In new samples taken on Mars last week, NASA's Curiosity rover has detected a high level of methane in the air, offering new evidence of possible life on the red planet and adding to the fascinating--and surprisingly contentious--mystery of sudden, unexpected bursts of methane gas on Mars.
NASA's Curiosity Rover Smelt it, But Who Dealt It?
NASA's Curiosity rover took air samples on Wednesday of this week that revealed a surprisingly-high level of methane, according to a new report by the New York Times, high enough to grab the attention of NASA's Curiosity rover team, who have reorganized this weekend's work to go back and have Curiosity take another sample to confirm this initial reading.
“Given this surprising result, we’ve reorganized the weekend to run a follow-up experiment,” wrote Ashwin R. Vasavada, a project scientist for the Curiosity mission, in an email to the science team obtained by The Times. Instructions were sent to the Curiosity rover on Friday, with results expected back by Monday. NASA has not formally announced any conclusions yet, but they did acknowledge the readings in a statement on Saturday.
“To maintain scientific integrity," an agency spokesperson said, according to The Times report, "the project science team will continue to analyze the data before confirming results.”
Why We Really Want to Know Who Keeps Farting on Mars
Why the level of methane so excited the scientists at NASA's Jet Propulsion Laboratory (JPL) is that methane here on Earth is predominantly a waste byproduct of various life processes, discharged by single-cell microbes and gassy mammals alike. The two are related as the bacteria in our gut produce the gas that builds up in our systems which are then expelled in concentrated bursts--usually in elevators, or when giving a presentation to a room full of your peers.
"Mars is about life, not geology, as interesting as that is." - Seth Shostak, SETI Institute
The same process might be at work on Mars if these readings are correct. Scientists have seen nothing to suggest that life exists on the surface of the planet. Below the surface, however, bacteria and other microbes could live, buried deep down in the Martian soil where we wouldn't be able to see them, but we might be able to smell them.
If they were anything like organisms here on Earth, they would be belching out methane just like the bacteria in our gut, methane that would eventually make its way up through the soil.
Methane isn't exclusively a biological process, however. So-called 'abiotic methane' can be produced as the result of natural chemical reactions between water and rocks composed, at least in part, of carbon. These pockets of gas would eventually seep up through the rock and soil above to the surface, where it would be released in a similar 'burst' that would characterize 'biotic methane', or that produced by biological organisms.
Mystery of Mars' Methane Is an Unexpectedly Intriguing Melodrama
One of the important things about methane is that it breaks down very quickly once it reaches the atmosphere and is exposed to sunlight, usually in just a few centuries. While centuries might be a long time in the lifespan of a human, geologically it barely measures at all. Significant levels of methane would mean that whatever produced it couldn't be more than a few centuries old, at its oldest.
The odds of finding a significant amount of methane on Mars emitted by microbes a few centuries ago who then all died off immediately afterward is even more improbable than that they ever existed in the first place, so we can be confident that if biotic methane were found, it would be very strong evidence of an active biosphere, not one that may have died out millions or even thousands of years ago.
In this context then, in 2003, Dr. Michael Mumma, of NASA's Goddard Space Flight Center, led a research project looking for methane on Mars when they made a startling discovery. Using two sets of Earth-based telescopes together with a Mars orbiter, they saw what looked like massive plumes of methane on the surface of Mars that measured as high as 45 parts per billion.
They followed the methane plume for about two years before it faded, finally vanishing below the sensitivity of their instruments in 2005. Since this methane should have persisted in the atmospheres for centuries, this led Mumma and his colleagues to the unusual conclusion, published in a 2009 paper, that something on Mars was producing a large amount of methane but that something else was destroying it almost as soon as it was produced, a phenomenon that hasn't been observed on Earth.
Anticipation Grows For Curiosity Mission
This caused a great deal of anticipation ahead of the Curiosity mission, which was outfitted with instruments that could initially detect as few as six parts per billion of methane when it landed on the surface of Mars in 2012. Not surprisingly, soon after landing, it began taking nice, deep robotic breaths and checking them for methane.
It reportedly found methane almost immediately, leading to a great deal of excitement--until someone realized that the methane Curiosity found was actually methane from Earth's atmosphere that rode along with Curiosity to Mars. Dispersing and pumping away the remaining Earth atmosphere around the rover, they measured again and the initial methane readings had disappeared.
The six parts per billion threshold was a crucial one, as this would have marked the level where the methane from Mumma's 45-parts-per-billion plume would have settled if it were distributed evenly throughout the Martian atmosphere, which is what we would naturally expect to happen. The Curiosity team refined their methane detection equipment even further, increasing its sensitivity to 1.3 parts per billion, and still, nothing.
Mumma's 2003 readings had been a major source of contention in the years since he and his team published their findings, so Curiosity was in many ways supposed to be the final arbiter of this controversy. Christopher R. Webster, a NASA JPL scientist on the Curiosity team, told The Times in 2013 that “[a] lot of people got excited and started working on [the 2003 measurements]. It was a very important result, because of the magnitude of methane.”
So the conclusion reached by the Curiosity team came as a major blow for many when they published their findings in the fall of 2013 that Curiosity could find no methane on Mars above 1.3 parts per billion, putting an end to the hopes of countless scientists and non-scientists alike that Mars could be home to an active biosphere of microbes.
The Academic Pile-On Begins
"The certainty that methane is there will go away," Kevin Zahnle, a planetologist at NASA's Ames Research Center who didn't work on Curiosity, told Science Magazine for a September 2013 story about the Martian-methane-flare-up then underway on Earth.
The publication of the Curiosity's report that there wasn't any detectable methane on Mars was the opening volley of a one-sided professional scuttling of Mumma's 2003 findings, and to some degree his reputation.
The Times reached out to Mumma for comment on the paper that sank his theory shortly after it was announced. Mumma stood by his findings, as he would for the next couple of years.
"[Mumma] said he now believed that methane on Mars was episodic," The Times report read, "released in large plumes and then quickly destroyed. He suggested, half-jokingly, that there could be huge colonies of methane-eating microbes on Mars that eliminated the gas from the air."
Dr. Sushil K. Atreya, a member of the science team from the University of Michigan, reportedly thought there might be something to Mumma's theory at the outset. From The Times report: "Dr. Atreya of the Curiosity team said he originally thought that highly reactive chemicals on the Martian surface could be destroying methane, as Dr. Mumma envisioned. But 'that’s not panning out,' Dr. Atreya said."
Mumma never claimed to know why large amounts of methane would suddenly appear and suddenly vanish in less than two years, only that this is what him and his team saw back in 2003.
"Dr. Mumma acknowledged," The Times reported, "that he could not identify any phenomena that would explain why methane plumes spurted out that year but not more recently, or how methane could be destroyed much more quickly on Mars than on Earth."
"'Mars may not be operating the same way,' he said. 'It’s a puzzle.'”
Mumma pointed out that nothing that Curiosity reported back to the Curiosity team actaully contradicted his theory, because if the methane were being broken down quickly--in as little as 0.4 years and in no more than 4 years, as his 2009 paper on the subject argues--the methane levels detected on Mars would be more or less as Curiosity found them.
"So far [Curiosity's Tunable Laser Spectrometer] results don't challenge anything we said in the 2009 paper," Mumma said, according to Science Magazine.
In an interview with National Geographic after the Curiosity team's 2013 announcement, Mumma said "[t]hese findings are actually consistent with our results. We reported that the methane releases are likely to be sporadic and that the methane is quickly eliminated in the atmosphere."
"The good news here," he added, "is that the rover instrument designed to detect methane is working, and we look forward to ongoing monitoring in the future."
Webster pushed back on this theory, telling National Geographic that "[m]ethane is a very well understood gas that is quite stable. We know how long it lasts and how it is destroyed over decades."
The Curiosity team's paper directly disputed Mumma's 2009 claim that methane must be breaking down at an accelerated rate on Mars by some unknown process, concluding that "[w]ith an expected photochemical lifetime of methane in the Martian atmosphere of hundreds of years, there currently remains no accepted explanation for the existence and distribution of the reported plumes, nor of the apparent disappearance of methane over the last few years."
Webster was confident that the methane Mumma and his team saw simply wasn't there. "Every time we looked, we never saw it," he told The Independent, later telling Science Magazine that Mumma's theory "requires physics and chemistry that is unknown." On this last point, Mumma completely agreed, Science Magazine dutifully pointed out at the time, since that was the logical conclusion to be drawn from his 2009 paper.
Dr. Atreya, who co-authored the 2013 paper, said in a NASA press release announcing the Curosity team's findings that without a process that they could actually see and study that takes methane out of the atmosphere quickly, all they could do was go by the readings from Curiosity, which Atreya felt put the question of methane on Mars more or less to rest.
"There's no known way for methane to disappear quickly from the atmosphere," Atreya said. "Methane is persistent. It would last for hundreds of years in the Martian atmosphere. Without a way to take it out of the atmosphere quicker, our measurements indicate there cannot be much methane being put into the atmosphere by any mechanism, whether biology, geology, or by ultraviolet degradation of organics delivered by the fall of meteorites or interplanetary dust particles."
At the current level of parts per billion they were seeing, no more than 10 to 20 tons of methane could be entering into Mars' atmosphere every year, which is 50 million times less than the amount entering into Earth's every year.
Webster concluded said finally, "It would have been exciting to find methane, but we have high confidence in our measurements, and the progress in expanding knowledge is what's really important. We measured repeatedly from Martian spring to late summer, but with no detection of methane."
Apparently not willing to leave a knife in the gut untwisted, The Times report in September 2013 kicked some more dirt on a quickly-filling grave--writing an epitaph for Mumma's theory of the sudden, localized eruption and disappearance of methane on Mars: "[a] simpler explanation would be that there was never much in the way of methane—or microbes—on Mars."
Meanwhile, Back on Mars...
Even before all of this was happening, however, the Curiosity team had already refined Curiosity's instruments further than the 1.3 parts per billion threshold, achieving a sensitivity of 0.7 parts per billion. In July 2013, just a few months before the publication of Curiosity team's paper, the Curiosity rover got its first whiff of methane.
At 0.7 parts per billion, the detected methane in the air was half the amount by volume that the Curiosity team were expecting to find.
Everybody expected there to be some level of detectable methane on Mars. Methane is a byproduct of chemical reactions in the atmosphere as falling space dust interacts with the sun's ultraviolet radiation, producing a background-level of methane in the air, which became the methane 'ceiling' in the paper the Curiosity team would publish just a few months later.
This ceiling also factors in the natural destruction of methane in the atmosphere by the sun over time, so while readings this low didn't indicate that anything was actively producing methane on Mars, it did appear to show something else: something other than the sun appeared to be breaking down and destroying the methane in the atmosphere faster than the sun would on its own.
Around this same time, they also picked up a higher level of methane than the assumed ceiling, but which fell by half in about a week. The margin of error for their readings made it difficult to tell for sure what they had just seen, so unable to make a determination about the lower-than-expected level of background methane and the anomolous blip in Curiosity's readings, as scientists, all they could really do was go with the data they knew was valid, which showed no significant levels of methane on Mars.
The Curiosity rover continued on its mission, inching slowly towards Mount Sharp and performing other important test along the way; but for the next four months, no other methane readings were taken.
Then, in November 2013--just two months after announcing that Curiosity had found no methane on Mars and starting a rather rough scientific smack down in the pages of popular science magazines--, the Curiosity team instructed Curiosity to take another measurement of the surrounding air.
Having just published a paper that declared that no methane had been found on Mars--and repeatedly smacking down the idea that significant, localized bursts of methane appeared on the surface of Mars sporadically--the Curiosity team was stunned by what Curiosity found.
Lobbing back a bombshell to the Curiosity team from Mars, Curiosity reported it had detected a tenfold spike in methane over the detectable background level, a reading of about 7 parts per billion.
The readings came back for the next two months, showing consistently-high levels of methane, before quickly falling off to below one part per billion in January 2014. It was like a massive cloud of methane gas had blown over the Curiosity rover--one might even call it a methane plume. The new readings were well beyond the margin of error and clearly indicated something on Mars was producing a scientifically significant amount of methane, the first direct evidence that there might be an active biosphere on Mars after all.
The Sweet Smell of Professional Vindication
Given that no measurements were taken between July and November 2013, it's possible that the methane gas pocket the Curiosity team recorded was even larger than their measurements reflected. Dr. Atreya told The Times in 2014 that if the anomalous readings the Curiosity team saw in July weren't just hiccups, but actual measurements of a spike in methane, those readings might have been part of the same cloud of methane gas they found in November, taken just as the gas cloud began wafting over the Curiosity rover while it sat in Gale Crater.
“It could have been over six months,” he said, “but we don’t know that.”
However long the gas pocket lingered, its quick emergence and subsequent disappearance indicates a sudden burst of methane, consistent with the findings by Mumma's team a decade earlier.
Taken together with the Curiosity team's measurements in July 2013 that indicated something on Mars other than the sun was breaking down methane at a remarkable rate, Dr. John Grotzinger, the Curiosity's mission scientist, told The Times that Mumma's theory could no longer be written off as it was following the Curiosity team's 2013 paper.
"It's back on the table," Grotzinger said.
As for Mumma, one gets the sense of his somewhat-lyconic satisfaction when he told The Times in 2014 that the new findings confirming the existence of his methane plumes were "pleasant."
After at least a year of having his work derided in several major popular scientific magazines and one esteemed peer-reviewed journal, you can hardly blame Mumma for taking a bit of a victory lap in the pages of the same newspaper that dedicated five paragraphs of its No-Methane-on-Mars story in 2013 to really bringing home the point to readers that Mumma's 2003 work was 'proven' invalid by the Curiosity team's first--and incorrect--paper.
These new findings, Mumma said, "confirmed this startling reality that methane is being released, sporadically, and it is being destroyed quickly. Both events are surprising.”
So, What Does All This Methane on Mars Mean?
Whether the methane on Mars is abiotic or biotic, it is definitely there, and it's weird. Its presence also tells us something new about Mars that we didn't know before Mumma first detected it back in 2003.
While the excitement is focused on the potential that it is biotic methane, even abiotic methane indicates that there may be vast reservoirs of underground water below the surface, interacting with carbon-bearing rock formations and under intense heat.
And moreover, if underground water deposits are coming into contact with hot, carbon-bearing rock and producing methane in the process, these underground reservoirs could host geothermal or even hydrothermal vents, thought by many to be ground zero for the genesis of microbial life here on Earth. The same could be true of Mars, so they would be exactly where we'd want to start looking for life.
Still, the biotic methane is clearly what everybody is hoping for, making this the one time identifying something by its fart would be a pleasant surprise, because that would mean that something on Mars is alive, and that we aren't alone in the universe. If two planets in the same solar system can give rise to life and they were independent of each other, then why not countless other planets in the galaxy?
“That’s the mythology,” said astronomer Seth Shostak, of the Search for Extra-Terrestrial Intelligence (SETI) Institute. “Mars is about life, not geology, as interesting as that is.”
As for last week's methane reading, it was even higher than the one Curiosity found in 2013, measuring at 21 parts per billion, but it has been only one of many spikes it has detected over the years. Even before last week's measurement, it has been clear that there is definitely something strange about the methane on Mars.
With several years of data now in hand, Webster and the Curiosity team reportedly found that the background level of methane can change over time, between about 0.5 parts per billion and 1.5 parts per billion, and that this variation in background methane may track the Martian seasons.
“It's very, very fascinating and puzzling,” Webster told The Times in a 2018 report on their lastest findings.
That report in The Times went on to note that Mumma believed "[the Curiosity team's] work was carefully done and confirms the low background levels, but that he was not yet convinced of the seasonality of the variations" of the background levels of methane on Mars that Webster and the Curiosity team was reporting.