Delivery of Volatiles to the Terrestrial Planets
Delivery of Volatiles to the Terrestrial Planets
Hans Rickman
Uppsala Astronomical Observatory
Hans Rickman
Uppsala Astronomical
Observatory
The Earth’s Water The Earth’s Water
More than 70% of Earth’s surface is covered by water
The mass of the oceanic and atmospheric H2O is
~1.51021 kg
The Earth’s interior is relatively dry, but there may be several additional ocean masses of H2O in the mantle rocks
The total amount of water in the Earth may be
~61021 kg (~0.001 ME)
More than 70% of Earth’s surface is covered by water
The mass of the oceanic and atmospheric H2O is
~1.51021 kg
The Earth’s interior is relatively dry, but there may be several additional ocean masses of H2O in the mantle rocks
The total amount of water in the Earth may be
~61021 kg (~0.001 ME)
Possible origins (standard) Possible origins (standard)
Endogenous water
- (“local water”, acquired from the Solar Nebula near Earth’s orbit)
Asteroidal water
- (brought into the growing Earth from the asteroid main belt)
Cometary water
- (brought into the full-grown Earth by a late cometary bombardment)
Endogenous water
- (“local water”, acquired from the Solar Nebula near Earth’s orbit)
Asteroidal water
- (brought into the growing Earth from the asteroid main belt)
Cometary water
- (brought into the full-grown Earth by a
late cometary bombardment)
Against Endogenous Water Against Endogenous Water
In an equilibrium
condensation model for the solar nebula, water ice and
hydrated minerals occur only at lower temperatures than expected for Earth’s formation region
In an equilibrium
condensation model for the solar nebula, water ice and
hydrated minerals occur only at lower temperatures than expected for Earth’s formation region
Water in asteroids (1) Water in asteroids (1)
Fraction of asteroids showing the 0.7 m band of the Fe2+Fe3+ transition in hydrated minerals (proxy for OH) versus orbital semimajor axis (Jewitt et al,
Protostars & Planets V, 2006)
Fraction of asteroids showing the 0.7 m band of the Fe2+Fe3+ transition in hydrated minerals (proxy for OH) versus orbital semimajor axis (Jewitt et al,
Protostars & Planets V, 2006)
Water in asteroids (2) Water in asteroids (2)
Three comets recently identified in the outer
Main Belt (Hsieh & Jewitt 2006) existence of
subsurface ice
Existence of dynamical routes from the Jupiter family into the Main Belt, in particular when the gas disk existed
Three comets recently identified in the outer
Main Belt (Hsieh & Jewitt 2006) existence of
subsurface ice
Existence of dynamical routes from the Jupiter family into the Main Belt, in particular when the gas disk existed
Hsieh & Jewitt (2006)
Water in meteorites Water in meteorites
Carbonaceous chondrites contain ~3-11% water as clay minerals and
serpentines. They come from the outer MB.
Ordinary and enstatite chondrites have only
~0.05-0.1% water or less.
They come from the inner MB.
Carbonaceous chondrites contain ~3-11% water as clay minerals and
serpentines. They come from the outer MB.
Ordinary and enstatite chondrites have only
~0.05-0.1% water or less.
They come from the
inner MB. Allende meteorite
(carbonaceous chondrite)
Geochemical Data:
D/H Ratios
Geochemical Data:
D/H Ratios
D/H in VSMOW is
~6 protosolar and
~1/2 cometary
D/H in Martian
meteorites is ~2
VSMOW (close to cometary)
D/H in VSMOW is
~6 protosolar and
~1/2 cometary
D/H in Martian
meteorites is ~2
VSMOW (close to cometary)
Drake & Righter (2002)
Geochemical Data:
D/H Ratios
Geochemical Data:
D/H Ratios
VSMOW may be a mixture of cometary and asteroidal H2O, possibly with some endogenous (protosolar?) contribution
Caveats:
- cometary D/H is measured only for Oort cloud comets and has important error bars
- possible D/H fractionation at diffusion and sublimation of cometary H2O
- martian D/H may have been raised by atmospheric erosion (cf. Venus)
VSMOW may be a mixture of cometary and asteroidal H2O, possibly with some endogenous (protosolar?) contribution
Caveats:
- cometary D/H is measured only for Oort cloud comets and has important error bars
- possible D/H fractionation at diffusion and sublimation of cometary H2O
- martian D/H may have been raised by atmospheric erosion (cf. Venus)
Geochemical Data:
Noble Gases
Geochemical Data:
Noble Gases
Kr/Ar in Earth and Mars is
>> solar
Xe/Kr in Earth and Mars is
<< chondritic
All noble gases are much more abundant on Earth than Mars (martian
atmospheric erosion)
Kr/Ar in Earth and Mars is
>> solar
Xe/Kr in Earth and Mars is
<< chondritic
All noble gases are much more abundant on Earth than Mars (martian
atmospheric erosion)
Owen & Bar-Nun (1996)
Geochemical Data:
Noble Gases
Geochemical Data:
Noble Gases
Owen & Bar-Nun (1995): mixture of rocks (devoid of noble gases) and cometary ice
(trapping ratios for condensation of amorphous ice at ~50 K)
Caveats:
- observations of comets are lacking; their formation temperature may be different
- the noble gases and the water may have different origins, if the former were degassed before the Moon-forming impact
Owen & Bar-Nun (1995): mixture of rocks (devoid of noble gases) and cometary ice
(trapping ratios for condensation of amorphous ice at ~50 K)
Caveats:
- observations of comets are lacking; their formation temperature may be different
- the noble gases and the water may have different origins, if the former were degassed before the Moon-forming impact
Endogenous water (Drake &
Campins 2006)
Endogenous water (Drake &
Campins 2006)
Pro – evidence for early H2O ocean (129Xe/132Xe ratio; zircons ~4.3-4.4 Gyr)
BUT: there is an exogenous source predating this ocean
Pro – adsorption of H2O onto nebular grains (Stimpfl et al. 2004): depends on S/V ratio of grains, ~1 Earth ocean adsorbed at
T~700 K for S/V~100(S/V)sph (Drake 2005)
BUT: the S/V ratio of real nebular grains is completely unknown
Pro – evidence for early H2O ocean (129Xe/132Xe ratio; zircons ~4.3-4.4 Gyr)
BUT: there is an exogenous source predating this ocean
Pro – adsorption of H2O onto nebular grains (Stimpfl et al. 2004): depends on S/V ratio of grains, ~1 Earth ocean adsorbed at
T~700 K for S/V~100(S/V)sph (Drake 2005)
BUT: the S/V ratio of real nebular grains is completely unknown
Exogenous water (Drake &
Campins 2006)
Exogenous water (Drake &
Campins 2006)
Con – Stern et al. (2000) found solar Ar/O ratio in comet Hale-Bopp (tentative detection)
BUT: Weaver et al. (2002) found <10% solar Ar in two other comets; impact erosion may cause loss of Ar from the atmosphere
Con – 187Os/188Os of Earth’s PUM agrees with anhydrous but not carbonaceous chondrites
BUT: the late veneer now appears to have been mainly due to water-poor asteroids, not comets
Con – Stern et al. (2000) found solar Ar/O ratio in comet Hale-Bopp (tentative detection)
BUT: Weaver et al. (2002) found <10% solar Ar in two other comets; impact erosion may cause loss of Ar from the atmosphere
Con – 187Os/188Os of Earth’s PUM agrees with anhydrous but not carbonaceous chondrites
BUT: the late veneer now appears to have been mainly due to water-poor asteroids, not comets
The Osmium evidence The Osmium evidence
The PUM (primitive upper mantle) has a chondritic abundance pattern of highly
siderophile elements
assumption that this is due to the late veneer, after core
formation
The PUM (primitive upper mantle) has a chondritic abundance pattern of highly
siderophile elements
assumption that this is due to the late veneer, after core
formation Drake & Righter (2002)
The cometary late veneer The cometary late veneer
This used to be the standard
explanation for the Earth’s water about 10 years ago but was first challenged by the discovery of an apparent discrepancy between the cometary and VSMOW D/H ratios
Later on, other more important shortcomings have been found
This used to be the standard
explanation for the Earth’s water about 10 years ago but was first challenged by the discovery of an apparent discrepancy between the cometary and VSMOW D/H ratios
Later on, other more important
shortcomings have been found
Giant Planet Formation:
Classical Recipe
Giant Planet Formation:
Classical Recipe
The cores of the giant planets formed in situ by planetesimal accretion
time scales: ~ 106 yr for Jupiter ~ 108 yr for Neptune
Gravitational scattering into Earth-
crossing orbits occurs on the same time scales
long-lasting bombardment (decay time constant
~108 yr) from Neptune’s accretion zone
The cores of the giant planets formed in situ by planetesimal accretion
time scales: ~ 106 yr for Jupiter ~ 108 yr for Neptune
Gravitational scattering into Earth-
crossing orbits occurs on the same time scales
long-lasting bombardment (decay time constant
~108 yr) from Neptune’s accretion zone
Lunar Cratering Rate Lunar Cratering Rate
Suppose lunar
cratering decayed exponentially with
~108 yr until the end of the heavy bombardment time-integrated
impactor mass, scaled to Earth, from 4.4 to 3.9 Gyr ago: ~ 21022 kg (Chyba et al. 1995)
Suppose lunar
cratering decayed exponentially with
~108 yr until the end of the heavy bombardment time-integrated
impactor mass, scaled to Earth, from 4.4 to 3.9 Gyr ago: ~ 21022 kg (Chyba et al. 1995)
However, … However, …
The picture of exponential decay has been increasingly challenged in favour of an
episodic Late Heavy Bombardment
There are problems explaining the amount of impacting cometary material on dynamical grounds
New planetary formation models involve radial migration due to scattering of the planetesimal disk
The picture of exponential decay has been increasingly challenged in favour of an
episodic Late Heavy Bombardment
There are problems explaining the amount of impacting cometary material on dynamical grounds
New planetary formation models involve
radial migration due to scattering of
the planetesimal disk
Late Cataclysmic Cratering Episode
Late Cataclysmic Cratering Episode
No trace of 600 Myr continual lunar cratering (no impact melt rock older than ~3.9 Gyr in
Apollo/Luna samples and lunar meteorites)
No crustal enrichment in siderophiles limits on total amount of impacts
No trace of large impact basins before ~4.0 Gyr ago
Early oceans on Earth speak against an extremely high impact rate at early epochs
The total mass of LHB Earth impactors yields <10%
of the Earth’s water
No trace of 600 Myr continual lunar cratering (no impact melt rock older than ~3.9 Gyr in
Apollo/Luna samples and lunar meteorites)
No crustal enrichment in siderophiles limits on total amount of impacts
No trace of large impact basins before ~4.0 Gyr ago
Early oceans on Earth speak against an extremely high impact rate at early epochs
The total mass of LHB Earth impactors yields <10%
of the Earth’s water
Dynamical Constraints (1) Dynamical Constraints (1)
Earth-crosser chance of hitting the Earth per orbital revolution:
~10-8
Earth-crossing lifetimes:
- for asteroids from the MB ~106 orbits total impact chance ~10-2
- for comets ~103104 orbits total
impact chance ~10-5-10-4 (depends on probability of decoupling from Jupiter)
Earth-crosser chance of hitting the Earth per orbital revolution:
~10-8
Earth-crossing lifetimes:
- for asteroids from the MB ~106 orbits total impact chance ~10-2
- for comets ~103104 orbits total
impact chance ~10-5-10-4 (depends on probability of decoupling from Jupiter)
Dynamical Constraints (2) Dynamical Constraints (2)
To acquire ~10
-3Earth masses of H
2O from comets, ~10100 Earth masses of potential impactors are needed
TOO MUCH, IF COMING FROM NEPTUNE; PROBABLY ALSO IF COMING FROM JUPITER-SATURN
To acquire the same from asteroids with
~10% water (CC), ~1 Earth mass of potential impactors is needed
MAYBE REASONABLE DURING THE FORMATION OF JUPITER
To acquire ~10
-3Earth masses of H
2O from comets, ~10100 Earth masses of potential impactors are needed
TOO MUCH, IF COMING FROM NEPTUNE; PROBABLY ALSO IF COMING FROM JUPITER-SATURN
To acquire the same from asteroids with
~10% water (CC), ~1 Earth mass of potential impactors is needed
MAYBE REASONABLE DURING THE FORMATION OF JUPITER
The “Nice Model” (1) The “Nice Model” (1)
Start with a condensed system of giant planets
They scatter away all
remaining planetesimals before the gas disk
dissipates
Later migration due to eating away of the
outer planetesimal disk
J-S 5:2 resonance crossing at the time of the LHB
Start with a condensed system of giant planets
They scatter away all
remaining planetesimals before the gas disk
dissipates
Later migration due to eating away of the
outer planetesimal disk
J-S 5:2 resonance crossing at the time of the LHB
Gomes et al. (2005)
The “Nice Model” (2) The “Nice Model” (2)
Strong dynamical instability!
Rapid clearing of outer disk heavy cometary bombardment
Migration of Jupiter and Saturn
sweeping of secular
resonances through the Main Belt heavy asteroidal
bombardment
The LHB crater size distribution on the Moon agrees with MB
size distribution of asteroids asteroids were likely dominant
Strong dynamical instability!
Rapid clearing of outer disk heavy cometary bombardment
Migration of Jupiter and Saturn
sweeping of secular
resonances through the Main Belt heavy asteroidal
bombardment
The LHB crater size distribution on the Moon agrees with MB
size distribution of asteroids
asteroids were likely dominant Gomes et al. (2005)
Early Exogenous Water Early Exogenous Water
Clearing of the region exterior to the Main Belt by the growth of Jupiter
Clearing of the space in and between the giant planet accretion zones
Time scales: less than or ~a few 106 yr
Clearing of the region exterior to the Main Belt by the growth of Jupiter
Clearing of the space in and between the giant planet accretion zones
Time scales: less than or ~a few 106 yr
Suggested delivery
mechanism for Earth’s water Suggested delivery
mechanism for Earth’s water
The growth of Earth-mass and larger planetary embryos in the Jupiter-Saturn zone led to the scattering of huge numbers of icy planetesimals (IPs) at a time when the gas disk (solar nebula) still existed
The IPs that were scattered into small perihelion distances got their orbits circularized by gas drag and ended up in the outer asteroid belt
Parts of the terrestrial planets may have been brought in from that region (CC meteorites)
The growth of Earth-mass and larger planetary embryos in the Jupiter-Saturn zone led to the scattering of huge numbers of icy planetesimals (IPs) at a time when the gas disk (solar nebula) still existed
The IPs that were scattered into small perihelion distances got their orbits circularized by gas drag and ended up in the outer asteroid belt
Parts of the terrestrial planets may have been brought in from that region (CC meteorites)
Dynamical transfer routes Dynamical transfer routes
Blue curves (bidirectional):
gravitational kicks at close encounters for different encounter velocities
Green curves (only toward the upper left): eccentricity damping by gas drag for
different values of the angular momentum
The time scales for the two phenomena are similar for a certain IP size range
Blue curves (bidirectional):
gravitational kicks at close encounters for different encounter velocities
Green curves (only toward the upper left): eccentricity damping by gas drag for
different values of the angular momentum
The time scales for the two phenomena are similar for a certain IP size range
Terrestrial planet growth (1) Terrestrial planet growth (1)
Wetherill (1992), Chambers &
Wetherill (1998):
Runaway accretion of
planetary embryos all over the inner SS contemporary with the growth of Jupiter
As Jupiter arises, the MB embryos are caught in resonances and their
eccentricities are increased
Some are accreted by the
embryos in the TP region, and the MB is efficiently depleted
Wetherill (1992), Chambers &
Wetherill (1998):
Runaway accretion of
planetary embryos all over the inner SS contemporary with the growth of Jupiter
As Jupiter arises, the MB embryos are caught in resonances and their
eccentricities are increased
Some are accreted by the
embryos in the TP region, and the MB is efficiently depleted
Terrestrial planet growth (2) Terrestrial planet growth (2)
Morbidelli et al. (2000):
Simulations show that, in general, the Earth should contain ~1-2 embryos from the region at >2.5 AU
If these are as hydrous as CC meteorites, then there is more than enough water for the
Earth’s budget
Caveats: small number statistics, loss of water during accretion of embryos, loss of water during the final giant impacts
Morbidelli et al. (2000):
Simulations show that, in general, the Earth should contain ~1-2 embryos from the region at >2.5 AU
If these are as hydrous as CC meteorites, then there is more than enough water for the
Earth’s budget
Caveats: small number statistics, loss of water during accretion of embryos, loss of water during the final giant impacts
Tentative Conclusions Tentative Conclusions
Some of the planetary volatiles may be of endogeneous origin
A rich exogenous source existed
while the giant planets formed; H2O could have been brought to Earth both from outside and inside the snow line
The late veneer, including the LHB, was apparently unimportant for the delivery of volatiles
Some of the planetary volatiles may be of endogeneous origin
A rich exogenous source existed
while the giant planets formed; H2O could have been brought to Earth both from outside and inside the snow line
The late veneer, including the LHB, was apparently unimportant for the delivery of volatiles
Final Question:
Final Question:
An early origin of the Earth’s volatiles makes them vulnerable to accretional giant
impacts, including the Moon-forming impact
But the lack of a relevant late veneer to replace the lost water means that these
impacts must not have desiccated the Earth
Can this be understood?
An early origin of the Earth’s volatiles makes them vulnerable to accretional giant
impacts, including the Moon-forming impact
But the lack of a relevant late veneer to replace the lost water means that these
impacts must not have desiccated the Earth
Can this be understood?
