EXTERNAL — All the planets in our solar system consist of two forms, namely rocky and gaseous. But all the moons in our solar system are rocky, even those orbiting the gas giant Jupiter.
So why aren’t all the moons in the solar system made of gas? More broadly, are there gas moons in the universe?
There are several plausible reasons why no moon is gaseous. “And while we haven’t found a gas moon outside our solar system yet, it might happen under the right conditions,” Jonathan Lunine, chair of the Department of Astronomy at Cornell University, told Reuters. Live Science, as reported by Space.com, Sunday, March 13, 2022.
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According to him, in particular it is influenced by the mass of the moon, the temperature around it, and the influence of its tidal forces, namely the gravitational pull of nearby objects such as its parent planet. To illustrate how these conditions can affect a gas moon, imagine that the rock composition of our moon is replaced with pure hydrogen.
“Hydrogen gas is much denser than rock, so soon, the moon will grow to the size of Earth,” Lunine said.
In fact, the size of a gas giant like Jupiter is the reason why it exists. If Jupiter were too small, the force of gravity would not be strong enough to hold the light elements together. But size isn’t the only factor at play, there’s also temperature.
“Let’s just think of the moon as it is, as a rock. Then we put a hydrogen atmosphere around it,” said Lunine.
We know that the hydrogen atmosphere will escape very quickly simply because of the thermal effect. In other words, the heat of the sun will cause the hydrogen to evaporate.
“So what I’m going to say is, (if) Earth’s moon that was entirely hydrogen at Earth’s distance from the sun would be unstable. It happens, even (if it’s a gas moon) as far as Pluto’s orbit,” Lunine said.
But even if an imaginary gas moon the size of Earth and the temperature around it was extremely cold, its parent planet might still tear it apart. “Remember, that our moon is subject to tidal forces from Earth. So it’s actually being pulled out a bit, but not ripped off because it has some material force associated with it,” Lunine said.
It wouldn’t have happened if the moon had formed out of gas. “Because it’s a gas and not a solid, if it’s orbiting around something else, it’s going to be chipped and torn apart by the tides,” Lunine said.
According to him, gas moons can form only if the lunar and planetary systems are very far apart and cold or very large like Jupiter. “If it were the size of our moon, anywhere in our solar system, it wouldn’t function properly (would tear/evaporate). Deep in the depths of interstellar space? Over there, that’s (still) a question mark,” said Lunine.
The second possibility is that if a gas moon were to form as large as Neptune around Jupiter, then it might survive. In this case, the gravitational force holding these massive bodies together would likely prevent tidal forces from destroying a Neptune-sized moon. “It can be very stable,” said Lunine.
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