Scientists Unveil a 5.7 Terapixel Global Image of Mars
A planet-spanning online image of Mars created at a scale of 5 meters per pixel was unveiled today. The image is freely available to the public and can be accessed online through Caltech's Bruce Murray Laboratory for Planetary Visualization. It will also be delivered to the NASA Planetary Data System for posting.
The mosaicked image—comprising more than 5.7 trillion pixels (5.7 terapixels)—was generated at the Murray Lab by merging 110,000 individual images taken by the Context Camera (CTX) aboard NASA's Mars Reconnaissance Orbiter (MRO). It covers 99.5 percent of the surface of Mars between 88° South and 88° North. (The remaining 0.5 percent either was not imaged at all or was not imaged at a high enough quality by the time the mosaic was created.)
The image took six years and tens of thousands of hours of labor to build. If it were printed out at 300 dpi (the standard printing resolution), the resulting image could be used as a sunshade for the Rose Bowl and a significant chunk of the Rose Bowl's parking lot.
"The scale of this is really unprecedented," says Jay Dickson, Murray Lab manager and research scientist in image processing. Dickson conceived and spearheaded the project soon after he was hired to establish the Murray Lab in 2016.
Individuals who access the image can view it on an interactive interface called SceneView developed by Esri, a geographic information system company. This interface allows for seamless exploration of the entire Red Planet at "outcrop resolution." Outcrops are individual small cliffs and buttes that may be of interest to scientists.
"Mission data are a gift that keeps on giving. NASA's research and analysis program allowed us complete this huge mosaic and make it universally available," says Bethany Ehlmann, professor of planetary science and associate director of the Keck Institute for Space Studies. "I can't wait to see everything people do with it and the science it enables." Ehlmann and Dickson were principal investigators of the NASA Planetary Data Archiving, Restoration, and Tools grant that funded much of the work.
While scientists have higher-resolution individual images of locations on Mars, the highest resolution available at a global scale before this was 100 meters per pixel compared to 5 meters per pixel for the new mosaic. This represents a 20-fold increase in resolution in both dimensions of the Martian surface, providing 400 times more information for a given area.
To construct the image, Dickson and his colleagues used a feature-matching algorithm that aligned all of the images and blended overlapping images together by calculating the path of least contrast between those two and joining them together like jigsaw puzzle pieces. "This is nondestructive processing," Dickson says. "We're not blurring lines to smooth out boundaries; we're finding the best possible line to join two images." Importantly, a map of this jigsaw puzzle of individual images has been released along with the image, allowing complete traceability of each pixel to its parent image.
The process worked well for the vast majority of the 110,000 CTX images Dickson pulled from MRO's mission archive. The remainder, some 13,000 images, had to be stitched together by hand—a labor-intensive process conducted over three years. Mars's thin atmosphere creates clouds and dust storms that affect image quality of the surface and prevent computers from automatically aligning some images, necessitating this manual effort.
Dickson came up with the idea for the mosaic while working on a project to map ice-related features on Mars, and he grew frustrated with existing interfaces for MRO images.
"I wanted something that would be accessible to everyone and without seams," Dickson says. "Schoolchildren can use this now. My mother who just turned 78 can use this now. The goal is to lower the barriers for people who are interested in exploring Mars."
A beta version of the image—which included seams between the 3,960 individual mosaics that comprised it—was rolled out in 2018 to get feedback from the planetary science community. It has already been cited more than 100 times in other work. NASA funded the project in 2019, and much of the updated work was done during the early days of the pandemic. Dickson used his time at home to refine the code for automation and painstakingly stitch remaining recalcitrant images together.
Though impressive, the image is really a testament to the value of data gathered by MRO, Dickson says. The Context Camera was built 20 years ago by Malin Space Science Systems in San Diego. It was installed on the MRO satellite, which was launched in 2005, and has been gathering images and other data from Mars for the past 17 years.
"I worked on this for six years, but the MRO team has spent the past couple decades making this possible in the first place," Dickson says. "And the spacecraft is still out there doing great science."
The new Mars CTX mosaic image viewer can be accessed at https://murray-lab.caltech.edu/CTX/V01/SceneView/3dViewer.html and more information and downloadable data are available at https://murray-lab.caltech.edu/CTX/.
