Did you know that the polarization properties of visible light can be used to detect the presence of life? That’s right, recent research has shown that the circular polarization of light can be used to detect the presence of biosignatures in the solar system and beyond with great sensitivity. In fact, the polarization properties of visible light scattered by atmospheric surfaces and aerosols can provide a lot of information on their structures and compositions. And recently, studies pointed out that the circular polarization in reflected light is a potential biomarker because its presence can be induced by the homochirality (uniformity of chirality) of biotic organic matter.
Polarization is a property of light inaccessible to the naked eye but rich in information about solid particles, suspended aerosols and surfaces, with which the light interacts. Polarimetric imaging is a powerful and yet under-exploited technique to retrieve quantitative properties of both surface materials and atmosphere aerosols on Earth, in the Solar System and beyond. Recent developments in polarimetric sensors offer new opportunities to build and operate powerful instruments for this purpose.
For instance, Earth-based polarimetric observations of Mercury’s surface had suggested the presence of a layer of fine particles similar to the one found on Moon’s surface long before this could be confirmed by dedicated missions.
Our experiment for the Bexus campaign will be composed of a compact and lightweight full-Stokes polarimetric imager that will be able to measure the four Stokes parameters continuously, ideally in different visible bands. The imager will be used throughout the entire balloon’s flight to image the surface and the atmosphere through the varying air column.
Our instrument will be with a core paired with compact cameras utilising modern colour and polarimetric commercial CMOS (complementary metal-oxide-semiconductor) sensors, which allow fast high-resolution polarimetric colour imaging. A first camera will record the linear polarization while the second one will be equipped with additional optics to record the circular polarization. The data will be analysed to assess the potential of polarimetric imaging for separating the contributions of the surface and the atmosphere to the signal. It will also gain experience with hardware which might later be used to develop instruments for future Solar System missions.
Ultimately, this dataset and the experience gained during the completion of this campaign will help guiding the next steps in the development of future ground-based and spaceborne polarimetric instrumentation.
Our project consists of taking pictures of Earth’s surface with a polarised camera which can give us information about the 4 stokes parameters. The goal is to test if these high-altitude pictures can give us enough information about the reflected light observing polarized light with a polarized camera.
The data will then be analysed by the team to determine the instrument’s performances, to test different dehazing techniques using both linear and circular polarization and better assess the potential of circular polarization as a biosignature. Thanks to this experimental and theoretical project, we will have at our disposal a set of data that characterize the inhabited and uninhabited environments of our planet.
To fully understand this experiment, it is important to understand all important factors:
Light is an electromagnetic wave. The figure below shows a single light wave on the very left, we observe that the wave can be projected on a plane where this single light wave represents a straight line. This is what we call linear polarization. Note that light waves can be superposed, and the resulting addition of light waves will be a vectorial sum of both waves at each point in time. The sum of two waves that have the same phase will also be linearly polarized but at a different angle. When two light waves have different phases the resulting vector for each point will be turning around the axis of propagation. This is what we call circularly polarized light. Light from the sun and most light sources is a superposition of many different waves and is therefore unpolarized or does not have a specific polarization.
To quantify or characterize light polarization four parameters are used. They are called the Stokes parameters. There is : I ,Q, U and V. I is the intensity of the light. Q and U give informations about linearly polarized light at two different angles. And V is the parameter for circularly polarized light which can be right- or left-handed. One parameter can be calculated from two others and is not independent.
Polarization and Life
The two pictures and corresponding graphs below show the Stokes parameters for the circularly polarized light when taking pictures of artificial turf and distant trees. At around 680 nm a spike in intensity can be observed on the distant trees. This is the red edge effect and is the biosignature.
The reason this phenomenon happens is homochirality. Chiral objects are objects that are not superposable with their mirrored object. Like the human hand for example. In chemistry this applies to molecules. These molecules can have left- or right-handed chirality. When all molecules have the same chirality, we talk about homochirality. Sugar molecules for example can be left- or right-handed too, what is interesting is that all sugar molecules present in nature are right-handed. Only artificially created sugar molecules can end up left-handed. This is due to the way these molecules are created in plants, the enzymes that build sugar molecules can only build right-handed ones. This means that this property is exclusive to life molecules. These chiral molecules induce circularly polarized light and when all molecules have the same chirality, the induced circular polarization is not cancelled out by another molecule that would have the opposed handedness. If all molecules have the same handedness, therefore homochiral, the effect of circularly induced light is detectable.
The illustration below shows the effect of a leaf on sunlight that hits it. The chiral molecules within the leaf (in this case not sugar), all have the same chirality and therefore induce the rotation. The reflected light will be mostly unpolarized as not every ray hit one of those molecules but also partly circularly polarized and partly linearly polarized.
Polarization to data
To capture the polarization two cameras are used. Each pixel on the camera captures the light information for a specific colour and polarization angle, a cluster of 16 pixels gives the full information about the light captured. An algorithm allows to extract the information of each pixel and calculates the value of each stoke parameter.
Of the two cameras one is equipped with a simple polarization filter the other one with a quarter wave plate added. The quarter wave plate de-phases certain waves by a quarter wavelength and thus linearizes the circularly polarized light (and vice versa depending on what way it is mounted). The direction of the light on the illustration below is the other way around for our experiment.
The intensity of the red edge effect is most likely going to be too small to be able to detect with the cameras used. The information about the parameters that affect this experiment are the main subject of study. Snow, clouds, trees, lakes are all aspects that cannot be influenced, neither here on earth or on exoplanets, it is important to be prepared to all kinds of scenarios. The experimental values of this project are going to help build the foundation for this new technology.