Mars is extraordinarily dry by Earth standards. Its surface has no standing lakes, rivers, or oceans, and its thin atmosphere holds so little moisture that even the most humid conditions on Mars would feel bone-dry compared to any desert on Earth. But “dry” isn’t the whole story. Billions of years of water loss have left Mars a frozen, desiccated world on its surface, yet significant amounts of water ice remain locked underground and at the poles.
How Dry the Surface Really Is
The Martian atmosphere contains water vapor measured in precipitable micrometers, a unit that describes how much liquid you’d get if you condensed all the moisture in a column of air. On Mars, that number ranges from less than 1 to about 36 micrometers depending on season and latitude. For perspective, Earth’s atmosphere typically holds 10,000 to 50,000 precipitable micrometers of water. The driest spot on Earth, the Atacama Desert, still carries roughly 1,000 micrometers. Mars, at its most humid, holds about 30 times less water vapor than the driest place on our planet.
Equatorial regions stay relatively stable at 2 to 20 precipitable micrometers year-round, while high northern latitudes peak near 36 micrometers during summer as polar ice sublimates into the atmosphere. Even at that seasonal peak, you wouldn’t notice any humidity if you were standing there.
Why Liquid Water Can’t Survive on the Surface
Mars has an average surface pressure about 160 times lower than Earth’s. At that pressure, liquid water behaves strangely. NASA modeling has shown that the mean annual surface pressure and temperature on Mars do not allow liquid water to remain stable on the surface for any significant period. Water at the surface would either freeze or boil away almost immediately, depending on local conditions. The temperature and pressure occasionally creep above water’s triple point (the minimum threshold where liquid can exist at all), but only briefly and in limited locations.
This is fundamentally different from Earth, where atmospheric pressure comfortably holds water in its liquid phase across most of the planet. On Mars, ice sublimates directly into vapor, skipping the liquid stage entirely, much like dry ice does on Earth.
Signs of Seasonal Moisture
Dark streaks called recurring slope lineae appear on steep Martian slopes during warm seasons and fade when temperatures drop. These features sparked excitement because they look like something wet trickling downhill. Some researchers have proposed they form from seasonal brine flows, where salts in the soil absorb tiny amounts of atmospheric moisture or melt subsurface ice into a salty liquid that can stay fluid at lower temperatures than pure water. Others argue the streaks are entirely dry, caused by granular flows of dust and sand. The debate remains unresolved, but even the most optimistic interpretations describe only thin films of extremely salty water, not anything resembling a stream or puddle.
Where Mars Still Holds Water
Despite its parched surface, Mars stores a substantial amount of water as ice. The northern and southern polar caps are 2 to 3 kilometers thick and stretch more than 1,000 kilometers across. Both caps contain water ice, though the southern cap is topped with a layer of frozen carbon dioxide.
Away from the poles, buried ice sheets exist across the mid-latitudes. U.S. Geological Survey researchers studied erosion scarps (natural cliff faces) that expose the internal structure of these deposits. They found water ice layers more than 100 meters thick, starting as shallow as 1 to 2 meters below the surface. These aren’t thin frost layers. They’re massive ice deposits blanketed by a thin mantle of soil and dust.
Even the loose surface dirt contains water. NASA’s Curiosity rover heated Martian soil samples and found that about 2 percent of the surface material by weight is water, bound into the mineral structure of the regolith. You couldn’t wring it out, but it’s chemically present throughout the ground.
How Mars Lost Most of Its Water
Early Mars held far more water than it does today. Climate modeling suggests the planet once carried enough water to form a global layer roughly 700 meters deep, with about half sitting in an ocean and half locked in ice sheets. That’s a modest amount compared to Earth’s oceans, but it was enough to carve the massive valley networks and river channels still visible from orbit.
Mars lost the bulk of this water to space. Without a strong global magnetic field, the solar wind gradually stripped away the upper atmosphere, and ultraviolet radiation broke water molecules apart. The freed hydrogen atoms escaped into space. This process continues today. Hydrogen escape rates vary dramatically with Mars’ orbit: near its closest approach to the Sun, roughly 800 trillion trillion atoms of hydrogen leave the atmosphere every second, about 40 times faster than during the opposite season. Over billions of years, this relentless leak drained most of the planet’s surface water into space. Some water also became locked into hydrated minerals in the crust, effectively absorbed into the rock itself.
Dry on Top, Icy Underneath
The short answer is yes, Mars is dry. Its surface is drier than any environment on Earth, and liquid water cannot persist there under current conditions. But calling Mars simply “dry” undersells the complexity. The planet holds enormous reserves of water ice at its poles and buried beneath mid-latitude surfaces, and even its dusty soil contains chemically bound water. Mars is better described as a frozen desert: a world that once had rivers and possibly an ocean, now locked in a state where its remaining water exists almost entirely as ice hidden from the surface or vapor in trace amounts too small to measure without instruments.