Shoichi Itoh

Hydrogen isotope evidence for loss of water from Mars through time

[1]xa0The high D/H of the Martian atmosphere (∼5–6 × terrestrial) is considered strong evidence for the loss of Martian water to space. The timing and magnitude of the loss of water from Mars can be constrained by measurements of D/H in Martian meteorites. Previous studies of Martian meteorites have shown a large range in D/H, from terrestrial values to as high as the current Martian atmosphere. Here we show that the ancient (∼4 Ga) Mars meteorite ALH84001 has a D/H 4 × terrestrial and that the young (∼0.17 Ga) Shergotty meteorite has a D/H 5.6 × terrestrial. We also find that the young Los Angeles shergottite has zoning in D/H that can be correlated to igneous growth zoning, strongly suggesting assimilation of D-enriched water during igneous crystallization near the Martian surface. In contrast to previous studies, we find higher and less variable D/H ratios in these three meteorites. Our results suggest a two-stage evolution for Martian water—a significant early loss of water to space (prior to 3.9 Ga) followed by only modest loss to space in the last 4 billion years. The current Martian atmospheric D/H has remained essentially unchanged for the last 165 Ma.

Oxygen Isotopic Compositions of Asteroidal Materials Returned from Itokawa by the Hayabusa Mission

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in 16O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.

Contemporaneous formation of chondrules and refractory inclusions in the early Solar System

Chondrules and calcium-aluminium-rich inclusions (CAIs) are preserved materials from the early history of the Solar System, where they resulted from thermal processing of pre-existing solids during various flash heating episodes which lasted for several million years. CAIs are believed to have formed about two million years before the chondrules. Here we report the discovery of a chondrule fragment embedded in a CAI. The chondrules composition is poor in 16O, while the CAI has a 16O-poor melilite (Ca, Mg, Al-Silicate) core surrounded by a 16O-rich igneous mantle. These observations, when combined with the previously reported CAI-bearing chondrules, strongly suggest that the formation of chondrules and CAIs overlapped in time and space, and that there were large fluctuations in the oxygen isotopic compositions in the solar nebula probably synchronizing astrophysical pulses.

Formation of a solid solution in the MgSiO3–MnSiO3 perovskite system

Experiments using laser-heated diamond anvil cells combined with synchrotron X-ray diffraction and SEM–EDS chemical analyses have confirmed the existence of a complete solid solution in the MgSiO3–MnSiO3 perovskite system at high pressure and high temperature. The (Mg, Mn)SiO3 perovskite produced is orthorhombic, and a linear relationship between the unit cell parameters of this perovskite and the proportion of MnSiO3 components incorporated seems to obey Vegard’s rule at about 50xa0GPa. The orthorhombic distortion, judged from the axial ratios of a/b and

Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon

sqrt{2},a/c,

Oxygen isotopic zoning of reversely zoned melilite crystals in a fluffy type A Ca-Al-rich inclusions from the Vigarano meteorite

2a/c,monotonically decreases from MgSiO3 to MnSiO3 perovskite at about 50xa0GPa. The orthorhombic distortion in (Mg0.5, Mn0.5)SiO3 perovskite is almost unchanged with increasing pressure from 30 to 50xa0GPa. On the other hand, that distortion in (Mg0.9, Mn0.1)SiO3 perovskite increases with pressure. (Mg, Mn)SiO3 perovskite incorporating less than 10xa0mol% of MnSiO3 component is quenchable. A value of the bulk modulus of 256(2)xa0GPa with a fixed first pressure derivative of four is obtained for (Mg0.9, Mn0.1)SiO3. MnSiO3 is the first chemical component confirmed to form a complete solid solution with MgSiO3 perovskite at the P–T conditions present in the lower mantle.

Isotopic compositions of asteroidal liquid water trapped in fluid inclusions of chondrites

High precision Mg isotope measurements have been performed to determine radiogenic 26Mg of coarse‐grained Ca‐Al‐rich inclusions (CAIs) in two carbonaceous chondrites by secondary ion mass spectrometry using faraday cup multi‐collection system. Minerals in each CAI show individual internal isochrones of 26Al‐26Mg with respect to each own oxygen isotopic composition of minerals. This indicates that 16O‐rich and 16O‐poor oxygen isotopic environments existed more than 0.45 Myrs in the solar nebula. The switching time scale between 16O‐rich and 16O‐poor gaseous environments surrounding CAIs was faster than 10 kyrs.