Its not the energy, its the entropy Saturns age is revealed in its moonsEarths water source is not in the meteorite collections...Is the Hadean Earth unavoidably anomalous?Methane vs. water cycles. Weve already found martian lifeThe best system to visit isnt JupitersThe skys the limitThe budgets the lmit

Lecture 4: Hadean Earth1. The absolute age of the Earth2. Hadean impact rates3. Delivery and survival of organic molecules on the Hadean Earth.

Halliday, 2003Late Heavy Bombardment

Properties of CAIsCalcium-Aluminium richFirst elements to condensate whencooling down from high temperaturesMust have formed at high temperature (~2000 K)Oldest solids in the solar system(1 to 4 Myear older than most chondrules)Their formation is still unclear!Refractory minerals

182Hf182W8.9 x 106Radioactive decay

ParentDaughterHalf-Life (years)Mode of Decay14C14N5.73 x 103b129I129Xe1.6 x 107b235U207Pb7 x 108chain238U206Pb4.5 x 109chain40K40Ar1.2 x 1010EC232Th208Pb1.4 x 1010chain87Rb87Sr4.9 x 1010b147Sm143Nd1.06 x 1011a

N0 is the initial number of parent atomsN is the number of parent atoms at time tRadioactive decay-0Let t be the time elapsed since the rock solidified:-

4.565 gyrRadioactive decay-0-

Undecayed hafnium is left in the silicate mantle of a differentiating planet like the EarthTungsten goes into the core.Hafnium left behind decays to tungsten with a half-life of 9 M.y.By knowing the original ratio in primitive bodies and the decay rate, one may determine the time of core formation.

Halliday, 2003Late Heavy Bombardment

Apollo 12 from Lunar Reconnaissance OrbiterOrigin of the Moon from a giant impact.

Apollo 12 from Lunar Reconnaissance OrbiterOrigin of the Moon from a giant impact.

Geochemical data from the Moon.

Canup, 2004Origin of the Moon from a giant impact

The Hadean impact rate

Impact crater density on the moon can, through the moon rocks, provide an absolute chronology for the impact rates

ZirconOldest rock

Coen et al 2000Impact melts

During a field expedition to Isua, we sampled three types of metasedimentary rocks, deposited 3.8 billion years ago, that contain information about the sedimentary river load from larger areas of surrounding land surfaces (mica-schist and turbidites) and of the contemporaneous seawater (BIF). Our samples show evidence of the LHB impacts that took place on Earth, by an average of a seven times enrichment (150 ppt) in iridium compared to present-day ocean crust (20 ppt). The clastic sediments show slightly higher enrichment than the chemical sediments, which may be due to contamination from admixtures of mafic (proto-crustal) sources. We show that this enrichment is in agreement with the lunar cratering rate and a corresponding extraterrestrial LHB contribution to the Earths Hadean-Eoarchean crust.The Earth-Moon system during the late heavy bombardment period - Geochemical support for impacts dominated by comets.Gre Jrgensen, Uffe; Appel, Peter W. U.; Hatsukawa, Yuichi; Frei, Robert; Oshima, Masumi; Toh, Yosuke; Kimura, Atsushi Icarus, V olume 204, 2009 p. 368-380.

Nice (Obs. Cote dAzur Nice) ModelSaturnJupiter2:3 resonance

Gomez et al. 2005.

Gomez et al. 2005.

The Hadean organic environment

50Existence of liquid water at/near surfaceDeclining ocean salinity?Morphological LHBArchean

~1020 kg

If embryos from the main belt or beyond were the source of water (carbonaceous, MB), then they could have also delivered organics. 6 x1024 kg x 0.01 x 0.02= 1021 kg, about 10 times Earths carbonate inventory

However, suppose instead that Earths water came from local water chemisorbed on grains. Then the only carbon introduced during accretion would be the (minor) refractory fraction (graphite, kerogens, PAHs...).How much could be introduced by the LHB?

Impactor Distribution during Late Heavy Bombardment Abramov and Mojzsis, 2009Assume 10% of the comet is organic material1019 kg of organics, about 10% of the present Earth carbonate inventory Mwater, Earth ~ 1021 kg

Temperature at depth of 4 km (surface of box) during Late Heavy BombardmentAbramov and Mojzsis, 2009

50Existence of liquid water at/near surfaceDeclining ocean salinity?Morphological LHBArcheanCarbon from embryosCarbon from LHB

ConclusionsThe timescale for Earth formation is fairly well known.

The presence of two major g.p. and possibly fortuitous timing set up a late heavy bombardment that may not be a general feature of terrestrial planet histories.

Carbon-bearing molecules were delivered either early by asteroid belt embryos, or late by LHB comets. The LHB cannot supply Earths water, which came early by embryos or chemisorbed local water. Therefore, we cannot exclude the possibility that there was an early period from 4.5-3.9 Ga, swhen the Earth was relatively impoverished in carbon-bearing species. This is probably oversimplified, since comets were striking Earth during its formation.

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