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PHIRE 1 & 2 - The Physikalisches Institut Radiometer Experiments

 

Introduction

PHIRE is a goniometer/reflectometer experiment. It is designed to measure the reflected radiance from a surface as a function of the angles of incidence and emission and the phase angle (i, e, and g, respectively, see figure). The concept for the experiment is similar to the Bloomsburg University Goniometer (BUG; run by Michael Shepard) and the Spectro-gonio-radiometer developed at the IPAG/University of Grenoble by Bernard Schmitt. Slightly different systems are used at JPL and the Astronomy Group (including Karri Muinonen) of the University of Helsinki also work in this field.

 

Why is it interesting?

Definitions of angles for photometric analysis.

It is possible to infer the properties of a surface from the way it scatters light. Surfaces which are quite rough tend to show a surge in the reflected radiance as the angles of emission and incidence reach zero. The sizes of the particles which make up the surface also play a role. The scattering is complicated but in recent years several theoretical developments have been made (notably by Bruce Hapke and Michael Mischenko) which we can now test by experiment. The results can be used to test model of the scattering properties of surfaces on, for example, Mars, Mercury and the Moon.


We are interested in this for several reasons. Firstly, we have been using the experiment to study the reflectance behaviour of simulants of the Mercury surface. This will be important for calculating signal to noise ratios for the BepiColombo Laser Altimeter (BELA). Secondly, the reflectance properties of Martian surface materials has been of interest to us in the past, particularly for Mars Pathfinder and for the MER missions. We are also interested in measuring photometric properties of icy samples containing impurities, analogs to cometary surfaces or icy satellites, in support of the ROSETTA (VIRTIS, OSIRIS) and JUICE missions (GALA).  Finally, the surface of Titan may have some rather bizarre surface materials. PHIRE can be used to study whether the scattering properties of some of these materials are unusual.

PHIRE-1

The first version of the instrument was fully described by Gunderson et al. in a paper in Planetary and Space Science in 2006. The system is sufficiently simple that students can use it in the advanced practicals at the University.

A basic CAD/CAM of the instrument in its simplest form.
Goniometer implementation. The system is sufficiently simple that students can use it in the advanced practicals at the University.

PHIRE-2

A second version of the instrument was built to permit measurements at sub-zero temperatures and hence to study the photometry of samples containing water ice. We have also applied a series of modifications to improve the general performances of the instrument (in particular the measurement speed) and to make the measurement procedure fully automatic, allowing the user to concentrate on the sample preparation and characterization.

CAD drawing of the PHIRE-2 instrument.
Picture of the PHIRE-2 installed in the low temperature freezer.

Both the hardware and the software parts of the PHIRE-2 instrument were entirely designed and developed at the Physikalisches Institut in Bern. The PHIRE-2 instrument shares some characteristics with the PHIRE-1 instrument, the BUG instrument operated at Bloomsburg University by M. Shepard and the Spectro-gonio-radiometer operated at Grenoble University by B. Schmitt.

An example of reflectance measurements performed with the PHIRE-2 instrument: clean snow (left), lunar regolith simulant (right) and snow contaminated by the regolith simulant (center). We display here as 3D surfaces the reflectance factor of the sample (I/F.cos i) for three different fixed incidence angles: 0, 40 and 60°.

The PHIRE-2 instrument is the central part of the Planetary Ice laboratory where analog samples are characterized using different complementary methods: photometry, microscopy, electric and thermal properties etc.

 

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