This pair of pictures from Viking Lander 1 at Mars' Chryse Planitia shows the only unequivocal change in the Martian surface seen by either lander. Both images show the one-meter (3-foot) high boulder nicknamed 'Big Joe.' Just to the lower right of the rock (right photo) is a small-scale slump feature. The picture at left shows a smooth, dust-covered slope; in the picture at right the top surface layer can be seen to have slipped downslope. The event occurred sometime between Oct. 4, 1976, and Jan 24, 1977. (Pictures taken before Oct. 4 do not show the slump; the first picture in which it appears was taken Jan. 24.) The surface layer, between one-half and one centimeter (one-fifth to one-third inch) thick, is apparently less cohesive than the underlying material. The layer that slipped formed a 30-centimeter-long (11.8-inch) 'tongue' of soil and a patch of exposed underlying material. The triggering mechanism for the event is unknown, but could have been temperature variations, wind gusts, a seismic event, or perhaps the lander's touchdown on July 20, 1976.