Transcript Proto Earth

Planetary system Earth
Rob de Meijer
Stichting EARTH/Dept. Phys. UWC
AAP2009, Angra dos Reis, Brazil, 19 March 2009
Geophysics and Geochemistry
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Geophysics tries to measure the present state
of the Earth.
Geochemistry tries to explain how we got
there.
Aim of this talk is hypothesing the past from
present knowledge and proposing ways to
measure the present state.
Focus area: Core-Mantle Boundary (CMB).
Origin of Earth
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Formed at the start of our solar system from
dust accumulation from the solar disk. The
dust is a left over of supernovae explosions.
Accumulation of mass heats up the dust ball
(conversion of potential to kinetic energy and
finally to heat).
Melting of the dust material causes heavy
materials to sink to the centre. (core-mantle
differentiation).
Earth’s interior at present
CMB
Dating core formation
on planet building blocks
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182Hf
 182W,
half-life 8.9 Ma
Lithophile (stone-loving)
Elements (e.g. Hf)
Siderophile
(iron-loving)
Elements
(e.g. W)
Core-mantle differentiation
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(Hf/W)mantle~40 (Hf/W)chond.
At differentiation remaining
182Hf in mantle decays to
182W and increases the
ε(182W) value.
Difference in ε is a measure
of time.
Conclusion: the core
separated from the mantle
30-70 Ma after the start of
the solar system.
Other processes in Earth
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There is another dating clock: 146Sm decaying to
142Nd (t =103 Ma). Both nuclides prefer the
1/2
mantle over the core and hence one expects no
deviation from meteoric ratios.
But there exists a difference and since the total
mantle should be “chondritic” the mantle should
consist of two reservoirs: one with a high ratio
and one with a low ratio. The surface samples
have a high ratio.
Separation of reservoirs occurred at t= 30 ± 10
Ma.
Also lunar rocks exhibit a high ratio.
So mantle-core differentiation and hidden
reservoir formation occur in the first 30 Ma. The
Moon followed shortly afterwards.
Where is the low-ratio (hidden) reservoir?
Properties of the hidden
reservoir
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Separated from the
rest of the mantle at
t= 30 Ma;
No exchange with the
mantle since then;
Only candidate: The
Core-Mantle Boundary
(CMB).
Enhanced U,Th and Pu
concentrations.
Likely consequences for the
hidden reservoir
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At t~30 Ma the hidden reservoir had
relatively high concentrations of 232Th, 235U
(24%), 238U (76%) and 244Pu. Presently Pu is
extinct and 235U abundance is 0.7%.
The reservoir contains a mineral CaPv, which
incorporates very selectively all U, Th and Pu.
Local concentrations of a factor of ten seem
to be sufficient to ignite one or more natural
nuclear reactors and keep them running.
These reactors are fast breeders which
produce their own fuel for many Ga.
Heat production of Earth.
Present heat-flow
distribution has been
determined from 23000
measurement sites. The
figure indicates the warm
mid-ocean ridges and the
cold spots under the
continents, reflecting the
up- and down welling of
material. The total power
is ~45 terawatt (1012 W).
Radiogenic processes
produce 21±4 TW (crust
~8TW and ~13TW from
mantle.
Arevalo et al (2009) estimate from K/U ratios that of this ~13TW only 5 TW is
accounted for by radioactive decay. Could ~8TW be produced by georeactors?
Evidence for georeactors?
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Yes, at t=30 Ma Earth was so hot that
various volatile elements have escaped
Earth’s atmosphere. The abundances of
Na, K and Cl are lower than chondritic.
Also helium should have escaped.
But we know there is both 3He and 4He
in the atmosphere and in gasses from
wells. How is that possible?
Production of Helium
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Helium has two stable isotopes: 4He atoms formed
from alpha-particles collecting electrons. Decay series
of Th en U produce 4He in this way.
In the atmosphere 3He mainly originate from solar
wind.
In the Earth’s interior 3He can only be produced by
nuclear reactors via the fission product tritium (3H),
which decays to 3He.
The observed 3He/4He ratios in wells are consistent
with potential georeactors in the hidden reservoir.
Other fission products?
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The isotope mass ratios in many fission
produced elements differ from the standard
values. The presence of georeactors is
consistent with the change towards heavier
isotopes in xenon.
Our atmosphere has compared to solar wind
already more heavier Xe isotopes. In well
gasses the distribution is shifted further
towards the heavies. These shifts can be
quantitatively described by the georeactor
concept!
What is the relation to the
infancy of Mother Earth?
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Differentiation of mantle and core,
formation of the hidden reservoir and
formation of the Moon all occur more or
less in the same narrow time window.
Are these events accidental or related?
Origin of the Moon?
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Prior to the Apollo-missions to the Moon
(1968-1972) three hypotheses were present
by the lack of data:
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Co-formation: Moon formed from collecting dust
from the solar nebula, like Earth;
Capture: Moon is a celestial body originating
somewhere from between Mars and Jupiter.
Segregation: The young Earth was a very fast
spinning planet that lost the Moon with help of
tidal bulges inflicted by the Sun. Early hypothesis
by George Darwin (Charles’ son).
Current hypothesis
A celestial body
with the size of
Mars collides
“gently” with
Earth. This Great
Impactor
hypothesis is the
standard
explanation at
present.
Problems
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Simulations show that the collision had to be very
gentle, otherwise the remnants of both bodies would
have been ejected into space.
Moon should be composed by 80% of impactor
material. But the O-isotope ratios then require that
the impactor was formed in almost the same orbit
around the Sun as Earth.
It is hard to understand that Moon, similar to Earth’s
mantle, has a hidden reservoir, that did not mix with
the rest of the Moon’s mantle in such a collision.
Almost coincidental occurrence of core-mantle
differentiation on Earth, formation of hidden reservoir
and Moon-formation is shear coincidence.
Recent dating of lunar metals
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Analysis of Hf/W and Sm/Nd
ratios in metal inclusions
indicate that the Moon has
the same Hf/W (ε=0) and
Sm/Nd ratios as the Earth. So
Moon is formed after coremantle differentiation and
the Moon-mantle is not
mixed with the hidden
reservoir.
The Moon-mantle and the
Earth mantle are very similar
in elemental and isotopic
composition of e.g. Cr, W and
Ti.
Possible explanations for
Earth-Moon similarities.
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To save the Great Impactor Hypothesis
it is assumed that the silicate vapour
atmosphere of the proto-Earth and the
lunar magma disk have equilibrated.
Hard to see how W and Ti (refractory
elements) fit in.
The Moon comes mainly from the
Earth’s mantle.
Alternative?
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Wim van Westrenen and I propose a rapidly energy producing
georeactor, located in the hidden reservoir, producing the missing
energy needed to expel the Moon.
The missing energy requires ~ 4% of the U and Th in the hidden
reservoir.
This energy is released in a geologically short period and raises the
temperature of its surroundings from 5000 to 13000 K.
At this temperature all materials become gaseous, which leads to the
formation of a bubble, which under buoyancy travels to the surface and
“spits out” the Moon.
The initial distance Moon – Earth is about 100 000 km. The Moon is
rapidly rotation-locked and gradually moves away from Earth due to
tidal interactions. The present rate is ~4cm per year.
Moon is mainly composed of Earth mantle material with some small
admixtures from the core and the hidden reservoir.
The process transfers so much energy and angular momentum from
Earth to Moon, that a second moon formation is practically inhibited.
Missing energy source
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Consider Earth-Moon as a bound two-body system.
Ground state: Moon inside the Earth; Excited state:
Moon orbiting the Earth. Moon and Earth have
90% of their present masses.
Estimate binding energy from lattice energy of
quartz: -2.3*1031J. Total gravitational energy:
γmE mM
1
2
W 
  mM vM .
2rEM
2
mM (kg)
mE (kg)
(rEM)gs (m)
(rEM)ex (m)
6.61*1022
5.38*1024
6.38*106
parameter
Ground and excited states
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Assumption on validity of angular momentum
conservation determines vM in gs and excited
state and hence the rotational and binding
energy.
For the excited state the initial separation is a
parameter. At present the rEM increases by ~4
cm/a.
GS should be more bound than excited state.
Missing-energy estimate
The sum of the rotational and gravitational energy, in units of
1030 J, of the Earth-Moon system 4.5 Ga ago as function of the
Earth-Moon distance and the angular momentum in units of its
present value (Lp).
rEM (108 m)
0.9 Lp
1.0 Lp
1.1 Lp
1.2 Lp
2.0 Lp
0.8
2.63
3.67
4.89
6.26
23.3
0.9
2.46
3.47
4.65
6.13
22.8
1.0
2.30
3.29
4.43
5.75
22.3
1.25
1.96
2.87
3.95
5.19
21.1
1.5
1.67
2.52
3.53
4.71
20.1
2.0
1.21
1.94
2.84
3.91
18.4
Proto Earth
1.87
2.87
3.97
5.17
18.6
For rEM=108 m then the missing energy is about ~0.5 1030 J.
Testing this hypothesis?
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In principal there are two ways. The first one
searches for footprints of the fission products in lunar
rocks: mainly helium, xenon and maybe krypton.
This requires lunar samples from some depth
because the surface is contaminated by solar wind
deposition. This holds in particular for helium.
At present the scarce information from lunar rocks
shows that isotope ratios of xenon are shifted
towards the heavy ones, as we expect.
The recent plans for missions to the Moon may bring
new test materials.
..and ?
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In radioactive decay and in nuclear fission
antineutrino’s are emitted with characteristic
energies.
Antineutrinos can cross Earth having hardly any
interaction. Hence they are difficult to detect.
Programme EARTH, started in NL, develops with SA
direction-sensitive antineutrino detectors, which
eventually put together will form a network of about
ten telescopes looking into the Earth and map the
radiogenic heat sources.
Each telescope contains ~ 4000 ton detection
material.
Conclusions-Earth
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Recent developments in geochemistry and
analysis of isotope ratios have created a new
vision on the processes that occurred in the
infancy of Earth.
Core-mantle differentiation and the formation
of a hidden reservoir all took place during the
first 30 Ma after the start of the solar system,
shortly afterwards followed by the formation
of the Moon.
In the hidden reservoir conditions allowed the
formation of georeactors.
Conclusions-Moon
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Similar to previous hypotheses also the Great
Impactor hypothesis has difficulties in
explaining a number of observables.
An active georeactor could have delivered the
missing energy for the hypothesis proposed
by Darwin (1876).
The georeactor hypothesis for the genesis of
the Moon in principal can be tested.
Conclusion-technology
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Developing direction-sensitive antineutrino
detectors requires beyond the state of the art
technologies.
These detectors will, prior to their scientific
goals, first result in practical applications.
Such a development strategy is required for
financing such an ambitious and extensive
project.
Speculative consequences
alternative Moon formation
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Moon formation created large turbulence in the
mantle, setting off formation of continental crust and
continental drift.
After Moon formation the system has insufficient
energy and angular momentum to produce a second
Moon.
Supercritical georeactors have to release energy in
another way. (kimberlite pipes and mantle plumes).
Energy may have been converted to create large
mineralisations (Au & Pt in South Africa) and
formation of abiotic natural gas (Brazil?).