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Speleothems are mineral deposits capable of recording detrital and/or chemical remanent magnetization at annual timescales. They can offer high‐resolution paleomagnetic records of short‐term variations in Earth's magnetic field, crucial for understanding the evolution of the dynamo. Owing to limitations on the magnetic moment sensitivity of commercial cryogenic rock magnetometers (∼10−11 Am2), paleomagnetic studies of speleothems have been limited to samples with volumes of several hundreds of mm3, averaging tens to hundreds of years of magnetic variation. Nonetheless, smaller samples (∼1–10 mm3) can be measured using superconducting quantum interference device (SQUID) microscopy, with a sensitivity better than ∼10−15 Am2. To determine the application of SQUID microscopy for obtaining robust high‐resolution records from small‐volume speleothem samples, we analyzed three different stalagmites collected from Lapa dos Morcegos Cave (Portugal), Pau d'Alho Cave (Brazil), and Crevice Cave (United States). These stalagmites are representative of a range of magnetic properties and have been previously studied with conventional rock magnetometers. We show that by using SQUID microscopy we can achieve a five‐fold improvement in temporal resolution for samples with higher abundances of magnetic carriers (e.g., Pau d'Alho Cave and Lapa dos Morcegos Cave). In contrast, speleothems with low abundances of magnetic carriers (e.g., Crevice Cave) do not benefit from higher resolution analysis and are best analyzed using conventional rock magnetometers. Overall, by targeting speleothem samples with high concentrations of magnetic carriers we can increase the temporal resolution of magnetic records, setting the stage for resolving geomagnetic variations at short time scales. Plain Language Summary: Earth has a magnetic field that is generated in the outer core, and through paleomagnetism we can retrieve information about the evolution of the field from rocks. Different types of rocks have been used to determine how Earth's magnetic field has changed over time. In this study, we use magnetic microscopy to analyze the magnetic record of speleothems, which are rocks that form inside caves and that can record magnetic fields in annual scales. This allows us to obtain magnetic records with higher resolution than previous studies. This is important because Earth's magnetic field changes in timescales ranging from thousands of years to a few years, and obtaining records from speleothems using magnetic microscopy can help us track these variations. We provide data demonstrating how the technique can be used and showing its limitations, and we discuss how the geologic context of the cave influences the robustness of the magnetic record. We also provide protocols for future studies using magnetic microscopy in speleothems. Key Points: Magnetic microscopy can be used to produce a 5× improvement in temporal resolution of magnetic records from speleothemsVariations in magnetic properties across speleothems can affect the temporal resolution of the magnetic recordProtocols for conducting high‐resolution magnetic measurements of speleothems with magnetic microscopy are presented [ABSTRACT FROM AUTHOR]

Although Mars today does not have a core dynamo, magnetizations in the Martian crust and in meteorites suggest a magnetic field was present prior to 3.7 billion years (Ga) ago. However, the lack of ancient, oriented Martian bedrock samples available on Earth has prevented accurate estimates of the dynamo's intensity, lifetime, and direction. Constraining the nature and lifetime of the dynamo are vital to understanding the evolution of the Martian interior and the potential habitability of the planet. The Perseverance rover, which is exploring Jezero crater, is providing an unprecedented opportunity to address this gap by acquiring absolutely oriented bedrock samples with estimated ages from ∼2.3 to >4.1 Ga. As a first step in establishing whether these samples could contain records of Martian paleomagnetism, it is important to determine their ferromagnetic mineralogy, the grain sizes of the phases, and the forms of any natural remanent magnetization. Here, we synthesize data from various Perseverance instruments to achieve those goals and discuss the implications for future laboratory paleomagnetic analyses. Using the rover's instrument payload, we find that cored samples likely contain iron oxides enriched in Cr and Ti. The relative proportions of Fe, Ti, and Cr indicate that the phases may be titanomagnetite or Fe‐Ti‐Cr spinels that are ferromagnetic at room temperature, but we cannot rule out the presence of non‐ferromagnetic ulvöspinel, ilmenite, and chromite due to signal mixing. Importantly, the inferred abundance of iron oxides in the samples suggests that even <1 mm‐sized samples will be easily measurable by present‐day magnetometers. Plain Language Summary: Mars today does not have a magnetic field, but laboratory studies of Martian meteorites and spacecraft observations of the Martian crust indicate that Mars's metallic core once generated a magnetic field, known as a core dynamo. Although the dynamo seems to have been active prior to ∼3.7 billion years ago, its strength, direction and lifetime are largely unknown. Determining these characteristics is important for understanding the evolution of the Martian core and to test the theory that the magnetic field played a key role in making ancient Mars habitable. The Perseverance rover is providing a unique opportunity to answer these questions by acquiring bedrock samples that may span the full lifetime of the field. However, to establish whether these samples could tell us about the ancient dynamo, it is important to show they contain minerals with a special property known as ferromagnetism that allows them to record and retain records of the ancient magnetic field. Here, we present evidence the samples contain iron oxide minerals, many of which are known to be ferromagnetic. As such, we expect that future laboratory studies of these samples following their return to Earth will provide powerful constraints on the history of Mars's magnetic field. Key Points: Iron oxides with titanium and chromium components are present in lithologies sampled by the Perseverance rover on MarsThese iron oxides may contain magnetizations from ancient Martian magnetic fieldsReturned samples of these lithologies will constrain the history and characteristics of the Martian dynamo [ABSTRACT FROM AUTHOR]

Recovering ancient records of Earth's magnetic field is essential for determining the role of the magnetosphere in protecting early Earth from cosmic radiation and atmospheric escape. We present paleomagnetic field tests hinting that a record of Earth's 3.7‐billion‐year (Ga) old magnetic field may be preserved in the northeastern Isua Supracrustal Belt as a chemical remanent magnetization acquired during amphibolite‐grade metamorphism in the banded iron formation. Multiple petrological and geochronological lines of evidence indicate that the northernmost part of Isua has not experienced metamorphic temperatures exceeding 380°C since the Eoarchean, suggesting the rocks have not been significantly heated since magnetization was acquired. We use "pseudo" baked contact tests (intrusions emplaced 3.26–3.5 Ga ago) and a fold test (folding 3.6 Ga ago) to demonstrate that some samples preserve a ca. 3.7 Ga record of the magnetic field. We recover a field strength of >15 µT. This suggests that Earth's magnetic field may have been weak enough to enhance atmospheric escape during the Archean. Plain Language Summary: Recovering ancient records of Earth's magnetic field is challenging because the magnetization in rocks is often reset by heating during tectonic burial over their long and complex geological histories. We show that rocks from the Isua Supracrustal Belt in West Greenland have experienced three thermal events throughout their geological history. The first event was the most significant, and heated the rocks up to 550°C 3.7‐billion‐years‐ago. The subsequent two events did not heat the rocks in the northernmost part of the area above 380°C. We use multiple lines of evidence to test this claim, including paleomagnetic field tests, the metamorphic mineral assemblages across the area, and the temperatures at which radiometric ages of the observed mineral populations are reset. We use these lines of evidence to argue that an ancient, 3.7 billion year old record of Earth's magnetic field may be preserved in the banded iron formations in the northernmost part of the field area. The magnetization was acquired during mineral transformation associated with the first thermal event and therefore only a lower limit on the strength of the ancient magnetic field was constrained. However, we are able to conclude that the ancient magnetic field was likely comparable with the strength of Earth's magnetic field today. Key Points: The north‐eastern part of the Isua Supracrustal Belt experienced two metamorphic events at 3.69 Ga and 2.85 Ga and one hydrothermal event at 1.5 GaBanded iron formations acquired a chemical remanent magnetization during the first thermal event that was not entirely overprinted by subsequent eventsPaleomagnetic results hint that a record of the Eoarchean geomagnetic field is preserved in the Isua Supracrustal Belt [ABSTRACT FROM AUTHOR]

A major objective of the Mars 2020 mission is to sample rocks in Jezero crater that may preserve organic matter for later return to Earth. Using an ultraviolet Raman and luminescence spectrometer, the Perseverance rover detected luminescence signals with maximal intensities at 330 to 350 nanometers and 270 to 290 nanometers that were initially reported as consistent with organics. Here, we test the alternative hypothesis that the 330- to 350-nanometer and 270- to 290-nanometer luminescence signals trace Ce 3+ in phosphate and silicate defects, respectively. By comparing the distributions of luminescence signals with the rover detections of x-ray fluorescence from P 2 O 5 and Si-bearing materials, we show that, while an organic origin is not excluded, the observed luminescence can be explained by purely inorganic materials. These findings highlight the importance of eventual laboratory analyses to detect and characterize organic compounds in the returned samples.