Rugged Martian terrain that resembles a giant spider’s web when viewed from orbit may contain important evidence of ancient Mars’ water history.
For about six months, NASA’s Curiosity rover has been exploring an area covered in geological features known as boxwork. These formations appear as narrow ridges approximately 3 to 6 feet (1 to 2 meters) high separated by sandy depressions. The criss-crossing ridges, which extend for miles across the landscape, suggest that groundwater once flowed through this region of Mars, slower than scientists previously thought. If that’s true, it raises new questions about how long microscopic life existed on Earth billions of years ago, before rivers and lakes disappeared and Mars became the cold desert we see today.
Viewed from orbit, the boxwork ridges create a pattern that looks like a giant spider’s web spread across the landscape. Researchers believe the shape was formed when groundwater moved through cracks in the rock and deposited minerals along the cracks. Over time, mineral deposits hardened the fracture zone to form a ridge. Lacking this reinforcement, the surrounding rock gradually eroded away, leaving behind the web-like network visible today.
Until Curiosity arrived in this region, scientists could only study the formation from orbital images, leaving many questions about its true structure and origin.
Explore the boxwork of Mars up close
Box formations also occur on Earth, but are usually only a few centimeters high and are commonly found in caves and dry sandy environments. The Mars version is much larger. To better understand them, the Curiosity team aimed to directly survey the ridges and collect detailed measurements.
Navigating the terrain was not easy. Engineers must carefully guide the SUV-sized Curiosity rover, which weighs about 1 ton (899 kilograms), along the top of a ridge that is only slightly wider than the rover itself.
“It feels like a freeway you can drive on, but then you have to go down into the cavity, and you have to be careful that Curiosity’s wheels slip or become difficult to turn on the sand,” said Ashley Stroup, an operations systems engineer at NASA’s Jet Propulsion Laboratory in Southern California, who developed Curiosity and is leading the mission. “There’s always a solution. All you have to do is try different avenues.”
Scientists are also working to understand how such a vast network of ridges formed on Mount Sharp, the 3-mile-high (5-kilometer-high) mountain that Curiosity climbs. Each layer of the mountain represents a different chapter in Mars’ ancient climate history. As the rover ascends, the landscape begins to show signs of water gradually disappearing over time, but rivers and lakes return during periods of occasional rain.
“The fact that the box was so high up in the mountain suggests that the water table was quite high,” said Tina Seeger of Rice University in Houston, one of the mission scientists who led the search for the box. “And that means the water needed to sustain life may have lasted much longer than we think from orbit.”
evidence of ancient groundwater
Previous satellite images revealed another interesting feature: a dark line running along a spiderweb-like ridge. In 2014, researchers suggested that these stripes may represent central fissures where groundwater once seeped through cracks in the rock, concentrating minerals.
Curiosity’s closer examination confirmed that these black lines were indeed cracks, supporting the idea that groundwater shaped the formation of the ridge.
The rover also discovered small irregular structures called nodules. These textures are commonly associated with ancient groundwater activity and have been observed by Curiosity and other Mars exploration missions in the past. Surprisingly, the tubercle was not located close to the central fracture. Instead, they appeared along the sides of the ridges and in the sandy depressions between them.
“We still don’t fully explain why nodules appear where they exist,” Seeger says. “Perhaps the ridge was first cemented by minerals, and subsequent groundwater development left nodules around it.”
Curiosity acts as a mobile chemistry laboratory
A key part of Curiosity’s mission involves collecting rock samples using a drill attached to the end of its robotic arm. The drill will grind the rock into powder, which will then be sent to advanced equipment on the rover for analysis.
Last year, scientists analyzed three samples taken from the boxwork area. One from the top of the ridge, another from the bedrock in the depression, and a third from the area Curiosity passed through before reaching the ridge. Using X-ray analysis and a high-temperature oven, the spacecraft detected clay minerals within the ridge and carbonate minerals within the cavity. These discoveries provide additional hints about the processes that formed the unusual landforms.
Recently, the rover collected a fourth sample for special analysis, reserved for a particularly interesting target. After the powdered rock was heated in the rover’s oven, chemical reagents were introduced to perform what scientists call wet chemistry. This method helps uncover specific organic compounds, carbon-based molecules, that play important roles in the chemistry of life.
Continue exploring Mars’ climate history
Curiosity is expected to move out of the boxwork realm sometime in March. This area lies within the Mount Sharp Formation, which is rich in salty minerals known as sulfates. These minerals formed as Mars’ water gradually disappeared.
Over the next year, the rover will continue to travel through this sulfate-rich layer, collecting new clues about how ancient Mars’ climate changed billions of years ago.
About Curiosity Rover
Curiosity was built by NASA’s Jet Propulsion Laboratory, managed by the California Institute of Technology in Pasadena, California. JPL operates missions for NASA’s Science Mission Directorate in Washington as part of NASA’s Mars Exploration Program portfolio.
