Transcript Slide 1
It’s not the energy, it’s the entropy Saturn’s age is revealed in its moons Earth’s water source is not in the meteorite collections... Is the Hadean Earth unavoidably anomalous? Methane vs. water cycles. We’ve already found martian life The best system to visit isn’t Jupiter’s The sky’s the limit The budget’s the lmit Lecture 4: Hadean Earth 1. The absolute age of the Earth 2. Hadean impact rates 3. Delivery and survival of organic molecules on the Hadean Earth. Late Heavy Bombardment Halliday, 2003 Properties of CAIs • Calcium-Aluminium rich – First elements to condensate when cooling down from high temperatures • Must 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 Radioactive decay Parent Daughter 14 182CHf 14N 182 W 129I 129Xe 235U 207Pb 238U 206Pb 40K 40Ar 232Th 208Pb 87Rb 87Sr 147Sm 143Nd Half-Life (years) 5.73 1063 8.9 xx10 1.6 x 107 7 x 108 4.5 x 109 1.2 x 1010 1.4 x 1010 4.9 x 1010 1.06 x 1011 Mode of Decay b b chain chain EC chain b a Radioactive decay N N0 e Rt N0 is the initial number of parent atoms N is the number of parent atoms at time t 1 1 R theaveragelifetimeof theradioactive atom 1.4t1/ 2 Let t be the time elapsed since the rock solidified: 87 Sr 86 Sr 87 87 Sr0 Rb (e 86 86 Sr0 Sr -t 1.4t1 / 2 1) Radioactive decay 4.565 gyr 87 Sr 86 Sr 87 87 Sr0 Rb (e 86 86 Sr0 Sr -t 1.4t1 / 2 1) •Undecayed hafnium is left in the silicate mantle of a differentiating planet like the Earth •Tungsten 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. Late Heavy Bombardment Halliday, 2003 Origin of the Moon from a giant impact. Apollo 12 from Lunar Reconnaissance Orbiter Origin of the Moon from a giant impact. Geochemical data from the Moon. Apollo 12 from Lunar Reconnaissance Orbiter Origin of the Moon from a giant impact Canup, 2004 The Hadean impact rate Impact crater density on the moon can, through the moon rocks, provide an absolute chronology for the impact rates Oldest rock Zircon Impact melts Coen et al 2000 The Earth-Moon system during the late heavy bombardment period - Geochemical support for impacts dominated by comets. Gråe Jørgensen, Uffe; Appel, Peter W. U.; Hatsukawa, Yuichi; Frei, Robert; Oshima, Masumi; Toh, Yosuke; Kimura, Atsushi Icarus, V olume 204, 2009 p. 368-380. 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 Earth’s Hadean-Eoarchean crust. Nice (Obs. Cote d’Azur Nice) Model Saturn Jupiter 2:3 resonance Gomez et al. 2005. Gomez et al. 2005. The Hadean organic environment Existence of liquid water at/near surface LHB Archean 5 0 Morphological ~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 Earth’s carbonate inventory • However, suppose instead that Earth’s water came from local water chemisorbed on grains. Then the only carbon introduced during accretion would be the (minor) refractory fraction (graphite, kerogens, PAH’s...). • How much could be introduced by the LHB? Impactor Distribution during Late Heavy Bombardment Abramov and Mojzsis, 2009 Assume 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 Bombardment Abramov and Mojzsis, 2009 Existence of liquid water at/near surface LHB Archean 5 0 Morphological Carbon from embryos Carbon from LHB Conclusions 1. The timescale for Earth formation is fairly well known. 2. 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. 3. Carbon-bearing molecules were delivered either early by asteroid belt embryos, or late by LHB comets. The LHB cannot supply Earth’s water, which came early by embryos or chemisorbed local water. 1. 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.