Happy sixth anniversary to the Mars Curiosity Rover, which tweeted out a little anniversary greeting to itself on Sunday. "I touched down on #Mars six years ago," the tweet reads. "Celebrating my 6th landing anniversary with the traditional gift of iron... oxide. (It puts the red in Red Planet.)" Curiosity landed at Bradbury Landing (named for the late sci-fi author Ray Bradbury) in Mars' Gale Crater back in 2012, and has been exploring Mars ever since. Fans of Curiosity marveled at the image it shared. Wrote one Twitter user, "I love how impressive it is that what appears to be stones and mud, something so familiar, happens to be in fact another planet that no human has ever stepped a foot in. But we can still get a picture as if it was taken with your phone camera. Science rocks." Some fans asked Curiosity about the widespread belief that the rover sings "happy birthday" to itself every year, but it turns out that's not quite right. "Did you stop and play happy birthday today?" asked one Twitter user. "(It's) moving to imagine Curiosity alone on Mars with happy birthday playing on a distant planet." But another pointed out, correctly, that this was a one-time song, back in 2013. "I just read that he only sung it once, it's too much to coordinate for it to happen every year and no one can hear it on Mars either way," Tom Manners tweeted. "Curiosity only sung it for its first birthday in 2013 I believe.." And that's correct: The rover only sang to itself for that first 2013 birthday/anniversary, using its sample analysis instrument to "hum" the tune. You can hear how it sounded in the following YouTube video shared by NASA. Curiosity itself debunked the annual singing rumor in a tweet sent out last year to mark year No. 5.