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.

Halliday, 2003

Late Heavy Bombardment

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

Parent DaughterHalf-Life (years)

Mode of Decay

14C 14N 5.73 x 103 129I 129Xe 1.6 x 107 235U 207Pb 7 x 108 chain238U 206Pb 4.5 x 109 chain40K 40Ar 1.2 x 1010 EC232Th 208Pb 1.4 x 1010 chain87Rb 87Sr 4.9 x 1010 147Sm 143Nd 1.06 x 1011

182Hf 182W 8.9 x 106

Radioactive decay

N0 is the initial number of parent atoms

N is the number of parent atoms at time t

RteNN 0

2/14.1

1

atom eradioactiv theof lifetime average the

1

tR

)1( 2/14.186

87

860

87

86

87

t

t

eSr

Rb

Sr

Sr

Sr

Sr

Radioactive decay

-

0

Let t be the time elapsed since the rock solidified:

-

4.565 gyr

Radioactive decay

)1( 2/14.186

87

860

87

86

87

t

t

eSr

Rb

Sr

Sr

Sr

Sr-

0

-

•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.

Halliday, 2003

Late 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, 2004

Origin 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

Zir

con

Old

est

rock

Coen et al 2000

Impact 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 Earth’s Hadean-Eoarchean crust.

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.

Nice (Obs. Cote d’Azur Nice) Model

Saturn

Jupiter2:3 resonance

Gomez et al. 2005.

Gomez et al. 2005.

The Hadean organic environment

5

0

Existence of liquid water at/near surface

Declining ocean salinity?

Morphological

LHB Archean

~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

5

0

Existence of liquid water at/near surface

Declining ocean salinity?

Morphological

LHB Archean

Carbon from embryos Carbon from LHB

Conclusions1. 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.