36 research outputs found
Geoastronomy: Rocky planets as the Lavosier-Lomonosov Bridge from the non-living to the living world
Life on Earth emerged at the interface of the geosphere, hydrosphere and
atmosphere. This setting serves as our basis for how biological systems
originate on rocky planets. Often overlooked, however, is the fact that the
chemical nature of a rocky planet is ultimately a product of galactic chemical
evolution. Elemental abundances of the major rock-forming elements can be
different for different stars and planets formed at different times in galactic
history. These differences mean that we cannot expect small rocky exoplanets to
be just like Earth. Furthermore, age of the system dictates starting nuclide
inventory from galactic chemical evolution, and past, present and future mantle
and crust thermal regimes. The bulk silicate mantle composition of a rocky
planet modulates the kind of atmosphere and hydrosphere it possesses. Hence,
the ingredients of a rocky planet are as important for its potential to host
life as proximity to the so-called habitable zone around a star where liquid
water is stable at the surface. To make sense of these variables, a new
trans-disciplinary approach is warranted that fuses the disciplines of Geology
and Astronomy into what is here termed, Geoastronomy.Comment: to appear in the Royal Society of Chemistry (e-Book), Prebiotic
Chemistry and the Origin of Life (2022) (13370 words) (254 references) (21
Figures) (1 Table
Onset of giant planet migration before 4480 million years ago
Immediately after their formation, the terrestrial planets experienced
intense impact bombardment by comets, leftover planetesimals from primary
accretion, and asteroids. This temporal interval in solar system evolution,
termed late accretion, thermally and chemically modified solid planetary
surfaces and may have impeded the emergence of life on the Hadean Earth. The
sources and tempo of late accretion are, however, vague. Here, we present a
timeline that relates variably retentive radiometric ages from asteroidal
meteorites, to new dynamical models of late accretion that invokes giant planet
migration. Reconciliation of the geochronological data with dynamical models
shows that giant planet migration immediately leads to an intense 30 Myr influx
of comets to the entire solar system. The absence of whole-sale crustal reset
ages after 4450 Ma for the most resilient chronometers from Earth, Moon, Mars,
Vesta and various meteorite parent bodies confines the onset of giant planet
migration to no later than ca. 4480 Ma. Waning impacts from planetesimals,
asteroids (and a minor cometary component) continue to strike the inner planets
through a protracted monotonic decline in impactor flux; this is in agreement
with predictions from crater chronology. Amended global 3-D thermal analytical
bombardment models derived from our new impact mass-production functions show
that persistent niches for prebiotic chemistry on the early Hadean Earth could
endure late accretion for at least the last 4400 Myr.Comment: Main text: 46564 characters with spaces/7549 words Tables: 3
Figures:7 References: 11
Abodes for life in carbonaceous asteroids?
a b s t r a c t Thermal evolution models for carbonaceous asteroids that use new data for permeability, pore volume, and water circulation as input parameters provide a window into what are arguably the earliest habitable environments in the Solar System. Plausible models of the Murchison meteorite (CM) parent body show that to first-order, conditions suitable for the stability of liquid water, and thus pre-or post-biotic chemistry, could have persisted within these asteroids for tens of Myr. In particular, our modeling results indicate that a 200-km carbonaceous asteroid with a 40% initial ice content takes almost 60 Myr to cool completely, with habitable temperatures being maintained for $24 Myr in the center. Yet, there are a number of indications that even with the requisite liquid water, thermal energy sources to drive chemical gradients, and abundant organic ''building blocks'' deemed necessary criteria for life, carbonaceous asteroids were intrinsically unfavorable sites for biopoesis. These controls include different degrees of exothermal mineral hydration reactions that boost internal warming but effectively remove liquid water from the system, rapid (1-10 mm yr À1 ) inward migration of internal habitable volumes in most models, and limitations imposed by low permeabilities and small pore sizes in primitive undifferentiated carbonaceous asteroids. Our results do not preclude the existence of habitable conditions on larger, possibly differentiated objects such as Ceres and the Themis family asteroids due to presumed longer, more intense heating and possible long-lived water reservoirs
Europium as a lodestar: diagnosis of radiogenic heat production in terrestrial exoplanets
Long-lived radioactive nuclides, such as K, Th, U and
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, 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 Cen A and B. We find that europium
is depleted with respect to iron by 0.1 dex and to silicon by
0.15 dex compared to solar in both binary components. To first order, the
measured Eu abundances can be converted to the abundances of Th,
U and U with observational constraints while the abundance of
K is approximated independently with a Galactic chemical evolution
model. We find that the radiogenic heat budget in an -Cen-Earth is
TW upon its formation and TW at the
present day, respectively % and % lower than that in the
Hadean and modern Earth. As a consequence, mantle convection in an
-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
The curious case of Mars' formation
Dynamical models of planet formation coupled with cosmochemical data from
martian meteorites show that Mars' isotopic composition is distinct from that
of Earth. Reconciliation of formation models with meteorite data require that
Mars grew further from the Sun than its present position. Here, we evaluate
this compositional difference in more detail by comparing output from two
-body planet formation models. The first of these planet formation models
simulates what is termed the "Classical" case wherein Jupiter and Saturn are
kept in their current orbits. We compare these results with another model based
on the "Grand Tack", in which Jupiter and Saturn migrate through the primordial
asteroid belt. Our estimate of the average fraction of chondrite assembled into
Earth and Mars assumes that the initial solid disk consists of only sources of
enstatite chondrite composition in the inner region, and ordinary chondrite in
the outer region. Results of these analyses show that both models tend to yield
Earth and Mars analogues whose accretion zones overlap. The Classical case
fares better in forming Mars with its documented composition (29% to 68%
enstatite chondrite plus 32% to 67% ordinary chondrite) though the Mars
analogues are generally too massive. However, if we include the restriction of
mass on the Mars analogues, the Classical model does not work better. We also
further calculate the isotopic composition of , ,
, , , and in the
martian mantle from the Grand Tack simulations. We find that it is possible to
match the calculated isotopic composition of all the above elements in Mars'
mantle with their measured values, but the resulting uncertainties are too
large to place good restriction on the early dynamical evolution and birth
place of Mars.Comment: 14 pages, 8 figures, presented in the 2017 DPS meeting, 2018 Solar
system symposium in Sapporo and 2018 AOGS annual meeting, Accepted for
publishing in A&
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
The chemical evolution of the solar neighbourhood for planet-hosting stars
Theoretical physical-chemical models for the formation of planetary systems
depend on data quality for the Sun's composition, that of stars in the solar
neighbourhood, and of the estimated "pristine" compositions for stellar
systems. The effective scatter and the observational uncertainties of elements
within a few hundred parsecs from the Sun, even for the most abundant metals
like carbon, oxygen and silicon, are still controversial. Here we analyse the
stellar production and the chemical evolution of key elements that underpin the
formation of rocky (C, O, Mg, Si) and gas/ice giant planets (C, N, O, S). We
calculate 198 galactic chemical evolution (GCE) models of the solar
neighbourhood to analyse the impact of different sets of stellar yields, of the
upper mass limit for massive stars contributing to GCE () and of
supernovae from massive-star progenitors which do not eject the bulk of the
iron-peak elements (faint supernovae). Even considering the GCE variation
produced via different sets of stellar yields, the observed dispersion of
elements reported for stars in the Milky Way disk is not reproduced. Among
others, the observed range of super-solar [Mg/Si] ratios, sub-solar [S/N], and
the dispersion of up to 0.5 dex for [S/Si] challenge our models. The impact of
varying depends on the adopted supernova yields. Thus,
observations do not provide a constraint on the M parametrization.
When including the impact of faint supernova models in GCE calculations,
elemental ratios vary by up to 0.1-0.2 dex in the Milky Way disk; this
modification better reproduces observations.Comment: 36 pages, 26 figures, 1 Table, 1 Appendix, Accepted for publication
in MNRA