Josep M. Trigo-Rodríguez Institute of Space Sciences, CSIC-IEEC

1. “An estimate of the flux of primitive bodies in all range of masses during the LHB and the source of terrestrial water” Josep M. Trigo-Rodríguez Institute of Space Sciences, CSIC-IEEC Workshop on Astrobiology in the Early Solar System, Paris, 18-20 April 2011

2. OUTLINE Solar system formation: water in the disk Chondrites: the building blocks of terrestrial planets Evidence for hydrothermal activity in primitive asteroids Aqueous alteration in chondrites Evidence for alteration depending of burial depth Recent findings on CM, CR, CI and CV groups. Nature and magnitude of the late veneers flux Fragile and volatile-rich nature of bodies approaching terrestrial planets Evidence from lunar and terrestrial materials Conclusions: astrobiological implications

3. THE INNER-DISK REGION • During the ESS gas drag produces fast migration of bodies • Icy bodies are evaporated in their quick approach to the Sun (gas drag) • The O availability depends on the formation region and time • Limited regions around giant planets had reducing environments Adapted from Van Boekel (2007) Giant embryo subnebula

4. THE BUILDING BLOCKS OF PLANETS • Anders & Grevesse (1989) compared the chemical abundances measured in the solar photosphere with primitive meteorites (chondrites) • Extraordinary fit with carbonaceous chondrites of CI group (meteorite-type: Ivuna) • In scientific literature is assumed that composition as “solar” since then • The CI chondrites are among the most fragile materials, but they have suffered important aqueous alteration • However, the enstatite and ordinary chondrites were probably the main building blocks of Earth on the basis of O isotope data CI chondrite Orgueil CI chondritic compared to solar Anders and Grevesse (1989) Adapted from Hutchison (2002)

5. OXYGEN CLUES ON EARTH’S BUILDING BLOCKS Wasson (2000) • Clayton et al. (1973) found that the O isotopic ratios are far from the line of Terrestrial fractionation (TF) • SMOW: standard mean ocean water • So the different planetesimals as a function of their formation region and instant of formation retained different isotopic abundances (Wasson, 1972) • Most carbonaceous chondrites (CCs) are falling into the CCAM (anhydrous minerals of CCs) • Some were significantly altered by water: CM & CR • Existed a gradient in O composition in the nebula, depending of heliocentric distance and time • Self-shielding (Lyons y Young, 2005) Proplyd (NASA)

6. CHONDRITE GROUPS Weisberg et al. (2006) • Different degrees of aqueous alteration and metamorphism are found in some chondrite groups • The CCs can be classified according the estimated T required to produce the petrographic types (Dotto et al., 2005) • The highest hydrated groups correspond to the carbonaceous chondrites, particularly: CMs, CRs, and CIs • At the Institute of Space Sciences (CSIC-IEEC) we work in the lab focusing on the role of aqueous alteration on primitive chondrites and performing remote studies of comets Dotto et al. (2005)

7. CM CHONDRITES • Members of this group are water-rich aggregates containing chondrules, inclusions and fine dust materials: • Until 10% H2O and 4% C in mass • CMs experienced aqueous alteration to different degrees (McSween, 1979) • Many mineral phases are alteration products (Zolensky & McSween, 1988): • phyllosilicates, sulfides, carbonates, oxides, and poorly characterized phases or “PCPs” • CM chondrites suffered secondary processes after accretion (Brearley & Jones, 1998): • Aqueous alteration • Brecciation (consequence of impacts) • Two main scenarios for aqueous alteration: • Parent body (Zolensky & McSween, 1988) • Preaccretionary, occurred in uncompacted precursors (Metzler et al. 1992; Bischoff, 1998) • Parent-body alteration is favored by current evidence Y791198 CM2 QUE93005 CM2

8. PROGRESSIVE AQUEOUS ALTERATION IN CMs QUE 97990 ColdBokkeveld • We studied nine CMs that span the range of moderate to extreme aqueous alteration • Mobilization of different chemical elements is the origin of different features present in CMs • Many CMs are breccias containing clasts of different alteration stages (e.g., Cold Bokkeveld, Murray) • Initially it is likely that all CMs had petrologic type 3.0 • They were altered to different degrees, possibly due to their burial depth • The samples were later fragmented and compacted by impacts • It is not surprising that asteroids could exhibit all a continuous diversity in reflectance spectra QUE 99355 LAP 02277 Murray QUE 93005 (Trigo-Rodriguez et al., GCA 4581 , 2006).

9. Fe-RICH VEINS IN MET 01070 • Trigo-Rodríguez & Rubin (2006) found several lenses rich in Fe,Ni sulfides and carbonates in MET 01070 • The lenses transect the matrix and wend their way among numerous chondrule pseudomorphs • The lenses are oriented in approx. the same direction as the lineation in the meteorite • MET 01070 exhibits extensive aqueous alteration. Mafic silicates are absent; metal grains are very rare. • These lenses are produced by mobilization of Fe, Si, Ca, etc, dissolved in an aqueous fluid • Ca sulfate veins in Cold Bokkeveld (Lee, 1993) • Fe oxy-hydroxide (Hanowski & Brearley, 2000) • Young (2001) suggested that fluid from asteroid interiors flowed upwards by capillary action • These lenses are likely evidence of water flow • Veins are more common in CIs

10. PROCESSEDCHONDRITES:PLAINVIEWC-RICHIMPACT CLASTS • We described a 5-mm-diameter unequilibrated chondritic clast that contains 13 wt.% C, mostly amorphous (Rubin et al., 2005; GCA) • PV1 inter-fused chondrules indicate that T briefly reached ~1100˚C • A nice example of an impact-processed material that could be representative of the surface of some collisionally processed asteroids (e.g. Trojans)

11. SEVERAL PHASES IN THE PLANETARY BOMBARDMENT Late-heavy bombardment period Hadean

12. SCALING THE LUNAR IMPACT RECORD TO EARTH • The energy of an object in solar orbit hitting the Earth Where Vesc is the escape velocity • The effective capture cross-section: • The average impact velocity: about 16 km/s for NEOs, 20 km/s for TNOs, and 24km/s from MB asteroids Moon forming impact (NASA) The Earth accreted about 20 more mass than the Moon!

13. Incoming mass on Moon(g) Inferred mass accreting Earth (g) * Nature/origin of incoming bodies REFERENCE 1022 21023 TNO region Gomes et al. (2005) 1021 21022 Main Belt, <50% from outer belt Morbidelli et al. (2010) 1022 21023 E-Belt Bottke et al. (2011) REALLY EXISTED THE LHB? • The Nice model (Gomes et al., 2005; Morbidelli et al., 2005) explains the LHB as produced by the migration of Jupiter and Saturn, but it is unclear that a spike existed. • Bottke et al. (2010) proposed that the Hungarias are the remaining bodies of an extinct “E-belt” population located in the inner main belt (allowed by a high-e Mars). • Such population should have contributed to the LHB, but they were enstatite-like so the origin of water and organics remains unclear. Bottke et al. (2011) LPSC 42, #2591

14. MORE PRISTINE OBJECTS: COMETS! Blum et al. (2006) ApJ • Comets are fragile objects, much less collisionally processed that chondritic meteorites (Trigo-Rodríguez & Blum, PSS, 2009)

15. FLUX OF TINY MATERIALS Comet SW3 imagedby HST (NASA) • Earth is currently accreting annually about 40,000 tons of interplanetary matter, most on the size range <1 mm (Love & Brownlee, 1994; Brownlee, 2001) • Such a flux scaled during the LHB could have been 4 or 5 times higher during the LHB • This flux can increase the final accreted mass • The LHB flux of impactorswas probably dominated by volatile-rich bodies tens of kilometers in diameter • Fragile + ice-rich objects experiencing fragmentation under thermal stress and planetary approaches (tidal forces) • New pathways for a more efficient delivery of water and organics to Earth • Disruption of comets in close approaches by tidal forces could have also produced a significant delivery of particles capable to survive for centuries before reaching the Earth

16. LATE VENEERS: COMETS AND WATER-RICH CHONDRITIC ASTEROIDS AS SOURCE OF VOLATILES • A significant flux of volatiles was carried on from comets and hydrated chondrites • Setting of N2 when UV flux decreased • Cometary abundances: Abundance relative to H2O in: • Hale-Bopp (Charnley et al., 2002): • 1P/Halley (Jessberger & Kissel, 1991) • Impact of disrupted weak bodies (Blum et al., 2006; Trigo-Rodríguez & Blum, 2009) • Cometary fragmentation: • Formation of dust trails. • Efficiency of low-Vg meteoroid streams in the delivery of organic matter

17. WATER+ORGANICS+CLAYS… • Hydrothermal activity in carbonaceous asteroids promoted increasing complexity in organics present in the matrix • Minerals participated in the synthesis and interconversion of organic species, particularly in presence of water and N (Glavin et al., 2010) • Clays are produced in chondrules and inclusions, by the progressive hydration from the matrix: • Clays to synthesize life (Cairns-Smith, 1982) • OsO4 vapor deposition demonstrated the relationship between clays and complex organics (Pearson et al., 2002; 2007) • Carbonaceous matrix is source of all biogenic elements, including P • Aqueous alteration of schreibersite (Fe3P) forms phosphates and phosphites (Lauretta et al.) • The matrix is source of biogenic elements • Consequently, a prebiotic synthesis could have occurred in first stages of chondritic accretion Extraterrestrial environments to synthesize water-soluble compounds, also rich in N • GRA 95229 (Pizzarello et al., 2011) GRA 95229 (NASA/JSC)

18. LUNAR SOIL EVIDENCE • Moon materials retained evidence on the nature of LHB impactors: • Lunar soils exhibit CI chondritic concentrations: • Variable: 1 to 3% by weight. • Is the abundant megaregolith evidence of a LHB flux of bodies? • Three components: • Ancient heavy bombardment • Young crater events • Continuous micrometeoroid flux • Earth evidence: • Plate tectonics incorporated volatiles from the building blocks to the mantle • Noble gases in atmosphere (Owen & Bar-Nun, 1993) Variation of iridium concentration in lunar regolith samples (Haskin & Warren, 1991 ; Warren, 2004). IAC

19. CONCLUSIONS • Primitive carbonaceous asteroids should be considered astrobiological key targets! • Sampling of carbonaceous materials unaffected by meteorite delivery biases is essential to complete our evolutionary picture: Marco Polo goal • These asteroids are sources of all biogenic elements and have minerals that could synthesize complex organics • Some CRs experienced hydrothermal activity where complex organics formed GRA 95229 (Pizzarello, 2011) • Carbonaceous asteroids plus MB and JF comets were a key source of organic material at the time life originated: • Models suggest that they delivered a total mass close to 1022 to 1023 g • Prebiotic synthesis as consequence of hydrothermal activity occurred in them • We need to come back to the Moon: • Terrestrial meteorites and nature of megaregolith

20. SPACE WEATHERING: REFLECTANCE DIFFERENCES The surfaces of the Moon and asteroids have been subjected to impact processing and solar wind bombardment since their formation • Moon and asteroidal materials: • Collisions. • Soils are mixed materials: breccias • Different regolith size and composition • Spectral reddening • It originates big differences with reflectance spectra of meteorites

21. Fe-RICH LENS COMPOSITION • 2 MURCHISON AUREOLES (Hanowski & Brearley, 2000)SiO2    5.61 TiO2    0.16 Al2O3    2.0 FeO    43.6Cr2O3    6.1 MnO    0.23 MgO    6.7 CaO    0.3 Na2O    0.23 NiO    7.6 S        8.3 total    80.9 • LENS COMPOSITION (n = 23 points) SiO2    27.0  ± 4.6 TiO2    0.10 ± 0.06 Al2O3    2.5 ±0.38 FeO    35.8 ±3.5 Cr2O3    0.42 ±0.18 MnO    0.25 ±0.04 MgO    16.4 ±2.2 CaO    0.15 ±0.19 Na2O    0.20 ±0.03 K2O    <0.04 ±0.01 P2O5    <0.04 ±0.07 NiO    0.72 ±0.44 S        1.5 ±0.97 total    85.0 • Low SiO2 and MgO in aureoles are consistent with production from altered metallic Fe • High SiO2 and MgOin lenses close to PCP composition present in matrix • No recognizable weathered metal blob suggests that lenses formed by PCP precipitation through parent-body aqueous flow 1 mm

22. LABORATORY EXPERIMENTS • Main goal: • To learn about the expected physical properties of primeval accretionary bodies • Study of macroscopic aggregates (a,b) built from different types of grains: • c) Spherical monodisperse SiO2 grains • d) Irregular diamonds • e) Irregular polydisperse SiO2 • Resulting porosities of the order of 80 to 67% for the maximum compression of planetesimals (relative velocities of 50 m/s) • Tensile strengths in the range of 0.2 to 1.1 kPa Tensile Strength Determination experiment Blum et al., ApJ (2006)

23. POROSITY AND DENSITY OF METEORITES • All meteorites arriving at Earth are compacted samples: • Materials are also biased during atmospheric transition towards the tougher objects (!)