Color Catalogue of Life in Ice: Surface Biosignatures on Icy Worlds.

With thousands of discovered planets orbiting other stars and new missions that will explore our solar system, the search for life in the universe has entered a new era. However, a reference database to enable our search for life on the surface of icy exoplanets and exomoons by using records from Earth's icy biota is missing. Therefore, we developed a spectra catalogue of life in ice to facilitate the search for extraterrestrial signs of life. We measured the reflection spectra of 80 microorganisms-with a wide range of pigments-isolated from ice and water. We show that carotenoid signatures are wide-ranged and intriguing signs of life. Our measurements allow for the identification of such surface life on icy extraterrestrial environments in preparation for observations with the upcoming ground- and space-based telescopes. Dried samples reveal even higher reflectance, which suggests that signatures of surface biota could be more intense on exoplanets and moons that are drier than Earth or on environments like Titan where potential life-forms may use a different solvent. Our spectra library covers the visible to near-infrared and is available online. It provides a guide for the search for surface life on icy worlds based on biota from Earth's icy environments.

Soil directioal (biconical) reflectance in the principal plane with varied illumination angle under dry and saturated conditions.

Change in directional (biconical) spectral reflectance was monitored for three soil samples under air dry and saturated conditions in the laboratory. The illumination angle was set consecutively at θi  = -10°, -40°, and -70° (left side of the sample on the principal plane), while the observation angle ranged from θo  = -60° to + 60° (both sides of the sample in the principal plane) in 5° increments. The soil samples were chosen to represent a variety of soil properties. Emphasis is on observations that illustrate the changes in the directional and spectral distribution of soil reflectance when the soil is dry or saturated.

Correction for reflected sky radiance in low-altitude coastal hyperspectral images.

Low-altitude coastal hyperspectral imagery is sensitive to reflections of sky radiance at the water surface. Even in the absence of sun glint, and for a calm water surface, the wide range of viewing angles may result in pronounced, low-frequency variations of the reflected sky radiance across the scan line depending on the solar position. The variation in reflected sky radiance can be obscured by strong high-spatial-frequency sun glint and at high altitude by path radiance. However, at low altitudes, the low-spatial-frequency sky radiance effect is frequently significant and is not removed effectively by the typical corrections for sun glint. The reflected sky radiance from the water surface observed by a low-altitude sensor can be modeled in the first approximation as the sum of multiple-scattered Rayleigh path radiance and the single-scattered direct-solar-beam radiance by the aerosol in the lower atmosphere. The path radiance from zenith to the half field of view (FOV) of a typical airborne spectroradiometer has relatively minimal variation and its reflected radiance to detector array results in a flat base. Therefore the along-track variation is mostly contributed by the forward single-scattered solar-beam radiance. The scattered solar-beam radiances arrive at the water surface with different incident angles. Thus the reflected radiance received at the detector array corresponds to a certain scattering angle, and its variation is most effectively parameterized using the downward scattering angle (DSA) of the solar beam. Computation of the DSA must account for the roll, pitch, and heading of the platform and the viewing geometry of the sensor along with the solar ephemeris. Once the DSA image is calculated, the near-infrared (NIR) radiance from selected water scan lines are compared, and a relationship between DSA and NIR radiance is derived. We then apply the relationship to the entire DSA image to create an NIR reference image. Using the NIR reference image and an atmospheric spectral reflectance look-up table, the low spatial frequency variation of the water surface-reflected atmospheric contribution is removed.

A Monte Carlo study of the seagrass-induced depth bias in bathymetric lidar.

A bathymetric lidar survey is the most cost efficient method of producing bathymetric maps in near shore areas where the ocean bottom is both highly variable and of greatest importance for shipping and recreation. So far, not much attention has been paid to the influence of bottom materials on the lidar signals. This study addresses this issue using a Monte Carlo modeling technique. The Monte Carlo simulation includes a plane parallel water body and a flat bottom with or without seagrass. The seagrass canopy structure is adopted from Zimmerman (2003). Both the surface of the seagrass leaves and the bottom are assumed to be Lambertian. Convolution with the lidar pulse function followed by the median operator is used to reduce the variance of the resultant lidar waveform. Two seagrass orientation arrangements are modeled: seagrass in still water with random leaf orientation and seagrass with a uniform orientation as would be expected when under the influence of a water current. For each case, two maximum canopy heights, 0.5 m and 1 m, three shoot densities, 100, 500, and 1000, and three bending angles, 5, 25, and 45 degrees, are considered. The seagrass is found to induce a depth bias that is proportional to an effective leaf area index (eLAI) and the contrast in reflectance between the seagrass and the bottom material.

Development of a laboratory spectral backscattering instrument: design and simulation.

A typical integrating sphere configuration for measuring backscatter includes a conventional cuvette with flat windows. This arrangement results in a significant amount of total internal reflection, preventing a large portion of the backscattered flux from entering the integrating sphere-detector system. Use of a semispherical cuvette overcomes this problem. Monte Carlo simulations of a semispherical cuvette window demonstrate that the detected signal varies monotonically with the attenuation, depending only on the probability of backscattering for a given single scattering albedo. That is, only the total backscattering probability matters, regardless of subtle differences in the scattering phase function in the backward direction. The system is calibrated by use of standard microspheres for which the size distribution and the refractive index are known; this makes it possible to compute the exact phase function based on Mie theory. We have performed Monte Carlo simulations for various measured volume scattering functions and for computed phase functions, using particle scattering codes. All the results indicate that the backscattering measurement errors are likely to be less than 10%.