Friday, July 17, 2015

Forming Graphite-Like Carbon in the Atmosphere of GJ 436b

GJ 436b is a Neptune-sized planet that orbits around a relatively small M-dwarf star located only 33 light years away. The planet is ~10 times closer to its parent star than Mercury is to the Sun and it completes one orbit around its host star in just 2.64 days. Being so close to its parent star, the planet is referred to as a hot-Neptune and its dayside is heated to a temperature of ~800 K. GJ 436b is estimated to have 22 times the mass of Earth and 4.3 times the radius of Earth. A rocky core is predicted to exist at the center of GJ 436b. Surrounding the rocky core is a mantle of exotic high pressure water-ice. Finally, an envelope of hydrogen and helium gas forms the outermost layer of GJ 436b.

By observing the thermal emission from the dayside of GJ 436b, Stevenson et al. (2010) found that the methane (CH4) to carbon monoxide (CO) ratio in the planet’s atmosphere is at least ~10,000 times smaller than predicted. This is puzzling because in a hydrogen-dominated atmosphere at the temperatures found on GJ 436b, carbon in the atmosphere should prefer CH4 over CO. A likely reason for the lower-than-expected abundance of atmospheric CH4 is that the process of photodissociation can decompose CH4 into fragments which then react with each other or with other CH4 molecules to form C2 hydrocarbons (i.e. hydrocarbon molecules containing 2 carbon atoms). Successive reactions can lead to the formation of C3 and C4 hydrocarbons.

Using laboratory experiments to mimic the chemical processes in hydrogen-dominated atmospheres containing C3 and C4 hydrocarbons at elevated temperatures, Dangi et al. (2015) show that the surfaces of silicon grains can act as a catalyst, allowing the conversion of C3 and C4 hydrocarbons to carbonaceous refractory matter with carbon content greater than 90 percent. On GJ 436b, silicon grains can be supplied by micrometeoroids. This explains the low CH4 to CO ratio in the atmosphere of GJ 436b, whereby CH4 photochemically converts to higher order hydrocarbons such as C3 and C4 hydrocarbons. The catalytic surfaces of micrometeoroids then convert these hydrocarbons to refractory graphite-like carbon.

- Stevenson et al. (2010), “Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b”, arXiv:1010.4591 [astro-ph.EP]
- Dangi et al. (2015), “Toward the Formation of Carbonaceous Refractory Matter in High Temperature Hydrocarbon-rich Atmospheres of Exoplanets Upon Micrometeoroid Impact”, ApJ 805:76 (7pp)