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.4.1 Impactor and Plume Dynamics
The centerpiece of the Europa Ice Clipper concept is the controlled impact on the surface and the resulting production of a plume of surface material which can be analysed and sampled. Because of the importance of this aspect of the mission we devote considerable detail to the description of our analysis of the plume formation and time evolution (see pullout number 2). Currently this analysis is based on existing laboratory data for cratering in sand, basalt and ice. Because the ice cratering data is limited, we have begun some small scale experimental studies of impacts into ice. More extensive simulations will then be conducted as part of the feasibility study. These studies will be conducted at Caltech and at Ames in the Vertical Gun facility. Our current designs are based on theoretical extrapolation of the available laboratory data.
To demonstrate the viability of ejecta collection from Europa by the Ice Clipper spacecraft, model calculations of ejecta dynamics and collection have been performed. An overview and results of the calculations are presented here; a detailed description of the calculations are given in the pullout at the end of Section D. The calculations assume a 10 kg projectile released by the spacecraft. The impact velocity is approximately the flyby velocity of Ice Clipper, assumed to be 10.6 km/s. Ejecta is collected as the spacecraft passes through the ejecta plume. Total collected ejecta is computed along the spacecraft trajectory discussed in section F. For this trajectory the impact angle is ~ 20 degrees with respect to horizontal. Because the porosity of Europa's surface is not known, calculations are presented for both gravity and strength- dominated cratering. A cohesive layer of water ice would crater in the strength regime, whereas a fluffy ice or a loose regolith may be gravity controlled. Specifically, cratering in wet sand, dry sand, and crystalline ice are considered. Abundant laboratory data exists for ejecta excavation in wet and dry sand, and provides a good proxy for gravity-controlled cratering. Strength- controlled cratering was represented by ice impact data. Estimates were also made of the ejecta particle size distribution, but limited data exists for impact ejecta distributions from ice targets. The effect of atmospheric drag on fines, and the total amount of Europa atmosphere sampled by the spacecraft, are also calculated.
Cratering scaling laws for wet and dry sand (Schmidt and Housen (1987) are used to determine the total mass of ejecta and crater dimensions. The total ejecta mass is 2.4 x 104 and 2.5 x 105 kg for dry sand and wet sand, respectively. For a non-porous surface, such as annealed ice, cratering is in the strength- regime, and the total ejecta mass depends on the tensile strength of the target material. Using a scaling law derived for cratering in ice targets (Lange and Ahrens 1987), and for a surface ice with a tensile strength of 200 bars (Europan surface ice could be significantly weaker), the total ejecta mass computed is 2.6 x 104 kg. Assuming a half-oblate spheroid crater with a depth-to-diameter ratio of 0.2 (Ahrens and Harris 1994), the crater diameter is 17 m (wet sand), 7.4 m (dry sand), and 6.2 m (ice, strength 200 bars). Thus, a sensitive measure of Europa's crustal strength will be available from imaging of the impact crater.
To determine the mass of ejecta collected by the spacecraft, the mass and velocity distributions of the ejecta at the flyby altitude of the spacecraft are required. Based on the semi-empirical expressions of Housen et al. (1983) and Holsapple (1993), the cumulative mass of ejecta with velocity > v has been calculated for cratering in the gravity and strength regimes. For the spacecraft trajectory considered here, ejecta originating with v > 0.5 km/s is capable of reaching the spacecraft altitude. The total mass of ejecta moving at v > 0.5 km/s is 510 kg (wet sand), 25 kg (dry sand), and 120 kg (ice, 200 bars). The actual ejecta mass collected is computed by integrating the ejecta mass distribution along the spacecraft trajectory. For a hemispherical ejecta cloud, and a collection area of 0.1 m2, the collected ejecta mass is 0.2 micrograms (ug) (wet sand), .01 ug (dry sand), and 0.08 ug (ice, 200 bars). The bulk of the ejecta is collected over a period of ~ 102 seconds, about 500 seconds after projectile impact. To refine our modeling results, light-gas gun experiments with ice targets are underway to provide direct data on total ejecta mass excavated and the mass and velocity distribution of ice ejecta particles.
An estimate of the distribution of particle sizes in the collected ejecta is made following the formalism of O'Keefe and Ahrens (1985) in which the cumulative mass distribution function of ejecta particles with mass > m and velocity v has the same functional form as the distribution function for ejecta fragments in an ejecta blanket. Via this formalism the number flux of particles collected as a function of particle size is computed (see pullout at end of Section D). For ice targets, calculations of this type are quite uncertain; even the minimum ejecta particle size for ice targets is unknown. Data from planned laboratory impact experiments into ice will greatly alleviate this uncertainty.
From the formalism of O'Keefe and Ahrens (1985), an assessment of the collision hazard of Ice Clipper with larger ejecta particles can be made. The largest particles capable of reaching the spacecraft have diameters of 0.52 cm (wet sand), 0.17 cm (dry sand), and 4.9 cm (ice). In a worst-case scenario (all ejecta mass is in particles ~ 0.1 to 1.0 cm in diameter), the probability of collision with a 10 m2 spacecraft is ~0.01. Thus, spacecraft shielding against particles ~1 cm is required.
Based on these calculations we determine that the 0.1 m2 area of the particle analyser (JEPA) will collect over 5,000 particles of size about 1 um (radius) over the course of the fly through. Collected ice ejecta may be in particles larger than 1 micron. Similarly the 0.01 m2 area of the particle collector (PC) will collect 0.3 nanograms of water, and the AVC will collect ?? molecules m- 2 of water vapor. Given the shield configuration discussed in Section F, we estimate that the probability of an impact serious enough to endanger the spacecraft is less than .01 (see above paragraph).