View Full Version : Terraforming: perhaps ice dwarfs could be our friends

Roger E. Moore
2018-Jul-26, 05:19 PM
A problem that crops up on papers about super-Earths is that so many of them are close-in to M-dwarf stars, subjecting some to intense radiation and making retention of their atmospheres unlikely. This makes colonizing said worlds difficult. No harm in thinking ahead.

However, why not terraform the most "mini" of mini-Neptunes, relatively low-mass ice dwarfs with an Earthlike core and atmospheres full of hydrocarbons, oxygen, nitrogen, and other things? Cyanobacteria or other microorganisms could be seeded into the atmospheres to convert the air into life-sustaining molecules. We could build an Earth-friendly ecosystem there, maybe (key word).

Papers on air content of ice dwarfs (emphasis mine)...

Effect of Pressure Broadening on Emission and Transmission Spectra of H2O Modeled for sub-Neptune/super-Earth exoplanets: An Application to JWST
Gharib Nezhad, Ehsan; Line, Michael R.; Lyons, James R.
"Water is the most readily detected molecule over a diverse range of exoplanet properties (solar composition hot-Jupiters to high metallicity super-earths/neptunes). It is also one of the most important sources of opacity that govern radiative energy balance. It is well known that pressure/collisional broadening significantly influences the opacity of a given molecule. Laboratory spectroscopic studies have shown that the line-broadening (i.e. Doppler, Lorentzian) is influence by several factors including temperature, pressure, dipole moment of the broadeners (or bath gases), and the rotational quantum numbers. Since absorption cross-sections (or opacities) are central to both atmospheric modeling and observational research, there is a critical need to investigate the effect of pressure line-broadening on the absorption cross-sections and subsequent influence on observed transmission and emission spectra of transiting exoplanets. Typical data-model comparisons (either forward modeling or retrieval's) generally rely upon pre-computed grids of absorption cross-sections that assume trace molecules are broadened by a solar composition mixture (e.g., mainly H2 and He as a bath gas). However, as the metallicity of a planetary atmosphere increases, as anticipated for smaller planets in the sub-Neptune-Super Earth range, broadening due to other gases (e.g., N2, CO2, H2O, CH4, CO) can become significant and the H2-He broadening is no longer appropriate. In this work, we assess the influence of different background broadeners on the absorption cross-section of water, and subsequent influence on observed transmission/emission spectra. Initial results suggest that the choice of foreign broadener can result in up to a factor of ~5 increase in pressure broadened wings of the absorption cross sections, resulting in a factor of 1.6x reduction in the atmospheric spectral modulation. Such a difference will be detectible in the hi-resolution/SNR spectra anticipated with JWST, and will certainly influence the interpretation of high-metallicity atmospheres."

Characterizing Transiting Exoplanet Atmospheres with JWST
Greene, Thomas P.; Line, Michael R.; Montero, Cezar; Fortney, Jonathan J.; Lustig-Yaeger, Jacob; Luther, Kyle
"We explore how well spectra from the James Webb Space Telescope (JWST) will likely constrain bulk atmospheric properties of transiting exoplanets. We start by modeling the atmospheres of archetypal hot Jupiter, warm Neptune, warm sub-Neptune, and cool super-Earth planets with atmospheres that are clear, cloudy, or of high mean molecular weight (HMMW). Next we simulate the lambda = 1-11 mum transmission and emission spectra of these systems for several JWST instrument modes for single-transit or single-eclipse events. We then perform retrievals to determine how well temperatures and molecular mixing ratios (CH4, CO, CO2, H2O, NH3) can be constrained. We find that lambda = 1-2.5 mum transmission spectra will often constrain the major molecular constituents of clear solar-composition atmospheres well. Cloudy or HMMW atmospheres will often require full 1-11 mum spectra for good constraints, and emission data may be more useful in cases of sufficiently high Fp and high Fp/F*. Strong temperature inversions in the solar-composition hot-Jupiter atmosphere should be detectable with 1-2.5+ mum emission spectra, and 1-5+ mum emission spectra will constrain the temperature-pressure profiles of warm planets. Transmission spectra over 1-5+ mum will constrain [Fe/H] values to better than 0.5 dex for the clear atmospheres of the hot and warm planets studied. Carbon-to-oxygen ratios can be constrained to better than a factor of 2 in some systems. We expect that these results will provide useful predictions of the scientific value of single-event JWST spectra until its on-orbit performance is known."