Noble gas isotopes were measured in three rocky grains from asteroid Itokawa to elucidate a history of irradiation from cosmic rays and solar wind on its surface. Large amounts of solar helium (He), neon (Ne), and argon (Ar) trapped in various depths in the grains were observed, which can be explained by multiple implantations of solar wind particles into the grains, combined with preferential He loss caused by frictional wear of space-weathered rims on the grains. Short residence time of less than 8 million years was implied for the grains by an estimate on cosmic-ray-produced (21)Ne. Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.

Phobos and Deimos occupy unique positions both scientifically and programmatically on the road to the exploration of the solar system. Japan Aerospace Exploration Agency (JAXA) plans a Phobos sample return mission (MMX: Martian Moons eXploration). The MMX spacecraft is scheduled to be launched in 2024, orbit both Phobos and Deimos (multiple flybys), and retrieve and return >10 g of Phobos regolith back to Earth in 2029. The Phobos regolith represents a mixture of endogenous Phobos building blocks and exogenous materials that contain solar system projectiles (e.g., interplanetary dust particles and coarser materials) and ejecta from Mars and Deimos. Under the condition that the representativeness of the sampling site(s) is guaranteed by remote sensing observations in the geologic context of Phobos, laboratory analysis (e.g., mineralogy, bulk composition, O-Cr-Ti isotopic systematics, and radiometric dating) of the returned sample will provide crucial information about the moon’s origin: capture of an asteroid or in-situ formation by a giant impact. If Phobos proves to be a captured object, isotopic compositions of volatile elements (e.g., D/H, 13C/12C, 15N/14N) in inorganic and organic materials will shed light on both organic-mineral-water/ice interactions in a primitive rocky body originally formed in the outer solar system and the delivery process of water and organics into the inner rocky planets.

We have developed a new nano-beam time-of-flight secondary neutral mass spectrometry system: laser ionization mass nanoscope or LIMAS. The primary ion beam column was equipped with a Ga liquid metal ion source and aberration correction optics. The primary ion beam was down to 40 nm in diameter under a current of 100 pA with an energy of 20 keV. The sputtered particles were post-ionized under non-resonance mode by a femtosecond laser. The post-ionized ions were introduced into a multi-turn mass spectrometer. A mass resolution of up to 40 000 was achieved. The vacuum of the sample chamber was maintained under an ultrahigh vacuum of 2 Â 10 À8 Pa. This instrument would be effective for ultrahigh sensitive analysis of nanosized particles such as return samples from asteroids, comets, and planets.

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Laser ionization mass nanoscope is a time-of-flight sputtered neutral mass spectrometer associated with laser post-ionization by tunneling effect. A spherical and chromatic aberration corrector is installed in the primary ion column. The lateral spatial resolving power of He imaging of solid surface has been evaluated by scanning image using a probe diameter of 90 nm from crater edge slope of a He ion-implanted Si substrate. Helium distribution from the scanning image is quantitatively equivalent with depth profiling analysis from surface of the same substrate, indicating that spatial resolving power of 20 nm for depth resolution has been achieved on the He scanning image through use of oblique incident effect of the primary beam.