We present SPEX, the Spectropolarimeter for Planetary Exploration, which is a compact, robust and low-mass spectropolarimeter designed to operate from an orbiting or in situ platform. Its purpose is to simultaneously measure the radiance and the state (degree and angle) of linear polarization of sunlight that has been scattered in a planetary atmosphere and/or reflected by a planetary surface with high accuracy. The degree of linear polarization is extremely sensitive to the microphysical properties of atmospheric or surface particles (such as size, shape, and composition), and to the vertical distribution of atmospheric particles, such as cloud top altitudes. Measurements as those performed by SPEX are therefore crucial and often the only tool for disentangling the many parameters that describe planetary atmospheres and surfaces. SPEX uses a novel, passive method for its radiance and polarization observations that is based on a carefully selected combination of polarization optics. This method, called spectral modulation, is the modulation of the radiance spectrum in both amplitude and phase by the degree and angle of linear polarization, respectively. The polarization optics consists of an achromatic quarter-wave retarder, an athermal multiple-order retarder, and a polarizing beam splitter. We will show first results obtained with the recently developed prototype of the SPEX instrument, and present a performance analysis based on a dedicated vector radiative transport model together with a recently developed SPEX instrument simulator.
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
We present a highly integrated payload suite which consists of the following instruments: a hyperspectral imager
covering the wavelength range from 0.7 μm up to 5μm, and a thermal infrared radiometric imaging spectrometer.
The payload design is the result of a design study that was performed in the context of the development of space
exploration technologies under ESA contracts. The payload is broadly applicable to environmental research and
for a number of remote sensing mission scenarios. All instruments have imaging capability and have been chosen
such that they profit from close integration. HIBRIS is a combination of the hyperspectral NIR spectrometer,
considered as generic instrument being part of many missions, and the radiometric micro-bolometer in the
thermal infrared spectrum. A linear variable filter (LVF) concept is implemented in the NIR range that avoids
the use of gratings which are usually limited to one decade of spectral range or less. The thereby rather compact
design does allow the integration of multiple instruments within a rather limited volume envelope. The suite
also makes use of a microcooler and the most advanced NIR detector technologies. The use of an LVF drives
the spectral resolution of the instruments to 1% of the wavelength. The SNR is satisfactory in the most part
of the spectrum for LEO EO missions. Current activities at cosine Research have focused on the design and
performance of uncooled microbolometers, linear filters, light shielding baffles, beam splitters for shared optical
paths, and the thermal design of HIBRIS.
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.