The Ol Doinyo Lengai Volcano, Tanzania, As an Analogue for Carbon Planets

Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3070.pdf

THE OL DOINYO LENGAI VOLCANO, TANZANIA, AS AN ANALOGUE FOR CARBON PLANETS. J. Radebaugh1 , R. Barnes2, and J. Keith1. 1Department of Geological Sciences, Brigham Young University, Provo, UT 84602, [email protected]. 2The University of Washington Department of Astronomy and Physics, Seattle, WA.

Introduction: Recent stellar observations and mod- The scenario of carbon planets orbiting M dwarf els of exoplanet compositions have revealed the possi- stars is particularly intriguing because of the possibility bility of carbon-rich nebular environments that could of an oxygen-rich atmosphere. The planet may spend lead to the creation of “carbon planets” (e.g., [1, 2]). millions or even billions of years closer to the star than Such objects would have high amounts of carbon in the HZ due to the slow luminosity decline of the star their interiors and on their surfaces, similar to the graph- during the pre-main sequence (e.g. [10]). During this pe- ite observed on Mercury, but very different from the riod, the planet is in a runaway greenhouse, and water vast majority of Earth’s surface. can reach the stratosphere and be photolyzed by high- The Ol Doinyo Lengai (ODL) volcano of Tanzania energy radiation. The liberated hydrogen may then es- is currently erupting carbon-rich lavas, or carbonatites, cape while the heavier oxygen atoms remain. This process can result in large, oxygen-rich atmospheres [11]. in the summit crater (Fig. 1). These lavas are unique on It is unknown how the oxygen interacts with carbon-rich Earth, but may be analogous to those that would erupt surfaces, so the longevity of a carbon planet oxygen at- onto the surface of a carbon planet, making ODL to our mosphere, the resultant surface features such as albedo, knowledge the first field analogue of an exoplanet. De- and the final composition of the atmosphere are also un- tailed studies of the lava flow textures, landscape, lava known. Studies of how the carbonatite lava flows of colors and albedos, and how these change with atmos- ODL are impacted by Earth’s oxygen-rich atmosphere pheric composition, would reveal characteristics ex- would yield important information for carbon planetary pected when observing carbon planets in various stages surfaces. of internal and atmospheric evolution. On the other hand, habitable planets orbiting G dwarfs may experience a similar atmospheric evolution as Earth, i.e. the early atmosphere may have been reduc- ing, like Earth’s Archaean atmosphere. Studies of how the carbon-rich lava flows of ODL would interact with an Archaean-like atmosphere could provide critical in- sight into how Archean chemistries led to conditions right for life. Carbon-rich lavas of ODL: The Ol Doinyo Lengai volcano of Tanzania is in the East African Rift Zone, where the crust is thin and heat flux is high. Quiet, fluid eruptions of carbonatites emerge from discrete centers on the generally flat caldera floor and gradually fill up the crater [12]. The carbonatite lavas are thought to be produced from melt segregation, either from Fig. 1. The caldera floor of ODL in 2001, with active strongly peralkaline nephelenite magmas through ex- (black), recently cooled (white) and older (brown) treme fractionation [13], or from a high-silica, low FeO, flows. Lavas are fluid, but some steep landforms occur. high K/Na source [14], or from more typical silicate fractional crystallization [15]. These carbonatite lava Carbon Planets: The possibility of carbon planets production mechanisms, along with direct, carbon-rich was first raised by Kuchner & Seager [3], who argued source melting, could all exist in carbon-rich planetary that a C/O ratio in excess of 0.98 could result in a con- interiors. densation sequence that favored the formation of C-rich The ODL lavas are dominantly composed of Na2O, planetesimals (see also [4]). Detailed cosmochemical K2O, CaO and CO2 with minor Ba, Sr, Cl, F, P, S [16]. and N-body simulations by Bond et al. [2, 5] found that They have similar flow morphologies as basalts and when C/O > 0.8 in the disk, then some planets may form tend to also erupt from fissures and cones (Fig. 1), but with large carbon abundances, including cases in which they are cooler, with a maximum eruption temperature the primordial carbon abundance of a planet in the hab- of 593° C [17], and are an order of magnitude less vis- itable zone (HZ) was larger than 75%! While these re- cous than basalt (0.3-120 Pa s, with gas-rich lavas hav- sults suggest that some planets may be extremely car- ing higher viscosity; [18]). The lavas are highly varied bon-rich, significant ambiguity remains (see, e.g. [6, 7, in color, erupting as black and changing within 24 hours 8, 9]). Nonetheless, the existence of carbon planets re- (not precisely measured) to white, presumably as the mains a viable possibility. lava is exposed to atmospheric oxygen [12] (Fig. 2). Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3070.pdf

More detailed field observations of how quickly this oxygen and an overabundance of CO2 and creates a hab- color change occurs, and if lavas protected from the at- itable environment. Foley & Smye [24] derived a model mosphere remain black (or if they devitrify and form for the geochemical evolution of such a “hot spot microlites, similar to obsidian), would reveal the level planet” in which water and CO2 are outgassed and then of atmospheric control on the carbonatite lava albedo. reincorporated into the mantle. Should carbon-rich planets be unlikely or unable to transport internal energy via plate tectonics or the heat pipe mode, gases may build in the atmosphere and ultimately trigger a runaway greenhouse that removes all water from the surface, rendering it uninhabitable. This is the case on Venus, where neither plate tectonics nor the heat pipe mechanism appear to have been operating for billions of years, and CO2 has been expelled to the atmosphere rather than recycled. Habitable Carbon Planets: The surface environment of a carbon planet depends on the atmospheric composition, surface composition, and stellar radiation. Bond et al. [2] suggest that carbon planets will be dark, which is also seen in the color of newly erupted carbon- Fig. 2. Carbonatite lava flow in ODL crater. Liquid lava atites at ODL (Fig. 2). Prior to the rise of O2 on Earth, is red, recently solidified lava is black. ODL-type lava flows may have in fact remained dark. On the other hand, the surfaces of oxygen-rich carbon Carbon Planet Tectonism and Heat Release: The planets may have whiter (high albedo) surfaces, as the atmospheric evolution of a carbon planet depends criti- initially black lavas gradually change to white (Fig. 2). cally on geochemical processes that are intimately tied An analysis of carbonatite’s chemical reactions and to the tectonic expression. On Earth, plate tectonics cy- color changes with different atmospheres could help us cles carbon dioxide into and out of the atmosphere to predict and interpret planetary albedo, possibly related maintain a quasi-stable mean surface temperature [19]. to oxygen levels. Crucially, free oxygen may prevent Plate tectonics appears to require a mobile mantle and the development of large biomolecules because it is so decoupling of the lithosphere by an asthenosphere. reactive. Therefore, it is critical to measure the rate at Strictly carbon-based lavas, such as the carbonatites of which carbon-rich surfaces can absorb oxygen, relative ODL, have low melting temperatures and viscosities, to the rate at which it is produced by water photolysis, and thus the solid form found in the deep mantle may to determine if habitable conditions can exist on carbon readily convect. If conditions are right for an astheno- planets. sphere, perhaps the lithosphere can move on top of this Exoplanet in our backyard: The ODL volcano is a layer and subduct. rare and unique opportunity to study potential materials In carbon planets with significant Si, SiC may exist and processes on carbon planets. Analyses of the lava as a dominant mantle and crustal component, which is a flow characteristics, and especially how those materials stiff, high-melting-temperature, and much more insulat- interact with Earth’s current and Archean atmospheres, ing material [17]. This material is not likely to be mobile will help us predict and interpret observations of the sur- enough to enable sufficient mantle convection to pro- faces of carbon planets. duce plate tectonics, nor would the crust readily move and bend. These worlds may be more like Io than Earth, References: [1] Seager & Kuchner (2005). [2] Bond in which there is heat pipe or advection-style heat re- et al. (2010). [3] Kuchner & Seager (2005). [4] Lodders lease [20, 21], expressed as isolated surface volcanic (2003). [5] Bond et al. (2008). [6] Teske et al. (2013). centers and hot spots [22]. Under this model of heat re- [7] Nissen et al. (2014). [8] Wilson et al. (2016). [9] lease, over time, the planet completely volcanically re- surfaces and crustal materials are returned to the mantle Brewer & Fischer (2016). [10] Baraffe et al. (2015). through burial and subsidence (similar to Io) to re-melt [11] Luger & Barnes (2015). [12] Dawson et al. and begin the process again. This enables significant (1995a). [13] Peterson et al. (1995). [14] Sweeney et al. transfer of crustal solids and volatiles from the surface (1995). [15] de Moor et al. (2013). [16] Dawson et al. to the interior and vice versa. (1995b). [17] Pinkerton et al. (1995). [18] Dawson et al. Carbon Planet Atmospheric Evolution: Plate (1990). [19] Walker et al. (1981). [20] O’Reilly & Da- tectonics and the heat pipe mechanism both cause the vies (1981). [21] Moore (2001). [22] Lopes et al. (1999). burial and recycling of atmospheric volatiles trapped in [23] Schenk & Bulmer (1998). [24] Foley & Smye the crust. This depletes the atmosphere of toxic levels of (2018).