Experiments of samples of molecules of exobiologic and planetologicinterest require designing cells exhibit. The samples we study are prepared in so-called "opened" or "closed” cells. Open cells are a legacy of exposure experiments that took place in Earth orbit in the 1990s and early 2000s. Closed cells have been developed to specifically meet the needs of our experiments concerning gaseous phase analysis and simulations of atmosphere. They were the subject of a phase of development in partnership with COMAT Aerospace (Toulouse), and then a phase of optimization with the Advanced Techniques Division of Air Liquide company. This original development has been funded by CNES.
Two types of open cells were used for experiments in Earth orbit. The molecules are deposited on the inside of the cell window. According to the type of atmosphere to simulate, windows are either MgF2 or quartz. MgF2 is a material which has a good transmission in the far UV field (transparent to UV > 110nm), which allows to have a photolysis including the Lyman band (122 nm), very intense in the VUV solar field. Quartz cuts UV radiation below 190 nm, which simulates the Martian environment where the atmosphere filters the UV < 190 nm. With open cells, we can conduct studies on kinetics of photochemical reactions. However, the volatile products of photo-destruction are evacuated by ventilation systems to the space vacuum and are therefore non-analyzable.
An open cell composed of a cylindrical aluminum rim and a window MgF2 or quartz on the cylinder. The organic molecule is deposited on the inside of the window. Such cells have been used to UVolution and PROCESS. (Figure of COMAT, France).
Design of an open cell such as those used in AMINO (schema: Kayser - Threde GmbH Germany)
During the three Uvolution, PROCESS and AMINO experiments, closed cells were first used for the first timein orbit exhibition experiments. They can be used to study the photolysis of a gas mixture (simulation of an atmosphere) or photolysis of a solid molecule by interaction with a simulated atmosphere (for example, a molecule on the surface of Mars), or even analyze gaseous products from the photo-dissociation of an organic solid molecule.
If the concept has not changed, the manufacturing process has significantly changed since their first use in UVolution and PROCESS in 2007. The first generation of closed cells consisted of two cylinders of aluminum (a male part and a female part) that screw into each other. A window (MgF2 or quartz) is pasted at each end of the cell. The tightness is ensured by an o-ring Viton between the two parties (design of cells: COMAT, Toulouse, France). It has been demonstrated that the leak level is sufficient to not disrupt short-term experiments as UVolution (14 days in space, and about a month between the sample preparation and analysis after the return), but insufficient to ensure more long-term PROCESS or AMINO experiments. Therefore, it is possible that closed cells mounted on the PROCESS experiment are almost empty when they are back on Earth.
A new generation of closed cells was manufactured for the AMINO experience, by Air Liquide (Grenoble, France). This time, the cell body is made of stainless steel, windows are brazed, and the sealed between the two parts of the cells is ensured by a laser welding. The results of leak tests (< 10-10 atm. L.s-1) show that this new design is consistent with the experiments of long duration in orbit (loss < 10% for 2 years in space).
Design of a closed cell. For the first generation of closed cells, two cylinders in aluminum are screwed into each other. Two MgF2 or quartz windows are glued to both ends, which allow the analysis of the samples within the cell by spectroscopy. The tightness is ensured by a Viton o-ring. For the new generation of closed cells, the body is stainless steel, windows are brazed and tightness is ensured by laser welding. (Schema: COMAT)
Gaseous samples are prepared in a specially developed analytical cell for these projects. This analytical cell is designed for both the fill and the reopening of closed cells. It consists mainly of two parts in stainless steel, in which the two parties (male and female) of a closed cell are mounted separately. The two parts of the analytical cell are then adjusted and sealed with a Viton o-ring. The gas mixture of the experiment may then be introduced into the analytical cell, then the two parts of the closed cell can be screwed one into each other, by means of a screw connecting the interior cell of analysis. Thus, the gas is trapped inside the closed cell.
Design of the analytical cell. It allows the filling of gas in a closed cell, and analysis at the end of the experiments. An integrated infrared chamber can be used for the analysis of the gas after opening of a closed cell. (Schema COMAT).