A Proposed Discovery Mission
Photo: NASA/JPL [Larger 99K GIF]
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.4.3. Jupiter Europa Particle Analyzer (JEPA)
In addition to the Camera, the principle instrument for in situ analysis will be the JEPA (Jupiter and Europa Particle Analyzer). JEPA will be a virtually identical instrument to CIDA (Cometary and Interstellar Dust Analyzer) which is being flown on the Stardust mission, and which is derived in turn, from the PIA (Particle Impact Analyzer) flown successfully on the Giotto spacecraft mission to Halley's comet.
As the spacecraft flies through the impact generated plume, particles in the plume hit the silver target of the JEPA instrument (as shown in the pullout). Upon impact the particles are vaporized and partly ionized. The resulting ions in the impact plasma are extracted by electrostatic fields for injection into a time-of-flight mass spectrometer. This technique was demonstrated on the Halley Flyby missions, with three different TOF-MS instruments of this type, one each on the Giotto (ESA) mission and the two Vega (Russian) spacecraft. JEPA and CIDA are improved versions of the Giotto/Vega versions of the instruments, with mass range now covering 1 to 330 Daltons. In addition, all data will be complete spectra, with no need for the highly lossy data compaction that was necessary for the Halley flybys. Mass resolution for this TOF-MS is 150 or better, using a transient recorder method with 10 ns sampling increments.
A complete compositional analysis of elemental composition of the particles is obtained, and because of the much lower flyby speeds than for Halley's comet (70 km/s), it is expected that much more information will be preserved, generating considerable data on molecular constituents, including the organic materials that may be present.
Our model of the plume density shows that the number of particles larger than 2 um in diameter impacting the JEPA target will nominally be 5,000 over the course of the fly through. Since the JEPA spectrum sweep is completed in 72.66 us, the instrument can accommodate particle impact event rates of several thousand per second. JEPA is sensitive to much smaller particles, 10- 16 g (less than 0.1 um), for which the event rates could be extremely high. A similar problem occurs for the Stardust mission. Using the same built-in adaptive technique for reducing the effective target area via electrostatic biasing, the JEPA instrument will prevent spectral overlaps if high event rates are encountered. The only limitation on data taking will be the storage capacity of the on-board mass memory. Baselining for the time being the Stardust memory size (1 Gbit total), the allocation for JEPA is 100 Mbits. This is adequate to store up to 15,000 particle spectra, with simple lossless data compression. The total number of events from pooling the three Halley missions was considerably less than this amount, and the number of lossless spectra obtained was less than 1% of this amount. Thus, we expect to acquire a wealth of in-flight compositional data on the particulates, which should also provide an excellent statistical base from which to ascertain the microscale compositional heterogeneity of the area sampled. For example, mineral or organic components may not be uniformly dispersed throughout the material volume sampled.
This large dynamic range capability of the JEPA instrument will also allow us to respond to changes in the expected particle concentration due to improved models, experimental simulations, or further data from Europa itself.
The JEPA instrument has the capability to analyze either positive or negative ions from a given particle event, but not simultaneously. It takes up to 30 seconds to switch from one mode to the other. Because the fly through time is short, it will not be possible to repeatedly switch modes in the instrument. Our strategy will be to set the mode for positive ions during cruise, to use both modes in the Jupiter zone prior to Europa encounter, and then for negative ions at the plume encounter. These strategies are related to the fact that the Halley data and the Stardust interstellar data are with positive ion extraction. However, for Europa both modes are desired. Our approach will be to set the instrument for the pre-determined optimum setting, and once 2,000 particles have been successfully detected and analyzed, it will automatically switchover to the oppositely charged ions. This assures that the switch, with its associated downtime, is not done prior to collection of a significant data set.
We expect that the particles analyzed will consist primarily of water ice but with measurable levels of organic and mineral components. The JEPA instrument will thus return the following information:
- overall elemental composition of all components
- characterization of the organic component
- characterization of the mineralogy of the dust particles
- range of elemental ratios
- light element abundance relative to CI Chondrites
- light carbon and other major isotopic anomalies
The concept of the dust-impact time-of-flight mass-spectrometer has proven its capabilities during the flyby missions to Comet p/Halley in 1986. The PUMA 1 and 2 and PIA dust impact mass spectrometers on board Vega1 and 2 and Giotto respectively provided several thousand mass spectra of individual dust particles released by the comet. In the meantime extensive data analysis and data interpretation has taken place. The consistency and the variety of the results obtained and published by various groups proves the versatility of this kind of instrument. In addition, the development of the instruments and the in situ measurements have lead to an improvement of the understanding of ion formation during particle impact, with continuing improvements (Hornung and Kissel, 1994). This better understanding together with earlier work like by Krueger and Kissel (1984) ensures that data obtained at different impact velocities can easily be compared. The Ice Clipper trajectory provides an opportunity to measure also at lower impact speeds (as compared to Halley), which as found by Knabe and Krueger (1982) provides a higher share of molecular ions in the mass spectrum of the individual particles. The major elements measured at comet Halley were: H C N O Mg Al Si S Ca Fe; the minor elements measured were: B Li Na P (Cl) K Ti Cr Mn Co Ni Cu; and the elements whose presence were only inferred were: V Zn Ga Ge Se Br Sr Y. Detection of mineral components was clearly successful at Halley (Brownlee and Kissel, 1989), and characterization of organic components was likewise successful, including the determination of CHON particles and subclasses of organic constituents (Clark et al., 1987). With the improvements in mass resolution and full spectral readout, it is to be expected that the JEPA instrument will be able to further improve upon the Halley measurement capabilities.