Apollo 17 SM contained the scientific instrument module (SIM) bay. The SIM bay housed three new experiments for use in lunar orbit: a lunar sounder, an infrared scanning radiometer, and a far-ultraviolet spectrometer. A mapping camera, panoramic camera, and a laser altimeter, which had been carried previously, were also included in the SIM bay. The lunar sounder was to beam electromagnetic impulses toward the lunar surface, which were designed with the objective of obtaining data to assist in developing a geological model of the interior of the Moon to an approximate depth of . The infrared scanning radiometer was designed with the objective of generating a temperature map of the lunar surface to aid in locating surface features such as rock fields, structural differences in the lunar crust, and volcanic activity. The far-ultraviolet spectrometer was to be used to obtain information on the composition, density, and constituency of the lunar atmosphere. The spectrometer was also designed to detect far-UV radiation emitted by the Sun that had been reflected off the lunar surface. The laser altimeter was designed to measure the altitude of the spacecraft above the lunar surface within approximately , providing altitude information to the panoramic and mapping cameras, which were also in the SIM bay. Light-flash phenomenon and other experiments Beginning with Apollo 11, crew members observed light flashes that penetrated their closed eyelids. These flashes, described by the astronauts as "streaks" or "specks" of light, were usually observed while the spacecraft was darkened during a sleep period. These flashes, while not observed on the lunar surface, would average about two per minute and were observed by the crew members during the trip out to the Moon, back to Earth, and in lunar orbit. The Apollo 17 crew repeated an experiment, also conducted on Apollo 16, with the objective of linking these light flashes with cosmic rays. Evans wore a device over his eyes that recorded the time, strength, and path of high-energy atomic particles that penetrated the device, while the other two wore blindfolds to keep out light. Investigators concluded that the available evidence supports the hypothesis that these flashes occur when charged particles travel through the retina in the eye. Apollo 17 carried a sodium-iodide crystal identical to the ones in the gamma-ray spectrometer flown on Apollo 15 and 16. Data from this, once it was examined on Earth, was to be used to help form