How close to the sun Ganymede would have to be in order to hold water in its atmosphere. (in AU)

To determine how close Ganymede would have to be to the Sun in order to hold water in its atmosphere, we need to consider several factors.

First, let's understand that the distance between celestial bodies in the solar system is often measured in astronomical units (AU), which is the average distance between the Earth and the Sun (about 93 million miles or 150 million kilometers).

Ganymede is the largest moon of Jupiter and the largest moon in the entire solar system. It is located about 484 million miles (778 million kilometers) away from the Sun on average, or about 5.2 astronomical units.

For liquid water to exist on a celestial body's surface or in its atmosphere, several conditions must be met. The most critical factors are temperature and pressure. The proximity to the Sun influences the temperature.

Water is a volatile substance, meaning it can exist as a liquid, gas, or solid at different temperatures and pressures. Densely packed, cool regions of a planet or moon are more likely to retain water in a liquid or solid state. However, if a celestial body is too close to the Sun, the increased solar irradiation can lead to high enough temperatures for water to evaporate and escape into space.

To estimate how close Ganymede would need to be to the Sun to hold water in its atmosphere, we need to consider its temperature. Ganymede's average surface temperature is around -171 degrees Celsius (-276 degrees Fahrenheit). At these extremely low temperatures, water is generally frozen as ice.

However, for water to exist as a gas or vapor in the atmosphere, it would require even higher temperatures. Suppose we consider a temperature range of around 0 to 100 degrees Celsius (32 to 212 degrees Fahrenheit), which is within the liquid water range.

To reach these temperatures, Ganymede would need to be closer to the Sun than its current distance of 5.2 AU. However, the exact distance depends on various factors, including its atmosphere, composition, and the degree of solar radiation received.

It is worth noting that Ganymede does not currently possess a significant atmosphere. It has a very thin exosphere, which is the outermost region where atoms and molecules escape into space. Therefore, even if Ganymede were closer to the Sun, it might not be able to hold a substantial amount of water in its atmosphere due to its weak gravitational field and lack of atmospheric retention mechanisms.

In conclusion, although Ganymede would need to be closer to the Sun than its current distance of 5.2 AU to hold water in its atmosphere, other factors such as its thin atmosphere and weak gravitational field make it unlikely for Ganymede to have a substantial gas envelope capable of retaining significant amounts of water vapor.