Active, controlled circular, and spin-rotational movement of optically trapped airborne micro-particles.

We present a novel method for actively controlling circular and/or spin-rotational motion of an optically trapped airborne micro-particle. A 532-nm Gaussian laser beam is shaped into an elliptical ring by a pair of axicons and a cylindrical lens. The shaped beam is then focused into an elliptic cone that produces an optical trap. As the cylindrical lens is rotated, a torque is exerted on the trapped particle, resulting in circular or spin-rotational motion. We show examples of the circular-rotational movement as a function of laser power and the rotation rate of the cylindrical lens.

Active remote sensing of atmospheric dust using relationships between their depolarization ratios and reflectivity.

The backscattered light from agglomerated debris particles shows that an approximate linear correlation exists between the logarithm of the geometric albedo $ \log(A )$ of polydispersions of agglomerated debris particles and their lidar linear or circular depolarization ratios, $ \unicode{x00B5}_L$ and $ \unicode{x00B5}_C$. The nature of the relationship depends on the complex refractive index of the particles in the distribution. This extension of the Umov law can be used for lidar and radar characterizations by placing constraints on the reflectivity of the particles. It suggests that an approximate inverse relationship exists between the lidar ratio and the lidar depolarization ratios whose scaling parameter depends on the refractive index of the aerosol population.

Interpolating light-scattering properties of irregularly shaped, absorbing particles.

Success in developing remote-sensing methods is largely based on adequate modeling of target-particle shapes. In various terrestrial and cosmic applications, submicrometer- and micrometer-sized dust particles appear to have a highly irregular morphology. Light scattering by such irregularly shaped particles can be computed only with a numerical technique that, in practice, is a time-consuming approach, demanding significant computational resources. In this Letter, we discuss an efficient way to accelerate light-scattering computations through interpolation of the numerical results obtained at different levels of material absorption. We find a nonlinear dependence of reflectance, degree of linear polarization, and linear and circular polarization ratios on the imaginary part of refractive index Im(m). Over the range of ΔIm(m)=0.05, the dependence can be satisfactorily described with a cubic polynomial function, whose determination requires exact computations at four different values of Im(m). The light-scattering characteristics at other intermediate values of Im(m) can be inferred with great accuracy via interpolation.

Formation of lunar swirls by magnetic field standoff of the solar wind.

Lunar swirls are high-albedo markings on the Moon that occur in both mare and highland terrains; their origin remains a point of contention. Here, we use data from the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer to support the hypothesis that the swirls are formed as a result of deflection of the solar wind by local magnetic fields. Thermal infrared data from this instrument display an anomaly in the position of the silicate Christiansen Feature consistent with reduced space weathering. These data also show that swirl regions are not thermophysically anomalous, which strongly constrains their formation mechanism. The results of this study indicate that either solar wind sputtering and implantation are more important than micrometeoroid bombardment in the space-weathering process, or that micrometeoroid bombardment is a necessary but not sufficient process in space weathering, which occurs on airless bodies throughout the solar system.

Global silicate mineralogy of the Moon from the Diviner lunar radiometer.

We obtained direct global measurements of the lunar surface using multispectral thermal emission mapping with the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment. Most lunar terrains have spectral signatures that are consistent with known lunar anorthosite and basalt compositions. However, the data have also revealed the presence of highly evolved, silica-rich lunar soils in kilometer-scale and larger exposures, expanded the compositional range of the anorthosites that dominate the lunar crust, and shown that pristine lunar mantle is not exposed at the lunar surface at the kilometer scale. Together, these observations provide compelling evidence that the Moon is a complex body that has experienced a diverse set of igneous processes.