The discovery proves that while Mars is inhospitable today, it previously allowed for "liquid water" to pool, meaning a water lake inside Mars' Gale Crater could have once supported life, NASA said in an announcement on Tuesday.
Organic molecules are the basic building blocks of life - and they are also left behind by living organisms.
That means they do not have compelling evidence for a biological origin of the carbon, but the possibility is not ruled out, either. After drilling, Curiosity heats the rock samples, releasing the compounds.
"The results convincingly show the long-awaited detection of organic compounds on Mars". "The first one would be life, which we don't know about".
Scientists hope to further the search for signs of life on Mars with the European and Russian rover, ExoMars, scheduled to land in 2021.
The scientists were surprised to find organic compounds, especially in the amounts detected, considering the harsh conditions, including bombardment of solar radiation on the Martian surface. And if life does exist elsewhere, it may be very different or even form differently from how we understand life on Earth.
"With these new findings, Mars is telling us to stay the course and keep searching for evidence of life", Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters, said.
It had to stop analysing samples on the Red Planet after a mechanical problem took the rover drill offline in December 2016.
The rover was able to heat the samples to between 932 and 1508 degrees Fahrenheit and study the organic molecules released through gas analysis.
Questions remain, however, as to how the organic material was formed. "While we don't know the source of the material, the unbelievable consistency of the results makes me think we have a slam-dunk signal for organics on Mars", Eigenbrode said.
The new results represent the longest systematic record of atmospheric methane, with measurements taken regularly over five years.
The team estimates the ancient sediments, where the complex organic molecules were found, were actually the remains of a vast lakebed that existed more than 3bn years ago. They are fairly certain that it comes from melting water-based crystals, called clathrates, buried just below the planet's surface.
A set of geological results recently delivered courtesy of Curiosity's drill bit provides a deeper understanding of the organic chemistry of the 300-million-year-old mudstone in two separate parts of Gale crater. Its two-year mission will explore Mars to see if it's "geologically alive", or active below the surface.
All of the outside sources I spoke with said it's important to be skeptical about claims of life, extinct or otherwise, on the Red Planet.
But National Aeronautics and Space Administration scientists emphasized there could be nonbiological explanations for both discoveries made by the Curiosity rover at a site called Gale crater, leaving the issue of Martian life a tantalizing but unanswered question.
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