Time in relation to life in the Jovian System

Question

I'd just like to preface this with the fact that I'm super new and I apologise if this has been discussed in some way previously (I have looked at similar topics but they haven't quite given me the answers I'm looking for).

So, I'm writing a story that is set in the Jovian system; Jupiter and it's primary moons Io, Callisto, Ganymede & Europa. Of course, the story takes place a long time in the future.

Perhaps it might be worth mentioning that I don't plan on adding fictional systems or moons here. I'm working with the information available on the mass and orbital velocity of the moons and planets I want to include - so my questions relate to the reality of the situation (a little ironic, maybe!).

My problem is working out time. I think tidal locking means that either the day and year or the day and month are equal? I need a little clarification on that one (forgive me for being a bit dim!).

But how would I work out seasons and days/months/years? And perhaps more importantly, how would tidal locking affect the lives of people living on these moons?

Answer 1

I'm assuming that you're talking about people living on the moons of Jupiter rather than the atmosphere of the planet itself, since you mention tidal locking.

The way days/months/years would work on a tidally locked moon would make days and orbits around the planet the same amount of time, so you would probably just end up calling a year an orbit of Jupiter around the Sun. This would make a "year" ~12 Earth years long.

Months aren't really an obvious unit of measurement like days and years are (Rotate once, orbit once), so you've got a lot of creative freedom with how you set them up. Months on Earth were originally based on the Moon's cycles, (Moon, Month, cycle= ~29 Days, Month= ~30 Days) but seeing as you have no moon orbiting around you, you don't exactly have that option. You could just find a nice number that divides out the number of days per year, or maybe you even use the cycles of your fellow moons. In the case of the Galilean moons, probably Callisto so you have more than a just couple days per month. Or you could do combinations of cycles of multiple moons. Like I said, lots of freedom with months, really any stable and repeating pattern that feels like a good length of time to you could work.

I'm fairly sure that seasons on the moons are basically nonexistent, because they return to exactly the same state in their orbit only a handful of days. Not even the long and frequent solar eclipses from Jupiter would make a practical difference because the same side of the moon is locked facing the planet. This would however cause the "Inner Face" to have a shorter day, interrupted by an eclipse, where the "Outer Face" would have equal times of daylight/darkness. Not exactly seasons, but possibly interesting!

Sorry if this answer is extremely long, it's my first one ever, but I just really enjoyed your question :D

Answer 2

You ask, "But how would I work out seasons and days/months/years?"

• Years, on the planet, are what they are — revolutions around the Sun.  "Years" on moons is not well defined, unless you take it to mean simply revolutions of the moon around the planet (i.e., months) or planetary years.
• Months (on Earth) are defined by the orbit of the Moon (Earth's natural satellite) and are not well defined on a planet with multiple moons.

• Seasons on Earth are a somewhat man-made concept, and are based on the Earth's orbit around the Sun and the tilt of the Earth's rotational axis relative to the planetary orbital plane, and how that affects energy received from the Sun at the surface of the planet.  I don't know offhand whether Jupiter's axis is tilted.  But it receives very little energy from the Sun, and so the lifeforms that live there may be oblivious to such issues.

  For that matter, Jovian folks might be oblivious to the passage of years.  Earthlings living there would probably keep track of time based on Earth's calendar.