Title: Theoretical Reflectance Spectra of Earth-Like Planets through Their Evolutions:
Astronomers: Yui Kawashima and Sarah Rugheimer
Date: April 3, 2019
April 10, 2019
Impact of Clouds on the Detectability of Oxygen, Water, and Methane with Future Direct Imaging Mission LUVOIR
As the search to find life elsewhere in the universe continues, scientists want to know more and more what the atmospheres of Earth-like planets (5 parsecs away) are like. “Earth-like” meaning a similar mass, radius, and orbital period to Earth’s. One way of identifying the molecular make-up of these atmospheres is reflectance spectroscopy. The astronomers in this study have plans to use the visible light and near-infrared telescope LUVOIR, which is intended to be used by 2021. The parameters set for this study are specific to the LUVOIR telescope, which doesn’t exist yet, so the data are relative and averaged in order to assume the overall general effects of cloud properties, namely altitude and coverage (see Table 2). These cloud properties greatly affect the detectability of important molecules on the reflectance spectra of Earth-like planets. The astronomers did this by creating a model of Earth at different geological epochs. Depending on the altitude of clouds and how much of the planet the clouds cover, they can hinder the detectability of molecules that are vital signs of potential life like O2, H2O, and CH4. These molecules are found in abundance throughout Earth’s history because they are the building blocks of oxygenic photosynthesis. However, this study was focused particularly on the abundance of O2 in Earth’s atmosphere, as it is a promising sign that these Earth-like planets are capable of harvesting life..
The three points in geological history at which the Earth was examined were: after the Great Oxidation Event (GOE), after the Neoproterozoic Oxidation Event (NOE), and the present day. These points in time were chosen because these are times in which Earth’s atmosphere was considered particularly active: during the GOE, O2 started to build up in the atmosphere that eventually lead to the NOE, which marked the beginning of multicellular life. This is why O2 is considered a target molecule for this study. In other words, the astronomers are looking at Earth during different periods of its evolution in order to have something to compare Earth-like planets to with atmospheres that have different cloud properties.
These astronomers tend to answer the question: How long would it take LUVOIR to detect O2 in a GOE atmosphere, an NOE atmosphere, and a present-day atmosphere?
- We expect to find different atmospheric compositions than that of Earth as we have already detected the existence of hot Jupiters and mini-Neptunes.
- We expect to find planets with atmospheres composed of no more than about 25% O2. Anything more would increase chances of widespread fires on the planet.
- We expect to find planets with seemingly featureless surfaces, which would indicate clouds and hazes that can absorb and scatter any light illuminating atmospheric composition.
- The astronomers used a line-by-line radiative transfer model to calculate altitudes of the clouds based on temperature and how disperse they are.
- The astronomers also considered the impact of noise on the detection of spectra of Earth-like planets. They did this using a coronagraph noise simulator modified to LUVOIR’s parameters.
Results: DETECTABILITY OF O2, H2O, AND CH4 WITH LUVOIR:
CH4: Earth-like planets with GEO-like atmospheres: 10 hours
Earth-like planets with NEO-like atmospheres: 30 hours
Earth-like planets with present day Earth-like atmospheres: > 6000 hours due to a low abundance in the present-day atmosphere, which is because of O2 destroying most of the methane.
H2O: The astronomers discovered that, generally, LUVOIR can detect H2O within 3-10 hours. Water for GEO and NEO epochs were less abundant because of the high surface temperature of the Earth, which caused evaporation.
CO2: Earth-like planets with GEO-like atmospheres: Up to 600 hours
Earth-like planets with NEO-like atmospheres: Up to 300 hours
Earth-like planets with present day Earth-like atmospheres: Up to 100 hours
It seems that when carbon dioxide is more abundant in the atmosphere, it takes longer to fully detect.
Results: INFLUENCE OF CLOUDS ON SPECTRA OF EARTH-LIKE PLANETS
At short wavelengths (more than 9 μm), the atmosphere is seemingly thick with mostly water, causing clouds to sink and have a lower altitude, thus decreasing their albedo. At long wavelengths (less than 9 μm), the atmosphere is thinner and thus a higher altitude, thus increasing their albedo. Despite this however, both wavelengths highlight a similar spectra, thus the astronomers concluded that altitude of clouds negligibly affected how well LUVOIR can detect molecules in the atmospheres.
The astronomers confirmed that detectability does vary when cloud coverage is concerned, but not by that much.