First off, let’s bust that myth once and for all: Liquid water can, and does, exist aboveground on Mars.
Mars’s atmospheric pressure varies enormously, by a factor of almost 40, from 30 pascals (.0044 psi) at the summit of Olympus Mons up to 1155 pascals (0.1675 psi) on the floor of the Hellas basin. Planet-wide average is 600 pascals (0.087 psi or about 1/170 of earth-normal). The boiling point of water at that average pressure is -5°C (23°F).
Further, as everyone knows, salty water is harder to freeze. Ordinary table salt can bring water’s freezing point down to -21.1°C (-6°F), though a certain combination of magnesium sulfate (Epsom salt) and calcium sulfate (gypsum), both of which exist abundantly in Martian soil, can keep it liquid beyond -60°C (-80°F). Minerals called perchlorates, also present there, can do even better.
Martian temperature averages −55°C (−67°F) but can go up as high as 20°C (70°F). Thus there are plenty of opportunities for liquid water on Mars, especially during spring and autumn when temperatures are moderate. So please, let’s not hear any more poppycock about water’s liquid phase being “impossible” on Mars’s surface because the air pressure down there is so low and all. The wet stuff may be transitory, but it's there.
Now back in 1999 I was getting emails in my Arizona lair tipping me off to various Mars Global Surveyor images coming out at the time that appeared to show spring thaw melt ponds at the bottoms of craters. Their locations lay between 60 and 80 degrees south. These bodies certainly exhibited the character of liquids, settling into the very lowest areas and maintaining smooth, pond-like perimeters. Liquid carbon dioxide was out of the question since it can only exist under pressures exceeding 517 kilopascals or over five times earth-normal. They had to be brine.
Not only that, but they hosted thousands of dark-colored objects. They were roundish, up to 200 meters or so in diameter. Many of them crowded up against the shorelines. For lack of a better term I dubbed them “floaters” and marveled at some of their internal features. Why this general subject wasn’t being discussed and speculated about widely — and still isn’t — I couldn't fathom.
I’m providing the following closeups from that now-ancient MGS batch without further comment other than data on their locations, acquisition dates, scales, and links to their original source images1. You’re seeing them in chronological order. The craters hosting these ponds were obscure and unnamed as far as I could determine, so I’m referring to them by their nearest named crater.
1. All MGS images shown courtesy of Malin Space Science Systems. They currently reside in an archive on its website. MSSS provided the Mars Orbital Camera used to capture these images. The $44 million unit consisted of a monochromatic narrow-angle camera and two wide-angle cameras.