Mars' Active Surface: Observing Changes with Orthorectified HiRISE Images

Active processes on Mars have been observed and documented with high resolution images from orbit. The HiRISE camera, operating on MRO since 2006, acquires images of the surface of Mars at up to 25 cm pixel scale, and has stereo capability. The use of HiRISE stereo images to produce digital terrain models (DTMs) allows for orthorectification of the stereo pair and other images targeted over the same area. HiRISE DTMs typically have 1 or 2 m horizontal resolution with vertical precision on the order of 10's of cm. DTM production with HiRISE requires that the source stereo pair has similar lighting and minimal surface differences. However, there is no similar requirement for orthorectifying other images taken of the same area. Changes on the surface can then be measured accurately in the orthoimages as topographic distortions have been minimized. Time sequences of surface changes can be constructed from a series of orthoimages, providing essential data for understanding their rate and magnitude. Some of the current areas of study using HiRISE DTMs and orthoimages include recurring slope lineae (RSL) (McEwen et al., Science 333, 740, 2011), dune and ripple migration (Bridges et al., Nature 485, 339, 2012), seasonal frost changes, and gully activity. In some cases, DTMs from before and after significant surface movement can be compared to measure volumetric changes. The demand for HiRISE DTMs and orthoimages has led to advances in techniques for their production and analysis. Improved image processing and terrain editing tools result in better DTMs and orthoimages, as well as minimize the time required to produce them. We present here ongoing work to refine these techniques and develop new methods for generating orthoimage sequences, including orthorectification of the HiRISE 3-band color strip. Results are illustrated using series of orthoimages over dunes, RSL sites and other areas of Martian surface activity. HiRISE DTMs and orthoimages are released to the Planetary Data System (PDS) on a regular basis via http://hirise.lpl.arizona.edu/dtm/.