Understanding the Interior of the Moon: A Missing Fundamental Dataset

Transcription

1 Understanding the Interior of the Moon: A Missing Fundamental Dataset Clive R. Neal 1 & the LuSeN Team, especially Yosio Nakamura??? 1 = Dept. of Civil Eng. & Geological Sciences University of Notre Dame Currently at: The Lunar & Planetary Institute Houston, TX neal.1@nd.edu

2 The Lunar-L An list server to facilitate communication within the lunar community. Currently 260 subscribers worldwide. Clive R. Neal at to subscribe.

3 Lunar Science Initiative New Views of the Moon Enabled by Combined Remotely Sensed and Lunar Sample Data Sets Reviews in Mineralogy Mineralogical Society of America Publication: Late 2005/Early 2006

4 Lunar Science Initiative Objectives remote sensing geophysics samples (1) To foster interdisciplinary communication among lunar scientists with diverse backgrounds. (2) To promote the use of integrated approaches to address fundamental issues in planetary science: interior structure crustal evolution impact history and age planetary volcanism regolith formation (3) To advance our understanding of the Moon and its history to a new level. (4) Synthesize global data sets and integrate them with one another and lunar-sample data base. (5) To show by example how to adopt a global perspective in exploring the constitution and history of a planet with diverse data. (6) To facilitate further Solar System exploration.

5 Information on the Interior of the Moon

6 Information on the Interior of the Moon Samples Highlands (intrusive) Ferroan Anorthosites (FANs) Mg-Suite Alkali Suite KREEP Glasses (µ < 100) Mare (extrusive) Basalts Crystalline Basalts (µ > 100) Very Low-Ti (VLT) Low-Ti High-Ti Experimental Petrology: Stratification of source regions. Lunar Magma Ocean.

7 Information on the Interior of the Moon Samples

8 Apollo 11 Geophysics Package deployed on the Moon

9 Information on the Interior of the Moon Geophysics Heat flow. Gravity. Apollo Passive Seismic Experiment:

10 An Example of Lunar Seismogram [from Nakamura, J. Geophys. 43, , 1977] [From Nakamura, JGR 88, , 1983]

11 What Do We Know About the Lunar Interior? Crust on near side is km thick - anorthositic. Upper mantle is hypothesized to be pyroxenitic, not peridotitic. Experimental Petrology: Stratification of source regions. Lunar Magma Ocean - cumulates formed the crust (flotation) and mare basalt source regions (sinking). Geochemical arguments require an overturn (mixing) of the cumulate pile. Magmatism was most active > 3 Ga.

12 What Do We Know About the Lunar Interior? There is a seismic discontinuity at ~500 km on the lunar nearside. There is probably a small ( km diameter) core. (Righter, 2002, Icarus 158, 1-13; Hood et al., 1999, GRL 26, 2327). [From Nakamura, JGR 88, , 1983]

13 Seismology of the Moon The Moon is NOT seismically dead! While it is a one-plate planet, seismicity is equal to intraplate seismicity on Earth. Oberst & Nakamura (1992) 2nd Conference on Lunar Bases & Space Activities

14 Four types of events induce seismicity on the Moon. Seismology of the Moon Nakamura et al. (1982) PLPSC 13th. 1) Deep Moonquakes ,000 km. >7,000 recorded. Originate from nests nests defined from Apollo seismic data to date. Small magnitude (<3). Associated with tidal forces.

15 Limiting Rays to Apollo Seismic Station Network Plastic/liquid zones?

16 2) Shallow Moonquakes - some >5 magnitude. Exact locations unknown. Indirect evidence suggests focal depths of km. May be associated with boundaries between dissimilar surface features. Exact origin unknown. Seismology of the Moon 3) Meteoroid Impacts - >1,700 events representing meteoroid masses between 0.1 and 100 kg were recorded Smaller impacts were too numerous to count. 4) Thermal Moonquakes - Associated with heating and expansion of the crust. Lowest magnitude of all Moonquakes.

17 Limitations to our Understanding of the Lunar Interior No direct sampling of the lunar mantle (i.e., no mantle xenoliths. Difficult to identify a primary melt (glasses are the best bet!) Very little of the lunar surface was sampled. Geophysical data only from limited sites on the nearside. Seismic network only of limited extent. There is a need for a global Lunar Seismic Network,

18 Unresolved Questions - Pure Science If there was a magma ocean, how deep was it? Is there a Moon-wide ~500 km discontinuity?

19 Unresolved Questions - Pure Science What is the nature of the deep lunar interior? Is the upper lunar mantle really pyroxenitic? 20 A11 (Old) A11 (New) A12 (old) A12 (New) A14 (Old) A14 (New) A15 (Old) A15 (New) Chondritic Ratio A17 (old) A17 (New) Glasses 200 A11 (Old) A11 (New) A12 (old) A12 (New) A14 (Old) A14 (New) A15 (Old) A15 (New) Chondritic Ratio A17 (old) A17 (New) Glasses 15 KREEP 150 KREEP Yb (ppm) 10 Y (ppm) Garnet in Residue 5% Garnet in Glass Souce 1-5% Partial Melting Sm (ppm) 50 Garnet in Residue 5% Garnet in Glass Souce 1-5% Partial Melting Zr (ppm)

20 Unresolved Questions - Pure Science MAY have a small core ~250 km. MAY be Fe, FeS, but MAY be ilmenite (FeTiO 3 ). Current models suggest that the core would be solid if Fe metal, but could still be liquid if it was FeS. Are there still plastic zones within the lunar mantle????

21 Unresolved Questions - Pure Science What are the structural and thickness variations in the lunar crust (nearside vs. farside)? Are crustal structure changes gradational or are distinct domains present? Do these extend into the lunar mantle? S.R. Taylor (1982) B. Jolliff et al. (2000) JGR 105, E2

22 Unresolved Questions - Pure Science Are there seismic nests on the lunar farside? What are the locations and origins of shallow Moonquakes, the largest lunar seismic events? Oberst & Nakamura (1992) 2nd Conference on Lunar Bases & Space Activities Nakamura et al. (1982) PLPSC 13th.

23 Unresolved Questions - Applied Science Where should the proposed lunar base be situated? Location(s) and origin(s) of Shallow Moonquakes are unknown. Need a better understanding of meteoroid impact locations.

24 The LuSeN Mission Concept A minimum of 8 (preferably 10) seismometers deployed around the Moon. Mission life = 5 years (minimum). Communication satellite required. 1. Fowler Crater. 2. Hertzsprung Crater. 3. Between Bjerknes & Clark craters. 4. Mare Imbrium. 5. Mare Cognitum. 6. Schickard Crater. 7. Mare Tranquilitatis. 8. Mare Marginis. 9. Kurchatov. 10. Between St. John & Mills craters.

25 Science Goals and Objectives Mission Goal: Investigate, in detail, the deep interior of the Moon Objective 1: Determine the thickness of the crust and its lateral variation. What is the mean crustal thickness? Establish the nature of the near side far side dichotomy. Measure the thickness beneath a mascon basin. Objective 2: Determine the size, and structure of the core. What is the core s radius and flattening? What is its density? Is it solid or liquid (or both)?

26 Science Goals and Objectives Objective 3: Investigate the structure and composition of the mantle. What discontinuities exist in the mantle? Nature of discontinuities: distinct vs. gradational? Is garnet present in the lower mantle? What is the nature of the seismogenic layer at 800 km? Do plastic zones exist? Objective 4: Map the seismicity of the Moon on the far side. What is the 3-dimensional distribution of seismic activity on the far side? Are there correlations with surface geology? Is there a component that is correlated with tides?

27 Science Goals and Objectives Objective 5: Get statistical data on the locations and magnitudes of shallow moonquakes and meteoroid impacts. Where is the safest place for a Moon base?

28 The LuSeN Mission Concept Technology Developments: Power Supply - mini-rtg/rps needed. Deployment - hard vs. soft landing of each package. Robust, yet sensitive seismometer package that can survive a hard landing, yet be able to detect the smallest of Moonquakes. Netlander?

29 LuSeN Team Clive R. Neal, PI: University of Notre Dame W. Bruce Banerdt: Jet Propulsion Lab Renee C. Bulow: IGPP, SCRIPPS, UC San Diego Hugues Chenet: ISAS/JAXA, Japan Lon Hood: University of Arizona Catherine L. Johnson: IGPP, SCRIPPS, UC San Diego Brad Jolliff: Washington University, St. Louis Amir Khan: IPGP, Paris Georgiana Y. Kramer: University of Notre Dame David J. Lawrence: Los Alamos National Lab Philippe Lognonne: IPGP, Paris Steve Mackwell: Lunar & Planetary Institute Kevin Miller: Ball Aerospace David Mimoun: IPGP, Paris Yosio Nakamura: University of Texas, Austin Jack Schmitt: University of Wisconsin Chip Shearer: University of New Mexico Mark Wieczorek: IPGP, Paris

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32 Top System Design Issues/Status Landing Trajectories with Respect to Lunar Terrain High probability of successfully defining trajectories within about 10 deg of target landing site while maintaining margin versus lunar terrain and complying with design constraints on landing shock Impact Attenuation / Probe Mechanical Design Rigid Body Impact with lunar surface results in 170 to 335 G Shock attenuation options include crushable metal foam, aerogel, shape memory materials, airbags Trade Study on Specific solution In Work Probe Data Collection Monitor seismometer output; collect and store data only during seismic event FIFO Memory Architecture Features 1 µsec Activation Time Power required is < 1 W standby, 1 W during event, 9 W transmit Investigating Power Reduction Options Clock synchronization on ground by measuring probe temperature and correlating with known oscillator characteristics Probe Telecom / Data Relay 16 kbps UHF link from probe to orbiter will work assuming 4:1 data compression Adequate number and duration of orbiter passes, with margin Capability to send limited commands to probe by tone The LuSeN Mission

33 Seismometer The Netlander Seismometer: a broad-band instrument already developed with sufficient sensitivity to record Moonquakes (usable sensitivity of better than 10-9 m/sec 2 ). Mass: 3.5 kg per seismometer (including electronics, communications, RHU, etc.). Power Requirements: 1 Watt (max.). Communications: Send (data) and receive (commands) on both near side and far side. Data Volume: Drives on-board storage and communication strategy. Seismometer generates about 30 mbit/hour; compressibility of seismic data is being studied.

34 Seismometer Temperature Range: -120 C to +40 C - requires insulation as for the Apollo seismometers. Deployment: Impact must be no more than 200 G (~20 m/s). Position: Must have good ground coupling. Relative position must be known relative to other seismometers (± 0.5 km). Orientation must be known to ~1 deg. The LuSeN Mission