There's an old vignette (under 400 word) sci fi story about an astronaut who goes to mars. He says mars has two moons and the martians say no, there are three

There's an old vignette (under 400 word) sci fi story about an astronaut who goes to mars. He says mars has two moons and the martians say no, there are three moons, Phobos, Deimos and Bottomos. There's a line of rocks on the ground that the martians always pause at, then step over quickly. The astronaut is about to ask them why when a puff of sand and dirt shoots out of a faraway cliff, then something shoots by just above ground level too fast to see. The Martians say see, that's the third moon. (the third moon was a tiny black hole orbiting mars just a foot off the ground)