26Al and Waterworlds

Steve Desch, ASU

Astrobiology Science Conference

Santa Clara, CA

April 15, 2008

Outline

The abundance of 26Al in a solar system controls volatile delivery to terrestrial planets.

• Earth's water came from asteroids.

• Heating by radioactive decay of 26Al devolatilizes asteroids.

• If our solar system had 10 x more 26Al, Earth likely would have < 0.5 oceans, "mostly" land

• If our solar system had 10 x less 26Al, Earth likely would have > 5 oceans, all ocean

Earth had the right amount of 26Al to be half land / half water

Origin of Earth's water

Earth has ~ 2.2 x 1024 g water

Earth is < 0.05 wt% H2O !

The Earth is DRY: (e.g., comets ~ 50% ice)

Mottl et al. (2007), Chemie der Erde 67, 253

Origin of Earth's waterEarth

D/H = 150 ppm

Comets?

D/H = 310 ppm

(Eberhardt et al. 1995; Bockelee-Morvan et al. 1998; Meier et al. 1998)

Solar nebula gas?

D/H = 21 ppm

(Lellouch et al. 2001)

Chondrites!

D/H ≈ 130-180 ppm

(Dauphas et al. 2000)Robert (2001)

Origin of Earth's water

Comets?

•Comet water comes with too much Ar, etc. (Swindle & Kring 1997; Owen & Bar-Nun 2000; Drake & Righter 2002)

Dynamically, < 10% of Earth's water delivered after Moon-forming impact (Levison 2001; Morbidelli et al. 2000)

Solar nebula gas?

•Adsorption of water onto grains? (Drake 2005) Can capture 1 - 3 oceans' worth. But why is the Moon so dry?

•Capture of an atmosphere possible if Earth grows to 0.1 M in <3 Myr. Invoked to explain He, Ne isotopic ratios (Harper &

Jacobsen 1996) Does enough H2O "ingas" into magma ocean?

Hale-Bopp (John Gleason)

Origin of Earth's water

Chondrites!

•Carbonaceous chondrites (CCs) up to 10 wt% water in hydrated silicates [clays] (Kerridge 1985)

•Ordinary chondrites (OCs) < 0.1 wt% H2O (Robert et al. 1987)

•Volatile loss during Earth-Theia impact reduces H2O content by factor > 3 (Melosh 2003) Earth must have accreted some CC.

•Geochemistry suggests Earth ~ 85% OC-like, 15% CC-like (Ringwood 1979; but see Drake & Righter 2002)

•Earth probably acquired its oceans during last stages of accretion, by capturing planetary embryo(s) ~ 0.1 M

• There's less CC material in Earth because it's not from 1 AU.

(Charnoz et al 2001, from Gradie and Tedesco 1982)

S asteroids: anhydrous silicates; source of ordinary chondrites; < 0.1 wt% water

C asteroids: 2/3 have hydrated silicates; source of carbonaceous chondrites; 10-15 wt% water

D and P asteroids: associated with anhydrous IDPs, but are probably 50 wt% water ice; never heated above 0 ºC

Water-rich

2.7 AU

Source Locations of Water-Rich Chondrites

Raymond et al. (2004)

e

a

•Asteroids inside 2.7 AU are generally dry, those outside 2.7 AU "wet"

•1 embryo 0.06 M, containing 6 x 10-3 M of water, impacting the Earth, would leave up to 2 x 10-3 M of water, or 4 x Earth's actual content.

•Earth quite possibly acquired its water from a single water-rich embryo beyond 2.7 AU (Morbidelli et al. 2000;

Chambers 2001; Raymond et al. 2004, 2006).

•But why are all the water-bearing planetary embryos stuck beyond 2.7 AU, anyway?

•Not a snow line: Planetesimals assembled during solar nebula (gas) stage: T(r ) = 150 (r / AU)-0.43 K (Chiang &

Goldreich 1997). "Snow line" at < 1 AU! (but see Lunine 2006)

Origin of Earth's water

achondrites

S / OCs C / CCs

D / P

McSween et al (2005), after Grimm & McSween (1993); Ghosh et al. (2001)

Heliocentric Zoning of Asteroid Belt due to 26Al!

Time to grow to 100 km diameter =

0.13 (r / AU)3 Myr

(Wetherill 1980)

2.6 Myr

achondrites

S / OCs C / CCs

D / P

2.6 Myr

(26Al / 27Al) = 2 x 10-5 4 x 10-6 5 x 10-7

(26Al/27Al)0 = 5 x 10-5

2.7 AU 4.6 AU

1 2 3 4 5 6 (7)CI

CMCR

CVCO

CK

CH

CB

H, L, LL

EH, EL

CC

OCEC

Petrologic Type

Aqueous Alteration Thermal Metamorphism > 400 °C

Van Schmus & Wood (1967) Classification Scheme, after Weisberg et al. (2006)

achondrites

S / OCs C / CCs

D / P

(26Al / 27Al) = 2 x 10-5 4 x 10-6 5 x 10-7

What if (26Al/27Al)0 = 5 x 10-4

3.4 AU 4.9 AU

5.0 Myr

achondrites

S / OCs C / CCs

D / P

(26Al / 27Al) = 2 x 10-5 4 x 10-6 5 x 10-7

What if (26Al/27Al)0 = 5 x 10-6

0.9 AU 4.3 AU

0.2 Myr

Raymond et al. (2006)

26Al/27Al = 5 x 10-4

26Al/27Al = 5 x 10-5

26Al/27Al = 5 x 10-6

< 0.5 oceans

> 5 oceans

= 1 ocean

Ability to accrete water-bearing planetesimals falls off rapidly as water-rich zone pushed outward (Morbidelli et al. 2000; Chambers 2001; Raymond et al. 2004, 2005, 2006)

0.5 oceans, 50% water

1 ocean, 75% water

5 oceans = 19 km deep, 100% water

Conclusions

The abundance of 26Al in a solar system controls volatile delivery to terrestrial planets.

Solar systems born with 10 x more 26Al have earths with < 0.5 oceans, "mostly" land

If our solar systems born with 10 x less 26Al have earths with > 5 oceans, all ocean

Our Solar System had the right amount of 26Al for Earth to be ~ half land / half water: IS THIS SIGNIFICANT?

Solar Systems without 26Al probably harbor Water Worlds

It's extremely important to quantify the odds that solar systems are born near massive stars, and quantify the likely amounts of 26Al they receive.