Someone has suggested that Mars used to have a moon about a third of the size of our Moon, to explain a range of odd features about the surface of Mars. I have to admit I'm intrigued:
New hypothesis suggests Mars's unique shape may be due to long lost moon
An astronomer at the U.S. Naval Observatory has developed a hypothesis to explain the unique shape and terrain of Mars. He suggests both are due to a long-lost moon. Michael Efroimsky has published a paper outlining the hypothesis on the arXiv preprint server.
phys.org