Grok 3
Marathon Crater, likely a confusion with Marathon Valley on Mars, wasn't "discovered" in the traditional sense as a distinct crater. Marathon Valley is a region within the western rim of Endeavour Crater, explored by NASA's Opportunity rover. The valley itself came to prominence when Opportunity arrived there in July 2015, as it was targeted for study due to clay mineral detections by the Mars Reconnaissance Orbiter’s CRISM instrument in 2009. Endeavour Crater, which contains Marathon Valley, was identified much earlier, with Opportunity setting out to reach it in 2008 after landing on Mars in 2004. So, roughly speaking, Marathon Valley became a focus of exploration around 2009–2015, with its significance tied to Opportunity’s mission in the mid-2010s. If you meant a specific "Marathon Crater" elsewhere, could you clarify?
Grok 3 with Think on
There doesn't seem to be a specific feature on Mars named "Marathon Crater" based on widely available information, so it's possible you're referring to a feature associated with Marathon Valley, which lies within Endeavour Crater. Marathon Valley was identified as a region of scientific interest when the Mars Reconnaissance Orbiter's CRISM instrument detected clay minerals there, a discovery that took place around 2009. Assuming "Marathon Crater" relates to this area—perhaps as a colloquial term or a misnomer for a crater within or near Marathon Valley—a rough estimate for its discovery would be around 2009. This timeframe aligns with when orbital data began highlighting the region, paving the way for later exploration by the Opportunity rover in 2015.