One of the primary goals of the Curiosity mission to Mars has been to search for and hopefully identify organic compounds — the carbon-based molecules that on Earth are the building blocks of life.
No previous mission had quite the instruments and capacity needed to detect the precious organics, nor did they have the knowledge about Martian chemistry that the Curiosity team had at launch.
Nonetheless, finding organics with Curiosity was no sure things. Not only is the Martian surface bombarded with ultraviolet radiation that breaks molecules apart and destroys organics, but also a particular compound now known to be common in the soil will interfere with the essential oven-heating process used by NASA to detect organics.
So when Jennifer Eigenbrode, a biogeochemist and geologist at the Goddard Space Flight Center and a member of the Curiosity organics-searching team, asked her colleagues gathered for Curiosity’s 2012 touch-down whether they thought organics would be found, the answer was not pretty.
“I did a quick survey across the the team and I was convinced that a majority in the room were very doubtful that we would ever detect organics on Mars, and certainly not in the top five centimeters or the surface.”
Yet at a recent National Academies of Sciences workshop on “Searching for Life Across Space and Time,” Eigenbrode gave this quite striking update:
“At this point, I can clearly say that I am convinced, and I hope you will be too, that organics are all over Mars, all over the surface, and probably through the rock record. What does that mean? We’ll have to talk about it.”
This is not, it should be said, the first time that a member of the Curiosity “Sample Analysis on Mars” (SAM) team has reported the discovery of organic material. The simple, but very important organic gas methane was detected in Gale Crater, as were chlorinated hydrocarbons. Papers by Sushil Atreya of the University of Michigan and Daniel Glavin and Caroline Freissinet from Goddard, along with other team members from the SAM team, have been published on all these finds.
But Eigenbrode’s work and her comments at the workshop– which acknowledged the essential work of SAM colleagues — move the organics story substantially further.
That’s because her detections involve larger organic compounds, or rather pieces of what were once larger organics. What’s more, these organics were found only when the Mars samples were cooked at over over 800 degrees centigrade in the SAM oven, while the earlier ones came off as detectable gases at significantly lower temperatures.
These latest carbon-based organics were most likely bound up inside minerals, Eigenbrode said. Their discovery now is a function of having an oven on Mars that, for the first time, can get hot enough to break them apart.
The larger molecules bring with them additional importance because, as Eigenbrode explained it, 75 to 90 percent of organic compounds are of this more complex variety. What’s more, she said that the levels at which the compounds are present, as well as where they were found, suggests a pretty radical conclusion: that they are a global phenomenon, most likely found around the planet.
Her logic is that the overall geochemistry of soil at Gale Crater as read by Curiosity instruments is quite similar to the chemistry of samples tested by earlier rovers at two other sites on Mars, Gusev Crater and Meridiani Planum.
Many Mars scientists are comfortable with taking these parallel bulk chemistry readouts — the sum total of all the chemicals found in the samples — and inferring that much of the planet has a similar chemical makeup.
Taking the logic a step further, Eigenbrode proposed to the assembled scientists that the signatures of carbon-based organics are also a global phenomenon.
“I think it just might be,” she told the NAS workshop, which was organized by the Space Studies Board. “We’ll have to find out more, but I think there’s a good possibility.”
That’s quite a jump — from a situation not long ago when no organics had been knowingly detected on Mars, to one where there’s a possibility they are everywhere.
And actually, they should be found everywhere. Not only do organic molecules rain down from the sky embedded in asteroids and interstellar dust, but they can also be formed abiotically out of chemicals on Mars and, just possibly, can be the products of biological activity.
The fact that Mars surely has had organics on its surface and elsewhere has made the non-detection of organics a puzzle. In fact, that conclusion of “no organics present” following the Viking landings in the mid 1970s set the Mars program back several decades. If there weren’t even organic compounds to be found, the thinking went, then a search for actual living creatures was pointless.
As is now apparent, the Viking instrument used to detect organics didn’t have the necessary diagnostic power that SAM has. What’s more, the scientists working with it did not know about a particular chemical on the Martian surface that was skewing the results. Plus the scientists may well have misunderstood their own findings.
First with the question of technological muscle. The oven associated with the search for organics is part of a Gas Chromatograph Mass Spectrometer (GCMS), and it heats and breaks apart dirt and rock samples for analysis of their chemical makeup. The oven on the Viking landers only went up to 500 degrees C. But the SAM oven on Curiosity goes hotter. It detected signs of organics between 500 and 850 degrees C.
In addition, NASA’s Phoenix lander discovered in 2008 that the Martian soil contained the salt perchlorate, which when burned in a GCMS oven can mask the presence of organics. And finally, the Viking landers actually did detect organics in the form of simple chlorinated hydrocarbons. They were determined at the time to be contamination from Earth, but the same compounds have been detected by Curiosity, suggesting that Viking might actually have found Martian, rather than Earthly, organics.
What makes carbon-based organic compounds especially interesting to scientists is that life is made of them and produces them. So one source of the organics in Martian samples could be biology, Eigenbrode said. But she said there were other potential sources that might be more plausible.
Organics, for instance, can be formed through non-biological geothermal and hydrothermal processes on Earth, and presumably on Mars too. In addition, both meteorites and interstellar dust are known to contain organic compounds, and they rain down on Mars as they do on Earth.
Eigenbrode said the organics being detected could be coming from any one source, or from all of them.
Asked at the workshop what concentrations of organics were found, she replied with a grin that more light will be shed on the question at next week’s American Geophysical Union meeting.
The detection of a growing variety of organics on Mars adds to the conclusion already reached by the Curiosity team — that Mars was once much wetter, warmer and by traditional definitions “habitable.” That doesn’t mean that life ever existed there, but rather that what are considered basic basic conditions for life were present for many millions of years.
Eigenbrode said that the detection of these carbon-based compounds is important in terms of both the distant past and the perhaps mid-term future.
For the past, it means that organics in a substantial reservoir of water like the one at Gale Crater some 3.6 billion years ago could have been a ready source of energy for microbial life. The microbes would then have been heterotrophs, which get their nutrition from organic material. Autotrophs, simpler organisms, are capable of synthesizing their own food from inorganic substances using light or chemical energy.
But Eigenbrode also sees the organics as potentially good news for the future — for possibly still living microbes on Mars and also for humans who might be trying to survive there one day.
“Thinking forward, the organic matter could be really important for farming — a ready energy source provided by the carbon,” she said.
Just what a human colony on Mars some day might need.