Tuesday, 28 February 2012

What Hayabusa brought back from 25143 (Itokawa).

In November 2005, the Japan Space Agency's probe Hayabusa touched down on the near Earth asteroid 25143 (Itokawa) in order to collect samples. The probe landed in the Muses Sea are of the asteroid, and was due to fire a projectile into the surface in order to dislodge material from the surface for collection. In the event this projectile did not fire, but the probe was able to collect a small number of mineral grains floating above the surface, of the asteroid, which has negligible gravity. This material was returned to Earth for analysis in June 2010.

Close up image of taken from Hayabusa. A & B indicate possible recent impact sites. Circles represent possible hydrological sinks. Arrows point to areas of talus (rubble). Curved lines indicate possible debris flow. Japan Space Agency.

25143 (Itokawa) is a 558 m long, 288 m diameter asteroid with a 556 day period (year) on an orbit that crosses that of the Earth. It is roughly bean-shaped and has a surface covered in rubble; in fact it may be rubble all the way through. The asteroid rotates on its axis every 12 hours, and has a gravity of about a millionth of the Earth's (though this varies from place to place, dependent on the local density of the asteroid). 25143 (Itokawa) has a number of areas on the surface that appear to be hydrological sinks.

These hydrological sinks are surprising in on an asteroid, which is not somewhere we would expect to find water, but are attributed to the former presence of ice. It is thought that the asteroid may have formed further out in the solar system, where chunks of ice (not necessarily water ice) were incorporated into its makeup. At some point it was shifted onto its current orbit, where it passes closer to the sun. This caused the asteroid to heat up, and the ice to sublimate (turn directly from a solid to a gas) in a similar way to material evaporating from the surface of a comet. After this happened the loose rocky material covering the new void subsided forming a sinkhole.

The orbit of 25143 (Itokawa). Bellatrix Astronomical Observatory.

On 27 February 2012 a paper was published in the Proceedings of the National Academy of Sciences, by a group of scientists lead by Eizo Nakamura of the The Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry at the Institute for Study of the Earth’s Interior at Okayama University, detailing the results of a study of five mineral grains brought back from 25143 (Itokawa) by Hayabusa, and the deductions made from these studies.

The grains were made of the minerals olivine, pyroxene, diopside and plagioclase, all common in igneous rocks from the Earth and the Moon, with small inclusions of other common minerals. They ranged in size from 30 × 40 μm to 90× 110 μm, and all were covered in tiny pits, apparently impact craters caused by the action of tiny grains 10-20 nm across. This is interesting as these may not have originated from 25143 (Itokawa), or a similar asteroid. In the Solar System objects larger than 5 μm tend to fall towards the sun, whereas those smaller tend to be carried outwards by the solar winds.

Scanning Electron Microscope images of an olivine grain from 25143 (Itokawa). (A) Detail of part of the grain as shown in inset. F1, F2 & F3 represent fracture plains (the plains along which a mineral will split). C, D & E are areas of magnification. (B) Back Scattered Electron Microscope image of (A) showing mineral textures; Ol is olivine, Pl is plagioclase. (C, D & E) Detail of (A) showing craters made by tiny impacts. These are 100-200 nm across, implying impactors 10-20 nm across. From Nakamura et al. (2012).

While the minerals from which 25143 (Itokawa) are made are not unusual they do tell us something about the history of the asteroid. The minerals present would generally form at a temperature of about 900°C, far hotter than the temperature likely to be reached during the formation of a 300 m radius asteroid. From this Nakamura et al. conclude that 25143 (Itokawa) was formed as part of a larger body, from which it has become separated at some point.

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