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.4.2 Camera Science Implementation
A crucial part of this activity will be Ice Clipper imaging system. It will have two main operational functions: (a) conduct optical navigation (opnav) exercises that will allow the close-in 50 km approach to Europa, and (b) provide geo-located data that will allow pinpointing of the actual impact site. In addition to and complementing the operational requirements are a variety of related science goals that will depend on the imaging system for their attainment. In general these are: (a) outbound and inbound stellar imaging for opnav, and for limb occultations to determine Europa's radius; (b) whole body imaging to accurately determine the optical figure of Europa at pixel scales of order 1-3 km, and thus substantively contribute to the discussion as to whether Europa possesses an ocean under an icy crust; (c) impact process imaging to attempt to capture the actual impact crater and to gauge the important parameters associated with the products of impacts (e.g., surface properties/site location/sample identification, ejecta plume characteristics, ejecta blanket characteristics) ; and (d) geomorphic imaging to address the actual formative processes of the surface as reflected in the surface textures related to bright interlinea areas and the lineaments themselves. All aspects of the imaging system will be geared to answer the primary question posed by this mission: does Europa possess an ocean under the ice, and are the ice - coloration materials ocean-derived products (salts, organics, etc)?
Optical Navigation will be essential for the Europa Ice Clipper. With precise optical navigation it will be possible to maneuver to achieve a 50 km Europa flyby distance.
Imaging of Impact Event and Site will be an important aspect of the planned mission, and will depend on imaging sequence timing and on adequate sensitivity of the detector. Seeing the impact event -- both the impact flash and the initial stages of ejecta plume formation -- will be important not only from the aspect of probing the mechanical and chemical properties of Europa's upper crust, but could be of great benefit in locating the site of the impact. This is particularly important, given that the small scale morphology and structure of the Europan surface is unknown below a pixel scale of 300 m/line pair. A posteriori determination of the impact site should be possible given the combination of wide-angle (low spatial resolution) and narrow angle (high spatial resolution) camera designs that we are implementing. Fresh bowl-shaped craters are rare at larger scales on Europa (Malin and Pieri, 1986) and resurfacing processes appear to soften and remove crater rim morphologies down to km scales. Thus, a new 100 m diameter crater with a fresh, rough, bright ejecta blanket will be clearly identified at the imaging resolutions of order 1 m, that will be possible during the impact site flyover from 137 km range, and that the relatively low planned sun angle (10-20 deg above the horizon) will aid in the detection of small scale topography, such as fresh crater rims. The challenge will be in targeting the impact site within the FOV of the NAC.
Imaging of Surface Materials will provide information at a spatial scale not accessible to either the Voyager or Galileo spacecraft. Maximum pixel scale at 50 km closest approach will be 0.5 m, yielding a "rule of thumb" isolated feature resolution of about 1.5 m, and a FOV of about 500 meters. This is about 10 times better than the maximum resolution expected from Galileo. This high resolution, narrow-angle data approximates terrestrial metric airphoto resolution, and will be comparable to Mars Global Surveyor highest- resoluion orbital data at Mars. Closest approach to the impact site will occur at 137 km with pixels only a little more than 1.2 m. Orbital data at these pixel scales can literally be used like airphotos for geological mapping on the Earth. Surface textures and discrete morphological features are easily accessible, and the geomorphic process signatures, prominent at scales of 1 - 10 m on the Earth and Mars, will be visible. Clearly, of particular interest will be any features diagnostic of ice floes, ice tectonism, or ice volcanism. With the several dozen Narrow/Wide-Angle nested and mosaic images planned for the flyby and close approach phases of Ice Clipper, a comprehensive sampling of the smallest scale Europan geomorphic features and processes should provide otherwise unattainable information on the nature of the formation, and composition, of the icy crust.
The impact site, of course, will merit special attention. Since it will be fresh, it will stand out in comparison to any other impact or endogenic geomorphic feature. Its ejecta blanket could show contrasting reversed layering, if the substrate itself is layered, and crater walls could reveal stratigraphic layering, which would undoubtedly yield valuable insights into the substrate and surface formation processes.
The characteristics of the plume and its dissipation timescales would provide important information on the mechanical properties of the Europan surface. While the smallest size fraction of the plume will tend to be grey spectrally, it is to be expected that the larger particles could show some spectral contrast, thus yielding information about the composition of the upper crust of Europa. And, even though the phase angle "parameter-space" observations will have a relatively modest angular excursion, there will be information on the particle size-frequency distribution embedded within these data.
Optical determination of the figure of Europa will also be possible using full disc images of the planet combined with partial limb-fits. The currently specified (see below) Cassini-derived telescope system with its 1024 x 1024 pixel array will allow us only approximately 3.1 km pixels when Europa fills the FOV of the Wide-Angle camera, with comparable pixel scale available from the Narrow-Angle camera at the point where its FOV is filled by Europa. During the feasibility study we will consider a larger (e.g., 3400x3400) CCD array on the Wide Angle camera, thus yielding much wider FOV, and at better determination of whole planet diameter, with a view toward more accurate radii values, for determining whether Europa possesses a hydrostatic figure, and thus possibly a liquid-water sub-ice ocean. Limb fitting routines can also be employed to fit limb segments, in the absence of a full disc image. On approach, Europa will exhibit a 120-130 degree phase angle, yielding a crescent Europa (we will impact into the sun-facing side just beyond the terminator). Nevertheless, our calculations show it will be possible to view the "unlit" side of Europa in reflected light from Jupiter in order to capture a full disc image.
Stellar occultations, if they can be identified and observed, will be useful to characterize (in a limited sense) the limb topography, but may possibly be useful to probe the lowermost (near surface) structure of Europa's tenuous oxygen atmosphere. In addition, since the limb of Europa will be overexposed for opnav purposes, images of such phenomena may be significant in detecting geyser plumes, similar to the Voyager-Io plume discoveries.
Finally, near-limb topography, as derived from the whole-disc images may indicate the presence of gravitationally uncompensated continental-sized terrain, but there is at present no topographic information to suggest this.
The NAC (Narrow Angle Camera) telescope design is derived from the space- qualified optics developed by Optical Corporation of America (OCA) Applied Optics Division for Lawrence Livermore National Laboratory. They were flown successfully on Clementine Mission for geological survey of the Moon. The high resolution telescope is low in mass and uses low-outgassing materials qualified for space usage. The optical designs use thermally compensating element cell designs for stability over wide temperature ranges necessary in remote sensing space applications. The NAC telescope uses a lightweight beryllium mirror and structure. The block diagram illustrates both the wide angle camera, WAC (150 mm focal length), and the NAC. Mass has been allocated to shield the CCD from the radiation environment that it will accumulate through the mission. The 1024 x 1024 12 micron pixel array CCDs will be provided by JPL. These CCDs were developed for the Cassini Camera, derived from the Galileo design, which is currently operational in the Jovian environment.
The camera design uses radiation tolerant electronic parts and shields those known to be radiation sensitive to a 10 krad total dose level. The radiation effects would be seen only as radiation noise in individual frames and quasi- permanent radiation damage to the CCDs know as "dark spikes". The CCDs on the Ice Clipper Cameras will have up to 0.5 to 1 cm of tantalum around the focal plane. The S/C spends much less time in the Jovian radiation environment than the Galileo S/C. The filter, primary and secondary mirror of the telescope will also shield the CCDs from the electron flux coming directly into the telescope aperture (the angular direction not shielded by the tantalum). We plan to store the CCDs at temperatures between -20° to +32° C for annealing and then to run them at about -20° C. The warm storage temperature will anneal out any dark spikes and the cool operating temperature will suppress the dark level and obtain optimium noise performance of ~10 electrons.