Would it be possible to move Ganymede into Mars' orbit causing them to fuse together like the two proto moons of earth?

The new planet "Marmede" would have a mass equal to 13.2 % of Earth.

How long this maneuver take if we could build an engine with 1 million tons of thrust on Ganymede's surface?

  • $\begingroup$ Hi Victor. I edited the tags to better categorize your question. $\endgroup$
    – user
    Jun 13, 2016 at 18:56

1 Answer 1



I calculate for desirable surface temperatures, the planet's mass needs to be close to 40% of the Earth's mass in order to retain Oxygen-Nitrogen with gaseous water over geological time. A colder surface temperature can lower that mass somewhat (perhaps down to 35%) but you're talking about permanent Arctic like conditions.

However, the collision of these two bodies will generate a great deal of heat. That will be enough to melt vast portions of the combined planet. In fact, it'll probably vaporize most the water provided by Ganymede and some will be lost to space. It is likely the surface will remain uninhabitable due to the high temperatures for many thousands to a million years or so.

Even worse, the plan suffers from another problem. Assume everything else works and you get a planet with the proper temperature, mass, etc. Ganymede possesses so much water (about 50x as much as on the Earth). Earth's water covers its surface to an average depth of 2,700 m or 2.7 km. Ganymede's water will cover Mars to an average depth of 521 km and will cover everything - including Olympus Mons.

So good news, you may remelt Marmede's core and generate a magnetic field. Bad news, don't expect to move in for the likely life of the human species and when you can move in, you'd better be sporting flippers.

  • $\begingroup$ Does that take into account the added distance from the Sun? At largish distances, there are small moon-sized bodies that have successfully retained atmospheres over the aeons, such as Titan. $\endgroup$ Jun 13, 2016 at 19:52
  • $\begingroup$ In Building Harliquin's Moon Niven had the population return to cryosleep for 60000 years or something like that, while the planet cooled. $\endgroup$
    – JDługosz
    Jun 15, 2016 at 3:21
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    $\begingroup$ You may want to revisit this answer in light of the OP's edits. $\endgroup$
    – user
    Jun 15, 2016 at 14:20
  • $\begingroup$ Hi Jim2B, thank you for your insightful comment. I didn't think about the amount of water Ganymede has. Even if only half 'd remain after the fusing and cooling, it is still a lot of water. However a "waterworld" kind of civilization or colony could potentially settle there after the cooling. I wonder if the cooling of the of the new small planet could be modeled, to include the mitigating fact that first you melt all the ice, then you heat it. The 4.5 bil yr old Earth, which is a lot more massive than Mars, has 4.45 billion year old crystals that needed liquid water to grow. $\endgroup$
    – victor
    Jun 21, 2016 at 20:53

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