^1,2,3Birger Schmitz, ¹Samuele Boschi, ¹Anders Cronholm, ^2,4Philipp R. Heck, ⁵Simonetta Monechi, ⁶Alessandro Montanari, ¹Fredrik Terfelt
¹Astrogeobiology Laboratory, Department of Physics, Lund University, Sweden
²Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, IL, USA
³Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, HI, USA
⁴Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL, USA
⁵Department of Earth Sciences, Florence University, Florence, Italy
⁶Geological Observatory of Coldigioco, Frontale di Apiro, Macerata, Italy

The onset of Earth’s present icehouse climate in the Late Eocene coincides with astronomical events of enigmatic causation. At ∼36 Ma ago the 90–100 km large Popigai and Chesapeake Bay impact structures formed within ∼10–20 ka∼10–20 ka. Enrichments of 3He in coeval sediments also indicate high fluxes of interplanetary dust to Earth for ∼2 Ma∼2 Ma. Additionally, several medium-sized impact structures are known from the Late Eocene. Here we report from sediments in Italy the presence of abundant ordinary chondritic chromite grains (63–250 μm) associated with the ejecta from the Popigai impactor. The grains occur in the ∼40 cm∼40 cm interval immediately above the ejecta layer. Element analyses show that grains in the lower half of this interval have an apparent H-chondritic composition, whereas grains in the upper half are of L-chondritic origin. The grains most likely originate from the regoliths of the Popigai and the Chesapeake Bay impactors, respectively. These asteroids may have approached Earth at comparatively low speeds, and regolith was shed off from their surfaces after they passed the Roche limit. The regolith grains then settled on Earth some 100 to 1000 yrs after the respective impacts. Further neon and oxygen isotopic analyses of the grains can be used to test this hypothesis.
If the Popigai and Chesapeake Bay impactors represent two different types of asteroids one can rule out previous explanations of the Late Eocene extraterrestrial signatures invoking an asteroid shower from a single parent-body breakup. Instead a multi-type asteroid shower may have been triggered by changes of planetary orbital elements. This could have happened due to chaos-related transitions in motions of the inner planets or through the interplay of chaos between the outer and inner planets. Asteroids in a region of the asteroid belt where many ordinary chondritic bodies reside, were rapidly perturbed into orbital resonances. This led to an increase in small to medium-sized collisional breakup events over a 2–5 Ma period. This would explain the simultaneous delivery of excess dust and asteroids to the inner solar system. Independent evidence for our scenario are the common cosmic-ray exposure ages in the range of ca. 33–40 Ma for recently fallen H and L chondrites.
The temporal coincidence of gravity disturbances in the asteroid belt with the termination of ice-free conditions on Earth after 250 Ma is compelling. We speculate that this coincidence and a general correlation during the past 2 Ga between K–Ar breakup ages of parent bodies of the ordinary chondrites and ice ages on Earth suggest that there may exist an astronomical process that disturbs both regions of the inner asteroid belt and Earth’s orbit with a potential impact on Earth’s climate.

Reference
Schmitz B, Boschi S, Cronholm A, Heck PR, Monechi S, Montanari A, Terfelt F (2015) Fragments of Late Eocene Earth-impacting asteroids linked to disturbance of asteroid belt. Earth and Planetary Science Letters 425, 77–83.
Link to Article [doi:10.1016/j.epsl.2015.05.041]

Copyright Elsevier

¹Yang Chen, ¹Yang Liu, ²Yunbin Guan, ²John M. Eiler, ²Chi Ma, ²George R. Rossman, ³Lawrence A. Taylor
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
³Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA

We report unambiguous chemical evidence for subsurface water activity in the martian crust at <600 Ma based on the data from Tissint, a fresh martian meteorite fall with minimal terrestrial weathering. The impact-melt pockets in Tissint contain abundant volatiles (H₂O, CO₂, F, and Cl), and their concentrations are positively correlated with each other. Higher H₂O concentrations also accompany higher deuterium contents. These correlations suggest mixing between two volatile sources. The first source is H₂O in the precursor basalt inherited from martian magma. Magmatic H₂O in the basalt had low deuterium concentration and was likely stored in the nominally anhydrous minerals. This source contributed little CO₂ or halogens to the impact melts. The second source is inferred to be aqueous-alteration products introduced to the basalt by water activity after the basalt erupted. These alteration materials contributed more volatiles to the impact melts than the magmatic source, and had high deuterium abundance, reflecting isotope equilibrium with recent martian atmosphere. The water activities occurred beneath the martian surface after ∼600 Ma (crystallization age), but before ∼1 Ma (ejection age). The chemical and isotopic signatures of the alteration products in Tissint resemble previously known martian samples associated with old water activities on Mars, which can be traced back to ∼4.2 billion years ago (e.g., the mudstone at Gale Crater). This similarity in chemistry and the wide age-span indicate that such water activities were common on Mars throughout its history, which had the potential to form habitable environment. However, the rarity of the volatile-rich zone in Tissint suggests that Martian crustal aqueous processes, where they have occurred are generally limited in their extent of water–rock reaction.

Reference
Chen Y, Liu Y, Guan Y, Eiler JM, Ma C, Rossman GR, Taylor LC (2015) Evidence in Tissint for recent subsurface water on Mars. Earth and Planetary Science Letters 425, 55–63
Link to Article [doi:10.1016/j.epsl.2015.05.004]

Copyright Elsevier