From Shane Jones, Ipswich, Queensland, Australia
Chelsea Whyte discusses why water on Mars still doesn't make sense (18 February, p 25). Is it not possible that 3.5 to 4 billion years ago, when Mars was both warmer and wetter, the planet was orbiting closer to the sun?
The editor writes:
• Several readers have asked this. Mars could indeed have been closer to the sun early in its history. But it must have been in its present orbit by about 4 billion years ago, when the water-carved features formed. After that time, there aren't any plausible mechanisms to move its orbit: the solar nebula would have long cleared out and really giant impacts would have ceased.
