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This topic is inspired by creative work done by Void in one of his many topics.
This topic is offered for NewMars members who might wish to contribute links or text (or perhaps images) to help our readers to understand the nature of the environment they will encounter if they visit Mars.
In post #3 I'll open the topic with a conversation with ChatGPT4o about what a visitor might expect if experimenting with water on Mars.
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This post is reserved for an index to posts that may be contributed by NewMars members over time.
Index:
Post #3: Conversation with ChatGPT4o about evaporation and sublimation on Mars.
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Conversation with ChatGPT4o about pressure needed to protect water in a pond under a dome on Mars.
Preventing Water Sublimation on Mars Under a Dome
Understanding the Conditions
The average atmospheric pressure on Mars is approximately **610 Pascals (Pa) (6.1 mbar)**, which is **less than 1%** of Earth's sea-level pressure (101,325 Pa or 1013 mbar).
At this low pressure, liquid water is unstable and will either **evaporate rapidly** or **sublimate directly from ice to vapor**.
To prevent this, a dome must provide sufficient internal pressure to maintain stable liquid water.
Minimum Pressure Required
The **triple point of water** (the lowest pressure at which liquid water can exist) is **611.657 Pa at 0.01°C**.
If the pressure inside the dome is **below 611 Pa**, water cannot remain liquid.
To prevent evaporation at common temperatures, the dome must maintain at least the saturation vapor pressure for that temperature:
Example Pressures for Stability
**0°C** → **611 Pa** (Triple point)
**10°C** → **1,228 Pa**
**20°C** → **2,341 Pa**
**30°C** → **4,246 Pa**
**40°C** → **7,384 Pa**
Experimental Evidence
**NASA Ames Research Center** has simulated Mars-like conditions to study water behavior under low pressure.
**University of Arkansas** has performed evaporation and briny water stability experiments under Martian conditions.
**Phoenix Mars Lander (2008)** observed frost formation and sublimation on Mars.
**Lunar and Planetary Institute (LPI)** has studied how brines can remain liquid under low-pressure environments.
Conclusion
To allow liquid water, the dome must maintain a pressure above 611 Pa.
To prevent excessive evaporation, pressure must match at least the saturation vapor pressure at the desired temperature.
For a **20°C pond**, a minimum of **2,341 Pa** is needed.
Additional considerations: dome transparency, heat management, and condensation control.
For those who might be curious, the value of 2,341 Pascal translates to 0.02341 bar
The Standard Habitat Pressure for Mars is 0.5 bar.
Thus, the pressure needed in a dome over a water pond to prevent evaporation is "low" compared to SHP.
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I like the solar pond idea. Assuming that the top of the pond achieves a maximum temperature of 10°C, the required pressure to prevent boiling is 1228Pa. That is effectively double atmospheric pressure (610Pa) at Mars datum. In the northern lowlands, atmospheric pressure is greater. So the structure covering the pond needs to be able to withstand a maximum differential pressure of 618Pa, or 13.1lb/ft2. That is low enough for a tempered glass paned, metal frame greenhouse structure to serve as cover. This has the advantage of being significantly more abrasion resistant than a polymer membrane. It is also insensitive to UV damage.
The pond is divided into two distinct layers. A dense brine layer sits at the bottom. A lighter brackish layer floats on top. The denser brine can be warmer, as it cannot rise by convection through the lighter brackish. A small ion exchange filter could be used to maintain the salinity gradient within the pond. A thin layer of polyethylene could provide a physical barrier between the two layers. This has the additional advantage that a convective boundary layer will form on both sides of it, providing additional insulation to the warm water underneath.
A solar pond would work better on Mars, as the lower gravity would tend to weaken thermal convection. The lower the gravity, the less efficient convection is at removing heat.
Last edited by Calliban (2025-03-15 17:41:22)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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