Martian chemistry: how the Curiosity rover detected organic compounds on Mars

NASA’s Curiosity Mars rover found organic molecules that have never been seen before on Mars. A rock that the rover drilled and analyzed in 2020 includes the most diverse collection of organic molecules ever found on Mars, NASA announced last week. Of the 21 carbon-containing molecules identified in the sample, seven of them had never been found on Mars before. 

Scientists are not sure whether these molecules were created by geological or biological processes, but this discovery increases confidence that ancient Mars might have had the right chemistry to support life. The findings are detailed in a paper in Nature Communications. 

“This is Curiosity and our team at their best. It took dozens of scientists and engineers to locate this site, drill the sample, and make these discoveries with our awesome robot,” the mission’s project scientist, Ashwin Vasavada of NASA’s Jet Propulsion Laboratory in Southern California, said in a statement. “This collection of organic molecules once again increases the prospect that Mars offered a home for life in the ancient past.”

Mary Anning 3

The rock sample, called Mary Anning 3 after an English paleontologist, was collected on a part of Mount Sharp thought to have been covered by lakes and streams billions of years ago. The water surged and dried up multiple times in the planet’s history, enriching the area with clay minerals that preserve organic compounds. 

Among the detected molecules was a nitrogen heterocycle, a molecular structure considered to be a predecessor to RNA and DNA. 

“That detection is pretty profound because these structures can be chemical precursors to more complex nitrogen-bearing molecules,” the paper’s lead author, Amy Williams of the University of Florida in Gainesville, said in a statement. “Nitrogen heterocycles have never been found before on the Martian surface or confirmed in Martian meteorites.”

The sample also contained benzothiophene, a carbon and sulfur-bearing molecule that has been found in meteorites that are thought to have seeded prebiotic chemistry in the early solar system. 

Chemistry on Mars

The findings were made with a minilab called Sample Analysis at Mars (SAM) that is located within Curiosity. A drill on the end of the rover’s robotic arm pulverized the sample into powder and then funneled it into SAM, where an oven heated the material, releasing gases that instruments within the rover analyzed to reveal the rock’s composition. 

SAM can also perform wet chemistry by dropping samples into a small cup of solvent. The resulting reactions break apart larger molecules that would otherwise be difficult to analyze. While the instrument has several such cups, only two contain tetramethylammonium hydroxide (TMAH), a powerful solution reserved for the highest-value samples. The Mary Anning 3 sample was the first to be exposed to TMAH.

To verify TMAH’s reactions with otherworldly materials, the paper’s authors also tested the technique on Earth with a piece of the Murchison meteorite, one of the most studied meteorites of all time. More than 4 billion years old, Murchison contains organic molecules that were seeded throughout the early solar system. A Murchison sample exposed to TMAH was found to break much larger molecules into some of the ones seen in Mary Anning 3, including benzothiophene. That result verifies that the Martian molecules found in Mary Anning 3 could have been generated from the breakdown of even more complex compounds relevant to life.