It’s important to keep in mind that, while rocky planets with large moons might be rare, gas giants with large moons are probably extremely common. This probably isn’t relevant for life developing on those gas giants, but don’t forget about the possibility of life developing on those moons. It may be that we’re unusual not in having life, but in having life on a primary planet.
I’ve also read that about Jupiter, which is a far more effective “gravity magnet” than the Moon could ever be.
Maybe. But given that evolution has a way of finding several different “solutions” to the same means (flight, for example), I’m guessing that aquatic life would eventually find a way to become terrestrial without tides. Maybe there’s a perfectly good alternative that never had a chance because tide-wash trumped it.
The difference is that that tidal effect is just stretching and squishing. It also comes about more due to their interaction with the other moons than because of Jupiter itself. Io is causing tides in Jupiter’s atmosphere, and that is speeding up Io’s orbit. Then that momentum is transferred to Europa, which undergoes its own stretching and squishing as it transfers that to Ganymede.
On the Earth, since it is rotating relative to the moon, those tides move around the planet.
The Earth would be a very different place without the moon. Our days would be less than half as long as they are now. Tectonic activity would be very dampened, or maybe non-existent. Tides would be around half of what they are, and would be very rapid. There would not be variations in the tide as we have now, where variations in the alignment of the sun and moon make higher or lower tides, nor the eccentricity of the moon’s orbit causing even those variations to be even more variable. The inclination of the moon’s orbit also means that those tides are sometimes greater in the northern or southern hemisphere.
Those constant variations in the tidal strength would mean that some areas would get innuadted with water only rarely. Enough to keep the area full of water, but not enough to promote regular mixing. This could concentrate chemicals that could act as precursors for life.
The stabilizing effect of the moon on the Earth’s axial tilt has also been mentioned, but what has not been mentioned is the effect of the tidal interactions with the Earth’s outer core. Some of the work of transferring the Earth’s rotational energy to the Moon’s orbital energy is transferred in the water and atmosphere, but some of it is also in the liquid outer core of the Earth. This has helped keep the core temperature higher than it would be otherwise, and has helped to maintain our magnetic field.
The early moon also apparently had a magnetic field for its first few hundred million years, which combined with the Earth’s weaker magnetic field at the time, and helped to protect our atmosphere during the earlier days of the sun, when it had more solar wind to strip it off.
Would there be no life on Earth without the Moon? I doubt it. I think that life is thermodynamically favored, and so will show up in any place where conditions are right. Would life as we know it be here? I doubt that as well. I would predict that there would be simple cellular life, with the most advanced structures being mats of algae.
One of the reasons to explore other worlds is to get a better idea on this. Right now we have a sample of 1 to extrapolate from. And extrapolating from a single sample requires a number of assumptions to be made that may not be substantiated.
If we find examples of life out there, on Europa or Mars, then I would say that Earth certainly would have had some form of life, with or without the moon. If we don’t, if we fairly conclusively determine that these are and always were lifeless worlds, then that certainty would be much more in doubt.
There would likely be far less land for life to move onto without a moon.
Asimov wrote an essay on this topic way back in 1972 or so, called “The Triumph of the Moon”. He paired it with another essay called “The Tragedy of the Moon” in which he argued that the existence of the moon set back astronomical knowledge by centuries
Don’t know where this idea came from. The moon (and to a lesser degree, the sun) causes tides - local twice-daily oscillations of sea level - but differential solar heating causes large-scale ocean currents. (That article points to several proximate causes - wind, salinity, temperature - but all of those trace back to the sun.)
That’s true enough. Warm water rises, cool water sinks, and the world rotates, so there is a certain amount of Coriolis effect mixed in.
The Sun itself does cause tides, so it is possible that even on a moonless world there could be enough liminal fluctuation to allow plants and animals to colonise the land surface. Especially since a fairly high fraction of life-bearing worlds might be orbiting K-type stars; the solar tides on a planet in the habitable zone of a K dwarf star would be significantly larger than the solar tides on Earth.
Also, while lunar tides are larger than solar tides, they’re both of the same order of magnitude. Tides about half as big as they are now would still be significant.
What do tides (whether solar or lunar) achieve that wind-driven wave action does not?
A tidal mudflat allows creatures to evolve that spend at least half the day out of the water. This could also happen in rivers with a seasonal flow, but the result would be different. That’s why I am optimistic that life, and particularly land life, might evolve on worlds with no moon, or with more than one moon; but it would probably be significantly different to life on Earth.
Most of the replies so far talk about the moon’s influence on how life on Earth evolved and got to where it is now. The referenced article however seems to be implying that the life we already have would suddenly go extinct if the moon were to disappear.
However, it’s not clear to me what the article is actually claiming. On one hand it says the moon will eventually float away over billions of years, implying a very slow gradual process that will take about as long to occur as it took life to evolve in the first place. To that I’d say no, life will easily evolve to match the slowly decaying tidal and other forces the moon has on the Earth. After all if the moon is moving away at 1.5 inches per year it would have been doing so all along, and would have been billions of inches closer billions of years ago. Life obviously has evolved and adapted to the slowly changing orbit distance so far, so why would anything change?
On the other hand the article describes what would happen with no moon in a way that seems to imply the moon suddenly disappearing overnight. Why else could ocean currents “grind to a halt”? Maybe the whole article is more clear, but I can’t tell from this short section what the actual claim is. To me it sort of reads as:
Your parents will eventually die.
Without your parents you’d not have come to be.
Therefore, if your parents die you cannot continue to exist.
I remember reading a theory back in the 60’s that claimed that the moon was responsible for siphoning off much of the earth’s atmosphere over eons, otherwise a planet the size of earth have a thick atmosphere like Venus. The tenuous atmosphere extending far out to the moon is accelerated into interplanetary space by the moon’s gravitational influence. Seems a bit of a stretch to me.
The thing with tides is they allow creatures like crabs and mussels to evolve - animals that are equally at home in water or land, but survive best when submerged. The mobile bottom-feeders become more able to support themselves without the buoyancy of water, the sessile ones survive the occasional loss of water supply without dying (but can be food for the former). This is perhaps the first step to being primarily land animals - they evolve in an environment where the water recedes regularly and grow a tolerance for land-based existence.
How stable does a seasonal variation need to be, considering our ice ages and concurrent climate variations have come and gone in the order or 100,000 years?
The tectonic activity within Earth is, I understand, due to the heat of radioactive decay. The moon may accentuate plate movement by deforming the semiliquid mantle and the covering plates, but does it follow that this is the primary and motivating force of plate movement?
Tides are most pronounced around large oceans. AFAIK the smaller land-locked bodies of water - Mediterranean especially - have minimal tides. OTOH, the Bay of Fundy west of Nova Scotia is peculiarly shaped such that the tidal variation is massive and powerful, despite it being the same tides found all along the Atlantic coast. The shape of land formations can have a major influence on the activity of tides, even if the total tidal effect is small. The distinction between Earth and Jupiter or Saturn is that Jupiter’s satellites do not have a liquid surface ocean, so we don’t see the same effect. Presumably what we see is an exagerrated influence of what the earth does to our geology, mitigated by the fact that the satellites are I think locked to Jupiter and Saturn. (Except Titan does have a surface liquid… we haven’t looked for tidal effects yet.)
Actually, we’ve sort of have. Titan’s seas are roughly the size of the Caspian Sea and the Great Lakes. (In fact one of them is even named after Lake Ontario.) They’re not going to have tides significantly larger than those Earth bodies of water. But there was a report not too long ago about detecting waves on a couple of those seas. Those waves are deduced to be in currents caused by tides in a couple narrow straits.
Last I heard, the idea wasn’t that the Moon itself was responsible for our atmosphere being so thin, but rather that the event which formed the Moon also stripped off most of the atmosphere suddenly.
Yeah, that was a theory in the 60s and 70s. Larry Niven used the idea in “Wrong Way Street”
It is very hard to tell what the “article” is claiming. It’s not really even an article, but a post on quora, apparently. A cite to this messageboard would be just as authoritative, and at least we’d be able to see the context, have some idea as to the reputation of the author.
Well… Certainly the Earth would be different if Theia hadn’t smacked us, but at the time of the collision the Earth was still a molten ball of slag, probably without any atmosphere to speak of, other than perhaps some hydrogen and helium which would have escaped into space.
The early Earth-Moon system consisted of two glowing hot objects orbiting just a few thousand miles from each other. If you think lunar tidal forces are strong now, imagine how strong they would have been when the Moon was only 5,000 miles overhead.
Over time, the Moon solidified and began to drift away from Earth. The Earth went through a period of both extreme volcanism and the late heavy bombardment of asteroids until Earth’s orbit was swept clean, both of which deposited atmospheric gases and water on the planet’s surface.
Earth’s first real atmosphere developed over a long period of time, long after the Theia collision, and we believe came mostly from volcanic eruptions, which emitted water vapor, CO2, ammonia, and other gases. The collision and tidal forces from the Moon no doubt contributed to the severity of the volcanism.
Life formed very early on Earth, and archaeobacteria consumed the bad stuff in the atmosphere… Then after a couple of billion years plants formed and consumed our excess CO2 and produced oxygen. It’s still not known exactly where all our water came from.
It’s safe to say that the Earth would be different today without the Moon, and life might look different, but there’s no reason to believe that life needed the Moon to exist.
By the way, during the Late Heavy Bombardment the Moon was thought to have had an atmosphere about twice as dense as Mars’ atmosphere today, and which likely lasted for tens of millions of years.
As I understand the Theia hypothesis, proto-Earth (for all that it was hot) had a substantial atmosphere that the collision wiped out. This article discusses Giant Impact That Formed the Moon Blew Off Earth's Atmosphere | Live Science
I believe the Earth had mostly a helium atmosphere (the article also mentiins neon), which the collision may have blown away. But without the collision, the helium would have gone anyway, since the Earth can’t hang on to a helium atmosphere.
As thr article says, Earth may have had a number of transitory atmospheres when it was still primarily a magma ocean. Impacts and release from rocks would create an atmosphere, then over time it would blow away.
Once the late heavy bombardment ended and collisions came less frequently, the Earth cooled down and a permanent atmosphere started to build as water and other volatiles that landed on Earth from comets and volatile rich asteroids could stick around.
That’s my understanding, anyway. But this is an area where we are learning new things almost daily.