Insights into Mars's magnetic history from meteorite paleomagnetism and large impact basin magnetism

Abstract

In September of 1997, Mars Global Surveyor (MGS) made the first measurements of Mars’s magnetic field. It was initially uncertain whether the fields it detected represented an active global dynamo field, like that present on the modern-day Earth, but was soon determined that MGS was instead measuring remanent crustal fields. These fossil magnetic fields implied Mars had a dynamo at some time in its past that terminated before the present day. By 1999, MGS had collected enough magnetic measurements to produce the first full maps of Mars’s crustal fields. In the first paper to publish such maps, Acuña et al. (1) noted that “The absence of crustal magnetism near large impact basins such as Hellas and Argyre implies cessation of internal dynamo action during the early Naochian [sic] epoch (~4 billion years ago).” Thus arrived the first estimate of Mars’s dynamo cessation age. Many subsequent studies took similar approaches to quantifying the dynamo’s longevity using the magnetic properties of large impact basins such as Hellas, Utopia, Argyre, and Isidis. Better-calibrated surface dating allowed the ages of these and other martian impact basins to be more precisely determined. Lillis et al. (2) combined these improved ages with techniques to more accurately characterize basins’ magnetization states to estimate that the Martian dynamo permanently shut down between 4.1 and 4.0 Ga. Vervelidou et al. (3) recovered a cessation age of 4.14 - 4.12 Ga following a similar approach. This idea was not without challenges, however. As early as 2000, Schubert et al. (4) proposed that rather than shutting down at ~4 Ga, Mars’s dynamo did not begin until after the formation of Hellas. It was also around this time that the first paleomagnetic studies of the martian meteorite ALH 84001 emerged, identifying strong magnetization in the meteorite that likely dated to ~4.0 Ga. However, the unusual pattern of strong but heterogeneous natural remanent magnetization (NRM) in ALH 84001 made it challenging to ascribe a specific magnetization age. Furthermore, a number of papers were published throughout the 2000s and 2010s identifying apparently magnetized volcanic features with ages between 3.9 and 3.5 Ga. Perhaps the most important of these studies was published in 2020; enabled by new, lower-altitude magnetic data from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, Mittelholz et al. (5) identified a demagnetized crater within the geologic region Lucus Planum. This implied that the underlying unit, dated to ~3.69 Ga, was magnetized in a dynamo field. The authors also identified crustal fields above the ~4.5 Ga Borealis basin, suggesting the dynamo was active after ~4 Ga but initiated well before that time. Having excluded that Mars’s dynamo could have started late, this brings us to the present question: did Mars’s dynamo shut down early (>4.0 Ga) or late (3.9-3.5 Ga)? The seemingly small difference between these cessation ages has important implications for Mars’s climate history. A common narrative is that the magnetic field of a planet shields it from solar radiation, thereby slowing atmospheric escape and helping the planet preserve a habitable climate over longer timescales. In this framework, it would be tempting to attribute the atmospheric loss Mars experienced, and the following shift to its present cold and arid climate, to the cessation of its dynamo. However, models of martian atmospheric escape find that a weak or very strong field could actually accelerate oxygen ion loss. Knowing the strength and longevity of Mars’s dynamo would accordingly inform our understanding of the relationship between planetary magnetic fields and atmospheric escape. The hypothesized >4.0 Ga dynamo cessation would pre-date the formation of Martian valley networks by at least 0.4 billion years, suggesting a global magnetic field was not necessary for Mars to maintain its early atmosphere. In contrast, a later dynamo cessation could imply a closer relationship between the early martian dynamo and the planet’s climate. The strength, geometry, and longevity of Mars’s magnetic field is also connected to the conditions and evolution of its deep interior. Because Mars’s dynamo was likely powered by convective cooling of its hot early core, its longevity is fundamentally limited by deep interior cooling timescales. These timescales in turn depend on interior physical properties (e.g. mantle reference viscosity, core conductivity, and initial core superheating), which govern the efficiency with which heat can be removed from the martian core. In general, longer dynamo lifetimes require lower core conductivities and/or higher degrees of core superheating. It is clear that stronger constraints on the strength, character, and longevity of Mars’s early dynamo would translate into improved understandings of Mars’s interior, atmospheric evolution, and climate. In this dissertation, I aim to couple novel paleomagnetic constraints on the martian dynamo’s longevity and strength with modeling of magnetization acquisition in impact basins to establish a martian magnetic history that is consistent with all available constraints. In Chapter 1, I describe paleomagnetic measurements of an ancient martian dynamo field preserved in 4.1-billion-year-old meteorite Allan Hills 84001 (ALH 84001). This study took advantage of novel techniques for net moment magnetometry based on micron-scale magnetic imagery from the quantum diamond microscope (QDM) to characterize the natural remanence in igneous Fe-sulfides. This analysis revealed that individual, 100-μm-scale ferromagnetic mineral assemblages in ALH 84001 are strongly magnetized in two nearly antipodal directions. These observations are most simply explained by a reversing martian dynamo that was active until 3.9 Ga, thereby implying a late cessation for the martian dynamo and potentially documenting reversing behavior in a nonterrestrial planetary dynamo. In Chapter 2, I discuss whether the weak magnetism observed above many martian imapct basins could naturally result from such a long-lived and reversing dynamo field. Modeling the cooling and magnetization of 200-2200 km diameter impact basins under a range of Earth-like reversal frequencies confirmed that magnetic reversals efficiently reduce field strengths above large basins, particularly if late remagnetization of near-surface material is efficient. This ultimately implies that Mars’s weakly magnetic basins do not require an early dynamo cessation. In Chapter 3, I assess whether the interiors of martian impact basins are likely to be systematically depleted in ferromagnetic minerals, potentially providing an alternative explanation for their weak magnetism. I modeled the crystallization of candidate impact melt compositions to quantify the expected magnetic properties of martian impact basins, ultimately finding that their interiors would have lower ferromagnetic contents than typical martian crust but would still produce stronger magnetic fields than observed if cooled in a non-reversing field. This suggests that a reversing or intermittent dynamo may be necessary to explain the weak magnetism of martian impact basins. Combined, these studies provide robust paleomagnetic evidence for a martian dynamo that persisted until at least 3.9 Ga and potentially reversed and demonstrate that such a dynamo is consistent with orbital magnetic observations above large martian impact basins. A martian dynamo which persisted until 3.9 to 3.7 Ga and reversed at rates >5 Myr-1 remains the most straightforward explanation for all available orbital magnetic observations and paleomagnetic measurements. In Chapter 4, I summarize the progress that these and other recent works have made towards improving our understanding of Mars’s dynamo within the last five years and reflect on prospects for expanding our knowledge of martian magnetism in the future.