The Sun was likely a Slow Rotator : Lunar Geochemical Constraints

Planetary and Space Science, 2012

The Moon is key to understanding both Earth and our Solar System in terms of planetary processes and has been a witness of the Solar System history for more than 4.5 Ga. Building on earlier telescopic observations, our knowledge about the Moon was transformed by the wealth of information provided by Apollo and other space missions. These demonstrated the value of the Moon for understanding the fundamental processes that drive planetary formation and evolution. The Moon was understood as an inert body with its geology mainly restricted to impact and volcanism with associated tectonics, and a relative simple composition. Unlike Earth, an absence of plate tectonics has preserved a well-defined accretion and geological evolution record. However recent missions to the Moon show that this traditional view of the lunar surface is certainly an over simplification. For example, although it has long been suspected that ice might be preserved in cold traps at the lunar poles, recent results also indicate the formation and retention of OH À and H 2 O outside of polar regions. These volatiles are likely to be formed as a result of hydration processes operating at the lunar surface including the production of H 2 O and OH by solar wind protons interacting with oxygen-rich rock surfaces produced during micrometeorite impact on lunar soil particles. Moreover, on the basis of Lunar Prospector gamma-ray data, the lunar crust and underlying mantle has been found to be divided into distinct terranes that possess unique geochemical, geophysical, and geological characteristics. The concentration of heat producing elements on the nearside hemisphere of the Moon in the Procellarum KREEP Terrane has apparently led to the nearside being more volcanically active than the farside. Recent dating of basalts has shown that lunar volcanism was active for almost 3 Ga, starting at about 3.9-4.0 Ga and ceasing at $ 1.2 Ga. A recent re-processing of the seismic data supports the presence of a partially molten layer at the base of the mantle and shows not only the presence of a 330 km liquid core, but also a small solid inner core. Today, the Moon does not have a dynamo-generated magnetic field like that of the Earth. However, remnant magnetization of the lunar crust and the paleomagnetic record of some lunar samples suggest that magnetization was acquired, possibly from an intrinsic magnetic field caused by an early lunar core dynamo. In summary, the Moon is a complex differentiated planetary object and much remains to be explored and discovered, especially regarding the origin of the Moon, the history of the Earth-Moon system, and processes that have operated in the inner Solar System over the last 4.5 Ga. Returning to the Moon is therefore the critical next stepping-stone to further exploration and understanding of our planetary neighborhood.

Icarus

Previous work has established that the solar wind and micrometeoroids produce spectral changes on airless silicate bodies. However, the relative importance of these two weathering agents, the timescales over which they operate, and how their effects depend on composition have not yet been well determined. To help address these questions we make use of the fact that solar wind and micrometeoroid fluxes vary with latitude on the Moon. Previous work has shown that this latitudinally varying flux leads to systematic latitudinal variations in the spectral properties of lunar soils. Here we find that the way in which a lunar soil's spectral properties vary with latitude is a function of its iron content, when we consider soils with 14-22 wt% FeO. In particular, a 50% reduction in flux corresponds to a significant increase in reflectance for 14 wt% FeO soils, while the same flux reduction on 21 wt% FeO soils is smaller by a factor of~5, suggesting that this brightening effect saturates for high FeO soils. We propose that lower iron soils may not approach saturation because grains are destroyed or refreshed before sufficient nano-and micro-phase iron can accumulate on their rims. We compare our results to the spectral variations observed across the Reiner Gamma swirl, which lies on a high-iron surface, and find it has anomalous brightness compared to our predictions. Swirls in Mare Marginis, which lie on a low iron surface, exhibit brightness differences that suggest reductions in solar wind flux between 20 and 40%. Our inferences suffer from the limited latitudinal extent of the maria and the convolution of micrometeoroid flux and solar wind flux changes with latitude. Superior constraints on how space weathering operates throughout the inner solar system would come from in situ measurements of the solar wind flux at lunar swirls. Hapke, 2001; Vernazza et al., 2009). There are at least two unresolved problems with previous studies of

Earth and Planetary Science Letters

We create the first quantitative model for the early lunar atmosphere, coupled with a magma ocean crystallization model. Immediately after formation, the moon's surface was subject to a radiative environment that included contributions from the early Sun, a post-impact Earth that radiated like a mid-type M dwarf star, and a cooling global magma ocean. This radiative environment resulted in a largely Earth-side atmosphere on the Moon, ranging from ∼10 4 to ∼10 2 pascals, composed of heavy volatiles (Na and SiO). This atmosphere persisted through lid formation and was additionally characterized by supersonic winds that transported significant quantities of moderate volatiles and likely generated magma ocean waves. The existence of this atmosphere may have influenced the distribution of some moderate volatiles and created temperature asymmetries which influenced ocean flow and cooling. Such asymmetries may characterize young, tidally locked rocky bodies with global magma oceans and subject to intense irradiation.

AIP Conference Proceedings, 2001

Solar wind implanted in surface layers (~ 0.03/^m) of lunar soil grains has often been analyzed to infer the history of the solar wind. In somewhat deeper layers, and thus presumably at higher implantation energies, a mysterious population, dubbed "SEP" for "solar energetic particle", accounts for the majority of the implanted gas-several orders of magnitude more than expected from the present-day flux of solar energetic particles. In addition, its elemental and isotopic composition is distinct from that of the solar system. While the heavy Ne isotopes are enriched relative to 20 Ne, 15 N is depleted relative to 14 N-a behavior that is hard to explain with acceleration of solar material. N is overabundant with respect to the noble gases (especially Ar). Here we show that interstellar pickup ions (PUIs) which are ionized and accelerated in the heliosphere and subsequently implanted in lunar regolith grains can account for the properties of the "SEP" population. This implies that lunar soils preserve samples of the galactic environment of the solar system and may eventually be used as an archive for solar system "climate".

Nature, 2006

Differences in isotopic abundances between meteorites and rocks on Earth leave unclear the true composition of the gas out of which the Solar System formed 1-4 . The Sun should have preserved in its outer layers the original composition, and recent work has indicated that the solar wind is enriched in 16 O, relative to Earth, Mars and bulk meteorites 5 . This suggests that self-shielding of CO due to photo-dissociation, which is a well understood process in molecular clouds, also led to evolution in the isotopic abundances in the early Solar System. Here we report measurements of oxygen isotopic abundances in lunar grains that were recently exposed to the solar wind. We find that 16 O is underabundant, opposite to an earlier finding 5 based on studies of ancient metal grains. Our result, however, is more difficult to understand within the context of current models, because there is no clear way to make 16 O more abundant in Solar System rocks than in the Sun.

Goldschmidt2021 abstracts, 2021

Space Science Reviews, 2019

The Moon is the only planetary body other than the Earth for which samples have been collectedin situby humans and robotic missions and returned to Earth. Scientific investigations of the first lunar samples returned by the Apollo 11 astronauts 50 years ago transformed the way we think most planetary bodies form and evolve. Identification of anorthositic clasts in Apollo 11 samples led to the formulation of the magma ocean concept, and by extension the idea that the Moon experienced large-scale melting and differentiation. This concept of magma oceans would soon be applied to other terrestrial planets and large asteroidal bodies. Dating of basaltic fragments returned from the Moon also showed that a relatively small planetary body could sustain volcanic activity for more than a billion years after its formation. Finally, studies of the lunar regolith showed that in addition to containing a treasure trove of the Moon’s history, it also provided us with a rich archive of the past 4.5 ...

Symposium - International Astronomical Union, 2004

We report on the results of a multi-wavelength program (X-rays to the near IR) of solar analogs with ages covering ∼0.1—9 Gyr. The chief science goals are to study the solar magnetic dynamo and to determine the radiative and magnetic properties of the Sun during its evolution across the main sequence. The present paper focuses on the latter goal, which has the ultimate purpose of constructing spectral irradiance tables to be used to study and model planetary atmospheres. The results obtained thus far indicate that the young Sun was extremely active, with large flares, massive winds, and high-energy emissions up to 1000 times stronger than presently. The strong radiation and particle emissions inferred should have had major influences on the photochemistry and photo-ionization of paleo-planetary atmospheres and also played an important role in the development of primitive life in the Solar System. Some recent results of the effects of the young Sun's enhanced radiation and partic...

Physics of the Earth and Planetary Interiors, 2008

We analyze published and new paleointensity data from Apollo samples to reexamine the hypothesis of an early (3.9-3.6 Ga) lunar dynamo. Our new paleointensity experiments on four samples use modern absolute and relative measurement techniques, with ages ranging from 3.3 to 4.3 Ga, bracketing the putative period of an ancient lunar field. Samples 60015 (anorthosite) and 76535 (troctolite) failed during absolute paleointensity experiments. Samples 72215 and 62235 (impact breccias) recorded a complicated, multicomponent magnetic history that includes a low-temperature (<500 • C) component associated with a high intensity (∼90 T) and a high temperature (>500 • C) component associated with a low intensity (∼2 T). Similar multi-component behavior has been observed in several published absolute intensity experiments on lunar samples. Additional material from 72215 and 62235 was subjected to a relative paleointensity experiment (a saturation isothermal remanent magnetization, or sIRM, experiment); neither sample provided unambiguous evidence for a thermal origin of the recorded remanent magnetization. We test several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples. Specifically, an overprint from exposure to a small magnetic field (an isothermal remanent magnetization) results in multi-component behavior (similar to absolute paleointensity results) from which we could not recover the correct magnitude of the original thermal remanent magnetization. In light of these new experiments and a thorough re-evaluation of existing paleointensity measurements, we conclude that although some samples with ages of 3.6 to 3.9 Ga are strongly magnetized, and sometimes exhibit stable directional behavior, it has not been demonstrated that these observations indicate a primary thermal remanence. Particularly problematic in the interpretation of lunar sample magnetizations are the effects of shock. As relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities, the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable. Consequently, current paleointensity measurements do not support the existence of a 3.9-3.6 Ga lunar dynamo with 100 T surface fields, a result that is in better agreement with satellite measurements of crustal magnetism and that presents fewer challenges for thermal evolution and dynamo models.

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2001

Nuclides with half-lives of 10 5 -10 8 yr permit the elucidation of nebula time-scales and the rates of accretion of planetesimals. However, the 182 Hf-182 W system with a half-life of 9 ± 2 Myr also provides new and very useful constraints on the formation of the terrestrial planets. This technique allows one to address the timing of metal-silicate equilibration in objects as different as chondrites and the Earth. With improvements in sensitivity and precision, very small time differences in metal segregation in asteroids should be resolvable from measuring iron meteorites. It is already clear that the formation and differentiation of some asteroidal-sized objects was completed in less than 10 Myr. Accretion and core formation were protracted in the case of the Earth (greater than 50 Myr) relative to Mars (probably less than 20 Myr). Indeed, the Martian mantle appears to retain both chemical and isotopic heterogeneities that are residual from the process of core formation. Such early features appear to have been eliminated from the Earth's mantle presumably because of 4.5 Gyr of relatively efficient convective mixing. Tungsten isotope data provide compelling support for the 'giant impact' theory of lunar origin. The Moon is a high Hf/W object that contains a major component of chondritic W. This is consistent with a time of formation of greater than 50 Myr after the start of the Solar System. New highly precise oxygen isotope data are unable to resolve any difference between the source of components in the Earth and Moon. Therefore, the giant impact itself may have produced some of the differences in moderately volatile element budgets between these objects. This finds support in precise Sr isotopic data for early lunar samples. The data are consistent with the proto-Earth and Theia (the impactor) having Rb/Sr ratios that were not very different from that of present day Mars. Therefore, the extended history of accretion, rather than nebular phenomena, may be responsible for some of the major differences between the terrestrial planets.