Lunar and Planetary Science XXXV (2004) 1987.pdf

MAKING WATER WORLDS: THE ROLE OF AL 26. S. J. Desch1 and L. A. Leshin2,3, 1 Dept. of Physics and Astronomy, 2 Dept. of Geological Sciences, 3 Center for Meteorite Studies, Arizona State University, Tempe, AZ 85287 ([email protected], [email protected]).

Planets such as Earth are the end products of region is a natural byproduct of planetary accretion complex formation histories. These formation his- that can account entirely for the Earth’s water [2]. tories render some planets habitable, especially if Spectrophotometry reveals the presence of hy- those planets inherit life enablers such as liquid wa- drated silicate absorption features at 3 µm in aster- ter. We seek to explore important astronomical and oids from C class asteroids, which typically orbit at planetary factors that affect the likelihood of form- 2.5 - 4 AU [5]. Their spectra are closely related to ing “waterworlds” like the Earth in other solar sys- those of carbonaceous chondrites, which show evi- tems, so that we may better assess the probability dence for only low heating, generally < 500 K [6], that other nominally Earthlike planets may harbor and contain water up to 10 % by weight [7]. Con- oceans capable of supporting life. In this abstract, versely, S class asteroids, which orbit from about 2 we consider the effect of the astronomical setting to 3 AU, have spectra that closely resemble those of of a forming solar system, and specifically its ef- ordinary chondrites, which show evidence for heat- fect on the abundance of 26Al, which, we argue, ing to temperatures in the range 500 - 1400 K, is a significant factor controlling the water content and which contain less than 0.1 % water by weight of Earthlike planets. We attribute the source of [8]. All suspected achondrite (differentiated) par- 26 the short-lived (t1/2 = 0.7 Myr) radionuclide Al ent bodies lie inside of 2.7 AU [5], including the in our solar system and others to a nearby super- HED parent body 4 Vesta, at 2.4 AU. Generally nova, so that a solar system’s initial abundance of speaking, relatively unheated, water-rich asteroids 26Al is set by the essentially random distance to the dominate beyond 2.7 AU, while heated, dry aster- supernova explosion. Our solar system very plausi- oids dominate inside of 2.7 AU. The models of [2] bly could have received much less 26Al than it did, indicate that roughly 15 % of the Earth’s mass, and which would have led to a wetter Earth. ≈ 90% of its water, derives from planetesimals that Earth is often perceived as especially wet and resided beyond 2.7 AU. It is tempting to associate life-promoting. In fact, Earth’s water makes up a the presence of water with a “snow line” in the so- relatively small fraction of the mass of the planet, lar nebula inside of which temperatures exceed 170 −4 about 2.8 × 10 ME of water on the surface, and K and ice sublimates, which in the standard solar −4 0.8 − 8 × 10 ME in the rest of the planet [1]. We nebula model lies at about 2.7 AU. This simple ex- −4 adopt 5 × 10 ME of water as a reasonable esti- planation does not, however, account for carbona- mate for the “bulk Earth”. In recent years, the case ceous chondrites being less water-rich than comets has been increasingly made that the Earth’s water or icy outer planet moons, and it ignores the ob- came from planetesimals in the asteroid belt region viously relevant fact that carbonaceous chondrites [2]. Comets cannot contribute more than 10 % of are generally not heated, but ordinary chondrites the Earth’s water, due to dynamical constraints [2] more often than not are. The clear correlation of in- and the high D/H ratio of comets [3]. While the late ternal heating with heliocentric distance implicates accretion of comets, especially from beyond 20 AU, thermal metamorphism as the determinant of water sets a minimum water content of the Earth as high content. as 10 % of its present value, the bulk of the Earth’s Models of asteroid heating [9] indicate that for water must have come from the asteroid belt region. asteroids larger than about 100 km in diameter Specifically, the water was present as hydrated sil- (i.e., planetary embryos), will either differentiate icates or ices in either asteroidal bodies (∼ 10 km completely or be severely heated (e.g., to 500◦ C), in size) or in planetary embryos (many hundreds of and presumably devolatilize and lose their water, if km in size). The accretion of smaller asteroids may the abundance of radioactive 26Al is high enough: have been an efficient process for delivering water 26Al/27Al > 6.5×10−6. These bodies form the very to the Earth during the first few Myr of the solar dry parent bodies of achondrites and ordinary chon- nebula, while the nebular gas was still dense [4], but drites. The same models show that even a slightly devolatilization of Earth during later impacts could lower 26Al abundance leads to heating no greater have removed this water [2]. On the other hand, ac- than a few hundred ◦ C. These bodies form the wet cretion of planetary embryos from the asteroid belt parent bodies of the carbonaceous chondrites. Our Lunar and Planetary Science XXXV (2004) 1987.pdf

26 27 −5 solar system began with Al/ Al ≈ 5 × 10 , as in a few Myr. A 25M supernova ejects about −5 26 measured from calcium-aluminum-rich inclusions or 7 × 10 M of Al [15]. If this ejecta intercepts CAIs [10], so any asteroids that accreted within 2.1 a solar-composition disk with surface density Σ at Myr of CAI formation would be dry, and bodies a distance r, the resulting 26Al/27Al ratio = (1.9 × that formed later could retain water. As accretion 10−5)(r/1017 cm)−2 (Σ/1000 g cm−2)−1. The sur- timescales scale as the cube of heliocentric distance face density at 3 AU in a minimum mass nebula is [11], early accretion is favored closer to the Sun. Ac- 300 g cm−2 [11], so the solar system 26Al/27Al ratio cretion of planetesimals in just 2.1 Myr at a distance is reproduced if it was 1 × 1017 cm from the super- of 2.7 AU seems reasonable: the initial 26Al/27Al nova. in the eucrite Piplia Kalan has been constrained at The parameters that can reproduce the initial ≈ 7 × 10−7 [12], implying that 4 Vesta (at 2.4 AU) 26Al/ 27Al ratio of the solar system (distance r < differentiated, and its crust resolidified, all within 5 1017 cm from the supernova, and surface density at Myr. 3 AU of Σ ∼ 300 g cm−2) happen to be represen- What if the solar system’s initial 26Al abun- tative of about 25 % of the proplyds in the Orion dance was a factor of 10 higher? Planetary em- Nebula [16]. However, the majority of the proplyds, bryos that accreted within the first 2.1+t1/e ln 10 = and the vast majority of protoplanetary disks in 4.5 Myr of the CAIs would then be dry. Given the the Orion Nebula that are not photoevaporating, cubic dependence on heliocentric distance of the ac- should lie at distances > 3 × 1017 cm from θ1Ori C. cretion timescale, we estimate that the boundary After θ1Ori C supernovas, these newly forming so- between the ordinary chondrites and carbonaceous lar systems should form planets with insufficient chondrites would be pushed out to 2.7(4.5/2.1)1/3 = 26Al to melt the ice in their asteroids. Any Earths 3.5 AU. As the Earth is very unlikely to accrete that form in these solar systems should be con- planetary embryos from beyond 3.5 AU [2], the siderably wetter (by an order of magnitude) than Earth would essentially gain no water from the plan- our own Earth. Further analysis of the injection etary embryos it accreted from. The Earth’s water of short-lived radionuclides into disks in the Orion would consist only of what would be accreted from Nebula is clearly warranted to better quantify these comets, about an order of magnitude less than the trends. Additionally, it is known that a fraction Earth’s current water content. ∼ 15% of solar systems form in regions without What if the solar system’s initial 26Al abun- massive stars, such as the Taurus-Auriga molecu- dance was a factor of 10 lower? All planetary em- lar clouds. These systems would acquire no 26Al bryos accreted at any time in the solar system would at their births, and any Earths to form in these re- then by water-rich, since they would not have enough gions would almost certainly be significantly more 26Al to be significantly heated. 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