Planetary Polarization Nephelometer
- Created: Tuesday, 31 December 2013
- Goddard Space Flight Center, Greenbelt, Maryland
Instrument provides more detailed information on aerosols encountered in a planetary environment.
Aerosols in planetary atmospheres have a significant impact on the energy balance of the planets, yet are often poorly characterized. An in situ instrument was developed that would provide more diagnostic information on the nature of aerosols it encountered if deployed on a planetary descent probe. Previous probe instruments only measured intensity phase functions, but much particle ambiguity remains with only this information. Adding the polarization phase function greatly reduces particle characteristic ambiguities, but also adds more challenges in designing a measurement approach. Laboratory instrumentation to measure intensity and polarization phase functions have existed since the early 1970s, but these instruments employed quarter-wave plates and Pockels cells to modulate the illuminating beam and the scattered light to isolate the intensity and polarization phase functions. Both of these components are unstable except under tightly controlled thermal conditions. This solution avoids the use of thermally sensitive components such as quarter- wave plates or Pockels cells, and avoids requiring the detectors to be placed around the sensing volume.
The approach allows detection of both the intensity and polarization scattering phase function from aerosols using no moving parts, and components that are suitable for deployment in an extreme environment. Further, the technique allows only passive optical components to be exposed to the harsh conditions, with all detection and illumination equipment located within the protected hull of a probe.
The novel features of the innovation are the use of a pair of semiconductor lasers arranged with their polarization axes orthogonal, and modulating their power in a sinusoidally alternating sense to mimic the effects one achieves with a carefully tuned Pockels cell to produce a light beam with polarization alternating between polarized vertically to polarized horizontally. This style of illuminating beam enables one to measure the time behavior of the intensity of the scattered light to determine the intensity and polarization ratio of the scattered light. Using this illuminating beam, and a set of detectors at various angles around a scattering volume, one is thus able to measure the intensity and polarization phase functions for those scatterers.
A further innovation was the introduction of fiber optics to deliver the illuminated beam to the scattering volume, as well as fiber optics to conduct the scattered light away from the ambient environment at the various angles needed to sample the phase function, back to the sensitive detectors. The use of the fiber optics enables location of the illuminating lasers and the detectors away from the harsh external environment, and instead places them within the hull of the descent probe and in a carefully controlled thermal environment. Because the lasers and the detectors are relatively thermally sensitive, this ability to relocate them away from the environment represents a significant hardening of this instrument.
This work was done by Don Banfield of Cornell University for Goddard Space Flight Center. GSC-16521-1