3.8m 新望遠鏡を用いた
惑星科学ネタあれこれ!

京都大学 宇宙物理学教室
佐々木貴教

主要観測テーマ（太陽系外惑星探査 -地球型惑星の直接撮像に向けて）
太陽系外縁部に潜む暗い惑星・衛星・リング・準惑星・小天体の発見的・統計的観測
太陽系内天体の継続的観測によるアストロバイオロジー
太陽系内に Habitable World を探る
地球型惑星形成の直接的証拠「ジャイアントインパクト」
新しい地球形成シナリオのサポート

太陽系の小天体・準惑星

太陽系形成標準理論（林モデル）
 







巨大氷惑星形成©Newton Press小惑星

メインベルト小惑星
火星と木星の間に存在トロヤ群小惑星
木星の軌道上に存在
地球に降ってくる隕石の母天体だと考えられている

太陽系外縁天体（エッジワース・カイパーベルト天体）

海王星の外側に散在冥王星もその一部短周期彗星（P＜200年）の巣だと考えられている

惑星形成過程の履歴を観測
太陽系小天体：始原的な天体であるが、軌道進化・衝突進化・物質進化を経験している
天体のサイズ・形状・自転周期・自転軸方向→ 天体の密度・空隙率・強度・反射率・熱慣性→ 衝突進化・物質進化の情報が得られる
大量の観測により、その進化を統計的に議論できる
日本は岩石・氷小天体の熱史・衝突破壊現象・宇宙風化に関する理論的・実験的研究が盛んであるはやぶさが明らかにしたイトカワの姿

太陽系の惑星・衛星

アストロバイオロジー
固体表層の構造・組成・温度場
イオの火山・エンセラダスの噴水・火星の極冠等
大気層の構造・組成・温度場
金星・木星・タイタンの雲層構造の発展・崩壊
大気層における微弱光とその機構
大気光・雷などの微弱発光現象
太陽系内における Habitable World の可能性を検証

イオには活発な火山活動が存在

木星や他のガリレオ衛星からの潮汐力がエネルギー源か？

エウロパの地下に広がる海

内部に液体(状)の水を持つ

赤っぽい地表とひび割れ→ 地下水が吹き出た跡？

衛星タイタン表面には石や氷塊液体メタンの湖が存在

衛星エンケラドス

氷粒と水蒸気からなる間欠泉“Tiger Stripes”

液体の水・有機炭素・窒素が存在惑星大気の構造と変動

大気散逸

様々なタイプの大気循環
木星の大赤斑いかにして “地球” をつくるか「ハビタブルゾーン」	

惑星表面に液体の水が存在できる領域

地球特有の事項
610

U. Mann et al. / Geochimica et Cosmochimica Acta 84 (2012) 593–613

on the parameters a, b and c in Eq. (20) as listed in Table 6.
In Fig. 6b–e the same relationships are shown as in Fig. 6a,except that log D uncertainty envelopes are included foreach element. In each figure, results for Ru, Rh, Re andPt respectively are plotted together with extrapolated partition coeﬃcients for Ir (the most siderophile element) and Pd(the least siderophile element). Including uncertainties, thepressure range where the Pd mantle concentration could
have been produced by metal–silicate equilibration is 30–
45 GPa. Total uncertainties are largest for Ir, because its
concentration in the silicate melt was below the detection
limit in several samples and fewer data were available forregression (Table 6). But even at the lower limit of uncertainty, the Ir mantle concentration could only have beenachieved at pressures of P53 GPa, based on the (unlikely)
assumption of oxidising redox conditions throughout accretion. Similarly, the Pt and Rh partition coeﬃcients could
core
only have the required D mantle
values at pressures of 50–80 GPa and 50–100 GPa respectively if the oxygen fugacity
remained at a constant value of IW -2. The latter value isinconsistent with the observed depletions of moderately siderophile elements (especially V and Cr) which require thatcore formation initially occurred under highly-reducing
conditions, such as IW -4 (Wade and Wood, 2005; Rubieet al., 2011). In the case of Ru and Re, at IW -2 even conditions of 75 and 85 GPa respectively could not have decreased their partition coeﬃcients suﬃciently (Figs. 6b, d).
At lower oxygen fugacities, the trends for all elements inFig. 6 would shift to even higher pressures such that at
60 GPa the partition coeﬃcients of Ru, Rh, Re, Ir and Pt
still would have been P103. The resulting mantle concentrations (normalized to CI chondrite) of Re, Ir, Ru, Pt,
Rh and Pd based on partition coeﬃcients calculated for
conditions of the peridotite liquidus at 5, 20, 40 and
60 GPa and IW -3 respectively, are plotted in Fig. 7. This

円盤ガスによる円軌道化？!
残存微惑星による円軌道化？

地球マントル高!
圧!
実!
験felsic melts, as indicated by multiple geochemical signatures6,8,9.
Oxygen isotopes, for example, demonstrate that the vast majority ofterrestrial Hadean zircons fall outside the zircon mantle equilibrium
field (d18OVSMOW 5 5.3 6 0.3 % (1s))5,20, which is a strong argument
for the input of supracrustal material into the source melts. Moreover,most zircon crystallization temperatures are consistent with formationin water-saturated or near-water-saturated melts7,8.
Conclusions about the oxidation state of the Hadean mantle basedon zircons of crustal affinity are admittedly indirect. However, a subsetof the Hadean zircon population does appear to be mantle-derived.
(We note that direct partial melts of the mantle cannot crystallize
zircon, but the fractionation products (residuals) of such melts doeventually saturate in zircon21.) Five of the Hadean crystals haved18O values that fall within the mantle equilibrium field, which has
been shown to be constant (60.2%) for the past 4.4 Gyr (ref. 20). If
these d18O values are primary, then these zircons crystallized fromuncontaminated, mantle-derived melts that did not interact with thehydrosphere. The five zircons in the d18O mantle equilibrium field—
including two crystals with U–Pb ages approaching 4,400 Myr—give acalculated oxygen fugacity of FMQ 1 1.4 (62) (Fig. 3). As a comparison, zircons crystallized from residual liquids of present-day
mantle-derived mid-ocean ridge basalts22–24 yield calculated fO2 valuesof FMQ 1 0.4 (62.6), which agrees with estimates of the oxidation
state of the upper mantle10. This also implies that the redox conditions
in residual, zircon-saturated liquids of present-day mid-ocean ridge
systems did not undergo significant changes in fO2 (relative to thebuffer curve) along the liquid line of descent from basalt. This is also
consistent with observations from lunar zircons. Finally, we note that
Martian basalts (for example, shergottite meteorite Dar al Gani 476)
exhibiting little or no interaction with a crustal component are only
about 1 log unit higher than the estimated fO2 of the upper mantle25.
Furthermore, calculated oxygen fugacities from the same meteorite are
all within a log unit of the average (FMQ 2 2.5), even though phases
cover a crystallization range of .300 uC.
As in all studies in which mantle fO2 is estimated from mantlederived glasses or minerals3,9, it is important to emphasize that thecalculated fO2 may not directly reflect that of the mantle source region.
We note, however, that mantle-derived Hadean zircons would have had
to crystallize from residual melts that underwent ,4 log units of change
in fO2 to be reconcilable with mantle source regions in equilibriumwith the IW buffer. At present and in the Archaean, relative fO2changes along magmatic liqui
order of ,1 log unit4,10,26,27. I
similarly constrained, then our
its present-day oxidation statepretation is consistent with coreand chemical isolation from theformation28.
If our deductions regarding t
are correct, then the speciation otime would have been dominat
An atmosphere of this compos
ance of sugars and especially am
reduced atmosphere is required
occurred exceptionally early on
outgassing of H2 coupled withatmosphere out of equilibrium‘late veneer’ may have served asalternative scenarios notwithsta
into the oxidation state of terrest
the data presented here open uption state of zircon-bearing rockcalibration will be especially val
eons, for which zircon is often
information.

水 0.023wt.%

Fig. 7. Highly siderophile element concentrations, normalised to
CI chondrite (Fischer-Gödde et al., 2010), predicted for theprimitive upper mantle (PUM, data from Becker et al., 2006; Rhfrom Fischer-Gödde et al., 2011) as a result of metal–silicate
parititoning at various P–T conditions in a magma ocean. Partitioncoeﬃcients were calculated at the pressures indicated and at the therespective peridotite liquidus temperatures (Herzberg and Zhang,
1996; Zerr et al., 1998) and at an oxygen fugacity of IW -3, using
Eqs. (19) and (20) together with the regression coeﬃcients from
6
Crust-derived
Table 6. From these partition coeﬃcients the magma ocean
Zircon in equil. with mantle (δ O)concentrations were calculated employing a mass balance equation
4and assuming a chondritic bulk composition for the Proto-Earth2
and a constant molar proportion of 16.6% for the core (all data0employed are based on molar proportions).

地球型惑星のマントル中に強親鉄性元素が過剰に存在!

METHODS SUMMARY

Starting materials were contained in
was buffered by placing ruthenium a
direct physical contact with the expe
chamber separated by a Pt or silve
MMO). Quenched glasses were ana
one measurement made at the top
homogeneity. Synthesized zircons w
the glass was dissolved with hydroflorder to obtain more accurate measu
For the inset plot of Fig. 1, Prxrepresents a normalized value, eithnatural zircons6,19,23–25,30 are calculate
text: (Ce/Ce*)CHUR 5 CeCHUR/(LaCH
represents normalization to the chonconcentration. Ce anomalies are plo
are $94% concordant, and with R
igneous chemistry5,6,17. Zircon crysta
assuming unity Ti-activity7; sub-uni
systematic shifts of all data to higher
the standard deviation of each pop
polymict breccias 14304, 14305 andfO2 of IW at 700 uC, for direct comp
distribution plot of Fig. 2. For the ifrom zircon Ce anomalies are plottebasaltic igneous rocks (IW 2 0.6)18,
and average oxygen fugacities returne
pairs (NNO)27.

コア形成後に強親鉄性元素を含む物質が少量降った（レイトベニア説）

ΔFMQ

18

Archaean and presentday upper mantle

Full Methods and any associated ref
the paper at www.nature.com/natur–2

Received 22 March; accepted 6 Octo

accretion of small amounts (e.g. for the Earth 0.09–0.5%of its mass) of some type of primitive meteoritic material
after core formation had ceased. The late veneer is addiFigure 3 | Oxygen fugacities of Hadean melts plotted against zircon
tionally discussed as a potential source of planetary water,
crystallization age. Errors for individual points are based on the standard
deviation of the entire data set (n 5 20; 1s 5 6 2.3 log units). Zircons withsignatures are within error of the estimated oxidation state of the presentcarbon
many
2 and other2highly volatile elements. Although mantle. On average, oxygen fugacities are lower for
day and Archaean upper mantle
younger zircons, though this trend is not robust given the present data set.
details of the late veneer are poorly understood, it provides

硫黄同位体異常 → 2.5Ga まで低O , CO 濃度–4–6
3,800IW

3,900

4,000

4,100

4,200207Pb/206Pb* age (Myr)

4,300

4,400

1.
2.
3.
4.
5.

Kasting, J. F. Earth’s early atmosp
Canil, D. Vanadium partitioning anmagmas. Nature 389, 842–845 (1Delano, J. W. Redox history of the E
prebiotic molecules. Orig. Life Evo
Burgisser, A. & Scaillet, B. Redox esurface. Nature 445, 194–197 (20Cavosie, A. J., Valley, J. W., Wilde, S
Magmatic d18O in 4400–3900 Ma
recycling of crust in the early Arche
Cavosie, A. J., Valley, J. W., Wilde, S
Correlated microanalysis of zircon
constraints on the igneous origin
Cosmochim. Acta 69, 637–648 (2マントルは酸化的@4.35Ga
3,10,26

6.

1 DECEMB

巨大天体衝突破片（GIF）
Giant Impact Fragments

巨大天体衝突



原始惑星




7
8
10 $10 

微惑星

106$107

地球型惑星形成理論




 

地球型惑星（スーパーアース・地球たち）

/*

sta;4<>7:5A:

rt&E5A:
input&

 // ,B#+@%@@C
01-2. / ,B@#+@@& C$)'3"
8A6!

4=?9GFD5A: / ,B#+@%@@C,B& @C!
/ / !
,B@& @#+C

input&/(3" 10  #  

GIFによる円軌道化10 #e0.01
#D;@3A:GBE@C0!
!'0!

<:%!9!
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>?F=/ !'264&87!
#$ !01!9 )5!
#$+/0<:*.*, 07!
#$D;@3A:!

原始海洋 ＋ GIF → 水素大気発生
金属鉄 Fe と原始海洋が反応して水素大気を生成↓原始海洋が大規模に失われる!
大量の水素大気をまとった原始地球の誕生
岩石金属鉄

H2大気の形成
H2O+ Fe → FeO + H2
原始海洋マントルへ

GIF降下後の大気進化
原始海洋と GIF が反応した後の水素の分配：f = H2O/(H2+H2O)!
→ GIF 降下後の原始地球における海洋質量が決まる!
!

H2 のハイドロダイナミックエスケープ：energy-limited escape!
（太陽 EUV のフラックスで散逸率が決まる [Zahnle et al. 1988]）

H2大気太陽EUV

2H2 + CO2 → H2O + CH2O
CO2ガス

海洋質量と水素大気保持期間
1.0A
0.9byr1=tionnetr
H 2 re2 byr
3 by

0.7
ocean vo

0.6
0.5
0.4

2lume =1

f = H2O / (H2+H2O)

0.8

0.5

0.3

A

0.20.10.0B

0102030

Initial ocean volume [x present]40

O2of FMQ 1 0.4 (62.6), which agrees with estimates of the oxidation
state of the upper mantle10. This also implies that the redox conditions
in residual, zircon-saturated liquids of present-day mid-ocean ridge
systems did not undergo significant changes in fO2 (relative to thebuffer curve) along the liquid line of descent from basalt. This is also
consistent with observations from lunar zircons. Finally, we note that
Martian basalts (for example, shergottite meteorite Dar al Gani 476)
exhibiting little or no interaction with a crustal component are only
about 1 log unit higher than the estimated fO2 of the upper mantle25.
Furthermore, calculated oxygen fugacities from the same meteorite are
all within a log unit of the average (FMQ 2 2.5), even though phases
cover a crystallization range of .300 uC.
As in all studies in which mantle fO2 is estimated from mantlederived glasses or minerals3,9, it is important to emphasize that thecalculated fO2 may not directly reflect that of the mantle source region.
We note, however, that mantle-derived Hadean zircons would have had
to crystallize from residual melts that underwent ,4 log units of change
in fO2 to be reconcilable with mantle source regions in equilibriumwith the IW buffer. At present and in the Archaean, relative fO2

地球は還元的大気を長期間保持
6

Crust-derived
Zircon in equil. with mantle (δ18O)

4ΔFMQ

2Archaean and present-0 day upper mantle

eons, for which
information.

METHODS SU

Starting materials w
was buffered by pla
direct physical contchamber separated
MMO). Quenched
one measurement
homogeneity. Synt
the glass was dissol
order to obtain moFor the inset plrepresents a norma
natural zircons6,19,23
text: (Ce/Ce*)CHUR
represents normaliz
concentration. Ce aare $94% concord
igneous chemistry5,assuming unity Tisystematic shifts ofthe standard devia
polymict breccias 1fO2 of IW at 700 uC
distribution plot of
from zircon Ce ano
basaltic igneous roc
and average oxygen
pairs (NNO)27.

Full Methods and athe paper at www.n–2

Received 22 March–4

–6
3,800

1.
2.

IW

3,900

4,000

4,100

4,200

4,300

4,400

207Pb/206Pb* age (Myr)Figure 3 | Oxygen fugacities of Hadean melts plotted against zircon
crystallization age. Errors for individual points are based on the standard
deviation of the entire data set (n 5 20; 1s 5 6 2.3 log units). Zircons withmantle signatures are within error of the estimated oxidation state of the presentday and Archaean upper mantle3,10,26. On average, oxygen fugacities are lower for
younger zircons, though this trend is not robust given the present data set.

3.
4.
5.

Kasting, J. F. Ear
Canil, D. Vanadi
magmas. Nature
Delano, J. W. Red
prebiotic molec
Burgisser, A. & S
surface. Nature
Cavosie, A. J., Va
Magmatic d18Orecycling of crus
Cavosie, A. J., Va
Correlated micr
constraints on t
Cosmochim. Act

硫黄同位体異常 → 2.5Ga まで低O2, CO2濃度

マントルはFMQ@4.35Ga[Farquhar et al., 2000][Trail et al., 2011]

6.

©2011 Macmillan Publishers Limited. All rights res

これまで：酸化的な原始地球マントル + 大気とマントルは平衡状態!
→ 2.5Ga まで大気を低O2, CO2濃度にしておくことは困難
本研究：大量の H2 大気が系を支配（大気とマントルは非平衡）!
→ 長期間 H2 大気持続, 低O2, CO2濃度を維持

いかにして “地球” をつくるか
・巨大天体衝突過程で大量の破片（GIF）が形成!
!

・惑星の円軌道化 & レイトベニアを説明可能!
!

・原始海洋 + GIF → 大量の水素大気の発生!
!

・初期海洋質量が適度に減少（→ 1海洋）!
!

・酸化的なマントルと還元的な大気が長期間共存

GIFの観測可能性

赤外線の超過としてGIF円盤を観測できるのでは？
地球型惑星の形成現場を観測できる

赤外超過（24μm)

Fobs/Fstar

100

Spitzer宇宙望遠鏡

101107108

時間or星の年齢[年]109赤外超過（24μm)

Fobs/Fstar

100

Spitzer宇宙望遠鏡

101107108

時間or星の年齢[年]109赤外超過（24μm)

Fobs/Fstar

100

Spitzer宇宙望遠鏡

101107108

時間or星の年齢[年]109