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  • Print publication year: 2016
  • Online publication date: March 2016

2 - Phobos and Deimos


Spirit and Opportunity

The Mars Exploration Rovers, Spirit and Opportunity, observed Phobos and Deimos frequently during their missions. Images were taken showing the little moons crossing the disk of the Sun, causing partial eclipses at the rover sites. Others were taken showing the satellites in the sky, rising or setting and entering eclipse. These images had scientific value as well as public relations interest, first, by improving knowledge of the satellite orbits and, second, in the rising, setting and eclipse observations, by measuring atmospheric opacity. As light from the moons passed through the atmosphere during rising or setting, it would be dimmed by layers of dust or haze, and during eclipses the moons probed the edge of the shadow of Mars, again probing variations in atmospheric opacity. Figure 204A shows some representative images of Phobos and Deimos from the MER missions. Phobos is resolved well enough to reveal its nonspherical shape and the large crater Stickney in a super-resolution composite of images taken just before entering eclipse on Spirit sol 675. Both satellites were imaged crossing the disk of the Sun on several occasions. Here Phobos is seen transiting on Opportunity sol 47 and Deimos on Spirit sol 420. Although not noted in Stooke (2012), the Viking Landers had also imaged Phobos and Deimos to measure atmospheric opacity at night, though the disks were not resolved (Pollack et al., 1977).

Mars Reconnaissance Orbiter

Mars Reconnaissance Orbiter (MRO) did not approach either satellite closely, but the exceptional resolution of HiRISE allowed for spectacular imaging of the two distant moons, though necessarily covering only the Mars-facing regions. CRISM observed Phobos and Deimos to collect calibration and compositional data.

HiRISE obtained two-color images of Phobos at a resolution of 6 m/pixel with stereo coverage for topographic mapping (Figures 204C and 204D). The two Phobos images are PSP_007769_9010 and PSP_007769_9015, taken 10 min apart on 23 March 2008. The enlarged section of PSP_007769_9015 in Figure 204D shows large rocks and grooves centered at 20° S, 340° E. Deimos was observed in color at 20 m/pixel in images ESP_012065_9000 and ESP_012068_9000 (Figure 204E), taken 5.6 hours apart on 21 February 2009. These HiRISE images are better than any other available coverage in some areas of each satellite. Figure 205A shows the surface coverage of HiRISE images on both satellites, including an area seen only in light reflected off Mars.

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