I saw a copy of this under its original title "Phobos The Robot Planet" at a jumble sale when I was about 9 and was intrigued by it. I think my brother bought it but gave it away later. Internet searches revealed the author and also that second-hand examples were quite pricey, but last year it finally got the digital print-on-demand reincarnation under the second title given to it.
I suppose Capon might have adjusted the name for the sake of strict scientific correctness (Phobos is a moon not a planet; but then we can argue what counts as a "planet", since the original notion of "wandering star" would cover the satellites as well as bodies of any size, such as Pluto). But never mind. This is a 1950s "thriller for boys" written just on the brink of the real space age. It has magnetic boots and Martian canals and pre-Mariner/Venera ideas of what Mars and Venus would be like. The plot is interesting (the Martians vacated their world long ago, leaving the robot brain of their biggest space station behind, and it now wants to study humans). Good to see that problems of radical translation aren't passed over too quickly, the "unemotional" nature of robotic intelliegence is sketched in, but it all ends a bit fast and I can't help feeling a bit of sympathy for the Phobotic mind. There's enough ideas here for any adult SF writer of the time, and it's a shame Capon didn't develop it at greater length for that marketplace, it might have been more famous.