In Situ Noble Gas-based Dating On Terrestrial Planet Surfaces

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1 In Situ Noble Gas-based Dating On Terrestrial Planet Surfaces Tim Swindle Lunar and Planetary Laboratory, University of Arizona Decadal Survey Terrestrial Planets Panel Irvine, California October 26, 2009

2 Outline What are the techniques? What is the status of development? What capability might reasonably be expected? What is needed to get to that capability?

3 U,Th-He similar to K-Ar

4 What can be dated? K-Ar Ar, U,Th-He date last thermal event Cratering events don t t do enough heating to reset most of the material involved Only melt sheets fully reset Solar heating not enough to cause loss of Ar, but can cause loss of He from surface material from 1 A.U. in CRE ages date time at very surface Can use to date cratering events Upper limit set by erosion/gardening rates 10s of Ma?

5 Advantages of noble gas geochronology Multiple techniques with single system Complementary (measure different identifiable events in many cases) Intrinsically simple using techniques that were used 50 years ago in terrestrial labs

6 Requirements for in situ noble gas geochronology system Measure noble gases Mass spectrometer MSL SAM, Beagle2, many others Measure major elements plus K, U, Th XRS (many flown) LIBS (Laser-Induced Breakdown Spectroscopy) MSL ChemCam This sounds easy! What s s needed to make those measurements?

7 The tough requirements Acquire sample (10 mg) Heat sample For CRE, many terrestrial labs use 1600ºC SAM, Phoenix TEGA ºC Weigh sample XRF, LIBS give fractional abundance; mass spectrometers give absolute Interpret the results

(_T( 4 ) Heating surrounding material Outgassing, stressing

8 Heating a sample Conduction (Phoenix TEGA, ASTID/LSSO VAPoR) Radiative (MIDP AGE) In both cases, fighting Radiative losses (_T( 4 ) Heating surrounding material Outgassing, stressing materials For ~ ºC, require W for mg Phoenix TEGA AGE oven

Heating a sample 2 Best alternative diode lasers Labs often use lasers, but not efficient enough for S/C Until recently, efficient lasers were low-power (mw( mw) Diodes spec ~40% efficiency, 10s

9 Heating a sample 2 Best alternative diode lasers Labs often use lasers, but not efficient enough for S/C Until recently, efficient lasers were low-power (mw( mw) Diodes spec ~40% efficiency, 10s of W (10mg requires ~20W) Development Instrument designers hesitant to use unproven technology Instrument development programs don t t see lasers as unproven Coherent diode array (80W, 32 mm long)

10 Weighing a sample Knife-edge balances unlikely to like vibe Calculate from volume of powder Packing fraction? Melt & measure volume, calculate density 7% relative uncertainty (1_) for MIDP AGE Requires melting Piezoelectrics? Vibration frequency? JPL reportedly now working on the problem Calibration factor (factor _V has to be multiplied by to get the correct mass) for two basalts, a chondritic meteorite, and three peridotites. The calibration factor is necessary because the molten sample developos a meniscus. From Fennema et al. (LPSC XXXVIII, #1772).

11 How precise can noble gas-based in situ ages be? Major sources of uncertainty (other than interpretation) compound in normal way Weight (7% 1_ 1 achieved by measuring melted volume, should be possible to do better) Elemental abundances (LIBS typically 5-7%) Noble gas abundances (10%?) Compounded uncertainty ~15%, might be able to do better

12 Ask again, how precise can noble gas-based in situ ages be? CRE ages age precision proportional measurement precision K-Ar ages 1.3 Ga halflife means that age uncertainty improves for older ages Logarithmic nature of age equation 150 Ma uncertainty at 4 Ga for 15% precision Percent Error T = (1/_) * ln [1 + C( 40 Ar K / 40 K)] Age Percent error in a K-Ar age determination, for a 15% error in 40 Ar K / 40 K ratio

13 Interpretation Basics Have to have right location to find right rock Have to find (and document) right rock Igneous easiest to find May be more interested in secondary alteration (Mars), age of crater (Moon, Mercury, asteroids) Harrison Schmitt at Apollo 17

14 Interpretation K-Ar problems at Mars Trapped atmosphere? Adsorption? 1% of P Earth, but lower temperatures Shock-implanted atmosphere unlikely to be problem Heavily shocked rocks likely to be uncommon Partially reset ages? Less likely to be problem than Earth (no plate tectonics, impacts not very effective at resetting) Martian meteorite Elephant Moraine 79001, with its shock-produced glasses (dark patches) full of Martian atmospheric gases.

15 Interpretation (continued) Mars Magmatic gases incorporated? Could be problem for 40 Ar, particularly for young samples Bogard Shergottites incorporate ~constant amount of 40 Ar, not constant 40 Ar/ 36 Ar ratio For very young samples, CRE age could be more accurate Need multiple samples Bogard (2008) LPSC XXXIX, #1100

16 Requirements for meaningful interpretation Need mobility and/or landing accuracy Need ability to characterize samples Chemical analysis part of chronological data Microscopic imaging systems exists Need ages on multiple samples

17 The bottom line (development) In situ noble gas geochronology is promising, but there are tough (not insurmountable) problems to solve Sample heating (10 mg to 1500ºC) Weighing a 10 mg sample These problems aren t t unique to noble gas systems Need integrated instrument(s) ) developed Only U.S. group currently working on development is JPL/CalTech (Keck funding) Concept studies from Germany, U.K.

18 Could we mount an in situ geochronology mission? Within 1-3 years TRL isn t t high enough for any instrument, though high for parts of some What s s needed Sustained funding to increase TRL for instrumental approaches for measurements using multiple and complementary isotope systems (e.g., Rb-Sr and K- Ar) Funding to develop sampling preparation approaches for unique challenges specific to in situ geochronology

In-situ geochronology General overview

In-situ geochronology General overview Timothy D. Swindle, University of Arizona, Tucson AZ F. Scott Anderson, Southwest Research Institute, Boulder CO October 26, 2009 Decadal Survey Inner Planets Panel