10
$\begingroup$

What would be the effect on climate on an Earth-like moon of a gas giant, which experienced regular ("monthly") total solar eclipses (by its gas giant parent) lasting approx. 40 hours.

I'm trying to recreate a realistic climate for this world. All things being equal, the moon is slightly warmer than Earth (its big enough, with sufficient magnetosphere to hang on to its atmosphere), it is not tidally locked, and rotates on its axis as well as orbiting its parent. So it has a fairly "normal" day and night cycle (notwithstanding light reflected by its parent during "night"). But I cannot find guidance anywhere on the web what kind of affect a monthly 40-hours of total darkness for the whole moon would have on its climate, and the development and behavior of its plants (maybe it has no appreciable affect, I don't know, but I'm assuming some cooling beyond what is usual for normal night, towards the end of that 40-hour period).

Any thoughts would be greatly appreciated.

The setting is the same as Captured Earth-Like Moons around Gas Giants

I used the formula provided by Michael Kjörling in this thread Earth-like Moon around the Gas Giant. Eclipse length? to calculate the eclipse length.

Same setting as: Captured Earth-Like Moons around Gas Giants

$\endgroup$
  • $\begingroup$ You might want to check out some of the details about heat sources of planetary bodies here. I suspect that a combination of tidal heating and the gas giant's incident sunlight reflected at infrared wavelengths would help offset the reduction in sunlight caused by being in its shadow. $\endgroup$ – Thriggle Jun 1 '17 at 22:00
3
$\begingroup$

Using moon that is eclipsed for 40 hours every month would receive roughly 1/18th (5.5%) less sunlight than a similar moon without such an eclipse. Since you have defined your planet to be earthlike we can assume that it's receiving more solar energy to compensate for this. There will be a monthly cycle to the average temperature, coldest at the end of the eclipse and slowly warming up again over the course of the month.

Life would have to evolve to cope with regular drops in temperature. This is something that life in higher latitudes, and altitudes has to deal with already.

$\endgroup$
2
$\begingroup$

The effect on climate would not be very strong. If your moon has Earth-like atmosphere, it would easily survive the effect of 40-hour eclipse. Temperature will plunge lower than during normal nights, but I don't think we'll see the difference ever exceeding 10 degrees C. Animals and vegetation should be able to easily adapt to such events.

P.S. I did not look at Michael Kjörling's formula, but the results do not seem right. Actual eclipse on Jupiter's Ganymede, for example, lasts for hours, not tens of hours.

P.P.S. Looked more into Michael Kjörling's answer. In comments, other people pointed to the error, using exactly the same Ganymede example. But there is no corrected formula still.

$\endgroup$
1
$\begingroup$

It wasn't clear to me if you wanted the 40 hours a month or if you calculated it and are now suffering with it. If it's the latter case, you might look at this wikipedia entry on Penumbra. You quote masses of planets necessary, but unless I'm mistaken the mass of a gas giant can be increased with a higher density, like a large solid core or something, without increasing it's size, which is what affects the eclipse length. You could probably find orbits, masses and sizes that work for any eclipse length. Heck, you could even have the moon orbit at a right angle to the sun and never really be in shadow. If you've considered this then. . . ummm, my apologies. Disregard. Click on this little number one. wiki on umbras1

$\endgroup$
1
$\begingroup$

Short answer:

On most or all habitable moons of gas giant planets the typical nights should last several times as long as the eclipses caused by the shadows of the planets the habitable moons orbit. Thus you should ask what the effects of those long nights are likely to be, instead of the effects of the much shorter eclipses.

Long answer:

40 hours per month of the moon? And how long is the month of the Earth like moon?

An Earth like astronomical body, habitable for beings like humans, should be at least 3,000,000,000 years old, and possible billions of years older.

However, considering an Earth-mass exomoon around a Jupiter-like host planet, within a few million years at most the satellite should be tidally locked to the planet—rather than to the star

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3549631/1

Thus the habitable moon's day should equal its monthly orbit around it's planet, while its year should its primary planet's orbit around their sun.

The synchronized rotation periods of putative Earth-mass exomoons around giant planets could be in the same range as the orbital periods of the Galilean moons around Jupiter (1.7–16.7 d) and as Titan's orbital period around Saturn (≈16 d) (NASA/JPL planetary satellite ephemerides)4.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3549631/1

The longest possible length of a satellite's day compatible with Hill stability has been shown to be about Pp/9, Pp being the planet's orbital period about the star (Kipping, 2009a).

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3549631/1

The planet's year as it orbits around the sun of their system should be at least nine times as long as the habitable moon's month and day as it orbits around the planet, and possibly several times nine times as long as a month/day.

If the habitable moon's month/day might be 1.7 to 16.7 Earth days, as suggested above, and the length of the year must be at least 9 times as long, the length of the year of the habitable moon should be at least about 15.3 to 150.3 Earth days.

Detected exoplanets considered to probably be within the habitable zones of their stars have days ranging in length from 6.1 Earth days to 384.8 days.

https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets1

We might arbitrarily assume that 2,000 Earth days is the longest possible year for a habitable world - Ceres, near the outer edge of the most optimistic habitable zone calculated for our sun has a year 1,683 Earth days or 4.60 Earth years long.

https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)2

https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#/media/File:Estimated_extent_of_the_Solar_Systems_habitable_zone.png3

And we might arbitrarily assume that 5.0 days is the minimum possible year for a habitable planet.

And thus the month/day of a habitable moon should be less than 0.5555 to 222.2222 Earth days in order to be less than one ninth the length of the planetary year.

An eclipse period of 40 Earth hours or 1.666 Earth days would require a month/day of considerable length.

The closer a moon orbits to its planet the more tidal heating will heat it. The closer a moon orbits to its planet the more sunlight reflected from its planet will heat it. Thus a moon orbiting too close to its planet will become too hot and suffer from runaway greenhouse heating and loss of its water into space.

Moons at distances between about 5 and 20 planetary radii from a giant planet can be habitable from an illumination and tidal heating point of view, but still the planetary magnetosphere would critically influence their habitability.

http://adsabs.harvard.edu/abs/2013arXiv1309.0811H4

The shadow cast by a planet upon its moon's orbit will have a diameter of 2 planetary radii. If the moon's roughly circular orbit has a radius of 5 to 20 planetary radii it will have a circumference of 31.4159 to 125.6636 planetary radii or 15.70795 to 62.8318 times the diameter of the shadow cast by the planet.

Thus the month/day of the habitable moon will be about 15.70795 to 62.8318 times as long as the period in eclipse - if the moon orbits the planet in the same plane, or close to it, as the planet orbits their sun. If the moon's orbital plane is more than slightly different from that of the planet, the moon will be eclipsed by the planet on rare occasions or will never be eclipsed by the planet.

If the habitable moon and its planet orbit in the same plane, the habitable moon's month/day should be about 15.70795 to 62.8318 times as long as the period in eclipse once per month/day.

If the of the planet orbited by the habitable moon should be at least nine times the length of the month/day of the habitable moon, there should be at least 9 eclipses during a year of the planet, and the year of the planet should be at least 141.37155 to 565.4862 times as long as an eclipse period.

If the eclipse period is about 40 Earth hours or 1.6666 Earth days as said in the original question, the habitable moon's month/day will be about 628.318 to 2,513.272 Earth hours, or 26.179916 to 104.7196 Earth Days. Thus the length of the planet's years should be about at least 235.61924 to 942.47694 Earth days or at least 0.6450 to 2.5803612 Earth years.

However, considering an Earth-mass exomoon around a Jupiter-like host planet, within a few million years at most the satellite should be tidally locked to the planet—rather than to the star

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3549631/1

Thus the month/day of the habitable moon should always be several times as long as the length of the period that habitable moon spends in the eclipse by its planet.

And if the calculation that a habitable moon's orbital distance should be five to twenty planetary radii is correct, the habitable moon's month/day should be about 15.70795 to 62.8318 times as long as the period in eclipse.

If the original question is hoping for an eclipse lasting longer than one day of the moon, the answer must be no, because a moon cannot have a day of arbitrary length. The length of a moon's day must equal the length of its month, since almost all moons will be tidally locked to keep one side always facing their planet and the other side always facing away from the planet.

There is some hope for a habitable moon to rotate faster than its orbital period around it planet.

Since the satellite's rotation period also depends on its orbital eccentricity around the planet and since the gravitational drag of further moons or a close host star could pump the satellite's eccentricity (Cassidy et al., 2009; Porter and Grundy, 2011), exomoons might rotate even faster than their orbital period.

The sources are:

Cassidy T.A. Mendez R. Arras P. Johnson R.E. Skrutskie M.F. Massive satellites of close-in gas giant exoplanets. Astrophys J. 2009;704:1341–1348.

http://iopscience.iop.org/article/10.1088/0004-637X/704/2/1341/meta5

Porter S.B. Grundy W.M. Post-capture evolution of potentially habitable exomoons. Astrophys J. 2011;736:L14.

http://iopscience.iop.org/article/10.1088/2041-8205/736/1/L14/meta6

But I have my doubts that the rotation period of a moon could ever be shorter that the time it spends in the shadow of its planet during an eclipse.

Thus on most or all habitable moons of gas giant planets the typical nights should last several times as long as the eclipses caused by the shadows of the planets the habitable moons orbit. Thus you should ask what the effects of those long nights are likely to be, instead of the effects of the much shorter eclipses.

$\endgroup$
0
$\begingroup$

As has already been said, the macro-effects of such eclipses would be relatively insignificant, yes, globally slightly colder with a warming period, but not much else, etc.

But the most interesting effect, to me, is on a more micro-scale, and that is changes in wind. On Earth, even in a relatively insignificant (compared to a full planet eclipse behind a gas giant) partial lunar eclipse, there are noticeable changes to winds, I've felt these changes personally, and it's weird and creepy feeling. What form these changes take would depend on the region of the moon, prevailing winds, Coriolis effects, and much more. What I felt during the lunar eclipse was first a cooling of the wind (not just the ambient temperature, but of the wind itself), followed by an unusual change in wind direction (far removed from normal prevailing winds in the area), then the wind became steady (as opposed to gusty), and then started slowing down steadily.

For an eclipse on a planetary scale, I would expect similar trends, overall. Though the restart of the wind could be quite drastic when the "sun" comes out again afterward. I imagine low elevation land areas would feel the wind almost die out completely, very quickly, and go cold. Higher elevations, and oceans could feel a more gradual change, because of factors like jetstreams and lack of obstructions, respectively. Areas with normally strong prevailing winds might feel the gusts align with jet streams or Coriolis-driven winds, and settle in to slow, cold, steady breezes.

When heat returns, it will start on just one edge of the moon, causing a massive updraft there, and major horizontal winds will move toward it to fill the space recently vacated by the rising air. This might even be drastic enough to cause a complementary downdraft on the night side of the moon, but at the very least it would cause a ring of downdrafts around the twighlight areas of the planet, all of which might eventually settle (days? a week?) in to whatever is the equivalents of 'normal' jetstreams and coriolis winds for this moon. Or the cycle of calm and chaos might be as 'normal' as it gets.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.