Results from MRO’s High Resolution Imaging Science Experiment (HiRISE), 2006-2022

NASA’s Mars Reconnaissance Orbiter (MRO) has been orbiting Mars since 2006; HiRISE has been imaging the surface with pixel scales down to 25-35 cm/pixel.  Resolution of 1-meter scale features is essential for choosing landing sites as well as for direct science. With MRO’s near-polar orbit and off-nadir pointing, any patch of ground on Mars’ surface can be imaged within a 2-3 week period. Given the high data volume of >70,000 images of about one gigapixel size, only 4 percent of Mars has been imaged, and much of that is repeat coverage for stereo or change detection, so the unique coverage is from 2-3 percent. Targets for HiRISE are carefully selected to cover exploration priorities (candidate landing sites, monitoring active surface missions) and science priorities.

            There have been more than 1500 peer-reviewed publications that include “Mars” and “HiRISE” in the main text, based on NASA ADS (see Figure).  Some of these are mentions of HiRISE that are not central to the main results, but there are others missed by ADS for which HiRISE is essential.  In any event, we cannot describe all of these results, but here are a few highlights:

 

• Mars Today. Prior to MRO there was much uncertainty about present-day activity on the martian surface beyond large-scale shifting dust and seasonal frost.   We now know that Mars exhibits diverse surface changes at all latitudes and seasons. Active processes include impact cratering, aeolian sand and dust transport, a variety of slope processes, changes in polar ices, and diverse effects of seasonal CO₂ The extent of surface change has been surprising and indicates that the present climate is capable of reshaping the surface (Dundas et al., 2021, JGR-Planets 126). 
• Ancient Mars. The past fifteen years of orbital infrared spectroscopy and in situ exploration have revised our understanding of the composition and history of Mars. Mars has a basaltic upper crust and many materials from ancient Mars bear the mineralogic fingerprints of interaction with water (Ehlmann and Edwards, 2014, AREPS 42).  HiRISE data has been essential to the geologic and stratigraphic interpretation of the mineralogic data, key to identification of geologic settings with fluvial and other processes and their successful in situ investigation with the Curiosity, Perseverance and other rovers. 
• Water on Mars. There have been multiple false alarms about liquid water on the surface of Mars today, although eutectic brines may be present in tiny quantities for short periods of time.  Mid-latitude gullies, recurring slope lineae, and slope streaks are all best explained as dry processes.  However, the case for geologically recent (Amazonian) water has been strengthened by observations of craters with alluvial fans and deltas (Wilson et al., 2021, GRL 48) and other studies.  HiRISE observations of new impact craters and scarps exposing ice have mapped the boundary of the subsurface water ice table and yielded information on its vertical structure.
• Polar processes. The condensation and sublimation of the seasonal CO₂ polar cap sculpts landforms, from the polar region to mid-latitude dunes, eroding furrows, gullies, and radially-organized channels, changes detected by HiRISE in repeat images.   Avalanches from blocks of ice falling from stretches of the permanent north polar cap edge occur every spring. HiRISE images and stereo DTMs have mapped polar stratigraphy enabling the recognition of climatic records Myr in length as well as more-ancient periods where polar ice was absent. Images of polar craters and their changes have allowed present-day ice accumulation rates to be deduced.
• Non-polar geologic processes. The list of geologic features studied with HiRISE includes impact craters, volcanic and tectonic features, channels, pit craters and potential caves, sedimentary volcanism, dune casts, boulder falls and fields, columnar jointing, dikes and water-cemented fractures, glacial and periglacial landforms, fractured mounds, landslides, polygonal terrains, inverted channels, bedrock layers, layered sedimentary deposits, and more.

 

What is the future of HiRISE? Given a worsening image data problem, HiRISE images may have limited use of full-resolution imaging within 3-4 years and greater use of 2x2 and 4x4 binned images. There are 3 central CCDs that have relatively minor issues, so narrow unbinned images could be acquired for some additional years, with 2x2 binned data for the 6 outer CCDs (plus the blue-green and NIR CCDs). Given the lack of current plans for new orbital imaging at this scale, there may be a gap in orbital sub-m resolution at Mars.

Figure: Refereed papers that include “HiRISE” and “Mars” according to NASA ADS on May 5, 2022 (1,685 total).