Explore the vast range of titles available.

Understanding the longevity of Mars’s dynamo is key to interpreting the planet’s atmospheric loss history and the properties of its deep interior. Satellite data showing magnetic lows above many large impact basins formed 4.1-3.7 billion years ago (Ga) have been interpreted as evidence that Mars’s dynamo terminated before 4.1 Ga—at least 0.4 Gy before intense late Noachian/early Hesperian hydrological activity. However, evidence for a longer-lived, reversing dynamo from young volcanics and the Martian meteorite ALH 84001 supports an alternative interpretation of Mars’s apparently demagnetized basins. To understand how a reversing dynamo would affect basin fields, here we model the cooling and magnetization of 200-2200 km diameter impact basins under a range of Earth-like reversal frequencies. We find that magnetic reversals efficiently reduce field strengths above large basins. In particular, if the magnetic properties of the Martian mantle are similar to most Martian meteorites and late remagnetization of the near surface is widespread, >90% of large ( > 800 km diameter) basins would appear demagnetized at spacecraft altitudes. This ultimately implies that Mars’s apparently demagnetized basins do not require an early dynamo cessation. A long-lived and reversing dynamo, unlike alternative scenarios, satisfies all available constraints on Mars’s magnetic history. Weak magnetic fields above Mars’s large impact basins are often interpreted as a signature of the dynamo’s early cessation. Here, the authors demonstrate that these weakly magnetic basins may instead have formed in a long-lived but reversing dynamo.

The Emirates Mars Mission (EMM) was launched to Mars in the summer of 2020, and is the first interplanetary spacecraft mission undertaken by the United Arab Emirates (UAE). The mission has multiple programmatic and scientific objectives, including the return of scientifically useful information about Mars. Three science instruments on the mission’s Hope Probe will make global remote sensing measurements of the Martian atmosphere from a large low-inclination orbit that will advance our understanding of atmospheric variability on daily and seasonal timescales, as well as vertical atmospheric transport and escape. The mission was conceived and developed rapidly starting in 2014, and had aggressive schedule and cost constraints that drove the design and implementation of a new spacecraft bus. A team of Emirati and American engineers worked across two continents to complete a fully functional and tested spacecraft and bring it to the launchpad in the middle of a global pandemic. EMM is being operated from the UAE and the United States (U.S.), and will make its data freely available.

Craters on Mars are a window into Mars' past and the time they were emplaced. Because the crust is heated and shocked during impact, craters can demagnetize or magnetize the crust depending on the presence or absence of a dynamo field at the time of impact. This concept has been used to constrain dynamo timing. Here, we investigate magnetic anomalies associated with craters larger than 150 km. We find that most of those craters, independent of age, exhibit demagnetization signatures in the form of a central magnetic low. We demonstrate a statistically significant association between such signatures and craters, and hypothesize that the excavation of strongly magnetic crustal material may be an important contribution to the dominance of demagnetized craters. This finding implies that the simple presence or absence of crater demagnetization signatures is not a reliable indicator for the activity of the Martian dynamo during or after crater formation. Plain Language Summary Craters on Mars allow studying the time at which they were emplaced and as such they are a window into Mars' past. Because the crust is heated and shocked during impact and thus recrystallization occurs, craters can demagnetize or magnetize the crust depending on the presence or absence of a dynamo field at the time of impact. A classic magnetization signature is expressed by a magnetic high in the crater interior and the youngest of those craters have been used to constrain dynamo timing. Here, we investigate magnetic anomalies associated with all craters larger than 150 km. We find that most of those craters and independent of age exhibit a demagnetization signature, and fewer a magnetization signature. In general, the largest craters show a demagnetization signature. To explain the dominance of demagnetization signatures we hypothesize that the excavation of strongly magnetic crustal material may be an important process. This finding implies that the simple presence or absence of crater magnetization signatures is not a reliable indicator for dynamo activity and magnetic signatures of craters are dependent on multiple parameters such as crustal thickness. Key Points Craters on Mars dominantly show demagnetization signatures Large craters and thin crust promote magnetic lows in the crater interior irrespective of dynamo activity Crustal excavation is likely an important process that promotes demagnetization signatures

Mars' dayside ionosphere is maintained primarily by ionization from solar ultraviolet photons and subsequent chemical reactions, with small contributions from other mechanisms such as impact ionization and charge exchange. In December 2023, the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observed the impact of an interplanetary coronal mass ejection (ICME) on Mars' ionosphere, including strongly enhanced fluxes of suprathermal electrons. We show that this enhancement in suprathermal electron fluxes increased ion production from electron impact, so that dayside electron impact ionization rates exceeded photoionization rates during the ICME. This change in ion production mechanisms led to unusually high densities of the minor ions C+${\\mathrm{C}}^{+}$and O++${\\mathrm{O}}^{++}$ . Space weather events are known to increase ion escape rates, so changes in ion composition during space weather events have important implications for atmospheric evolution. We show that scaling nominal loss rates to account for space weather may underestimate carbon loss from Mars' atmosphere. Plain Language Summary Dayside planetary ionospheres are primarily produced through interactions between atmospheric neutral gases and sunlight. Impact by energetic particles, especially electrons, typically only contributes a small amount of plasma. We observed unusually high densities of the minor ions C+${\\mathrm{C}}^{+}$and O++${\\mathrm{O}}^{++}$in Mars' ionosphere during a space weather event in December 2023, when a large bubble of magnetized plasma launched from the Sun impacted the planet. This plasma bubble compressed the Mars magnetosheath, pushing suprathermal electrons to lower altitudes, where they impacted the atmosphere and significantly increased plasma production through impact ionization. Changes in the production mechanisms of the ionosphere during space weather events lead to changes in its density and composition. This is important because space weather events are known to increase the amount of atmospheric gas escaping from a planet, which can have important implications for how the atmosphere evolves over millions of years. Key Points Unexpected increases in C+ and rarely detected O++ densities were observed during the December 2023 space weather event at Mars These ions were produced by electron impact ionization from the intense electron fluxes associated with the space weather event Electron impact ionization exceeded photoionization during the event, which temporarily altered the composition of ions escaping Mars

Planetary auroras reveal the complex interplay between an atmosphere and the surrounding plasma environment. We report the discovery of low-altitude, diffuse auroras spanning much of Mars’ northern hemisphere, coincident with a solar energetic particle outburst. The Imaging Ultraviolet Spectrograph, a remote sensing instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, detected auroral emission in virtually all nightside observations for ~5 days, spanning nearly all geographic longitudes. Emission extended down to ~60 kilometer (km) altitude (1 microbar), deeper than confirmed at any other planet. Solar energetic particles were observed up to 200 kilo­–electron volts; these particles are capable of penetrating down to the 60 km altitude. Given minimal magnetic fields over most of the planet, Mars is likely to exhibit auroras more globally than Earth.

The vertical structure of the nightside ionosphere of Mars and its dependence on solar zenith angle are currently poorly determined, as is the importance of two key sources of nightside plasma, electron precipitation and transport of dayside plasma. We examined 37 electron density profiles of the ionosphere of Mars at solar zenith angles of 101°–123° obtained by the Mars Express Radio Science Experiment (MaRS) between 18 August and 1 October 2005. In general, solar activity was low during this period, although several solar energetic particle events did occur. The results show that (1) trends in peak electron density and altitude with solar zenith angle are consistent with transport of dayside plasma as an important plasma source up to 115°, but not higher; (2) peak altitudes of around 150 km observed at larger (>115°) solar zenith angles are consistent with simulated plasma production by electron precipitation; and (3) peak altitudes observed during solar energetic particle events are at 90 km, consistent with accepted models. Solar energetic particle events can be the main source of nightside plasma. These results challenge current models of the nightside ionosphere, including their implications for plasma sources. The total electron content is correlated with peak electron density, requiring explanation. Due to the geographical distribution of this data set (latitudes poleward of 38°N), we do not explore the influence of crustal field strength and direction on the nightside ionosphere. Key Points We examine 37 profiles of electron density in nightside Mars ionosphere Peak densities affected by solar zenith angle to 115 degrees Total electron content highly correlated with peak electron density

Although Mars currently has no global dynamo‐driven magnetic field, widespread crustal magnetization provides strong evidence that such a field existed in the past. The absence of magnetization in the younger large Noachian basins suggests that a dynamo operated early in Martian history but stopped in the mid‐Noachian. Within a 100 Ma period, 15 giant impacts occurred coincident with the disappearance of the global magnetic field. Here we investigate a possible causal link between the giant impacts during the early and mid‐Noachian and the cessation of the Martian dynamo at about the same time. Using three‐dimensional spherical mantle convection models, we find that impact heating associated with the largest basins (diameters >2500 km) can cause the global heat flow at the core‐mantle boundary to decrease significantly (10–40%). We suggest that such a reduction in core heat flow may have led to the cessation of the Martian dynamo.

Two of the primary goals of the MAVEN mission are to determine how the rate of escape of Martian atmospheric gas to space at the current epoch depends upon solar influences and planetary parameters and to estimate the total mass of atmosphere lost to space over the history of the planet. Along with MAVEN’s suite of nine science instruments, a collection of complementary models of the neutral and plasma environments of Mars’ upper atmosphere and near-space environment are an indispensable part of the MAVEN toolkit, for three primary reasons. First, escaping neutrals will not be directly measured by MAVEN and so neutral escape rates must be derived, via models, from in situ measurements of plasma temperatures and neutral and plasma densities and by remote measurements of the extended exosphere. Second, although escaping ions will be directly measured, all MAVEN measurements are limited in spatial coverage, so global models are needed for intelligent interpolation over spherical surfaces to calculate global escape rates. Third, MAVEN measurements will lead to multidimensional parameterizations of global escape rates for a range of solar and planetary parameters, but further global models informed by MAVEN data will be required to extend these parameterizations to the more extreme conditions that likely prevailed in the early solar system, which is essential for determining total integrated atmospheric loss. We describe these modeling tools and the strategies for using them in concert with MAVEN measurements to greater constrain the history of atmospheric loss on Mars.

Despite extensive study, we do not yet fully understand the origins of the unique lunar crustal magnetism. The strength of surface fields and their relation to local geology are crucial pieces of the puzzle. However, only a few surface measurements exist, and spacecraft magnetometers cannot detect magnetization with wavelengths much smaller than the orbital altitude. Meanwhile, electron reflectometry (ER) enables a remote measurement of surface fields, but its sensitivity to magnetization with different spatial scales is not well understood. In this paper, we report on new simulations of the ER technique and its sensitivity to magnetic fields produced by simulated crustal magnetization with various strengths and spatial distributions, utilizing full particle tracing simulations and the same data analysis techniques used for space data. We find that the ER technique reliably detects surface fields from magnetization with wavelengths larger than ∼10 km but has increasingly less sensitivity to smaller wavelengths. Since the few surface measurements we have imply very incoherent near‐surface magnetization, this implies that the ER technique may seriously underestimate the strength of lunar fields in some areas. Our results imply that small‐scale impact‐related crustal magnetization may prove even more important than previously thought.

NASA’s two spacecraft ARTEMIS mission will address both heliospheric and planetary research questions, first while in orbit about the Earth with the Moon and subsequently while in orbit about the Moon. Heliospheric topics include the structure of the Earth’s magnetotail; reconnection, particle acceleration, and turbulence in the Earth’s magnetosphere, at the bow shock, and in the solar wind; and the formation and structure of the lunar wake. Planetary topics include the lunar exosphere and its relationship to the composition of the lunar surface, the effects of electric fields on dust in the exosphere, internal structure of the Moon, and the lunar crustal magnetic field. This paper describes the expected contributions of ARTEMIS to these baseline scientific objectives.