Brownish black and lumpy, manganese oxides aren’t minerals you’d find dangling from a pendant or set in a ring. But what they lack in style, they make up for in significance.
On Earth, scientists consider the drab minerals to be signatures of atmospheric oxygen. When rovers in the Gale and Endeavour craters spotted them on Mars, some scientists even concluded that Mars’s atmosphere must have contained oxygen in the past. However, what’s true on Earth isn’t always true on Mars. And there’s more than one way to cook up manganese oxides.
In a new study published in December in Nature Geoscience, researchers discovered that manganese oxides can form without atmospheric oxygen under Martian conditions. Using reaction chambers that emulated the acidic fluids of ancient Mars, the researchers discovered that reactions involving chemicals called oxyhalogens, which contain oxygen atoms bound to a halogen like bromine or chlorine, can also produce manganese oxides—and do so even faster than atmospheric oxygen does.
“The experiment presented in the paper is important, because this type of experiment is really missing in the field,” said planetary geoscientist Yang Liu of NASA’s Jet Propulsion Laboratory, who was not involved in the study. “Whether the Martian atmosphere contained oxygen in ancient times, whether the oxygen content changed with time—those are all key questions about Mars.”
To Follow Energy, Read the Rocks
To find alien life, it’s not enough just to “follow the water,” said geochemist and coauthor Jeffrey Catalano of Washington University in St. Louis. Life needs an energy source, too.
“The key way of determining that important part of habitability is by looking at the sediment record, the rock record, and what minerals are there,” he said. “They can tell you about the energy sources that might have been available for life.”
Life usually gets energy by shuttling electrons from one chemical to another in what’s called a redox reaction. Animals eat organic carbon and shuttle its electrons to the oxygen they breathe for energy. But although many microbes can get energy by “eating” and “breathing” more exotic substances, including minerals, oxygen is special. Redox reactions involving this reactive gas usually release a lot of energy, so it makes a great fuel for biology.
That’s one reason the manganese oxides discovered in the Gale and Endeavour craters are so interesting to astrobiologists—on Earth, manganese oxides reflect atmospheric oxygen, so their presence on Mars could hint that it was once present there, too.
“At first, we were like, maybe that’s a mistake!” said planetary scientist Nina Lanza of Los Alamos National Laboratory. Lanza was on the team that discovered manganese oxides in Gale crater and was not involved in the new study. “Now the question is, How do they form? And what does that mean about Mars?”
Bringing Martian Chemistry to the Lab
Although atmospheric oxygen reacts with manganese to form manganese oxides on Earth, it’s not clear the same thing would happen on Mars. The reaction would have been much slower in the weakly acidic fluids of early Mars, Catalano said, and may not have happened at all depending on the conditions. What’s more, manganese oxides could have formed from compounds other than atmospheric oxygen, too.
“If we’re going to make interpretations about habitability, we want to do it with the best scientific foundation,” said Catalano. “And so, we set out to see…are there other chemical compounds that maybe could have made these things and that they aren’t actually indicators of substantial oxygen in the atmosphere?”
Catalano and his team wondered whether more reactive chlorine- and bromine-containing chemicals called oxyhalogens might be responsible for Mars’s mysterious manganese oxides. Mars is rich in chlorine and bromine compared with Earth, and chlorates—a group of oxyhalogens that contain chlorine bound to three oxygen atoms—are abundant on the planet’s surface. It just wasn’t clear how readily these chemicals would produce manganese oxides in acidic fluids of early Mars.
To find out, the research team reacted manganese with the oxyhalogens chlorate and bromate in reaction chambers that mimicked the conditions on early Mars. They allowed reactions to go on for weeks, months, or even years.
Chlorate didn’t produce any manganese oxides within the time frame of the experiments. However, the reaction chambers containing bromate developed brownish-black veneers of manganese oxide minerals within 6–8 weeks. Under acidic conditions, bromate produced manganese oxides hundreds of thousands of times faster than oxygen.
The findings mean that manganese oxides might not reflect atmospheric oxygen on Mars. But it’s still too early to say how exactly these minerals formed and what they say about the Red Planet’s ancient habitability.
There are several kinds of manganese oxides, and knowing which ones appear on Mars could hint at the conditions that produced them, said Liu. However, that probably won’t be possible until samples from Mars can be brought to Earth, she continued. Catalano agreed that sample return will be critical for decoding what manganese oxides mean for the Red Planet’s ancient habitability.
Finally, although chlorate is abundant on Mars, it didn’t produce manganese oxides quickly in Catalano’s experiments. Bromate worked much faster, but Lanza and Liu both said it’s not clear where bromate would come from on Mars—one possible source would be a reaction involving atmospheric oxygen.
“There are many potential pathways, and the Earth and Mars are not identical,” said Lanza, “so we do need to think broadly about it. Because of this really strong tie between life and manganese minerals, it’s a really important question to answer.”
—Elise Cutts (@elisecutts), Science Writer