Why can't liquid water survive on the surface of Mars today?

Because the air is far too thin. Mars does have an atmosphere, mostly carbon dioxide, but its average surface pressure is only about 0.6 percent of Earth's. At such low pressure, pure liquid water is unstable: it freezes, evaporates or boils. That thin atmosphere prohibits liquid water on the surface today, although many studies suggest Mars once had a much thicker one.

We learned this in 1965. As NASA's Mariner 4 passed behind the planet, scientists used variations in its radio signal to calculate the atmosphere's density. The pressure came out below 1 percent of Earth's at sea level, effectively ruling out liquid water. One loophole remains: salty brines, like the salt spread on icy roads on Earth, may have freezing points low enough to stay liquid.

The cold makes it worse. Mars is farther from the Sun and receives less solar energy, and although its air is almost all carbon dioxide, there is so little of it that the greenhouse effect adds only about 5°C of warming, compared with 33°C on Earth. With little air to hold heat, temperatures near the surface can swing from about −75°C to near 0°C in a single day in some regions.

And the air keeps changing. It thins by about 25 percent in winter as carbon dioxide freezes onto the polar caps, and it has been leaking into space since the planet's core slowed down. What remains still lifts plenty of dust: planet-encircling dust storms occur about every 5.5 Earth years and can threaten rovers, and in 2018 a storm changed the planet's colour and brightness enough to be noticed with the naked eye.