Origin and age of the earliest Martian crust from meteorite NWA 7533 Humayun et al., 2013 Philip Reger Topics in Planetary Sciences 10. April 2014 Overview 1. Introduction to Mars and Meteorites 2. Methods 3. Results 4. Conclusions 5. Discussion 1.Introduction: Mars Semi-major axis 228 million kilometers (1.52 AU) Eccentricity 0.093 (Earth: 0.0167) Orbital period 687 Earth days Rotational period 1.026 Earth days Radius 3400 kilometers (53% of Earth’s) Axial tilt 25.2° (Earth: 23.5°) Mean density 3.9 g/cm3 (Earth: 5.5 g/cm3) Mass 0.107 Earth masses 1. Introduction: Mars • Differentiated planet • No current magnetic field, some remanent magnetized crust • Has a tenuous atmosphere (95 % CO2, mean surface pressure 6 mbar) • Traces of methane (10 ppb) • Polar ice caps made of dry ice and water ice 1. Introduction: Mars • Hemispheric dichotomy: cratered highlands in the south versus less- cratered plains in the north • Huge shield volcanoes in Tharsis (Olympus Mons: 22 km height) • Large impact basins (Hellas basin: 1800 km diameter) • Valles Marineris canyon system (4000 km long, depths up to 11 km) 1. Introduction: Exploration of Mars • Soviet and U.S. Mars probe/orbiter attempts starting in the 1960s • Mariner 4 (1961-67): provides first pictures of another planet • Mariner 9 (1971-72): first spacecraft orbiting another planet, first observation of a planet-wide dust storm (simultaneously with two Soviet orbiters) • Viking missions (1975-1980): high-resolution pictures, atmosphere and surface structures, search for biosignatures • Mars Pathfinder mission (1996-97): first rover on Mars, use of an Alpha Particle X-ray Spectrometer to find andesites, basalts and rocks eroded by wind 1. Introduction: Exploration of Mars • Mars Global Surveyor (1996-2006): mapping and water ice • Mars Exploration Rovers Spirit and Opportunity (2003 ongoing): continuing the search for evidence of past water, among others • Mars Science Laboratory Curiosity (2011 ongoing): investigation of mineralogical, isotopical and chemical composition of the surface 1. Introduction: Curiosity 1. Introduction: Future Mission • ESA ExoMars: planned for 2018 launch, should obtain samples from 2 meter depths, extensive search for organic and biochemical substances • Discovery program: Mars Geyser Hopper and InSight (both 2016 launch) • Sample return missions: delayed until at least 2024 • Several proposals for manned missions 1. Introduction: Meteorites Classification 1. Introduction: Martian Meteorites • SNC Meteorites • Shergottites (96) • Nakhlites (13) • Chassignites (2) • Others • Orthopyroxenite (ALH 84001) • Basalt Breccia (NWA 7034) • NWA 7533 1. Introduction: Martian Meteorites • ALH 84001: Orthopyroxenite, found in Antarctica in 1984. Source of controversy over possible fossilized remains of bacteria-like lifeforms • NWA 7034: Basaltic Breccia, found in the Sahara Desert in 2011, probably paired with NWA 7533, contains the most water of any Martian meteorite 2. Methods • Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) • Sensitive High-Resolution Ion MicroProbe (SHRIMP) U-Pb dating 2. Methods LA-ICP-MS: • Laser beam is focused on sample surface to generate fine particles (Laser Ablation) • Ablated particles are transported to the plasma torch where the sample gets separated into individual atoms and ionized • Ionized atoms get introduced into a mass spectrometer for elemental and/or isotopic analysis • Benefits: very sensitive (down to ppb), little to no preparation time, fast analysis, versatile application (bulk analysis, inclusions, elemental and isotopic mapping, depth profiling) 2. Methods SHRIMP U-Pb dating • First devised and built in the late 1970s • Great instrumentation for the use of the U-Th-Pb decay systems • Used most effectively on zircons (Jack Hills zircons, lunar zircons) • Also useful for titanium, hafnium and sulfur isotopic systems 2. Methods U-Pb dating in zircons: concordia and discordia lines 206Pb/238U = eλ238t – 1 207Pb/235U = eλ235t – 1 3. Results NWA 7533: • Polymict breccia, clasts in a fine-grained (1 μm) interclast crystalline matrix (ICM) • Main clast component: fine-grained (5-20 μm) clast-laden impact melt rock (CLIMR) • Other clasts: melt rock, melt spherules, fine-grained basaltic clasts (20-100 μm), crystal (mostly pyroxene, feldspar) and lithic fragments (some leucrocratic rocks, some of noritic and monzonitic composition) 3. Results NWA 7533: • High Mg-content (roughly 7.5%), but hardly any olivine • Exsolution in pyroxenes and alkali feldspar indicate plutonic origin • ICM and CLIMR (chemical uniformity and fine-grained textures) contain important amounts of wind-blown dust • 0.6% water content 3. Results • Ni and Ir content is higher than those of SNC meteorites, comparable to lunar breccias -> large meteoritic component • Relative abundances of Ru, Rh and Os to Ir are in chondritic ratios in ICM and CLIMR • Siderophile element contents require 5% CI chondrite mixed into the regolith 3. Results 3. Results • Leucocratic rocks are high in Ni content even in with low Mg: crystallized from impact melts with enriched siderophiles, similar concentrations to ICM and CLIMR • Chemical similarity between Gusev rocks and soils and meteorite NWA 7034 are confirmed through the analysis of NWA 7533: evident in major element concentrations as well as in Ni, Ti and K • ICM and CLIMR show no enrichment of S, Cl and Zn compared to modern Martian soils where they’re likely to be in water-soluble phases • Lack of enrichment in NWA 7533 may be due to transportation as salts by liquid water at the time of formation of ICM and CLIMR 3. Results • ICM and CLIMR identical REE patterns: similar precursor material • Chemical composition of wind-blown dust may indicate the original igneous processes that formed the crust: modelling of partial melting • Model of a 4% partial melt of a fertile mantle with less than 1% garnet fits the ICM and CLIMR REE patterns 3. Results • Extraction of this melt from the Martian mantle would lead to a uniform global layer with a thickness of 50 km -> equals average crustal thickness inferred by gravity measurements • Crust formation (enriched and depleted reservoirs) within the first 100 Ma of the planet’s history implied by Nd isotopes • Removal of the primary melt produces a depleted residue that would yield the composition of Tissint, a depleted shergottite • Crustal assimilation of depleted shergottite magma produces intermediate and enriched shergottites 3. Results • Discordia line intersects at 4,428 ± 25 Ma and 1712 ± 85 Ma • Spot problems: analysed spots overlapping matrix are excluded • Martian, lunar and terrestrial ages for oldest zircons very similar, implies coeval crust formation • Cause of lower intercept unknown 4. Conclusions • NWA 7533 is a Martian regolith breccia, originated from the earliest crust, brecciated by impacts • High Ni content in modern soils and in CLIMR implies little crustal resurfacing • Further evidence for early crustal differentiation (> 4.4 Ga ago) forming leucocratic rocks (can retain K and Th signatures), distributed globally • Early magmatic build-up requires volatile release, forming the atmosphere and hydrosphere 5. Discussion • Issues with methodological approach? Other ideas? • Uniform crustal thickness of 50 kilometers? • Partial melting model: Is it accurate? Trouble with REE? • Future Mars missions: what to look for? • … .