NASA’s Needs for Advanced Broad Band Seismometers Bruce Banerdt Jet Propulsion Laboratory Pasadena, CA

Agency’s Perceived Need NASA is not a particularly seismology-friendly agency. On the planetary side, there have been only four seismic experiments in the history of spaceflight: –Ranger (Moon, cheap, unsuccessful) –Apollo (Moon, expensive, successful) –Viking (Mars, cheap, unsuccessful) –Mars ‘96 (Mars, expensive, unsuccessful) In each case, the instrument was largely designed from scratch. In addition, there have been about a half-dozen canceled Mars seismic experiments in the past 15 years. Agency perspective: seismology is an expensive bother.

Scientific Needs Strong scientific priorities have been consistently stated by many advisory groups for understanding the interiors of the planets. –Implications for the formation, evolution and current state of the solar system. Clearly, the only effective way to do this is through seismology. –Most of what we know of the interior of the Moon comes from the Apollo PSE. Seismic measurements can also contribute to studies of current processes. –Meteorite flux –Level of endogenic activity (volcanism, tectonism, etc.)

Specific Targets Mars –Almost nothing is known about the deep interior (loose bounds on size, state of core; factor of 3 uncertainty in average crustal thickness). –Seismic network (4-16 stations) has been identified as a viable near-term mission (2011 earliest launch). Europa –Probable structure consists of ice shell overlying liquid water ocean. –Thickness of shell, depth of ocean are critical parameters in theories of its evolution. –Implications for the possible existence of life. –Single seismometer on a lander has been baselined for future JIMO mission (2014 earliest launch).

Specific Targets Moon –Large gaps still exist in our knowledge of the interior. –Any new experiment would have to significantly improve on Apollo. –Lunar science priorities are in a state of flux from recent Presidential initiative. –An improved lunar seismic network is being studied for a possible spot in the mission queue (earliest launch 2009). Venus, Mercury, … –Lots of fascinating science questions. –Formidable technical and financial hurdles.

Requirements for Planetary Seismometers Many of the requirements for a planetary seismometer overlap with those for terrestrial instruments: –Frequency range –Dynamic range –Stability –Etc. However, there are many requirements which are specific to the planetary arena. –Measurement requirements –Technical requirements

Measurement Requirements Generally, planetary seismometers require a level of sensitivity that might be considered unnecessary on the Earth due to its relatively high noise floor. –Noise levels on most planetary bodies are predicted to be 1-3 orders of magnitude lower than the Earth. Apollo PSE did not detect the noise floor at the 0.3 nm level. No subareal oceans No massive, turbulent atmospheres No interstate highways or trains –Levels of seismic activity are also expected to be many orders of magnitude lower than the Earth, making the detection of very small events critical to the success of an experiment.

Technical Requirements The cost of a flight experiment depends (roughly exponentially) on mass and power. –Lunar seismometers weighed 11.5 kg and required about 7.5 W. This would be a non-starter today. 1-3 kg and W might be considered reasonable. A planetary seismometer must be fairly rugged. –Typical shock specs are ~200g over a few 10s of ms. –This is equivalent to knocking an instrument off a table, or comparable to what UPS inflicts. –It has to always work. Always. Must be radiation-hard Insensitivity to temperature, orientation, EMI is a plus.

Conclusions We just might not get everything we want in terms of performance. –But, may only get one shot in a professional lifetime; want to get everything possible. The technical requirements tend to be non-negotiable. –If it’s heavy or power-hungry, it doesn’t happen. Measurement requirements generally must be sacrificed to some extent. –Sensitivity –Instrument noise –Low-frequency response But installation is poor anyway…