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We present an analysis of the remaining three candidate landing sites for Mars 2020, Columbia Hills (CH), Northeast Syrtis (NES) and Jezero (JE) from the perspective of understanding Mars' crustal magnetic field. We identify how the different sites can address each of six community‐defined paleomagnetic science objectives for Mars return samples. These objectives include understanding the early dynamo field and its variability, identification of magnetic minerals that carry magnetization in the samples, and characterization of any thermal and chemical alteration of samples. Satellite data have provided global and regional constraints on crustal magnetization, indicating strong magnetizations at CH and weak to no magnetization at JE and NES. However, the primary paleomagnetic interest—understanding the early dynamo—requires ground truth from a landing site at which pre‐Noachian and Early Noachian deposits are accessible. This requirement is most likely met by the site NES, which contains meggabreccia deposits, and it is therefore the highest priority landing site for magnetic field investigations. Importantly, a sample return mission has never been done, and so any of the three landing sites will provide critical, new data that will contribute to understanding the history of Mars' magnetic field and crustal mineralogy and, in turn, yield constraints on the planet's evolution. Plain Language Summary: Mars 2020, a mission to Mars, will for the first time collect rock samples for subsequent return to the Earth. Here we evaluate how each of the last three candidate landing sites can shed light on the Martian magnetic field and its history. We evaluate which landing site is best suited to address each of six science objectives regarding the origin, variability, and evolution of the magnetic signatures on Mars. We first summarize what is known about magnetization around the landing sites based on satellite data. Because we are primarily interested in the history of the magnetic field, an ideal site should offer ancient (∼4 billion years old and older) accessible outcrops that might be recorders of Mars' ancient global magnetic field. We combine current knowledge about the surface age, geology, and magnetic field, and chose Northeast Syrtis as the preferred landing site, followed by Columbia Hills and Jezero. However, any of the three sites will provide us with new data that will contribute to understanding Mars' magnetic field history and, in turn, its evolution. Key Points: Mars 2020 offers the opportunity to acquire samples that record the intensity and direction of the ancient Martian magnetic fieldLaboratory paleomagnetic measurements of returned samples can address questions about the history of the Martian dynamo, thermal evolution, and climateWe recommend Northeast Syrtis as preferred site for magnetic investigations, followed by Columbia Hills and Jezero [ABSTRACT FROM AUTHOR]

Abstract: Martian magnetic anomalies have been revealed by the Mars Global Surveyor (MGS) mission in the south hemisphere of Mars. The present study models anomalies located in the ancient Terra Sirenum area between latitudes 26°S and 40°S and longitudes 185°E and 210°E using forward and inverse approaches. While the high-altitude measurements reveal the presence of two main magnetic anomalies, three are detected by low-altitude data. They are modeled as uncorrelated dipolar sources. Forward models predict large magnetizations between 30 and 60A/m. A generalized non-linear inversion is used to determine the characteristics of the dipoles, based on different subsets of data. Low-altitude measurements inversion leads to more reliable results than those obtained by the inversion of high-altitude measurements only. Inversion of both low- and high-altitude data together provides with three dipoles that explain more than 57% of the signal, within this 106 km2 area. All dipoles have large magnetizations. Serpentinization of the early martian crust can explain such remanent magnetizations. Two resulting dipoles are 56km deep, which suggests a locally thick martian crust. The last one is shallower (31km). This indicates different origins and/or magnetization processes. Paleomagnetic poles are calculated and located around the Tharsis bulge. It suggests that Tharsis formed at high latitudes and moved toward its present location by polar reorientation. [Copyright &y& Elsevier]

Launched on 23rd February 1999, the Ørsted satellite opened the decade of geopotential field research. This is the first satellite to measure the three components of the Earth’s magnetic field since MAGSAT (1979–1980). Ørsted orbital parameters are very similar to those of MAGSAT, allowing a first-order comparison of the 1979 and 2000 magnetic fields. Using the available vector and scalar data over the first 14 months of the Ørsted mission and applying classical selection criteria (local time, external magnetic activity), we compute a 29-degree/order main-field model and a 13-degree/order secular-variation model for the period 1999–2000. The applied method and the accuracy of the derived model are discussed. We compare the resulting main-field model to a similar one derived from MAGSAT data. Results of this comparison are presented, such as (i) morphology and energy spectrum of the secular variation and (ii) morphology of the crustal magnetic field at MAGSAT and Ørsted epochs. [Copyright &y& Elsevier]

Abstract: The hemispheric dichotomy in the crustal magnetic field of Mars may indicate that the planet’s past dynamo was influenced by a degree-1 heterogeneity on the outer boundary of its liquid metallic convecting core. Here we use numerical dynamos driven by purely volumetric internal heating with imposed degree-1 heat flux heterogeneities to study mantle control on the past dynamo of Mars. We quantify both south–north and east–west magnetic field dichotomies from time-average properties that are calculated according to two different end member crust formation scenarios. Our results indicate that a moderate heat flux anomaly may have been sufficient for obtaining the observed dichotomy. Because of the excitation of a strong equatorial upwelling in the dynamo, the efficiency of a mantle heterogeneity centered at the geographical pole in producing a south–north dichotomy is much higher than that of an heterogeneity centered at the equator in producing an east–west dichotomy. These results argue against a significant True Polar Wander event with major planet re-orientation after the cessation of the dynamo. [Copyright &y& Elsevier]

Abstract: Mars is thought to have hosted large amounts of water and carbon dioxide at primitive epochs. The morphological analysis of the surface of Mars shows that large bodies of water were probably present in the North hemisphere at late Noachian (3.7–4Gyr ago). Was this water solid or liquid? For maintaining liquid water at this time, when the Sun was (likely) less bright than now, a CO2 atmosphere of typically 2 bars is required. Can sputtering, still presently acting at the top of the Martian atmosphere, have removed such a dense atmosphere over the last 3.5–4Gyr? What was the fate of the 100–200m global equivalent layer of water present at late Noachian? When did Martian magnetic dynamo vanish, initiating a long period of intense escape by sputtering? Because sputtering efficiency is highly non-linear with solar EUV flux, with a logarithmic slope of ≈7:Φ sput≈Φ EUV 7, resulting in enhanced levels of escape at primitive epochs, when the sun was several times more luminous than now in the EUV, there is a large uncertainty on the cumulated amount of volatiles removed to space. This amount depends primarily on two factors: (i) the exact value of the non-linearity exponent (≈7 from existing models, but this value is rather uncertain), (ii) the exact time when the dynamo collapsed, activating sputtering at epochs when intense EUV flux and solar wind activity prevailed in the solar system. Both parameters are only crudely known at the present time, due the lack of direct observation of sputtering from Martian orbit, and to the incomplete and insufficiently spatially resolved map of the crustal magnetic field. Precise timing of the past Martian dynamo can be investigated through the demagnetisation signature associated with impact craters. A designated mission to Mars would help in answering this crucial question: was water liquid at the surface of Mars at late Noachian? Such a mission would consist of a low periapsis (≈100km) orbiter, equipped with a boom-mounted magnetometer, for mapping the magnetic field, as well as adequate in situ mass and energy spectrometers, for a full characterization of escape and of its response to solar activity variations. Surface based observations of atmospheric noble gas isotopic ratios, which keep the signatures of past escape processes, including sputtering for the lightest of them (Ne, Ar), would bring a key constraint for escape models extrapolated back to the past. [Copyright &y& Elsevier]

The martian surface exhibits a strong dichotomy in elevation, crustal thickness and magnetization between the southern and northern hemispheres. A giant impact has been proposed as an explanation for the formation of the Northern Lowlands on Mars. Such an impact probably led to strong and deep mantle heating which may have had implications on the magnetic evolution of the planet. We model the effects of such an impact on the martian magnetic field by imposing an impact induced thermal heterogeneity, and the subsequent heat flux heterogeneity, on the martian core-mantle boundary (CMB). The CMB heat flux lateral variations as well as the reduction in the mean CMB heat flux are determined by the size and geographic location of the impactor. A polar impactor leads to a north–south hemispheric magnetic dichotomy that is stronger than an east–west dichotomy created by an equatorial impactor. The amplitude of the hemispheric magnetic dichotomy is mostly controlled by the horizontal Rayleigh number Ra h which represents the vigor of the convection driven by the lateral variations of the CMB heat flux. We show that, for a given Ra h , an impact induced CMB heat flux heterogeneity is more efficient than a synthetic degree-1 CMB heat flux heterogeneity in generating strong hemispheric magnetic dichotomies. Large Ra h values are needed to get a dichotomy as strong as the observed one, favoring a reversing paleo-dynamo for Mars. Our results imply that an impactor radius of ∼1000 km could have recorded the magnetic dichotomy observed in the martian crustal field only if very rapid post-impact magma cooling took place. [ABSTRACT FROM AUTHOR]