Looking for life on Mars is one thing, but you still have to know where to look, which means knowing the chronology of Martian history.
Life on Mars is a subject that fascinates both the general public and the scientific community. Some clues lead us to believe that the Red Planet could well have hosted biological activity in the past, but this remains to be proven. Work carried out on a Martian meteorite could well provide us with new elements of explanation.
While waiting for a spectacular discovery that would make it possible to decide once and for all, there is only one solution left: to go back as far as possible in the geological history of the planet, hoping to find there the signature of a biological activity. This is one of the main activities of the Mars rover Perseverance. But it is not necessarily necessary to go there to study Mars; it is also possible to wait for it to come to us via a meteorite.
A piece of Mars delivered to your home
It is on one of these objects that researchers from the University of Perth (Australia) have focused their attention. They are lucky to have access to NWA 7034, a very interesting specimen. Indeed, in most cases, these meteorites are composed of igneous rocks such as basalt. But in this specific case, it is a fragment directly from the surface of Mars. An extremely rare feature that makes this object very precious.
Unsurprisingly, NWA 7034 has therefore been the subject of almost uninterrupted analyzes since its discovery on Earth in 2013. First conclusion: the isotopic composition of this piece of rock corresponds well to that of our neighbour. No doubt, it is indeed a former resident of Mars. The researchers then analyzed the mineral structure of the sample, each fragment of which represents a chapter in Martian history.
Zircon, a real mineral archive
Among these elements, the Australian researchers noticed the presence of zircon. A find that must have made them paw the ground with excitement. Indeed, this mineral is known for its special properties which allow it to be used as a geochronometer. Very briefly, this means that there is an analytical technique which makes it possible to precisely determine the time elapsed since its crystallization. Ideally, we can thus deduce the age of the other elements of the meteorite, and consequently establish a chronology of the events of which the rock has kept traces.
A very exciting coincidence for researchers, because it turns out that these zircons are also very useful for studying the impacts of meteorites. Indeed, during the shock, the zircon grains tend to take an accordion shape under the effect of the compression; this is called “shocked zircon” (SZ). It is a structure that also exists on Earth; they are regularly found at the impact points of the largest meteorites.
A small grain of matter with large implications
However, even if NWA 7034 has been scrutinized on a daily basis for years, this SZ had gone completely unnoticed until now. And if it could seem anecdotal, the presence of this very, very small grain of matter has in fact considerable implications.
Indeed, about 4.5 billion years ago, Mars was permanently bombarded by a veritable rain of asteroids of all kinds. The absence of SZ in this meteorite has therefore led other researchers to conclude that this bombardment would have slowed down markedly around 4.2 billion years ago. But the discovery of this particular zircon in NWA 7034 seems inconsistent with this timeline. “The zircon we describe provides evidence of massive impacts at this time”, advance the researchers. If there was ever life on Mars, this work therefore suggests that it could have arisen later than expected, in a period between -3.9 and -3.7 billion years.
This difference seems anecdotal, but it is in fact crucial information. Indeed, it is impossible to comb through the entire planet. To hope to find traces of past life, it is necessary to carry out a sorting upstream to identify the zones chronologically compatible with these discoveries; otherwise, it would be about as useful as looking for traces of Roman artifacts from the time of the dinosaurs.
This information is therefore a new piece of a gigantic puzzle; the more we have at our disposal, the easier it becomes to have a clear idea of the overall situation. This allows researchers to focus on the most promising areas to maximize their chances of finding traces of life, assuming there ever was.
The researchers’ article is available here, and their research paper at this address.