Detecting Volcanic Activity on Exoplanets

On Earth, the majority of volcanic gases released into the atmosphere occur non-explosively near the surface, either along the mid-ocean ridges or from the venting of low-viscosity magmas like in the case of the continuous non-explosive eruption of Kilauea in Hawaii. Volcanic gases released in this manner typically do not reach the stratosphere and have short residing times in the atmosphere as they are subjected to rapid washout at lower altitudes. Only large explosive volcanic events such as the eruption of Krakatoa in 1883 and Mount Pinatubo in 1991 are powerful enough to inject volcanic gases directly into the stratosphere, where residing times are much longer. In addition, far less frequent eruptions involving super-volcanoes such as Yellowstone and Toba can inject greater quantities of volcanic gases into even higher altitudes.

L. Kaltenegger et al. (2010) show that large explosive volcanic events can be detected on transiting Earth-sized to super-Earth-sized exoplanets using sulfur dioxide gas as a proxy. In an Earth-like stratosphere, high quantities of sulfur dioxide can only be present following a large volcanic eruption. For example, the eruption of Mount Pinatubo injected 17 ± 2 million tons of sulfur dioxide into the stratosphere. As a result, by looking for the telltale presence of sulfur dioxide, observations of transmission spectra (i.e. light from the host star passes through the atmosphere of a transiting planet, some light is absorbed by the planet’s atmosphere and the shape of the transmission spectrum can reveal the planet’s atmospheric constituents) and emergent spectra (i.e. electromagnetic radiation emitted by the planet itself) of Earth-like exoplanets can allow recent major volcanic events on such planets to be detected.

Figure 1: Artist’s impression of an Earth-like exoplanet.

Synthetic transmission and emergent spectra were generated by L. Kaltenegger et al. (2010) to show the detectability of sulfur dioxide, thus indicating the presence of large volcanic eruptions on Earth-like exoplanets. The injection of sulfur dioxide from a volcanic eruption into the stratosphere is investigated under 2 cases. Case 1 involves a single main explosion resulting in a global distribution of sulfur dioxide within a 2 km stratospheric layer at 12-14 km in altitude (thin layer). Case 2 involves a complex eruption consisting of multiple explosions over multiple days or hours, leading to a global distribution of sulfur dioxide within a 13 km stratospheric layer at 12-25 km in altitude (thick layer).

Figure 2: Case 1, thin layer (top 2 panels). Case 2, thick layer (bottom 2 panels). Synthetic transmission and emergent spectra of an Earth-like exoplanet involving a cloud-free atmosphere for 3 volcanic sulfur dioxide concentrations (black: no eruption; red: 10 times Pinatubo; blue: 100 times Pinatubo). Case 2 (thick layer) shows a stronger feature in the transmission spectra than Case 1 (thin layer) because sulfur dioxide is distributed in a thicker layer of the atmosphere (12-25 km), which increases the effect on a transmission spectrum. (L. Kaltenegger et al., 2010)

Figure 3: Case 1, thin layer (top 2 panels). Case 2, thick layer (bottom 2 panels). Synthetic transmission and emergent spectra of an Earth-like exoplanet with water clouds at 1 km, 5 km and 12 km for 3 volcanic sulfur dioxide concentrations (black: no eruption; red: 10 times Pinatubo; blue: 100 times Pinatubo). (L. Kaltenegger et al., 2010)

Figure 4: Case 2, thick layer. Synthetic transmission spectra of an Earth-like exoplanet with 60 percent cloud cover at 12 km (1st panel), 5 km (2nd panel), 1 km (3rd panel) and no cloud cover (4th panel) for 3 volcanic sulfur dioxide concentrations (black: no eruption; red: 10 times Pinatubo; blue: 100 times Pinatubo). The transmission spectra show how cloud altitude has little effect on signal strength because the sulfur dioxide is in the stratosphere (i.e. above the clouds). (L. Kaltenegger et al., 2010)

Figure 5: Synthetic emergent spectra for both Case 1 and Case 2 of an Earth-like exoplanet with 60 percent cloud cover at 12 km (1st panel), 5 km (2nd panel), 1 km (3rd panel) and no cloud cover (4th panel) for 3 volcanic sulfur dioxide concentrations (black: no eruption; red: 10 times Pinatubo; blue: 100 times Pinatubo). Cloud altitude is shown to have a strong impact on overall emergent spectra, but little impact on transmission spectra (Figure 4). (L. Kaltenegger et al., 2010)

The results by L. Kaltenegger et al. (2010) show that sulfur dioxide features start to become detectable in an Earth-like atmosphere for an eruption event between 1 to 10 times Pinatubo. An eruption event ~10 times Pinatubo can be achieved by a Tambora-class event since Mount Tambora erupted in 1815 and injected ~200 million tons of sulfur dioxide into the stratosphere. However, larger eruption events tend to occur less frequently (~1 per 1000 years for a Tambora-class event) and detecting such an eruption event will require observing a larger sample of Earth-like exoplanets over a longer period of time. Detecting volcanism on exoplanets will improve our understanding of the diversity and prevalence of volcanism Furthermore, higher volcanic activity is expected for young exoplanets, super-Earths and tidally heated bodies.

Reference:

L. Kaltenegger et al. (2010), “Detecting Volcanism on Extrasolar Planets”, ApJ 140: 1370