Freeman says finding organic molecules only a few centimeters below the surface of Mars is an encouraging sign for finding possible life.
Organic molecules are considered one of the basic building blocks of life. Organic molecules contain carbon, the chemical element central to life.
Although tantalizing, the two findings remain far from definitive when it comes to past or present life on Mars.
NASA's Curiosity rover landed inside Mars's 96-mile-wide Gale Crater on August 6, 2012, and since then it has been probing the Red Planet's geology, climate and the question of whether or not it has ever supported microbial life.
Big news from Mars today: NASA's Curiosity rover found ancient traces of organic matter embedded in Martian rocks and detected a "seasonal variation" in atmospheric methane on the Red Planet - an annual pulse of the gas, nearly as if something out there were breathing.
Curiosity is the latest rover to trundle across the gritty sands of Mars. The 2020 rover will include an advanced spectrometer to scan for organic molecules. Mars2020 will shed light on the organic molecules-and prepare a sample that some future mission could bring back to Earth. Some of these fragments contain sulfur, which could have helped preserve them in the same way sulfur is used to make vehicle tires more durable, according to Eigenbrode.
On Mars, organic molecules could have been produced by some form of either present or past lifeforms. "And it makes us more confident that if biomarkers" - or direct evidence of biologic activity - "are there, we might find them".
NASA's Curiosity Mars Rover snaps a self-portrait at a site called Vera Rubin Ridge on the Martian surface in February 2018 in this image obtained on June 7, 2018. Close up, the veins have the appearance and chemistry of material that has been produced by reaction of water with the rocks, at a time when water was stable at the surface for extended periods of time. On Earth, living things (especially bacteria) produce lots of methane, though the gas also has plenty of non-living sources.
While we know that Mars was habitable in the past, the case demonstrates just how hard it will be to ever prove the existence of past life on its surface. They could even have been transported from elsewhere in the solar system.
One of NASA's aims with the Curiosity Rover is to search for signs of ancient life on the red planet.
That Mars possesses organic molecules is not surprising. About 3.5 billion years ago, research suggests, this pockmark on the Martian surface was brimming with water.
NASA will use both these discoveries to inform the design of its upcoming Mars 2020 rover.
"This is a significant breakthrough because it means there are organic materials preserved in some of the harshest environments on Mars", said lead author of one of two studies in Science, Jennifer Eigenbrode. It follows the discovery of organic molecules in 2014 that appeared to be much more sparse.
But the scientists can not say what the larger molecules were or how they formed. "I think we're moving toward extant [the opposite of extinct] life detection again".
It "defines how questions will be asked and pursued in the next stage of Mars exploration", Anbar, who was not involved in the study, told AFP by email.
Potential contaminants were analyzed and accounted for, so the results are the most conclusive yet.