History and Future of the Martian Dynamo and Implications of a Hypothetical Solid Inner Core

Abstract

Planetary magnetism is a fundamental but imperfectly understood phenomenon that has the power to inform our understanding of the deep interiors and thermal histories of planetary bodies of all kinds (e.g., Connerney, 2015;Stevenson, 2003;Stevenson et al., 1983). For terrestrial planets, convective motions in the electrically conductive liquid metallic core can drive a global scale self-sustaining magnetic field, in a process known as a magnetohydrodynamic dynamo. This field shields the planet's atmosphere and surface from the impinging solar wind and imparts thermoremanent magnetization to rocks as they cool at or near the planet's surface, preserving a record of how the dynamo field may have changed over time (e.g., Purucker & Whaler, 2015;Weiss & Tikoo, 2014). Paleomagnetic evidence has established that the Earth's dynamo field has been active for at least the last ∼3.5 Gyrs, and possibly longer (e.g., Tarduno et al., 2010Tarduno et al., , 2015. By contrast, the case of Mars is interesting because it has no global dynamo field operating today and yet magnetometer and electron reflectometer data from the Mars Global Surveyor

Influence of Thermal Stratification on the Structure and Evolution of the Martian Core

“…In all of our models the core is far hotter than the liquidus temperature observed by Gilfoy and Li (2020), with the successful models giving 𝐴𝐴 𝐴𝐴c > 1940 K at the present-day. Our successful models are also above the liquidus curve given by Stewart et al (2007) and those used by Rivoldini et al (2011) and Hemingway and Driscoll (2021) for sulfur concentrations >10 wt% S. Note that we assumed an isothermal mantle except in thermal boundary layers, and so, accounting for the roughly 100 K adiabatic increase in temperature across the convecting mantle would yield CMB temperatures roughly 100 K higher than we obtain.…”

“…(2007) and those used by Rivoldini et al. (2011) and Hemingway and Driscoll (2021) for sulfur concentrations wt% S. Note that we assumed an isothermal mantle except in thermal boundary layers, and so, accounting for the roughly 100 K adiabatic increase in temperature across the convecting mantle would yield CMB temperatures roughly 100 K higher than we obtain.…”

“…Second, the previous parameterized models did not account for thermal stratification, which likely affects core temperature estimates. These previous models (Breuer & Spohn, 2006;Hemingway & Driscoll, 2021;Williams & Nimmo, 2004) predict that the Martian core was heavily sub-isentropic ( 𝐴𝐴 𝐴𝐴c < 𝐴𝐴a ) for a significant part of its history, including at present. When sub-isentropic, the thermal state of the core deviates away from a convective state toward a conductive one, resulting in a stable, thermally stratified layer that grows from the top of the core downwards, as investigated for Earth's core (Greenwood et al, 2021;Labrosse et al, 1997;Nimmo, 2015).…”

“…The growth of a large solid inner core (bottom‐up crystallisation) would have likely restarted the dynamo (Williams & Nimmo, 2004), and is therefore not favored. Growth of a small inner core might not yet be able to restart the dynamo (Hemingway & Driscoll, 2021) but would be undetectable by current seismic data from InSight (Stähler et al., 2021). However, depending upon the relative slopes of the core temperature and its melting temperature, the Martian core may have started to solidify from the top‐down rather than bottom‐up (Stewart et al., 2007).…”

Seismic detection of the martian core

“…Strong crustal remnant magnetization in the southern hemisphere of Mars (72) and observations of further magnetized units suggest a dynamo that was active between 4.5 Ga and 3.7 Ga (9). The dynamo would have been thermally-driven in the first few hundred Myr (49, 50) and possibly followed by a compositionally-driven dynamo that may resuscitate through FeO exsolution (73) or inner-core crystallization (71,74,75). This, however, depends critically on the light element content and thermal state of the core.…”