Question

Why isn't liquid water stable at the martian surface today? What happens to water ice that melts on Mars?

Short answer

Liquid water on Mars is not stable due to its low atmospheric pressure and temperature, causing ice to sublimate or rapidly evaporate if it melts.

Step-by-step solution

Introduction

To understand why liquid water is not stable on Mars today, we need to explore the environmental conditions of Mars, primarily its atmospheric pressure and temperature.

Analyze Atmospheric Pressure

Mars has a thin atmosphere compared to Earth. Specifically, the atmospheric pressure on Mars is less than 1% of Earth's average pressure, around 600 pascals (0.6% of Earth's 101,325 pascals). This low pressure significantly affects the phase state of water.

Study Temperature Range

The average temperature on the Mars surface is about -80 degrees Fahrenheit (-62 degrees Celsius), with even the hottest days reaching just above freezing. These low temperatures prevent stable liquid water as it's primarily in solid or vapor form.

Explore Phase Diagram of Water

In a phase diagram, the boundary lines between solids, liquids, and gases depend on both temperature and pressure. The low pressure and temperature of Mars mean that water most often transitions directly from solid (ice) to vapor, a process known as sublimation, rather than melting into liquid.

What Happens to Melting Water Ice

If water ice on Mars's surface were to melt due to temporary warming, the resulting liquid would not remain stable. It would rapidly evaporate or boil away due to the low atmospheric pressure, reverting to vapor before significant pooling can occur.

Conclusion

In summary, conditions on Mars—highly specific low pressure and low temperature—result in liquid water not being stable or long-lasting. Ice sublimates directly into vapor, or melts and then quickly evaporates.

Key concepts

Martian Atmospheric Pressure

Mars has a very thin atmosphere when compared to Earth. The pressure on Mars is less than 1% of Earth's average atmospheric pressure. This translates to about 600 pascals compared to Earth's 101,325 pascals. Such low pressure has profound implications for the stability of water on the Martian surface.

In a high-pressure environment like Earth, water can exist in liquid form at a wide range of temperatures. However, on Mars, the low atmospheric pressure doesn't allow for liquid water to be stable on its surface. Instead, it forces water to exist predominantly as ice or vapor.

The thin atmosphere lacks the pressure required to keep water molecules close together in a liquid state. This scarcity of pressure leads to water on Mars often sublimating directly from ice to vapor. Understanding this feature of Mars is crucial to grasp why liquid water struggles to thrive today.

Phase Diagram of Water

A phase diagram is a tool used to show the state of a substance based on temperature and pressure. For water, the phase diagram is instrumental in understanding how it behaves under different conditions.

On Earth, the phase diagram depicts scenarios where water can comfortably exist in solid, liquid, and gaseous states, thanks to Earth’s balanced atmospheric pressure and moderate temperatures. However, on Mars, this diagram looks quite different.

Due to Mars' low atmospheric pressure, the conditions most often lie outside the range for liquid water. Instead, the Martian environment predominantly supports conditions conducive to water existing as either a solid or a gas. This means even if ice melts, it is more likely to quickly transition into vapor rather than remain as liquid.

Without stable parameters for liquid water, Mars continues to be an environment where water struggles to exist in its most life-harboring form.

Sublimation on Mars

Sublimation is a process where a solid directly transitions into a gas without going through the liquid phase. This phenomenon is commonplace on Mars due to its unique atmospheric conditions.

Because of the planet’s low pressure, the ice on Mars doesn’t melt into liquid water. Instead, when the conditions are right, it sublimates directly into water vapor. Mars' environment accelerates this process, especially when sunlight hits the ice, creating localized heating.

Sublimation is significant in shaping Martian landscapes. Features like the polar ice caps and certain enigmatic valleys and channels observed on the surface are partially attributed to this rapid transition of ice into vapor. Understanding sublimation on Mars provides insight into both current geological features and the planet's climatic history.

Mars Temperature Effects

Temperatures on Mars play a crucial role in the behavior of water. The Martian surface is generally very cold, with average temperatures around -80 degrees Fahrenheit (-62 degrees Celsius). The warmest times, which barely reach above freezing, are rare and localized.

This cold environment is inhospitable to liquid water, as even in optimal conditions, the temperatures remain low enough to keep it primarily frozen. The surface temperature fluctuation also adds to the volatility of conditions where water could exist as a liquid.

When brief warming occurs, which could potentially melt ice, the result is fleeting. The low atmospheric pressure, combined with these chilling temperatures, ensures that any resultant liquid water rapidly evaporates or sublimates. Mars' chilly climate is a dominant factor in maintaining its water in forms less conducive to hosting life as we know it.