Since NASA began driving rovers around the surface of Mars people on Earth have been anxious to find signs of life. According to NASA researchers however, the evidence isn’t there yet.

In a recent paper published in the journal Astrobiology Old Dominion University geologist Nora Noffke pointed to signs of possible microbial life. Noffke claimed that sedimentary structures in the sandstone of Gillespe Lake bore a resemblance to the interaction between microbial matts and their environment.

“Though there may not be life on the surface of Mars at the present, this does not exclude the possibility that life may have thrived earlier on the Red Planet,” said Noffke

After two decades of investigating microbial matts Noffke believes that the first step in finding signs of life is to search lake beds and other former aquatic environments for signs of microbial interaction. If the Earth and Mars shared similar environmental histories the microbially induced sedimentary structures (MISS) could bear a resemblance to similar structures on Earth.

However, it is not immediately clear that Mars and Earth have similar environmental histories. Being further from the Sun Maris is a much colder planet than Earth. A November, 2014 article published in Nature Geoscience suggests that Mars may have only had substantial water flow during periods of intense volcanic activity, lasting only tens or hundreds of years at a time.

In the case of the sandstone structures researchers with NASA’s curiosity rover team believe that they can be explained by normal patters of erosion.

“We really didn’t see anything that can’t be explained by natural processes of transporting that sand in water, and the nature of the rocks suggested that it was just a fluvial sandstone. We do have several members of our team who are always keen to look out for things that might be caused by biological processes, but there was no reason, we felt, to explore that [option] at that site. It came down to nothing exceptional, from our point of view, that wasn’t just a consequence of erosion of this sandstone,” mission project scientist Ashwin Vasavada, of NASA’s Jet Propulsion Laboratory in Pasadena, told Space.com.

Prior to Dr. Noffke’s paper the most recent possible signs of life on Mars came a month ago when methane spikes were detected. That discovery is still promising and could point to signs of microbial life. Methane, or natural gas, on Earth is usually produced as a byproduct of microbes digesting food.

However that is not the only way that methane is produced. Sunlight cooking off debris from meteorites or interactions between water and rock beneath the surface could also produce the spikes detected by Curiosity.

Little can be said for sure at this point about the methane spikes without further testing or, possibly, additional missions to Mars.

“Right now, it’s too much of a single-point measurement for us really to jump to any conclusions. So all we can really do is lay out the possibilities. And we certainly should have an open mind. Maybe there are microbes on Mars cranking out methane, but we sure can’t say that ​with any certainty. It’s just speculation at this point,” Paul Mahaffy of NASA’s Goddard Space Flight Center in Greenbelt, Maryland told Space.com at the time.

Amateur Mars watchers have also seen signs of life in rocks that look like a coffin,  a skull, a crocodile and a jelly donut. However the search for actual, definitive signs of alien life goes on unrewarded, for now.