A rugged Martian landscape that resembles a giant spiderweb when seen from orbit may hold important evidence about the history of water on ancient Mars.
For roughly six months, NASA’s Curiosity rover has been studying an area covered with geological features known as boxwork. These formations appear as narrow ridges about 3 to 6 feet (1 to 2 meters) tall separated by sandy depressions. Stretching across the terrain for miles, the crisscrossing ridges hint that groundwater once flowed through this region of Mars later than scientists previously believed. If that is true, it raises new questions about how long microscopic life might have survived on the planet billions of years ago, before its rivers and lakes disappeared and Mars became the cold desert we see today.
From orbit, the boxwork ridges create patterns that look like massive spiderwebs spread across the landscape. Researchers believe the shapes formed when groundwater moved through fractures in the bedrock, depositing minerals along those cracks. Over time, the mineral deposits hardened the fractured zones into ridges. Surrounding rock that lacked this reinforcement gradually eroded away, leaving behind the web-like network visible today.
Before Curiosity reached this region, scientists could only study the formations from orbital images, leaving many questions about their true structure and origin.
Exploring Martian Boxwork Up Close
Boxwork formations also exist on Earth, but they are usually only a few centimeters tall and often appear in caves or dry sandy environments. The Martian versions are far larger. To understand them better, the Curiosity team aimed to investigate the ridges directly and collect detailed measurements.
Navigating the terrain has not been easy. Engineers must carefully guide Curiosity, an SUV-size rover weighing nearly a ton (899 kilograms), along ridge tops that are sometimes only slightly wider than the rover itself.
“It almost feels like a highway we can drive on. But then we have to go down into the hollows, where you need to be mindful of Curiosity’s wheels slipping or having trouble turning in the sand,” said operations systems engineer Ashley Stroupe of NASA’s Jet Propulsion Laboratory in Southern California, which built Curiosity and leads the mission. “There’s always a solution. It just takes trying different paths.”
Scientists are also working to understand how such an extensive network of ridges formed on Mount Sharp, the 3-mile-tall (5-kilometer-tall) mountain that Curiosity has been climbing. Each layer of the mountain represents a different chapter in Mars’ ancient climate history. As the rover ascends, the landscape increasingly shows signs that water gradually disappeared over time, although occasional wetter periods allowed rivers and lakes to return.
“Seeing boxwork this far up the mountain suggests the groundwater table had to be pretty high,” said Tina Seeger of Rice University in Houston, one of the mission scientists leading the boxwork investigation. “And that means the water needed for sustaining life could have lasted much longer than we thought looking from orbit.”
Evidence of Ancient Groundwater
Earlier satellite images revealed another intriguing feature: dark lines running through the spiderweb-like ridges. In 2014, researchers suggested these streaks might represent central fractures where groundwater once seeped through cracks in the rock and concentrated minerals.
Curiosity’s close examination has confirmed that these dark lines are indeed fractures, supporting the idea that groundwater shaped the formation of the ridges.
The rover also spotted small, bumpy structures called nodules. These textures are commonly linked to ancient groundwater activity and have been observed by Curiosity and other Mars missions in the past. Surprisingly, the nodules were not located near the central fractures. Instead, they appeared along the sides of the ridges and within the sandy hollows between them.
“We can’t quite explain yet why the nodules appear where they do,” Seeger said. “Maybe the ridges were cemented by minerals first, and later episodes of groundwater left nodules around them.”
Curiosity Acts as a Mobile Chemistry Lab
A key part of Curiosity’s mission involves collecting rock samples with a drill attached to the end of its robotic arm. The drill grinds rock into powder, which is then delivered to sophisticated instruments inside the rover for analysis.
Last year, scientists analyzed three samples taken from the boxwork region. One came from the top of a ridge, another from bedrock inside a hollow, and a third from an area Curiosity passed through before reaching the ridges. Using X-ray analysis and a high-temperature oven, the rover detected clay minerals within the ridge and carbonate minerals in the hollow. These discoveries provide additional hints about the processes that formed the unusual terrain.
More recently, the rover collected a fourth sample for a specialized analysis reserved for particularly interesting targets. 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 reveal certain organic compounds, carbon-based molecules that play an important role in the chemistry of life.
Continuing the Search for Mars’ Climate History
Curiosity is expected to move on from the boxwork region sometime in March. The area lies within a layer of Mount Sharp rich in salty minerals known as sulfates. These minerals formed as water on Mars gradually disappeared.
Over the coming year, the rover will continue traveling through this sulfate-rich layer, gathering new clues about how the climate of the ancient Red Planet changed billions of years ago.
About the Curiosity Rover
Curiosity was built by NASA’s Jet Propulsion Laboratory, which is managed by Caltech in Pasadena, California. JPL operates the mission for NASA’s Science Mission Directorate in Washington as part of NASA’s Mars Exploration Program portfolio.