question:  What, within the context of the passage, is a 'marker'?
context: The ice-cap, by its existence and circular shape, proved that the planet rotated at a not unreasonable rate. A large cap would have meant a large area with arctic and sub-arctic temperatures, with small temperate and tropical climate-belts. The fact was verified by the thick, dense cloud-masses which covered most of the surface,—all the surface, in fact, outside the ice-cap. In short, the ice-cap proved that a man could endure the air and temperature conditions he would find. It wasn't an identification beacon, such as are placed on certain worlds for the convenience of interstellar skippers who need to check their courses on extremely long runs. They didn't seem to resent what he'd tried to do, or that he'd brought them an indefinite distance in an indefinite direction from their last landing-point, and they had still to re-locate themselves. With clearance-papers in order, they could land unquestioned at any other space-port and take off again—provided the other space-port was one they had clearance for. Wherever the yacht landed, such a disparity between its documents and its crew would spark an investigation. It could have travelled at almost any speed in practically any direction for a length of time that was at least indefinite. A liner could re-locate itself without trouble. The process would be to find a planet and check its climate and relationship to other planets, and its flora and fauna against descriptions in the Directory. It marked a first-landing spot on some planet or other, so the people could find that spot again. If they left a beacon, they may have left an identification of the planet. But space-travel regulations are especially designed to prevent such escapes. One of the controls on space-traffic required a ship on landing to deposit its fuel-block in the space-port's vaults. The fuel-block was not returned until clearance for departure had been granted.
output:  A safe space for a spacecraft to land on an alien planet
options:  A microchip inserted into a person, designating them as a fugitive  A safe space for a spacecraft to land on an alien planet  A sound picked up on a radar that reveals the closest habitable planet  A location on the alien planet that indicates high predatory activity