The cool and distant formation of Mars

With approximately one ninth of Earth's mass, Mars is widely considered to be a stranded planetary embryo that never became a fully-grown planet. A currently popular planet formation theory predicts that Mars formed near Earth and Venus and was subsequently scattered outwards to its present location. In such a scenario, the compositions of the three planets are expected to be similar to each other. However, bulk elemental and isotopic data for martian meteorites demonstrate that key aspects of Mars' composition are markedly different from that of Earth. This suggests that Mars formed outside of the terrestrial feeding zone during primary accretion. It is therefore probable that Mars always remained significantly farther from the Sun than Earth; its growth was stunted early and its mass remained relatively low. Here we identify a potential dynamical pathway that forms Mars in the asteroid belt and keeps it outside of Earth's accretion zone while at the same time accounting for strict age and compositional constraints, as well as mass differences. Our uncommon pathway (approximately 2% probability) is based on the Grand Tack scenario of terrestrial planet formation, in which the radial migration by Jupiter gravitationally sculpts the planetesimal disc at Mars' current location. We conclude that Mars' formation requires a specific dynamical pathway, while this is less valid for Earth and Venus. We further predict that} Mars' volatile budget is most likely different from Earth's and that Venus formed close enough to our planet that it is expected to have a nearly identical composition from common building blocks.Comment: Accepted in Earth and Planetary Science Letter

Tungsten isotope composition of the Acasta Gneiss Complex

AbstractHigh-precision tungsten (182W/184W) isotope measurements on well-characterised mafic and felsic samples of the ca. 3960 Ma Acasta Gneiss Complex (AGC; Northwest Territories, Canada) show radiogenic ε182W values between +0.06 to +0.15. Two ca. 3600 Ma felsic samples from this terrane have ε182W ∼ 0 and are the oldest samples so far documented to have a W isotopic composition indistinguishable from that of the modern mantle. The ε182W data are correlated with ε142Nd (Roth et al., 2014) and we attribute this variability to incomplete metamorphic homogenisation of the 3960 Ma protolith with more recent material in a 3370 Ma tectono-thermal event. Notably, the value of the positive ε182W anomalies seen in the 3960 Ma AGC samples that are least affected by metamorphic homogenisation is comparable to that observed in other early Archean rocks (Isua Supracrustal Belt, Greenland; Nuvvuagittuq Supracrustal Belt, Canada) and the late Archean Kostomuksha komatiites (Karelia). This demonstrates a globally constant signature. We infer that the presence of a pre-late veneer mantle represents the most straightforward interpretation of a uniform distribution of εW182∼+0.15 value in Archean rocks of different ages. We show that such a notion is compatible with independent constraints from highly siderophile element abundances in mafic and ultra-mafic Archean mantle-derived rocks. The absence of anomalous ε182W and ε142Nd so far measured in samples younger than ca. 2800 Ma suggests progressive convective homogenisation of silicate reservoirs. The downward mixing of an upper mantle rich in late-delivered meteoritic material might account for these combined observations

Analysis of terrestrial planet formation by the grand tack model:system architecture and tack location

The Grand Tack model of terrestrial planet formation has emerged in recent years as the premier scenario used to account for several observed features of the inner solar system. It relies on early migration of the giant planets to gravitationally sculpt and mix the planetesimal disc down to ~1 AU, after which the terrestrial planets accrete from material left in a narrow circum-solar annulus. Here we have investigated how the model fares under a range of initial conditions and migration course-change (`tack') locations. We have run a large number of N-body simulations with a tack location of 1.5 AU and 2 AU and tested initial conditions using equal mass planetary embryos and a semi-analytical approach to oligarchic growth. We make use of a recent model of the protosolar disc that takes account of viscous heating, include the full effect of type 1 migration, and employ a realistic mass-radius relation for the growing terrestrial planets. Results show that the canonical tack location of Jupiter at 1.5 AU is inconsistent with the most massive planet residing at 1 AU at greater than 95% confidence. This favours a tack farther out at 2 AU for the disc model and parameters employed. Of the different initial conditions, we find that the oligarchic case is capable of statistically reproducing the orbital architecture and mass distribution of the terrestrial planets, while the equal mass embryo case is not.Comment: Accepted for publication in The Astrophysical Journa

Europium as a lodestar: diagnosis of radiogenic heat production in terrestrial exoplanets

Long-lived radioactive nuclides, such as $^{40}$K, $^{232}$Th, $^{235}$U and $^{238}$U, contribute to persistent heat production in the mantle of terrestrial-type planets. As refractory elements, the concentrations of Th and U in a terrestrial exoplanet are implicitly reflected in the photospheric abundances in the stellar host. However, a robust determination of these stellar abundances is difficult in practice owing to the general paucity and weakness of the relevant spectral features. We draw attention to the refractory, $r-$process element europium, which may be used as a convenient and practical proxy for the population analysis of radiogenic heating in exoplanetary systems. As a case study, we present a determination of Eu abundances in the photospheres of $\alpha$ Cen A and B. We find that europium is depleted with respect to iron by $\sim$ 0.1 dex and to silicon by $\sim$ 0.15 dex compared to solar in both binary components. To first order, the measured Eu abundances can be converted to the abundances of $^{232}$Th, $^{235}$U and $^{238}$U with observational constraints while the abundance of $^{40}$K is approximated independently with a Galactic chemical evolution model. We find that the radiogenic heat budget in an $\alpha$-Cen-Earth is $73.4^{+8.3}_{-6.9}$ TW upon its formation and $8.8^{+1.7}_{-1.3}$ TW at the present day, respectively $23\pm5$ % and $54\pm5$ % lower than that in the Hadean and modern Earth. As a consequence, mantle convection in an $\alpha$-Cen-Earth is expected to be overall weaker than that of the Earth (assuming other conditions are the same) and thus such a planet would be less geologically active, suppressing its long-term potential to recycle its crust and volatiles. With Eu abundances being available for a large sample of Sun-like stars, the proposed approach can extend our ability to make predictions about the nature of other rocky worlds.Comment: Accepted for publication in Astronomy & Astrophysics. 11 pages, 4 figures, and 4 table

A radiogenic heating evolution model for cosmochemically Earth-like exoplanets

a b s t r a c t Discoveries of rocky worlds around other stars have inspired diverse geophysical models of their plausible structures and tectonic regimes. Severe limitations of observable properties require many inexact assumptions about key geophysical characteristics of these planets. We present the output of an analytical galactic chemical evolution (GCE) model that quantitatively constrains one of those key properties: radiogenic heating. Earth's radiogenic heat generation has evolved since its formation, and the same will apply to exoplanets. We have fit simulations of the chemical evolution of the interstellar medium in the solar annulus to the chemistry of our Solar System at the time of its formation and then applied the carbonaceous chondrite/Earth's mantle ratio to determine the chemical composition of what we term ''cosmochemically Earth-like'' exoplanets. Through this approach, predictions of exoplanet radiogenic heat productions as a function of age have been derived. The results show that the later a planet forms in galactic history, the less radiogenic heat it begins with; however, due to radioactive decay, today, old planets have lower heat outputs per unit mass than newly formed worlds. The long half-life of 232 Th allows it to continue providing a small amount of heat in even the most ancient planets, while 40 K dominates heating in young worlds. Through constraining the age-dependent heat production in exoplanets, we can infer that younger, hotter rocky planets are more likely to be geologically active and therefore able to sustain the crustal recycling (e.g. plate tectonics) that may be a requirement for long-term biosphere habitability. In the search for Earth-like planets, the focus should be made on stars within a billion years or so of the Sun's age

Evidence for Life on Earth before 3,800 Million Years Ago

It is unknown when life first appeared on Earth. The earliest known microfossils (approx. 3,500 Myr before present) are structurally complex, and if it is assumed that the associated organisms required a long time to develop this degree of complexity, then the existence of life much earlier than this can be argued. But the known examples of crustal rocks older than approx. 3,500 Myr have experienced intense metamorphism, which would have obliterated any fragile microfossils contained therein. It is therefore necessary to search for geochemical evidence of past biotic activity that has been preserved within minerals that are resistant to metamorphism. Here we report ion-microprobe measurements of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences a approx. 3,800 Myr-old banded iron formation from the Isua supracrustal belt, West Greenland, and a similar formation from the nearby Akilia island that is possibly older than 3,850 Myr. The carbon in the carbonaceous inclusions is isotopically light, indicative of biological activity; no known abiotic process can explain the data. Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present

THE STRUCTURE AND HYDRATION OF THE HUMITE MINERALS

The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned disciple. iii Hirner, Sarah Marie (M.S., Geology, Department of Geological Sciences) The structure and hydration of the humite minerals Thesis directed by Professor Joseph R. Smyth The entire water budget of the mantle may be dominated by nominally anhydrous minerals. The local structural environment of H in the humite minerals could provide a valuable model for the incorporation of H into olivine due to their structural similarities. It also thought that humites may play a significant role in the transport of water into the mantle. Four crystals of chondrodite, clinohumite, norbergite, and humite, both natural and synthetic, have been analyzed via Raman spectroscopy and electron microprobe analysis. Their structures have been refined by single-crystal X-ray diffraction analysis. The new data confirms earlier studies of cation ordering and hydration geometry, and adds new insight into the crystal chemistry of the humite minerals, particularly the geometry of the H position. In humite, hydrogen was found to occupy the H1 site. iv ACKMOWLEDGEMENTS This research was supported in part by National Science Foundation grants to Joseph R. Smyth