Modeling Radiometric and Polarized Light Scattering From Exoplanet Oceans and Atmospheres
Document ID: 9
Doctoral Dissertation
The Pennsylvania State University
The Graduate School
College of Engineering
Abstract
Of all substances, water appears to be the most vital to the appearance and sustenance of life as we know it. Proposed space-based observatories such as NASA’s Terrestrial Planet Finder – Coronograph (TPF-C) may make it possible in the near future to detect the presence of oceans on nearby extrasolar planets (exoplanets) by studying the polarization of visible, infrared, or ultraviolet radiation reflected from the planet. In this dissertation, we model and analyze light scattering properties of various notional exoplanets, including brightness and polarization state versus wavelength and position in the orbit (orbital longitude, OL) in order to predict the potential observability of distant oceans. Our initial working hypotheses are:
- With simulations we can anticipate terrestrial planet signatures in light scattered from distant star systems;
- Future instruments will gather sufficient information on terrestrial exoplanets to draw useful conclusions about the planet surfaces and atmospheres;
- TPF-C will have a baseline wavelength range of 500 – 1000 nm, and will have some capability to observe in spectral sub-bands;
- Polarized and unpolarized orbital light curves, possibly combined with broad-band spectral information, will provide enough information to discriminate between terrestrial class planets with and without large oceans;
- Observation of polarized light curves of exoplanet systems may provide other useful information about the systems beyond that from unpolarized brightness curves.
We find that total flux light curves from Lambertian and Rayleigh scattering dominated planets peak at full phase, OL = 180°, whereas ocean planets with thin atmospheres exhibit peak flux in the crescent phase near OL = 30°. The polarized results for ocean planets show that clouds, wind-driven waves, aerosols, absorption, and Rayleigh scattering in the atmosphere and within the water column, dilute the polarization fraction and shift it away from the OL = 74° predicted by Fresnel theory. On planets for which Rayleigh scattering dominates, the polarization peaks near an orbital longitude of 90°, but clouds and Lambertian surfaces dilute and shift this peak to smaller OL, and a shifted Rayleigh peak might be mistaken for a water signature unless data from multiple wavelength bands are available.
When observing over the baselined TPF-C wavelength range (500–1000 nm), Rayleigh scattering alone from an atmosphere as thick as Earth’s is enough to shift the polarization peak to an orbital longitude of 83°, closer to the Rayleigh peak at 90° than to the Fresnel peak at 74°. Ocean radiance in this wavelength band caused by scattering within the water column also dilutes the polarization peak, limiting the polarization fraction to a maximum of slightly over 0.9. Water aerosols shift the peak to even higher OLs and add a rainbow peak near OL = 140°. Clouds also have a strong effect in masking the ocean surface polarization, and water clouds can exhibit the rainbow peak as with water aerosols. The high albedo and depending on composition and particle size and shape. Wind over an ocean surface causes waves and sea foam, both of which tend to dilute the water polarization signal. The magnitude and polarization of exozodiacal light (exo-zodi) is another large unknown, because we have only limited measurements of zodiacal light in our own Solar System, and instrumentation is not yet sensitive enough to measure exo-zodi in mature exoplanet systems. Exo-zodi is expected to be polarized, so will likely contribute an additional competing “noise” polarization peak.
In addition to end-member planets dominated by Lambertian or Rayleigh scattering, we simulate variations of “water Earths”; these are planets similar to Earth but completely covered by oceans, disturbed only by light winds. For these models, we also include US 1962 Standard Atmosphere absorption, and maritime aerosols using the standard 5 km low visibility and 23 km high visibility aerosols, and some other much higher visibilities for comparison. The polarization fraction for cases with Earth-like aerosols peaks at only about 0.15 at OL = 100° for the 5 km case, and at less than 0.35 at OL = 95° for the 23 km case. With more and more transparent aerosols, the polarization peak for water Earth cases approaches that of a Rayleigh-only atmosphere over an ocean surface.
Our model supports the idea that Rayleigh effects are mitigated by observing at longer wavelengths, taking advantage of the dependence of Rayleigh scattering on the inverse fourth power of wavelength. For example, our simulations show that the polarization fraction for an ocean surface hidden by a Rayleigh-only atmosphere can be increased by using only the longer wavelength portion of the TPF waveband, from 900-1000 nm. However, this would result in a loss of much of the available signal, and it would do nothing to reduce the dilution of the polarization signal by other factors. In particular, the aerosols included in our higher fidelity water Earth model dominate scattering for visibility of 23 km, which represents a clear day on Earth. Still, if multiple wavebands are available on TPF-C, as baselined, then comparing the results of different wavebands from an exoplanet observation, with the above in mind, may be useful.
The net effect of clouds, aerosols, absorption, atmospheric and oceanic Rayleigh scattering, waves, and exo-zodi may severely limit the percentage of ocean planets that would display a significant polarization signature, and may also generate a significant number of false positives on dry planets. Attempting to use the orbital position and strength of the polarization peak to determine whether or not an exoplanet surface is water-covered or dry is risky because so many factors can reduce the strength of the polarization peak, or shift it to higher or lower orbital longitudes. The result is an inversion problem which, for many planets, may be ill-posed. All of this suggests that polarization measurements by a TPF-C type telescope may not provide a positive detection of surface liquid water on exoplanets. On the other hand, the strength and placement of the polarization peak in the orbit relative to the cases we model, combined with the magnitude of the total flux and shape of the orbital flux curve, may give strong evidence of exoplanet surface and atmospheric composition for some nearby Earth-like planets, if they exist.
We have also suggested that polarization could be used to help determine whether an object which appears to be near a star is in fact a planet in orbit around the star, or a background object. If the object exhibits polarization perpendicular to the line between object and the star, then the object probably is a planet and the polarization is likely due to Rayleigh scattering in the planet’s atmosphere, reflection from a liquid surface, or a combination of the two effects. This method has the advantage that it can be used to show probable association with a single observation. For some exoplanet observations, this technique could be implemented immediately by ground-based observatories.
This work is novel in including a more complete atmosphere in simulations of exoplanet light scattering than previous models. The resulting simulations of Earth-like and diffuse scattering planets highlight the potential difficulties in detecting exoplanet oceans, which are not as apparent when using simpler models. The work also includes new ideas on using polarization to help determine planetary association, as well as a novel graphical method of showing different planetary polarization types.
Keywords: exoplanets, optical scattering
| Citation: | M. E. Zugger, "Modeling Radiometric and Polarized Light Scattering From Exoplanet Oceans and Atmospheres", The Pennsylvania State University, Doctoral Dissertation, December 2011, 188 pages |