Detection of in-situ particles in planetary atmospheres using a Laser Nephelometer.

Proposed Venus and Saturn missions called out in the NASA Decadal Survey include atmospheric observations of clouds and aerosols. Remote sensing techniques to derive cloud properties, including density and particle size, are limited by the optical density of the respective atmospheres. A nephelometer is an instrument that makes in-situ measurements of the cloud particles it encounters as it descends, using a two-color backscattering process. A new nephelometer derived from the Galileo version has been prototyped; the design benefits from lower volume and increased power efficiencies of near infrared diode lasers. Using two wavelengths, as opposed to one used in Galileo, allows measuring particle size distribution. Two near IR (NIR) lasers (753 and 1556nm) with 0.15W output are used, with a receiving diode sensor and a laser output energy pickoff sensor. We are also evaluating a laser-cavity approach for single particle counting and providing scattering phase information. One laser module measures the particles that are outside of the descent probe boundary layer via the remote piece, and the laser-cavity module pulls particles into the probe for single-particle detection. Both techniques have pros and cons, and are complementary; combining the two would provide both ranged measurements and single-particle counting precision. An engineering development unit (EDU) for a NIR laser nephelometer is currently designed and built. A particle chamber has been constructed and tested, which verifies laser control and sensor design (figure 1); and model sensitivity calculations. The test data is used to determine the applicability of Dynamic Light Scattering technique (DLST) for determining the particle size distributions. Initial data tests the possibility of operating the lasers in a growing-period pulse mode, allowing for DLST to determine the number of particles, and their size distribution while minimizing total data downlink. Additional measurements are needed with calibration standard to verify sensitivity and SNR calculations. This system has multiple uses beyond Saturn: Mars, Venus, Titan, Earth clouds and aerosols. The estimated mass and power for this nephelometer are 1.7 kg and 1.5W, respectively. The system is suitable for a range of in-situ atmospheric missions, including descent probes and landers.