If Mars were tidally locked to the Sun, what would the temperatures on its surface be like?

Since there's very little atmosphere on Mars, it doesn't matter much whether the Sun is in the zenith or hanging just above the horizon: without the atmospheric "filter," the sunlight would have roughly the same effect on all sunlight areas.

What would the temperatures be like on the sunlit portion of the planet?

• It does matter if the Sun is at it's Zenith or Horizon. See pretty picture: windows2universe.org/earth/climate/images/… and some mathematics on same principal, using the Moon. lunarpedia.org/index.php?title=Lunar_Temperature The atmosphere actually does the opposite of what you suggest, it regulates the temperature away from the extremes, and it also acts as a kind of blanket, trapping more heat than it filters or reflects. Commented Aug 23, 2016 at 11:04
• @userLTK I think this question is subjective, namely the "doesn't matter as much". You interpret the difference in energy imparted by the Sun versus angle as "mattering", but the questioner considers such a small difference as "roughly the same effect". And really, the decrease based on angle is insignificant when compared to the decrease from the atmosphere. For Earth, the Sun on the horizon is effectively shining through 11 atmospheres of air, causing a significantly larger difference in surface temperature than caused by the angle change. I believe that's the root of their question. Commented Aug 23, 2016 at 12:36