The history of a planet that formerly had the conditions necessary to support life can be pieced together by researching the chemical elements present on Mars now, such as carbon and oxygen.
Weaving this story, element by element, from roughly 140 million miles (225 million kilometers) away is a painstaking process. However, scientists are not the kind to be easily intimidated. Due to signs like dry riverbeds, old shorelines, and salty surface chemistry, Mars orbiters and rovers have proved that the planet formerly possessed liquid water.
Using NASA’s Curiosity Rover, scientists have found evidence for long-lived lakes. They’ve also dug up organic compounds, or life’s chemical building blocks. The presence of liquid water and chemical compounds on Mars motivates researchers to continue looking for evidence of past or contemporary life.
The scientific understanding of Martian history is still developing, and numerous important questions are still up for debate, despite the enticing evidence discovered thus far. For one, was the ancient Martian atmosphere thick enough to keep the planet warm, and thus wet, for the amount of time necessary to sprout and nurture life?
And the organic compounds: are they signs of life or of chemistry that happens when Martian rocks interact with water and sunlight?
In a recent Nature Astronomy report on a multi-year experiment conducted in the chemistry lab inside Curiosity’s belly, called Sample Analysis at Mars (SAM), a team of scientists offers some insights to help answer these questions.
The scientists discovered that some minerals in the rocks at Gale Crater might have developed in a lake that was covered in ice. These minerals may have developed after Mars lost the majority of its atmosphere and started to become permanently frigid, or during a cold stage wedged between warmer phases.
Gale is a crater the size of Connecticut and Rhode Island combined. It was chosen as Curiosity’s 2012 landing site because it contained clay minerals that may have helped trap and preserve prehistoric organic molecules and other evidence of past water.
Indeed, while exploring the base of a mountain in the center of the crater, called Mount Sharp, Curiosity found a layer of sediments 1,000 feet (304 meters) thick that was deposited as mud in ancient lakes.
According to some scientists, for millions to tens of millions of warm, humid years, a tremendous volume of water would have flowed down into those lakes in order to build that much silt. However, several geological features in the crater also allude to a past with frigid, cold weather.
“At some point, Mars’ surface environment must have experienced a transition from being warm and humid to being cold and dry, as it is now, but exactly when and how that occurred is still a mystery,” says Heather Franz, a NASA geochemist based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Franz, who led the SAM study, notes that factors such as changes in Mars’ obliquity and the amount of volcanic activity could have caused the Martian climate to alternate between warm and cold over time. Chemical and mineralogical alterations in Martian rocks that demonstrate that some strata evolved in colder environments and others in warmer ones support this theory.
Franz claims that the variety of information gathered by Curiosity to date indicates that the crew is detecting indications of Martian climate change recorded in rocks.
Nevertheless, the carbon cycling is still happening and is still important because it’s not only helping reveal information about Mars’ ancient climate. It’s also showing us that Mars is a dynamic planet that’s circulating elements that are the buildings blocks of life as we know it.
Paul Mahaffy
Carbon and oxygen star in the Martian climate story
Franz’s team found evidence for a cold ancient environment after the SAM lab extracted the gases carbon dioxide, or CO2, and oxygen from 13 dust and rock samples. Curiosity collected these samples over the course of five Earth years (Earth years vs. Mars years).
One carbon atom is bound to two oxygen atoms to form the molecule CO2, which is a crucial witness in the case of the enigmatic Martian environment. In fact, in the hunt for life elsewhere, this straightforward yet adaptable ingredient is just as important as water. On Earth, carbon is continuously transported through the air, water, and surface in a cycle that is dependent on life.
For instance, plants take up carbon dioxide (CO2) from the environment. In exchange, they generate oxygen, which is used for respiration by humans and the majority of other living things. This process results in the release of carbon into the atmosphere, this time in the form of CO2, or into the Earth’s crust when living things perish and are buried.
The carbon cycle is also present on Mars, and scientists are striving to comprehend it. The Red Planet’s carbon cycle is significantly different from Earth’s because there hasn’t been any water or surface life there for at least the last 3 billion years.
“Nevertheless, the carbon cycling is still happening and is still important because it’s not only helping reveal information about Mars’ ancient climate,” says Paul Mahaffy, principal investigator on SAM and director of the Solar System Exploration Division at NASA Goddard. “It’s also showing us that Mars is a dynamic planet that’s circulating elements that are the buildings blocks of life as we know it.”
The gases build a case for a chilly period
After Curiosity fed rock and dust samples into SAM, the lab heated each one to nearly 1,650 degrees Fahrenheit (900 degrees Celsius) to liberate the gases inside. Scientists were able to identify the kind of minerals the gases were originating from by observing the oven temperatures that emitted the CO2 and oxygen. They can better comprehend how carbon is cycled on Mars with the aid of this information.
Several studies have hypothesized that Mars’ prehistoric atmosphere, which was primarily composed of CO2, may have been denser than the atmosphere of Earth at the time. The majority of it has been lost to space, but some of it may still be present in rocks on the surface of the planet, particularly in the form of carbonates, which are carbon and oxygen-containing minerals.
When CO2 from the air is absorbed in the oceans and other water bodies and subsequently mineralized into rocks, carbonates are created on Earth. The identical process, which occurred on Mars, according to scientists, may have contributed to the loss of some of the Martian atmosphere.
Yet, missions to Mars haven’t found enough carbonates in the surface to support a thick atmosphere.
However, the carbon and oxygen isotopes found in the few carbonates that SAM did find provided some intriguing information about the Martian environment. Isotopes are versions of each element that have different masses.
Because these isotopes are used in various ratios during various chemical processes, including the formation of rocks and biological activity, scientists can learn more about a rock’s formation by examining its heavy-to-light isotope ratios.
Scientists noticed that the oxygen isotopes in several of the carbonates SAM discovered were lighter than those in the Martian atmosphere. This shows that the carbonates did not originate long ago as a result of a lake absorbing atmospheric CO2. The oxygen isotopes in the rocks would have been a little bit heavier than those in the air if they had.
While it’s possible that the carbonates formed very early in Mars’ history, when the atmospheric composition was a bit different than it is today, Franz and her colleagues suggest that the carbonates more likely formed in a freezing lake.
In this case, the ice might have absorbed the heavier oxygen isotopes while leaving the lighter ones behind to eventually create carbonates. Other Curiosity researchers have also offered proof that ice-covered lakes may have once existed in Gale Crater.
So where is all the carbon?
The low abundance of carbonates on Mars is puzzling, scientists say. If Gale Crater doesn’t have a lot of these minerals, the early atmosphere might have been less than expected. Or perhaps another source is holding onto the missing atmospheric carbon.
Based on their findings, Franz and her coworkers hypothesize that some carbon may be stored in other minerals, such as oxalates, which do not have the same structure as carbonates and can store both carbon and oxygen. Their hypothesis is based on the temperatures at which CO2 was released from some samples inside SAM too low for carbonates, but just right for oxalates, and on the different carbon and oxygen isotope ratios than the scientists saw in the carbonates.
A model of a carbonate molecule next to an oxalate molecule
The most prevalent kind of organic mineral made by plants on Earth is oxalates. Oxalates can, however, also be made in the absence of life. Abiotic photosynthesis is one method, which involves the interaction of atmospheric CO2 with surface minerals, water, and sunshine. This type of chemistry is hard to find on Earth because there’s abundant life here, but Franz’s team hopes to create abiotic photosynthesis in the lab to figure out if it actually could be responsible for the carbon chemistry they’re seeing in Gale Crater.
On Earth, abiotic photosynthesis may have paved the way for photosynthesis among some of the first microscopic life forms, which is why finding it on other planets interests astrobiologists.
Franz and her colleagues want to examine soil and dust from various regions of Mars to determine whether their findings from Gale Crater accurately represent the situation throughout the planet. This is true even if it turns out that abiotic photosynthesis locked some carbon from the atmosphere into rocks at Gale Crater. They may one day get a chance to do so. NASA’s Perseverance Mars rover, due to launch to Mars between July and August 2020, plans to pack up samples in Jezero Crater for a possible return to labs on Earth.
