SPEX (Spectropolarimeter for Planetary Exploration) was developed in close cooperation between scientific institutes
and space technological industries in the Netherlands. It is used for measuring microphysical properties of aerosols and
cloud particles in planetary atmospheres. SPEX utilizes a number of novel ideas. The key feature is that full linear
spectropolarimetry can be performed without the use of moving parts, using an instrument of approximately 1 liter in
volume. This is done by encoding the degree and angle of linear polarization (DoLP and AoLP) of the incoming light in
a sinusoidal modulation of the intensity spectrum.
Based on this principle, and after gaining experience from breadboard measurements using the same principle, a fully
functional prototype was constructed. The functionality and the performance of the prototype were shown by extensive
testing. The simulated results and the laboratory measurements show striking agreement.
SPEX would be a valuable addition to any mission that aims to study the composition and structure of planetary
atmospheres, for example, missions to Mars, Venus, Jupiter, Saturn and Titan. In addition, on an Earth-orbiting satellite,
SPEX could give unique information on particles in our own atmosphere.
We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarimeter
measuring from 400 to 800 nm (with 2 nm intensity resolution), that is compact (~ 1 liter), robust and
lightweight. This is achieved by employing the unconventional spectral polarization modulation technique, optimized
for linear polarimetry. The polarization modulator consists of an achromatic quarter-wave retarder and
a multiple-order retarder, followed by a polarizing beamsplitter, such that the incoming polarization state is
encoded as a sinusoidal modulation in the intensity spectrum, where the amplitude scales with the degree of
linear polarization, and the phase is determined by the angle of linear polarization. An optimized combination
of birefringent crystals creates an athermal multiple-order retarder, with a uniform retardance across the field
of view. Based on these specifications, SPEX is an ideal, passive remote sensing instrument for characterizing
planetary atmospheres from an orbiting, air-borne or ground-based platform. By measuring the intensity and
polarization spectra of sunlight that is scattered in the planetary atmosphere as a function of the single scattering
angle, aerosol microphysical properties (size, shape, composition), vertical distribution and optical thickness can
be derived. Such information is essential to fully understand the climate of a planet. A functional SPEX prototype
has been developed and calibrated, showing excellent agreement with end-to-end performance simulations.
Calibration tests show that the precision of the polarization measurements is at least 2 • 10-4. We performed
multi-angle spectropolarimetric measurements of the Earth's atmosphere from the ground in conjunction with
one of AERONET's sun photometers. Several applications exist for SPEX throughout the solar system, a.o. in
orbit around Mars, Jupiter and the Earth, and SPEX can also be part of a ground-based aerosol monitoring
SPEX (Spectropolarimeter for Planetary EXploration) is an innovative, compact instrument for spectropolarimetry,
and in particular for detecting and characterizing aerosols in planetary atmospheres. With its ~1-liter volume
it is capable of full linear spectropolarimetry, without moving parts. The degree and angle of linear polarization
of the incoming light is encoded in a sinusoidal modulation of the intensity spectrum by an achromatic
quarter-wave retarder, an athermal multiple-order retarder and a polarizing beam-splitter in the entrance pupil.
A single intensity spectrum thus provides the spectral dependence of the degree and angle of linear polarization.
Polarimetry has proven to be an excellent tool to study microphysical properties (size, shape, composition) of
atmospheric particles. Such information is essential to better understand the weather and climate of a planet.
The current design of SPEX is tailored to study Martian dust and ice clouds from an orbiting platform: a compact
module with 9 entrance pupils to simultaneously measure intensity spectra from 400 to 800 nm, in different
directions along the flight direction (including two limb viewing directions). This way, both the intensity and
polarization scattering phase functions of dust and cloud particles within a ground pixel are sampled while flying
over it. We describe the optical and mechanical design of SPEX, and present performance simulations and initial
breadboard measurements. Several flight opportunities exist for SPEX throughout the solar system: in orbit
around Mars, Jupiter and its moons, Saturn and Titan, and the Earth.
Future planetary missions will require advanced, smart, low resource payloads (P/Ls) and satellites1,2 to enable the exploration of the solar system in a more frequent, timely and multi-mission manner with reasonable cost. The concept of highly integrated payload architectures was introduced during the re-assessment of the payload of the BepiColombo Mercury Planetary Orbiter3. Considerable mass and power savings were achieved throughout the instrumentation by better definition of the instruments design, higher integration and identification of resource drivers4. Higher integration and associated synergy effects permit optimisation of the payload performance at minimum resource requirements while meeting demanding science requirements. This promising concept has been applied to a set of hypothetical Planetary Technical Reference Studies11 (PTRS) on missions to Venus5, Jupiter/Europa6, Deimos7, Mars8 and the investigation of the Interstellar Heliopause9. The needs on future instrumentation were investigated for these mission concepts and potential instruments were proposed10. A demonstration programme is now proposed in form of an elegant breadboard that consists of a photon counting laser altimeter, a stereoscopic high resolution camera, and a broadband radiometric mapping spectrometer. The aim of the activity is to demonstrate to feasibility of such a miniaturised, low resource and highly integrated payload based on innovative instrument designs. The activity shall thereby provide a clear detailed definition of the technical and managerial aspects for implementation into potential future planetary space science missions.