Featured Image - 03/03/2009
Ina, the D-shaped collapse feature
Apollo 15 cameras first spotted the remarkable D-shaped Ina structure in Lacus Felicitatis (Figure 1; Apollo metric frame AS15-M-2302), and it was also photographed by the Apollo 17 crew from orbit using their handheld cameras. Ina, named for a Polynesian goddess who in mythology is responsible for the first coconut, is significant because it challenges general preconceptions of a currently geologically inactive Moon. Calibration of crater and spectral data with samples collected and returned by the Apollo astronauts indicate that the Ina structure is one of the youngest features on the lunar surface with an age of approximately 1 to 10 million years. For comparison, the youngest rock collected by the Apollo astronauts is about 3 billion years old, and recent crater counting studies of young lava flows (Hiesinger et al. 2003) indicate that active volcanism on the Moon might have continued until until about 1 billion years ago.
Ina is a depression containing numerous hills, but only contains two confimed impact craters. The scarcity of impact craters indicates a very young age for Ina, as do the sharp, high relief edges of the feature. Older features do not have sharp edges due to the continuous mixing of the regolith, impact degradation processes, and infilling by mass wasting. Furthermore, a study by Schultz et al. (2006) used multispectral data from the Clementine camera to suggest that the Ina structure is less mature and therefore more recent than other surrounding features.
Figure 1. Apollo 15 metric photograph showing Ina, located at 18.65 N and 5.29 E in Lacus Felicitatis, a mare region. Ina lies on the summit of a 15 kilometer wide, 300 meter high dome. Inside Ina there are dark hills 5 to 25 meters high, surrounded by brighter, rougher floor materials. The inset image is linearly stretched to enhance details in the image. The black smudge to the lower right of the feature is a piece of debris floating within the camera itself that got stuck within the camera's field of view while in lunar orbit. (Apollo Image AS15-M-2302 [NASA/JSC/Arizona State University])
These indications of recent activity make the Ina structure a high-priority site for future human lunar exploration. Ina is also a top priority for investigation with the Lunar Reconnaissance Orbiter Camera, which will provide high-resolution images (50/cm pixel) of Ina and other areas of potentially young geologic activity.
Hiesinger, H., et al., (2003). Ages and stratigraphy of mare basalts in oceanus Procellarum, Mare Nubium, Mare Cognitum, and Mare Insularum. J. Geophys. Res., 108, doi:10.1029/2002JE00198
Hiesinger, H., et al., (2006) New Ages for Basalts in Mare Fecunditatis Based on Crater Size-Frequency Measurements. Lunar and Planetary Science Conference. #1151.
Schultz, P.H., Staid, M.I., Pieters, C.M. (2006) Lunar activity from recent gas release. Nature, 444, pp.184-186.
Whitaker, E.A. (1972) Apollo 15 Preliminary Science Report 25-84-25-85. NASA SP-289.