Complex sentence: On February 19,2008, images obtained by the HiRISE camera on the Mars Reconnaissance Orbiter showed a spectacular avalanche, in which debris thought to be fine-grained ice, dust, and large blocks fell from a high cliff.
Split into simpler sentences: On February 19,2008, a geologic event was captured by the HiRISE camera on the Mars Reconnaissance Orbiter. Images that captured a spectacular avalanche thought to be fine grained ice, dust, and large blocks are shown to have fallen from a high cliff.