Vazan, Re’em Sari+1 more|arXiv (Cornell University)|Nov 1, 2020

Astro and Planetary Science7 references6 citations

TL;DR

This paper challenges the conventional layered interior model of ice-rich planets by demonstrating that ice and rock remain extensively mixed under high-pressure conditions due to miscibility, leading to a homogeneous interior structure rather than distinct layers. Using high-pressure experimental data and thermal evolution models, it shows that migration and mass loss influence atmospheric composition but not deep interior mixing, with over 99% of the planet's mass remaining mixed for gigayears.

ABSTRACT

Ice-rich planets formed exterior to the iceline and thus are expected to contain substantial amount of ice (volatiles). The high ice content leads to unique conditions in the interior, under which the structure of a planet may be affected by ice interaction with other metals. We use experimental data of ice-rock interaction at high pressure, and calculate detailed thermal evolution for possible interior configurations of ice-rich planets. We model the effect of migration inward on the ice-rich interior by including the influences of stellar flux and envelope mass loss. We find that rock and ice are expected to remain mixed, due to miscibility at high pressure, in most of the planet interior (>99% in mass) for a wide range of planetary masses. We also find that the deep interior of planetary twins that have migrated to different distances from the star are usually similar, if no mass loss occurs. Significant mass loss results in an interior structure of a mixed ice and rock ball, surrounded by a volatile atmosphere of less than 1% of the planet's mass. In this case, the mass of the atmosphere of water / steam is limited by the ice-rock interaction. We conclude that when ice is abundant in planetary interiors the ice and rock tend to stay mixed for giga-years, and the interior structure differs from the simple layered structure that is usually assumed. This finding could have significant consequences on planets' observed properties, and it should be considered in exoplanets characterisation.

Motivation & Objective

• To re-evaluate the internal structure of ice-rich planets formed beyond the iceline, where high ice content may alter expected layering.
• To investigate how ice-rock interactions at high pressure affect planetary thermal evolution and interior configuration.
• To assess the impact of inward migration and mass loss on the interior structure and atmospheric composition of such planets.
• To challenge the standard assumption of a layered interior (rock core, ice mantle) in favor of a long-lived mixed state.

Proposed method

• Utilization of high-pressure experimental data on ice-rock interactions to inform phase behavior and miscibility at planetary interior conditions.
• Development of thermal evolution models that incorporate variable envelope mass loss and stellar flux during inward migration.
• Numerical simulation of planetary interiors under varying masses and migration histories to assess structural stability.
• Application of thermodynamic principles to determine the stability of mixed versus layered configurations over gigayear timescales.
• Comparison of interior structures in planetary twins that migrated to different orbital distances to assess structural similarity.

Experimental results

Research questions

• RQ1 To what extent do ice and rock remain miscible in the deep interiors of ice-rich planets under high-pressure conditions?
• RQ2 How does inward migration affect the thermal evolution and internal structure of ice-rich planets?
• RQ3 What role does mass loss play in shaping the atmospheric composition and internal configuration of these planets?
• RQ4 How do the interiors of planetary twins that migrated to different distances compare in terms of structure and composition?

Key findings

• Over 99% of the planet's mass remains in a mixed ice-rock state due to high-pressure miscibility, challenging the assumption of layered interiors.
• The deep interiors of ice-rich planets are largely similar regardless of migration distance, provided no significant mass loss occurs.
• Significant mass loss leads to a volatile atmosphere of less than 1% of the planet's mass, limited by ice-rock interaction processes.
• The ice-rock mixture remains stable for gigayear timescales, indicating that mixed interiors are a long-term configuration.
• The observed properties of exoplanets may be significantly influenced by this mixed interior structure, necessitating revision in exoplanet characterization models.