ChemistryofEarth’sEarliest
Atmospheres
BruceFegley,Jr.
PlanetaryChemistryLaboratory,Department
ofEarth&PlanetarySciencesandMcDonnell
CenterfortheSpaceSciences,Washington
University,St.LouisMO63130USA
HIFOLColloquium,MPIA,Heidelberg,
Germany
5October2016
ConclusionsI.
•  LunarrocksgiveoxidaTonstateofearlyEarthatTme
oflunarformaTon–morereducedthanpresentBulk
SilicateEarth(BSE)byseverallogunits
•  EarlyEarthwasmorereduced–consistentwithlarge
amountofreducedmaterialneededtomaketheEarth
andtheidenTcaloxygenisotopiccomposiTonofhighly
reducedenstaTtemeteorites
•  OutgassedvolaTlesincludeand/ordominatedbyCH4,
NH3,H2andgiveanearlyatmospherethatisfavorable
fororganiccompoundformaTonviaMiller-Urey
reacTons
ConclusionsII.
•  EarthoxidizedduringLateHeavy
Bombardment(~4.2–3.8Gyrago)when
moreoxidizedmateriallikeordinaryor
carbonaceouschondritesdeliveredtoEarth
•  ReducingearlyEarthconceptexplainslowfO2
oflunarrocksandreducingatmosphereon
earlyEarththatisfavorableforMiller-Urey
reacTons(ulTmately)leadingtooriginoflife
OutlineofthisTalk
• 
• 
• 
• 
• 
• 
• 
SecondaryoriginofEarth’satmosphere
SourcesforitsVolaTles
OutgassingofchondriTcmaterial
OutgassingofpresentBSE
OriginofMoonandOriginofLife
Chemistryofitsearlygaseousatmosphere
Briefdiscussionoftheearliersilicatevapor
andsteamatmospheres
Secondaryoriginofatmosphere
•  Notcapturedfromthesolarnebula(protoplanetaryaccreTondisk)
•  Formedbyoutgassingofsolid&moltenEarth
materialduring&aceraccreTon
•  SupportedbylargenoblegasdepleTonsin
observableparts(atm,oceans,crust,upper
mantle)–Aston,H.Suess,H.Brown
•  3He&othersolarnoblegasesfrommantlenot
primordial,duetosolarwindimplantedgases
Chemistry of Earth’s Earliest Atmosphere
arth’s Atmosphere
100
10-1
10-2
10-3
Depletion factor
and originated by chemical
volatile-bearing solids durrth’s atmosphere is not prif solar nebula gas during its
ed on the large depletions of
Ne, Ar, Kr, Xe) relative to the
as H (as water), C, and N at
nsidered in this comparison
o space with an atmospheric
and 0.4–0.8 Ma for 3He
onsidered because its longest
half life of about 3.8 days.
en, we are concerned only
of hydrogen as H2 gas.
the enormous disparity beble gases and other volatile
elemental abundances as a
mber of 142 and noted the
relative to that of the other
earth has only one millionth
rt gases.’
mical and geochemical data
ng 25 years, Brown (1949)
smic abundances and Earth’s
2O), C, N, Ne, Ar, Kr, and Xe.
ments are depleted on Earth
However, the noble gases are
73
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
H
C
N
F
S Cl Ne Ar Kr Xe
Volatiles
Figure 1 Depletion factors for inert gases and chemically reactive
volatiles on Earth relative to their abundances in the solar nebula. See
Table 1 and the text for details.
of Lodders et al. (2009) are used in Table 1 because they
updated the abundances of C, N, O, F, Cl, S, and the noble
ExamplesofNobleGasDepleTons
Ra#o
Solar
BulkSilicateEarth
Ne/N2
3.1
2.3×10-5
36+38Ar/S*
0.25
5×10-8
36+38Ar/Cl*
0.055
2×10-7
*massraTo
grains accreted by Earth during its formation, but the noble
gases were not.
The differences in the depletion factors of the chemically
reactive volatiles arise from their solar elemental abundances
and the stability of different minerals in the grains accreted by
Earth. For example, the solar elemental abundances of F, Cl,
and S are small enough for these three elements to be
completely incorporated into various minerals such as halite
(NaCl), apatite [Ca5(PO4)3(F,Cl,OH)], and troilite (FeS)
found in chondritic meteorites (see Table 2, which lists some
of the volatile-bearing phases in chondrites). This is relevant
because chondritic meteorites are relatively unaltered samples
of solid material from the solar nebula and their composition
is a guide to that of the solid grains accreted by Earth.
Table 1
H (as H2O), C, and N into minerals formed with Mg, Si, Fe,
and other rock-forming metals because the solar elemental
abundances of H, C, and N are much larger than those of the
rock-forming metals. For example, the C/Si atomic abundance
ratio is ! 7.0 in solar composition material but only 0.76 in CI
chondrites, the most volatile-rich meteorites. Likewise, the
N/Si atomic abundance ratio is ! 2.1 in solar composition
material but only 0.055 in CI chondrites. Finally, the H2O/Si
molar abundance ratio is ! 12.7 in solar composition material
but only 5.1 in CI chondrites.
A small fraction of H (as H2O), C, and N occurs in meteorites as organic matter, for example, the insoluble organic
matter (IOM) in CI chondrites with a bulk composition of
C100H72N2O14S4 (Hayes, 1967), carbides, carbonates, diamonds,
Volatile inventories and depletion factors on the Earth
Volatile
Solar abundancea
mg g"1 in BSEb
Inventory (kg)
Depletion factor
Notesc
H (water)
1.27 # 107
1072
4.32 # 1021
6.2 # 10"4
C
N
F
Ne
S
Cl
7.19 # 106
2.12 # 106
804
3.29 # 106
4.21 # 105
5170
9.27 # 104
55.8
5.46
100
2
25
1.6 # 10"5
124
30
6.0 # 10"6
4.2 # 10"6
5.0 # 10"7
4.03 # 1020
8.06 # 1018
1.01 # 1020
6.50 # 1013
5.00 # 1020
1.21 # 1020
2.40 # 1013
1.69 # 1013
2.03 # 1012
1.5 # 10"4
1.9 # 10"5
0.22
7.6 # 10"11
1.2 # 10"3
2.2 # 10"2
2.3 # 10"10
1.2 # 10"7
9.3 # 10"8
Solar Awater ¼ AO " AMg " 2ASi, adjusted for O in rock
BSE water calculated from 120 mg g"1 H in BSE
C in BSE is 46–250 mg g"1, see Table 6.9 of LF98
Atmosphere !50% of total N in BSE
F in BSE is 19–28 mg g"1, see Table 6.9 of LF98
Taking atmospheric Ne as the total inventory
S in BSE is 13–1000 mg g"1, see Table 6.9 of LF98
Cl in BSE is 8–44 mg g"1, see Table 6.9 of LF98
Taking atmospheric Ar as total inventory of 36þ38Ar
Taking atmospheric Kr as the total inventory
Taking atmospheric Xe as the total inventory
36þ38
Ar
Kr
Xe
a
Solar abundance per 106 Si atoms, Table 1.2 of Lodders and Fegley (2011).
Concentrations in the bulk silicate Earth (BSE) for H, C, N, F, and Cl are from Palme and O’Neill (Chapter 3.1). Sulfur is from Table 4.4 of Lodders and Fegley (2011).
c
The range of published estimates for H, C, N, F, S, and Cl are in Table 6.9 of Lodders and Fegley (1998). The solar abundance of water is calculated from the oxygen abundance
adjusted for the amount of oxygen in rock (MgO þ SiO2). Other values that are used in the calculations are the Si concentration in the BSE (21.22%), the mean molecular weight of
Earth’s atmosphere (28.97 g mol"1), total atmospheric mass (5.137 # 1018 kg), mass of the BSE (4.03 # 1024 kg), and the concentrations of Ne, Ar, Kr, and Xe in dry air (18.18 ppmv,
9340 ppmv, 1.14 ppmv, and 87 ppbv). The Ar abundance in air is corrected for 40Ar, which is 99.6% of terrestrial Ar. Calculations compare terrestrial and solar abundances of 36Ar
and 38Ar.
b
Table 2
Some volatile-bearing phases in chondrites and potential outgassed volatiles
Name
Ideal chemical formula
Chondritea
Potential volatilesb
Apatite
Calcite
Cohenite
Dolomite
Graphite
Gypsum
Halite
Insoluble organic matter
Nierite
Osbornite
Sinoite
Ca5(PO4)3(F, Cl, Br, OH)
CaCO3
(Fe, Ni)3C
CaMg(CO3)2
C
CaSO4& 2H2O
NaCl
C100H72N3O22S4.5
Si3N4
TiN
Si2N2O
Many
C
Many
C
C
C, OC
C, OC
C, UOC
E
E, CH
E
HF, HCl, Cl2, HBr, Br2, H2O, H2, O2
CO, CO2
CH4, CO, CO2
CO, CO2
CH4, CO, CO2
SO2, H2S, OCS, Sx
HCl, Cl2
CH4, CO, CO2, H2O, H2, N2, NH3, Sx, H2S, OCS, SO2
N2, NH3
N2, NH3
N2, NH3
SourcesofVolaTles
•  Earthaccretedmixtureofreduced&oxidized
materialfromrangeofradialdistanceinsolar
nebula(e.g.,modelsofAnders,J.S.Lewis,
Lodders,Ringwood,Rubie,Wänke)
•  Large%ofFemetalintheEarthrequireslarge
amountsofreducedmaterial,e.g.,60-70%EHchondriTclikematerial
•  ChondriTcmaterialgoodsourceofvolaTles,
achondriTcmaterialpoorinvolaTles
CC
OC
EC
Earth
HED
5
micrograms/gram
4
3
2
1
0
-1
Water
C
Volatile
Figure 4.24
N
F
Ne
S
Cl
36þ38
Ar
Kr
Xe
804
3.29 # 106
4.21 # 105
5170
9.27 # 104
55.8
5.46
25
1.6 # 10"5
124
30
6.0 # 10"6
4.2 # 10"6
5.0 # 10"7
1.01 # 10
6.50 # 1013
5.00 # 1020
1.21 # 1020
2.40 # 1013
1.69 # 1013
2.03 # 1012
0.22
7.6 # 10"11
1.2 # 10"3
2.2 # 10"2
2.3 # 10"10
1.2 # 10"7
9.3 # 10"8
F in BSE is 19–28 mg g , see Table 6.9 of LF98
Taking atmospheric Ne as the total inventory
S in BSE is 13–1000 mg g"1, see Table 6.9 of LF98
Cl in BSE is 8–44 mg g"1, see Table 6.9 of LF98
Taking atmospheric Ar as total inventory of 36þ38Ar
Taking atmospheric Kr as the total inventory
Taking atmospheric Xe as the total inventory
a
Solar abundance per 106 Si atoms, Table 1.2 of Lodders and Fegley (2011).
Concentrations in the bulk silicate Earth (BSE) for H, C, N, F, and Cl are from Palme and O’Neill (Chapter 3.1). Sulfur is from Table 4.4 of Lodders and Fegley (2011).
c
The range of published estimates for H, C, N, F, S, and Cl are in Table 6.9 of Lodders and Fegley (1998). The solar abundance of water is calculated from the oxygen abundance
adjusted for the amount of oxygen in rock (MgO þ SiO2). Other values that are used in the calculations are the Si concentration in the BSE (21.22%), the mean molecular weight of
Earth’s atmosphere (28.97 g mol"1), total atmospheric mass (5.137 # 1018 kg), mass of the BSE (4.03 # 1024 kg), and the concentrations of Ne, Ar, Kr, and Xe in dry air (18.18 ppmv,
9340 ppmv, 1.14 ppmv, and 87 ppbv). The Ar abundance in air is corrected for 40Ar, which is 99.6% of terrestrial Ar. Calculations compare terrestrial and solar abundances of 36Ar
and 38Ar.
b
Table 2
Some volatile-bearing phases in chondrites and potential outgassed volatiles
Name
Ideal chemical formula
Chondritea
Potential volatilesb
Apatite
Calcite
Cohenite
Dolomite
Graphite
Gypsum
Halite
Insoluble organic matter
Nierite
Osbornite
Sinoite
Serpentine
Sodalite
Talc
Troilite
Ca5(PO4)3(F, Cl, Br, OH)
CaCO3
(Fe, Ni)3C
CaMg(CO3)2
C
CaSO4&2H2O
NaCl
C100H72N3O22S4.5
Si3N4
TiN
Si2N2O
(Mg, Fe)3Si2O5(OH)4
Na4Al3Si3O12Cl
(Mg, Fe)3Si4O10(OH)2
FeS
Many
C
Many
C
C
C, OC
C, OC
C, UOC
E
E, CH
E
C
C
C
Many
HF, HCl, Cl2, HBr, Br2, H2O, H2, O2
CO, CO2
CH4, CO, CO2
CO, CO2
CH4, CO, CO2
SO2, H2S, OCS, Sx
HCl, Cl2
CH4, CO, CO2, H2O, H2, N2, NH3, Sx, H2S, OCS, SO2
N2, NH3
N2, NH3
N2, NH3
H2O, H2, O2
HCl, Cl2
H2O, H2, O2
Sx, H2S, OCS, SO2
a
The abbreviations denote the following types of chondrites: C, carbonaceous chondrites; CH, CH chondrites; E, enstatite (EH, EL) chondrites; OC, ordinary (H, L, LL) chondrites; UOC,
unequilibrated ordinary chondrites, or many for phases found in many types of chondrites.
b
The nature of the potential outgassed volatiles depends on several factors including the temperature, pressure, and oxygen fugacity during outgassing. Elemental fluorine does not
form because it is too reactive. Hydrogen and oxygen are generated via equilibria of water vapor with Fe-bearing phases such as metal, magnetite, and FeO-bearing silicates.
OutgassingofchondriTcmaterial
•  Lookatafewexamplesofgaseous
atmospheresproducedbyheaTngupand
outgassingchondriTcmaterial(computer
calculaTons)
•  Showagreementofcalculatedandmeasured
oxygenfugacity(fO2)valuesformeteorites
wheremeasurementsareavailable-ordinary
chondrites(H,L,LL)
Outgassing of ordinary chondritic material
471
The chemical equilibrium oxygen fugacity of average H-chondritic material as a function of temperature compared to different solid-state oxyge
e points are intrinsic oxygen fugacities of the Guareña (H6) and Ochansk (H4) chondrites measured by Brett and Sato (1984). Also shown are th
acities of the Guareña (H6) chondrite measured by Walter and Doan (1969) and the calculated oxygen fugacities for average H4–H6 chond
and Labotka (1993). (b) The chemical equilibrium oxygen fugacity of average L-chondritic material as a function of temperature compared t
Pressure (bars)
1
0
10
CH4
20
30
a
H2
H2O
NH3
CO
C
O
2
N2
H 2S
-3
2H
6
-4
-5
-6
l
HC
C
log mole fraction
-1
-2
40 50 80
400
600
800
Temperature (K)
1000
1200
Pressure (bars)
1
0
10
20
40 50 80
CH4
H2
-1
H2O
CO
N2
C
O
2
-2
H 2S
-3
NH3
-4
6
C 2H
HC
l
log mole fraction
30
-5
-6
400
600
800
Temperature (K)
1000
1200
(1988) who modeled outgassing of a carbonaceous chondritic
planet. Schaefer and Fegley (2005) also studied outgassing
from H chondritic material depleted in Fe metal and FeS (see
Figure 8). A comparison of Figures 7 and 8 shows that a highly
reduced atmosphere is produced in both cases, even after Fe
metal and FeS have been removed, for example, by core formation on the early Earth.
and on rocky exoplanets.
The oxidation states of the silicate, steam, and ‘traditional’
volatile atmospheres on the early Earth are another unanswered question. This can be addressed by a combination of
thermochemical and photochemical models of the outgassing
of different types of chondritic and achondritic material. The
redox state of the proto-Earth during its accretion is an
Average H (falls) – no Fe metal or FeS (S = 10% total S)
from Jarosewich (1990)
Average H (falls) composition
from Jarosewich 1990
Water
atic
Lithost
H 2O
NH3
H2
2
2
N2
CO
0
N2
l
CO
log Pi (atm)
NH 3
0
HC
S
CO
H2S
HF
HF
H
2S
log Pi (atm)
CH 4
H 2O
2
CH 4
H2
Lithostatic
CO
4
4
-2
CO
2
CO
S
HC
l
-2
-4
400
600
800
1000
Temperature (K)
-4
1200
1400
Figure 7 Chemical equilibrium abundances of gases produced by
heating average H chondritic material along a terrestrial geotherm.
400
600
800
1000
Temperature (K)
1200
1400
Figure 8 Same as in Figure 7, but after removal of all Fe metal and FeS
before doing the computations.
84
Chemistry of Earth’s Earliest Atmosphere
Table 5
Major gas compositions of impact-generated atmospheres from chondritic planetesimals at 1500 K and 100 bars
Gas (vol. %)
CI
CM
CV
H
L
LL
EH
EL
H2
H2O
CH4
CO2
CO
N2
NH3
H2S
SO2
Othera
Total
4.36
69.47
2 # 10$7
19.39
3.15
0.82
5 # 10$6
2.47
0.08
0.25
99.99
2.72
73.38
2 # 10$8
18.66
1.79
0.57
2 # 10$6
2.32
0.35
0.17
99.96
0.24
17.72
8 # 10$11
70.54
2.45
0.01
8 # 10$9
0.56
7.41
1.02
99.95
48.49
18.61
0.74
3.98
26.87
0.37
0.01
0.59
1 # 10$8
0.33
99.99
42.99
17.43
0.66
5.08
32.51
0.33
0.01
0.61
1 # 10$8
0.35
99.97
42.97
23.59
0.39
5.51
26.06
0.29
9 # 10$5
0.74
3 # 10$8
0.41
99.96
43.83
16.82
0.71
4.66
31.47
1.31
0.02
0.53
1 # 10$8
0.64
99.99
14.87
5.71
0.17
9.91
67.00
1.85
5 # 10$5
0.18
1 # 10$8
0.29
99.98
a
‘Other’ includes gases of the rock-forming elements Cl, F, K, Na, P, and S.
either H2-rich (H, L, LL, EH chondritic material) or CO-rich
(EL chondritic material). As noted earlier, several geochemical
models predict that Earth formed from large amounts of ordinary and enstatite chondritic-like material, for example, up to
70% enstatite chondrite-like material and up to 21% H
chondrite-like material. The proto-Earth (and also the impactor) may have been more reducing than the present day Earth,
and thus the resulting ‘steam’ atmosphere may have been more
reducing. If this were the case, the ‘traditional’ volatiles left
after collapse of the steam atmosphere may have been more
reducing. For example, Table 5, based on Schaefer and Fegley
(2010), shows that the gases remaining after collapse of the
steam atmospheres on an ordinary or enstatite chondritic-like
early Earth may have been H2, CO, CO2, and trace water vapor.
This point will be taken up later when discussing outgassing on
H2-rich atmospheres with CO and H2O being the second and
third most abundant gases at high temperatures, and
increasing amounts of CH4 at lower temperatures. Degassing
of low iron enstatite (EL) chondritic material gave a CO-rich
atmosphere with H2, CO2, and H2O being the next most
abundant gases. Finally, degassing of CV carbonaceous
chondritic material gave a CO2-rich atmosphere with H2O
being the second most abundant gas. Their results at 1500 K,
100 bar total pressure are listed in Table 5, where the major gas
from degassing of each type of chondritic material is in
boldface.
Schaefer and Fegley (2010) did calculations over wide P, T
ranges and showed that the results in Table 5 are generally
valid at other pressures and temperatures. Water vapor, CO2,
and N remain the major H-, C-, and N-bearing gases in
OutgassingofChondriTcMaterial
•  Ordinary&enstaTtechondriTcmaterial
producesCH4-bearing&CH4-richatmospheres
•  CIandCMcarbonaceouschondriTcmaterial
producesCO2-bearing&CO2-richatmospheres
OutgassingoftheBSE
•  CO2-bearing&CO2-richatmospheres
producedbyoutgassingofthebulksilicate
Earth
•  Oneexampleonthenextslide
•  ThetransiTonfromreducingtooxidizingtook
placeearlyinEarthhistory,priorto3.9Gyr
agobasedonCrandVabundancesinancient
rocks(Delano2001)
cal Journal, 755:41 (16pp), 2012 August 10
Schaefer, Lo
0
0
H2O
O
SO2
Mg~Fe
-1
-1
-3
KO
(O
Mg
N2
1000
2000
3000
Temperature (K)
4000
2000
H) 2
3000
2
F
S iO
H
Na
2
NaCl
T iO
SO
NH 3
CO
-2
K
O~
F
H
KF
Fe
KCl
H
C
l
-2
Na
O
Na
Mg
SiO O
O
H
KO
H
O
H2
O
H
log mole fraction
log mole fraction
CO2
Na
CH 4
2
Na
+
-3
4000
Temperature (K)
as composition as a function of temperature at 100 bars for the bulk silicate Earth: (a) volatile elements H, C, N, S and (b) rock
, Ca, Al, and Ti.
f this figure is available in the online journal.)
he graph, with maximum abundances of 20 ppmv
composition material, e.g., as discussed by Lod
CO2 ratio is unity increases slightly with increasing total pressure and is ( 700 K at 100 bars total pressure. Ammonia has
qualitatively similar behavior, but at the same total pressure,
Temperature (!C)
[35]
250
, we see that at any given temessure (fugacity) corresponding
CH4 and CO2 is given by the
$1=2
1200 1500
IW
[36]
ions, PT is the total pressure of
equilibrium constant. Making
O mole fraction is unity and that
get a simple expression for the
and CO2 are equally important
1
KP
1000
-20
WM
$1=2
NN
O
2
XH
P2
2O T
¼
KP
BSE
Kilauea
Convergent plate
Divergent plate
-30
[37]
mperature at Kilauea, the oxygen
and CO2 to be equivalent. This
fO2 for the quartz–fayalite–
or ordinary chondritic material
see Figure 8 in Schaefer and
hows that the estimated fO2 is
fraction and the total pressure.
nt and total pressure of primorwn, but they are not essential to
ng of CH4 was probably insigmilar constraint can be applied
M
#
800
-10
QF
$1=2
500
Log10 fO2
PT2
300
MH
um constant is given by
"
2
PCO2 PH
Go
2O
¼
2
T
PCH4 PO2
[34]
-40
20
IM
44T log T & 24:6166T
15
10
5
104/ T (K)
Figure 6 Temperature-dependent oxygen fugacities for modern day
volcanic gases and the heated bulk silicate Earth (Schaefer et al., 2012)
are compared to oxygen fugacities of several common buffers denoted
by the following abbreviations: MH, magnetite–hematite; NNO, Ni–NiO;
he gas has a variable bulk O/S ratio that varies
ugacity decreases. The labeled arrows show the
buffers at this temperature.
occur only for the thermal dissociation of pure SO2 below 1100–
1200 K (Fig. 3b). These SO2 -rich, low-temperature conditions
were suggested for Prometheus-type plumes by McEwen and
Soderblom (1983). Therefore, hematite could be stable in lava
and pyroclastics from these volcanoes. In addition, Fe3+ -bearing
um abundances in ionian volcanic gas as a functal pressure and the f O2 of the QFM buffer. The
that is controlled by the temperature-dependent
mixing ratios <10−10 are not shown.
FIG. 10. The oxygen fugacities of terrestrial volcanic gases are plotted
as a function of vent temperature. Mineral buffer f O2 curves are shown for
comparison. The calculated f O2 values and vent temperatures for the volcanic
gases are from Symonds et al. (1994).
OriginoftheMoonandOriginofLife
fromBulkSilicateEarth
fromBulkSilicateEarth
•  LunaroxidaTonstate=thatofBSEatTmeof
Moon-formingimpact
•  SignificantlymorereducedthanBSE(~IWversus
~QFM)
•  BSEbecamemoreoxidizedatsomelaterTme
•  ExplicitlypostulatethiswasAFTERtheabioTc
originoflifeviaMiller-UreytypereacTonsina
48
ZOLOTOV AND FEGLEY
could be ultramafic mantle melts, in
temperatures inferred for a number o
Oxidizing conditions around the M
for terrestrial volcanic gases (Symond
observed in terrestrial mantle magma
natural occurrence of titanium in hem
solid solutions) lowers the f O2 for tha
pure hematite (e.g., Rumble 1976, Zol
MH buffer as an upper limit for the red
gases and their respective magmas. In
for SO3 in the Loki plume (Pearl et a
amount of SO3 frost tentatively identi
et al. 1995) do not indicate conditions
calculated for dominantly SO2 gase
reasonable buffer assemblage in silica
more oxidizing conditions than the M
leads to a predicted upper limit for
ionian volcanic gases of 3 × 10−6 to 5
1000–1400 K. This is consistent wit
for the SO3 mole fraction. The assum
significantly affect the predicted SO3
magnetite buffer would give lower S
FIG. 11. Typical oxygen fugacity ranges for terrestrial (Carmichael 1991,
Ballhaus 1993) and lunar igneous rocks (Papike et al. 1991).
compounds such as hematite and ferric sulfate might be partially
responsible for the observed color of Io’s surface (Nash and
Fanale 1977). On the other hand, the Fe/Na ratio in the plasma
torus is <0.04 (Na et al. 1998), which could indicate that Fe
Pele-type plumes. These plumes
comparison with Prometheus-type
Soderblom 1983). At the same tem
more S-rich plumes correspond to mo
mas. The proposed high tempera
(McEwen and Soderblom 1983) also
ditions in comparison with mineral ox
ues calculated for volcanic gases wi
ImplicaTonsforAtmospheric
Chemistry
•  LowerfO2leadstovolcanicoutgassingof
reducedgasessuchasH2,CH4,andNH3
•  (1)CalculaTonsatfixedfO2ofFe-FeObuffer
withBSEabundancesforvolaTles–showthis
examplenext
•  (2)CalculaTonsusingFe3+-freeBSE:MgO,SiO2,
FeO,CaO,Al2O3,Na2O,K2O,TiO2,Cr2O3,MnO,
NiO,etc.butwithoutthefew%Fe3+inupper
mantle–producesgraphiteatlowT
0
H2
CH4
-1
log mole fraction
-2
CO
H2O
CO2
-3
N2
-4
NH3
-5
-6
BSE volatiles outgassing at
Fe-FeO buffer 1 bar pressure
-7
600
800
1000
1200
Temperature (K)
1400
1600
0
BSE - no Fe
3+
- 100 bars
H2
CO
-1
log mole fraction
H2O
CO2
-2
CH4
-3
N2
-4
NH3
-5
-6
-7
400
600
800
1000
1200
Outgassing Temperature (K)
1400
1600
Silicatevaporatmosphere
•  HightemperaturesduringEarth’saccreTon
canleadtosilicatevaporatmosphere
•  Drymoltensilicatevaporatmosphere(BSE
composiTon)innextslide
•  Appliedtohotrockyexoplanetssuchas
CoRoT-7b,Kepler-10b
forsterite, fayalite, silica, and bulk silicate Earth magma are compared to
the total partial pressure of H2O þ C, S gases corresponding to the BSE
inventory of these volatiles.
Composition of saturated vapor
bulk silicate earth magma
0
Na
-1
SiO
O2
SiO
O
Log mole fraction
SiO2
-2
FeO
Fe
Si
Mg
NaO
-3
-4
-5
2000
Si
AlO
MgO
K
3000
Lower temperatures an
temperatures and low
monoxide SiO, O2, an
vapor over solid and
Nesmeyanov, 1960; K
Shornikov et al., 1998
of SiO, SiO2, which i
less abundant, are cou
4000
5000
6000
Surface temperature (K)
Figure 4 Temperature-dependent chemical equilibrium composition of
the saturated vapor in equilibrium with bulk silicate Earth magma.
Monatomic Mg an
MgO, and MgO gas i
and Srivastava, 1976a;
Monatomic Ca and O
and CaO (g) is much
1976b; Samoilova and
is more complex and o
to the elements and th
Chervonnyi et al., 19
1972; Ho and Burns, 1
Al, O, O2, AlO, Al2O
pressure in the BSE
AlO to be the major A
Al
Gas phase equilibr
rapid because of the
leads to chemical and
SteamAtmosphere
•  Impact-inducedoutgassingofH2Oandother
volaTles(e.g.,Arrheniusetal1974,Lange&
Ahrens1982,Abe&Matsui1985,1987)
•  InteresTngaspectissolubilityofSiO2and
otherrock-formingoxidesinsteam
•  Twoexamplesonnextslides
Surface Temperature (Celsius)
15
100
200
400
800 1500 3700
0
-10
log10 PΣSi (bar)
-20
-30
silica solubility in steam
red - this work
white - Plyasunov (2012)
-40
vapor pressure
of pure SiO2 (s,liq)
-50
-60
-70
35
30
25
20
15
10,000/T (K)
10
5
-1
2000
AB1965
10
1000
-2
1
100
-3
30
0.1
10
-4
3
0.01
1
-5
10
-3
10
-4
mass % SiO2 in gas
log10 Mole Fraction
300
-6
Silica Solubility Isobars
-7
1000
1500
2000
2500
3000
Surface Temperature (K)
H4SiO4pp.spw
ExoplanetObservaTons
•  ImpossibletogobackinTmeonEarth
•  Eventuallypossibletoobserveatmospheresof
rockyexoplanetsthatareindifferent
evoluTonarystagescomparabletothose
postulatedfortheearlyEarth
•  ExoPlanetaryTimeMachinetotheEarlyEarth
•  “Thus,ideasaboutEarth’searlyatmosphere,
whichcannotbeconstrainedbybiologicalor
geologicalevidence,maybeindirectlycon-
strainedinthenearfuturebyastronomical
observaTons.”Fegley&Schaefer2014TOG