22 research outputs found
Late veneer and late accretion to the terrestrial planets
It is generally accepted that silicate-metal (`rocky') planet formation
relies on coagulation from a mixture of sub-Mars sized planetary embryos and
(smaller) planetesimals that dynamically emerge from the evolving circum-solar
disc in the first few million years of our Solar System. Once the planets have,
for the most part, assembled after a giant impact phase, they continue to be
bombarded by a multitude of planetesimals left over from accretion. Here we
place limits on the mass and evolution of these planetesimals based on
constraints from the highly siderophile element (HSE) budget of the Moon.
Outcomes from a combination of N-body and Monte Carlo simulations of planet
formation lead us to four key conclusions about the nature of this early epoch.
First, matching the terrestrial to lunar HSE ratio requires either that the
late veneer on Earth consisted of a single lunar-size impactor striking the
Earth before 4.45 Ga, or that it originated from the impact that created the
Moon. An added complication is that analysis of lunar samples indicates the
Moon does not preserve convincing evidence for a late veneer like Earth.
Second, the expected chondritic veneer component on Mars is 0.06 weight
percent. Third, the flux of terrestrial impactors must have been low (
<=10^(-6) M_earth/Myr) to avoid wholesale melting of Earth's crust after
4.4~Ga, and to simultaneously match the number of observed lunar basins. This
conclusion leads to an Hadean eon which is more clement than assumed
previously. Last, after the terrestrial planets had fully formed, the mass in
remnant planetesimals was ~10^(-3) M_earth, lower by at least an order of
magnitude than most previous models suggest. Our dynamically and geochemically
self-consistent scenario requires that future N-body simulations of rocky
planet formation either directly incorporate collisional grinding or rely on
pebble accretion.Comment: Accepted for publication in Earth and Planetary Science Letter
Origin of Life
The evolution of life has been a big enigma despite rapid advancements in the
fields of biochemistry, astrobiology, and astrophysics in recent years. The
answer to this puzzle has been as mind-boggling as the riddle relating to
evolution of Universe itself. Despite the fact that panspermia has gained
considerable support as a viable explanation for origin of life on the Earth
and elsewhere in the Universe, the issue remains far from a tangible solution.
This paper examines the various prevailing hypotheses regarding origin of life
like abiogenesis, RNA World, Iron-sulphur World, and panspermia; and concludes
that delivery of life-bearing organic molecules by the comets in the early
epoch of the Earth alone possibly was not responsible for kick-starting the
process of evolution of life on our planet.Comment: 32 pages, 8 figures,invited review article, minor additio
Sluggish Hadean geodynamics: Evidence from coupled 146,147 Sm– 142,143 Nd systematics in Eoarchean supracrustal rocks of the Inukjuak domain (Québec)
International audienc
Geological constraints on detecting the earliest life on Earth: a perspective from the Early Archaean (older than 3.7 Gyr) of southwest Greenland
At greater than 3.7 Gyr, Earth's oldest known supracrustal rocks, comprised dominantly of mafic igneous with less common sedimentary units including banded iron formation (BIF), are exposed in southwest Greenland. Regionally, they were intruded by younger tonalites, and then both were intensely dynamothermally metamorphosed to granulite facies (the highest pressures and temperatures generally encountered in the Earth's crust during metamorphism) in the Archaean and subsequently at lower grades until about 1500 Myr ago. Claims for the first preserved life on Earth have been based on the occurrence of greater than 3.8 Gyr isotopically light C occurring as graphite inclusions within apatite crystals from a 5 m thick purported BIF on the island of Akilia. Detailed geologic mapping and observations there indicate that the banding, first claimed to be depositional, is clearly deformational in origin. Furthermore, the mineralogy of the supposed BIF, being dominated by pyroxene, amphibole and quartz, is unlike well-known BIF from the Isua Greenstone Belt (IGB), but resembles enclosing mafic and ultramafic igneous rocks modified by metasomatism and repeated metamorphic recrystallization. This scenario parsimoniously links the geology, whole-rock geochemistry, 2.7 Gyr single crystal zircon ages in the unit, an approximately 1500 Myr age for apatites that lack any graphite, non-MIF sulphur isotopes in the unit and an inconclusive Fe isotope signature. Although both putative body fossils and carbon-12 enriched isotopes in graphite described at Isua are better explained by abiotic processes, more fruitful targets for examining the earliest stages in the emergence of life remain within greater than 3.7 Gyr IGB, which preserves BIF and other rocks that unambiguously formed at Earth's surface