Two samples from Mars together provide the smoking gun in a new study revealing the origins of organic material on Mars. The study provides solid evidence for a prediction made more than a decade ago by researchers at the University of Copenhagen that could be key to understanding how organic molecules, the basis of life, first formed here on Earth.
Inside a meteorite crater on the Red Planet, a lone robot moves about. Right now, it’s likely collecting soil samples with a drill and a robotic arm, as it often does. NASA’s Curiosity rover has been working on Mars for nearly 12 years as the extended arm of science, and it continues to make discoveries that amaze and challenge scientists’ understanding of Mars and our own world here on Earth.
The discovery of sedimentary organic material with unusual properties has recently given many researchers pause for thought. The properties of these carbonaceous materials, especially the ratio of carbon isotopes, surprised researchers.
Organic materials with such properties, if found on Earth, would usually be a sign of microorganisms, but they could also be the result of non-biological, chemical processes. The discovery clearly sent researchers scrambling for a clear answer, but nothing seemed to fit.
For the research collaboration behind a new study published in 2014, Nature Geosciences, there is little hair scratching and much enthusiasm.
For this group of researchers from the University of Copenhagen and the Tokyo Institute of Technology, the discovery on Mars was the missing piece of the puzzle that made everything fall into place.
According to co-author and chemistry professor Matthew Johnson, this is “the evidence” needed to confirm his decade-old theory about so-called photolysis in the Martian atmosphere.
With the Curiosity sample, the new research can prove with reasonable certainty that the Sun broke down CO2 in the Martian atmosphere billions of years ago — as old theory predicted. And that the resulting carbon monoxide gradually reacted with other chemicals in the atmosphere to synthesize complex molecules — thus providing Mars with organic materials.
“Such complex molecules based on carbon are the prerequisite for life, the building blocks of life, you might say. So this is a bit like the old debate about which came first, the chicken or the egg. We show that the organic material found on Mars was formed by atmospheric photochemical reactions — without life. This is the 'egg,' a prerequisite for life. It still has to be shown whether this organic material has given rise to life on the Red Planet,” Johnson said, continuing:
“Moreover, because Earth, Mars and Venus had very similar CO2 levels2 “The photolysis occurred in rich atmospheres long ago, but it could also be important for our understanding of how life originated on Earth,” said Professor Matthew Johnson from the Department of Chemistry at the University of Copenhagen.
Two pieces separated by 50 million kilometers – one puzzle solved
12 years ago, Johnson and two colleagues used simulations based on quantum mechanics to determine what happens when a CO2 rich atmosphere is exposed to the sun's UV light, in a process called photolysis.
On Mars, in principle, about 20% of the CO2 is split into oxygen and carbon monoxide. But carbon has two stable isotopes: carbon-12 and carbon-13. They are usually present in a ratio of one carbon-13 for every 99 carbon-12. However, photolysis works faster on the lighter carbon-12, so the carbon monoxide produced by photolysis has less carbon-13 (is depleted) and the remaining CO2 has more (is enriched).
This allowed Johnson and his colleagues to make very precise predictions about the ratio of carbon isotopes after photolysis. And it gave them two distinctive fingerprints to look for. One of them was identified years ago in another Martian sample.
“We have here on Earth a piece of Mars that was knocked off that planet by a meteorite and became a piece itself when it landed here on Earth. This meteorite, called Allan Hills 84001 after the place in Antarctica where it was found, contains carbonate minerals that form from CO2 in the atmosphere. The evidence is that the ratio of carbon isotopes in it matches exactly what we predicted in the quantum chemical simulations, but there was a missing piece in the puzzle. We were missing the other product of this chemical process to confirm the theory, and that's what we've got now,” says Matthew Johnson.
The carbon in the Allan Hills meteorite is enriched with carbon-13, mirroring the depletion of carbon-13 now measured in the organic material Curiousity has found on Mars.
The new study links data from two samples that the researchers say share a common origin in Mars's infancy, but were found more than 50 million kilometers apart.
“There is no other way to explain both the decrease in carbon-13 in the organic material and the enrichment in the Martian meteorite, both in relation to the composition of volcanic CO2 “The amount of dust ejected on Mars has a constant composition, similar to that of volcanoes on Earth, and serves as a baseline,” Johnson said.
I hope we find the same evidence on Earth
Because the organic material contains this isotopic “fingerprint” of where it came from, researchers can trace the source of the carbon in the organic material to the carbon monoxide formed in the atmosphere by photolysis. But this also reveals a lot about what happened to it in the meantime.
“This shows that carbon monoxide is the starting point for the synthesis of organic molecules in these types of atmospheres. So we have an important conclusion about the origin of the building blocks of life, although this has only happened on Mars so far,” says Matthew Johnson.
Johnson says researchers hope to find the same isotopic evidence on Earth, but that hasn't happened yet. And it could be much more challenging, because our geological evolution has significantly changed the surface compared to Mars.
“It's reasonable to assume that the photolysis of CO2 was also a prerequisite for the emergence of life here on Earth, in all its complexity. But we haven't found this 'smoking gun' material here on Earth to prove that the process has happened. Perhaps because the Earth's surface is much more alive, geologically and literally, and therefore constantly changing. But it's a big step that we've now found it on Mars, from a time when the two planets were very similar,” says Matthew Johnson.
Facts: Organic matter
The sample found on Mars contains deposits of so-called organic material. To the layman, this may sound more exciting than it is. Organic material in a chemical context does not necessarily mean something living, as you might normally think. The term includes molecules that contain carbon and at least one other element and can easily exist without life. These molecules are rather the building blocks of life.
Facts: What is photolysis
Photolysis means that the sun's UV rays provide molecules with energy to carry out a chemical transformation. According to the research, this happened in the atmosphere of Mars, where 20% of the CO2 There the molecules were split into oxygen and carbon monoxide.
In previous research, Johnson and colleagues showed that carbon dioxide containing the carbon-12 isotope is photolyzed faster than the heavier isotope carbon-13.
Over time, CO is produced which is depleted into 13C, and 13C builds up in the remaining CO2. This results in so-called isotopic enrichment in CO2 and a decrease in CO, like mirror images of each other or the two halves of a broken record.
It is the fractionation ratio in carbon that serves as evidence for photolysis in the two Martian samples.
Facts: Oxygen turned Mars red
Photolysis of a CO2 molecule yields carbon monoxide (CO) and an oxygen atom (O). On Mars, only carbon monoxide remains, which is converted into the organic material found by the Curiosity rover.
But where the oxygen has gone is also no secret. The oxygen combines into O2which interacts with iron on the surface of Mars. The Red Planet is rusty red because of oxidized iron.
Facts: Isotopes have different weights
Isotopes are variants of the same element with different weights, because the nucleus contains more or fewer neutrons.
Carbon has two stable isotopes – Normally, about 99% of carbon has 6 protons and 6 neutrons in the nucleus (12C). About 1% has 6 protons and 7 neutrons (13C). The ratio can serve as a chemical fingerprint showing what reactions the carbon has undergone.
Photolysis has a beneficial effect on carbon-12, and a high concentration of the isotope may therefore indicate this process.
Additional info: The famous Martian meteorite
The discovery of organic sediments on Mars with a low carbon-13 ratio completes the puzzle of empirical evidence for the photolysis theory, as researchers discovered the other piece of that puzzle years ago in the famous meteorite, Allan Hills 84001. The meteorite contains carbonate with an elevated concentration of heavy carbon-13 isotopes.
The meteorite was discovered in Antarctica 40 years ago by Roberta Score and is believed to have originated from the Red Planet. It is best known for containing some of the deposits that led NASA researchers to announce in 1996 that they suspected they had found traces of microscopic fossils of bacteria from Mars.
Today it is generally agreed that these deposits are abiotic, that is, they are the result of non-biological processes.
Additional info: Mars, Earth and Venus had the same atmosphere
According to researchers, billions of years ago, Earth had roughly the same atmosphere as our neighboring planets Mars and Venus.
When the early planets Venus, Earth, and Mars eventually formed solid surfaces, researchers believe they began releasing large amounts of CO2 through extreme volcanic activity, forming their first atmospheres with large concentrations of the gas. Oxygen had not yet become part of the atmosphere; this happened later on Earth, after the emergence of life.
The photolysis theory states that UV rays from the sun then start a chain of chemical reactions. A chain that begins with the breakdown of CO2 into carbon monoxide, which is the building block for a multitude of other chemical compounds.
Thus, with the help of the sun, the basis was laid for the many carbon compounds and complex molecules that we know today. In the case of the earth, this laid the basis for life.
“Since then, the fates of the three planets have been significantly different. Earth's carbon dioxide reacted with our large amount of surface water and much of it was deposited over time as carbonate rocks such as limestone, creating the nitrogen-dominated atmosphere we have today. Life emerged and microorganisms produced oxygen, which among other things created our ozone layer, whereas Mars and Venus today still have atmospheres that are heavily dominated by CO2,” explains Matthew Johnson.
Today, Venus has a very dense and toxic atmosphere composed mainly of CO2, causing the surface temperature to be around 450 degrees Celsius.
On Mars, the atmosphere has become much thinner compared to that on Earth, creating a desert landscape.
