A Compact and Robust Method for Spectropolarimetry: Application to Space Exploration

A compact and robust method for spectropolarimetry is described which lends itself, in principle, to application in the field and in space. With space-based polarimetry, profoundly important astronomical topics may be addressed, including the search for extrasolar planets, their characterization and the presence of life. Within the Solar System, exploration and characterization opportunities are greatly enhanced. Polarization observations provide fundamental and unique insights into planetary characteristics, and may be invaluable in establishing the presence and nature of extrasolar planets. Within the Solar System, time resolved spectropolarimetry may probe aerosol, surface, plasma and atmospheric scattering processes, and cometary and zodiacal dust and dusty ring systems. Beyond the Solar System, in the search for extrasolar planets seen by reflected light, polarization can play a critical role in the recognition of planets, the characterization of their environment, and of the planets themselves. We may identify Rayleigh scattering atmospheres, liquid oceans, clouds, rocks and ices from their polarimetric signatures. A powerful new dimension is introduced by the addition of circular polarization. Through the homochirality of biological molecules, circular polarization may offer one of the purest biosignatures available. Homochirality arises as a consequence of self-replication hence is likely to be generic to all forms of biological life. We have shown that a variety of photosynthetic microbial organisms and macroscopic vegetation, yield distinctive signatures in their circular polarization spectra, hence circular polarization may prove to be an effective way to remotely sense photosynthesis. These topics are best tackled from space; a planetary probe or a space-based telescope. However, there are serious challenges to precision polarimetry in space, some applicable to polarimetry in general, which include: fragile components such as photoelastic modulators or ferro-electric liquid crystals which also have limited wavelength coverage; moving parts such as rotating waveplates; rapid modulation requiring sophisticated electronics. Classical astronomical polarimeters using rotating waveplates require sequential data acquisition, and when the target is in motion or variable, or the instrument is in motion, this limits the achievable accuracy. Sparks et al. (2012) Applied Optics, http://arxiv.org/abs/1206.7106, present an approach which provides full Stokes spectropolarimetry on a single two-dimensional data frame, such as may be acquired with a CCD detector. The method is to encode the polarimetry as an amplitude modulation orthogonal to the spectrum, and can be configured to provide either linear or full Stokes measurement. That is, on the two-dimensional data frame, one dimension gives the spectrum, and the other, the polarization. With this approach, all required information is available on a single frame hence time dependent concerns are absent, there are no moving parts, the technique is implicitly highly sensitive, and multiwavelength coverage is available at whatever spectral resolution is desired. The methods can, in principle, work equally well in the UV, visible or IR. The promise for greatly simplifying space-based spectropolarimetry is substantial.