A Proposed Discovery Mission
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Photo: NASA/JPL [Larger 99K GIF] | NOTE: The following information was provided to the Astrobiology Web by the Prinicpal Investigator on this proposed mission with their permission to distribute it freely as we deemed appropriate. This is not an official NASA or JPL website and, as such, the information presented here is in no way endorsed or validated by NASA or JPL. Neither the Astrobiology Web or Reston Communications has any contractual or bidding relationship with NASA, JPL, or any of the proposing parties. We just think these are exciting ideas and wanted to share what we have learned with our readers. |
D. SCIENCE
D.4.7. Active Volatiles Collector (AVC)
One of the key science goals for the Europa Ice Clipper Baseline Mission is the return of water from Europa for D/H and oxygen isotope analysis. These isotopic analyses will provide information on the sources of water in the Galilean satellites and the conditions in the jovian sub-nebulae. Water collection will be accomplished by the Active Volatiles Collector (AVC). In addition, if the heavier noble gases (Ar, Kr and Xe), and possibly Ne, are present in the ice on Europa at approximately solar concentrations then they also will be captured in sufficient abundance in the AVC to be carefully studied. Elemental concentrations and isotopic compositions of these noble gases can then be obtained. Noble gases, because of their low natural abundances, simple chemistry and multiple isotopes, have proven to be quite useful in developing our understanding of the early evolution of the solar system, the energetic particle environment of the early sun, the evolution of planetary atmospheres and, for the case of Europa, could provide further information on the source of the volatiles for the Galilean satellites.
Collection of volatiles is a challenge on the Ice Clipper, because the low encounter velocity with the Europa plume (8-10 km/s) is too slow for capture by implantation and gases held in absorption beds would not survive the return trip and entry at Earth. Our AVC method is based on a simple lightweight and low-power method of actively capturing cometary volatiles by the co-deposition of low-Z metal (Al or Mg) onto a sapphire substrate. As volatile molecules impact the substrate, some of them are held at the surface or near-surface for times much longer than the (10-13 s) collision time. and are covered up by the continuously deposited metal film before they are lost. Storage of the volatiles in the metal films is permanent until the film is removed. A schematic of the instrument is shown in the figure on the pullout. Upon return to the Earth, the metal films are easily removed by laser volatilization, releasing the entrapped volatiles for analysis by well - proven techniques.
The metal films are inherently clean since only local volatiles during encounter (when the metal film is deposited) are captured. This will be dominated by the Europa plume as the spacecraft will have been well degassed prior to encounter.
The Active Volatiles Collector will capture Europa volatiles in a thin metal film deposited during encounter. The metal film protects the volatiles from post-encounter contamination in the spacecraft, entry and terrestrial environments. After sample return, the thin metal film can be removed by a single laser pulse, liberating the contained volatiles for analysis. Some analyses, such as spectral absorption, can be done through the film on parts of the collector where thinner films are deposited without altering the sample.
Most of the returned material (75%) will be deposited in the JSC curatorial facility for use by other investigators. The volatiles in the metal films could be analyzed in many ways by a variety of techniques. Examples of the techniques that could be used on these samples include: (i) detection of volatile molecular species by UV and IR spectroscopy (absorption and fluorescence), (ii) measurement of isotope ratios for the more abundant species like H and O, and the heavy noble gases and (iii) laser desorption noble gas and 2-step organic mass spectrometry, as well as the more conventional static mass spectrometry. The stability of the metal films ensure that the samples will remain available for analysis for future generations of analytical techniques.
The most desirable metal for coating is magnesium, as it requires only a few watts of electrical power for the few minutes of actual metal deposition during plume encounter. In recent laboratory simulations we have demonstrated a capture efficiency of approximately 0.3% (for 14 eV krypton).
In support of volatiles collection in the outer solar system a breadboard of the Active Volatiles Collector has been constructed and tested for the collection of noble gases. Most of the elemental comparisons were made using aluminum as the co-evaporated metal, since Al films are known to capture with a nearly 100% efficiency at solar wind energies (1000 eV/amu) and tenaciously retain the captured gases. We have detected no sample loss after 8 years of storage in air of noble gas implants in Al films. In experiments with this breadboard, capture efficiencies for Ar (at 7.5 eV), Kr (at 14 eV), and Xe (at 24 eV) ranged between 0.1% and 1%. The thickness of the metal film used in the simulation runs was 200 nm at a deposition of 4 nm/s.
Mass discrimination for krypton and xenon has been measured on a suite of six different samples, co-implanted under different conditions and with different metals (Al, Mg and Zn). There appears to be an approximately 2%/amu mass discrimination independent of the experimental conditions. The capture efficiencies and the mass discrimination are experimental parameters that will be exhaustively determined in a matrix of normal (and abnormal) conditions prior to the mission. Initial studies show these to be well-behaved over a variety of conditions. As discussed below, if the Active Volatiles Collector encounters a plume from Europa of about 1016 H2O molecules cm-2, ample water will be collected for D/H and oxygen isotope measurements using SIMS. Assuming solar ratios, we should have sufficient collection of Ar, Kr and Xe for conventional mass spectrometry: (about 1011, 109, and 109, impacting per cm2, respectively). With the capture efficiencies given above, 1 cm2 of recovered collector will yield about 3 million Xe and Kr atoms and 30 million Ar atoms, well within current state-of-the-art mass spectrometry, where typical blanks for Kr and Xe are 10,000 atoms. If the column densities of volatiles is very much lower than this, or if the active volatiles collector should fail part way through the encounter, resonance ionization techniques can be used that have sensitivities significantly greater than conventional mass spectrometry.
In addition to the noble gases, which we have used in the simulations thus far, and represent the "worst case" capture candidates, the whole suite of volatiles expected from Europa should be captured and returned for laboratory analysis. For instance, water should be captured with high efficiency and with a H column density of more than 1016/cm2, a capture efficiency of only 1 percent will make the D/H ratio a possible measurement.
The plume model discussed above suggests that the total column mass of water vapor intercepted by the spacecraft is ?? molecules/cm2, adequate for the AVC. Power to evaporate the Mg, uniformly coating the wires, comes from a power-regulated DC-to-DC converter. This converter takes 28 V DC from the spacecraft bus and converts it to a 3 VDC, regulated by the total power supplied to the evaporator wire. Power required will less than 10 watts for less than 10 minutes. The weight of the total unit is 350 grams.
The key science goal for the Active Volatiles Collector is the collection of sufficient water vapor to allow for the determination of the D/H and O isotopes. We propose to do this analysis by Secondary Ion Mass Spectroscopy (SIMS) on a fraction (1/8) of the returned sample.
Current detection limit by SIMS is ~200 ppba for O in pure synthetic materials (such as semiconductors). This does not include the problems due to surface contamination, which must be eliminated in other ways.
If we can measure and correct for background with 10% accuracy, with no more than a 0.5% background correction we can achieve 0.5 per mil overall uncertainty due to background. This translates into collecting 40 ppma oxygen in the near-surface layers of the target (at the top 2 um). If we have a unit square centimeter of target this represents collecting about 5 x 1014 atoms of O (or molecules of water, since this may be the only source of oxygen). With a useful yield of 1 per mil by SIMS this gives enough detected 17O ions to achieve a statistical precision of order 0.1 per mil. With the background correction and other sources of instrumental uncertainty, it could still be possible to achieve an overall precision of 1 per mil (or maybe slightly better) in both del-17O and del-18O by analyzing an entire cm2 of collector (laborious, but achievable). One per mil provides an acceptable level of precision and useful results from the oxygen isotopes. It may be possible to reach 0.5 per mil with technology available in the near future, but in any case it would require about 5 times more collected sample. Note that the best current measurements on rocks (where O abundance is not a problem) can only achieve 0.5 per mil with extraordinary effort, (i.e., it is not at all routine to get to this precision level and it cannot always be achieved even with unlimited sample).
The SIMS measurements will require instrumental development well beyond the current state of the art, but most of these advancements are in the planning stages. The major issues are (1) a working, high mass resolution multicollector instrument capable of simultaneous measurements of all 3 oxygen isotopes (this is critical), (2) an UHV vacuum system with about factor of 10 improvement over existing commercial machines (< 10-10 Torr range required), and (3) some method for removal of surficial oxygen aside from sputtering (laser desorption) in vacuum prior to analysis. The first point is under development at CAMECA and UCLA (co-I: McKeegan) will receive a multicollector sometime next year. The second issue is reasonably straightforward and will be supported as part of this project. The third issue is not yet seriously addressed and a plan for its resolution will be developed during the feasibility study.