It took fifteen years of filming and nearly three years of merging the pieces to create the largest image ever made, a mosaic of 8 billion trillion pixels from the surface of Mars. Now, the first study to take full advantage of the picture provides an unprecedented insight into the ancient river systems that once covered the vast plains of the planet’s southern hemisphere. These three-billion-year-old sedimentary rocks, like those in Earth’s geological record, could prove valuable targets for future exploration of past climates and tectonics on Mars.
The work was published this month in Geology, complements existing research on the hydrological history of Mars by mapping ancient fluvial (river) reefs, which are essentially inverse to the bed. “If you have a river channel, it’s the erosional part of the river. So, by definition, there are no deposits there that you could study, ”explains Jay Dickson, lead author. “You have rivers biting rocks, so where did those rocks go? These ridges are the other half of the puzzle. “Using a mosaic, as opposed to localized images, let researchers solve this puzzle on a global scale.
Mars used to be a wet world, as evidenced by the stone records of lakes, rivers and glaciers. River reefs formed between 4 and 3 billion years ago, when large, flat rivers deposited sediments in their channels (instead of just cutting off water at the surface). Similar systems can be found today in places like southern Utah and Death Valley in the United States, and the Atacama Desert in Chile. Over time, sediment accumulated in the canals; when the water dried up, those reefs were all that was left of some rivers.
Reefs are present only in the southern hemisphere, where some of the oldest and harshest terrains of Mars are, but this pattern is probably an artifact for preservation. “These ridges used to be all over the planet, but later processes buried or eroded them,” says Dickson. “The northern hemisphere is very smooth because it has been rebuilt, primarily by lava flows.” In addition, the southern mountains are “some of the flattest surfaces of the solar system,” says Woodward Fischer, who was involved in the business. This remarkable plane created good sedimentary deposition, enabling the creation of records that are being studied today.
Whether the region has a fluvial reef or not is a basic observation that was not possible until this high-resolution image of the planet’s surface was compiled. Each of the 8 trillion pixels represents 5 to 6 square meters, and the coverage is almost 100 percent, thanks to the “spectacular engineering” of NASA’s context camera, which enables it to work continuously for more than ten years. An earlier attempt to map these ridges was published in 2007 by Rebecca Williams, co-author of a new study, but that work was limited by the scope and quality of the images.
“The first list of fluvial reefs using meter images was derived from data collected between 1997 and 2006,” Williams says. “These bands of images sampled the planet and gave enticing images of the surface, but there was uncertainty about the lack of fluvial reefs in the data gaps.”
The resolution and coverage of the Martian surface by the mosaic eliminated much of the team’s uncertainty, filling in the gaps and providing context to the features. The mosaic allows researchers to explore issues globally, rather than being limited to patchy, localized studies and extrapolating the results to the entire hemisphere. Many previous studies of Mars hydrology have been limited to craters or individual systems, where both the sediment source and destination are known. This is useful, but a larger context is better to really understand the history of the planet’s environment and be more confident in how a particular characteristic was created.
In addition to identifying 18 new fluvial ridges, the use of a mosaic image allowed the team to reconsider features that had previously been identified as fluvial ridges. Upon closer inspection, however, some did not form rivers, but lava flows or glaciers. “If you only see a small part [a ridge], you may have an idea of how it came to be, ”says Dickson. “But then you see it in a broader context – like, oh, it’s the side of a volcano, it’s a lava flow. So now we can determine with more confidence which are the fluvial ridges, in relation to the ridges formed by other processes. “
Now that we understand the distribution of ancient rivers on Mars globally, future research – either by rover or by astronauts – could use these stone records to explore what the climate and tectonics were like. “One of the greatest achievements of the last twenty years is the recognition that Mars has sedimentary records, which means that today we are not limited to studying the planet,” says Fischer. “We can ask questions about his history.” At the same time, he says, we learn not only about the past of a planet, but we also find “truths about how the planets evolved … and why the Earth is habitable”.
As this research is only the first to use a full mosaic, Dickson is looking forward to seeing how the next one is used. “We expect to see more and more studies of similar scale as we are doing here, from other researchers around the world,” he says. “We hope that this scientific study of the ‘first journey’ will give an example of the scale of science that can be achieved with such a large product.”