Did Venus ever have a moon?

One of the defining characteristics of the planet Venus, other than suffocating atmosphere, extreme temperature variations, and slow rotation, is that it doesn't have a moon. Observations have confirmed that this planet has no moon, basically not even a pebble, on a stable, long-term orbit anywhere in the planet's entire hillsphere. In fact, Venus is the largest object in the solar system that doesn't have a moon. This is pretty weird, especially because Venus is very similar in size to Earth, and Earth famously has a moon. Even Mars, an object Venus is over seven times more massive than, has two small asteroids orbiting it. Not having a large moon like Earth's is one thing, but Venus doesn't even have a small asteroid or anything around it. The common explanation for this is that Venus is too close to the sun to host a moon, as the sun's gravity would rip it away. However, this is untrue. The Venusian hillsphere, the area around it where a moon's orbit would be stable, is about a million kilometers wide, similar to Earth's hillsphere of 1.4 million kilometers. For comparison, Earth's moon orbits at 385,000 kilometers, and Phobos and Deimos orbit Mars at 9,400 and 23,500 kilometers respectively. The point is, Venus has more than enough space around it for moons to form. The hillsphere of Venus is actually larger than the hillsphere of Mars, 1 million kilometers versus 983,000. So this explanation doesn't actually explain the problem, so the question still remains. Why doesn't Venus have a moon? There are two major possibilities here. One is Venus just never formed any moons, or they were somehow destroyed sometime before the present day. We've already established that scenario one may be unlikely. Venus is 80% the mass of Earth, and has more than enough space around it to form moons. As I've already said, Venus not having a large moon isn't terribly surprising, maybe it just never underwent a planetary collision like Earth did. But Venus not even having so much as a small asteroid is interesting. The second scenario is actually pretty likely. Venus today would destroy any moon that formed around it. So the question becomes then, did Venus ever have a moon, and if so, what happened to it? The second part of that question is the easiest to answer. If Venus did have a moon, we know what would have happened to it. It would have spiraled inward and gotten destroyed. This is because of Venus's extremely slow rotation, taking 243 days to make a full rotation. This is longer than its year of 224 days. You may know that the moon is currently migrating away from Earth very slowly. This has to do with the difference between the rotation of the Earth and the orbit of the moon. Earth takes 24 hours to rotate, while the moon takes 28 days to orbit. Because Earth spins faster than the moon orbits, the tidal bulges created by the moon are always slightly ahead of the moon in its orbit. Because all mass has gravity, this means that the moon is being pulled on slightly harder in the direction it orbits, which over billions of years gradually causes the moon to speed up and thus move outward. This only happens because Earth spins so relatively fast relative to the moon's orbit. But for Venus, with its 243 day rotation, the opposite would be true. No moon would take longer than 243 days to orbit Venus, meaning whatever orbit any hypothetical moon would have, it would be shorter than Venus's rotation period. So the opposite thing happens compared to Earth. The tidal bulges this moon would exert on Venus's atmosphere and crust will be slightly behind the moon in its orbit, slowing it down and causing it to spiral inward. The moon will eventually stop migrating away from Earth as the two become tidally locked to one another, because as the moon moves further away, Earth's rotation also slows down. At the same time as the moon moves further away, tides weaken, slowing down the migrating. For Venus, the opposite would happen. Over time, any hypothetical moon around Venus would spiral in faster and faster. It would eventually be torn apart by the planet, creating a ring system that itself would also begin spiraling inward, or collide with Venus itself. So if Venus did have a moon, we know what probably happened to it. But that doesn't answer the question in the title of this video. Did Venus ever actually have a moon? Let's look at the evidence and also what we know about other planets to make a guess. As I've already said, moons forming around Venus doesn't seem to be impossible. But there's actually another planet in the solar system where a similar situation is Mars's moon Phobos takes 7 hours and 40 minutes to orbit Mars, which is less than Mars's rotation period of about 23 hours. This is causing Phobos to spiral inward toward Mars, and will likely be destroyed in about 43 million years due to this. However, based on another paper, it's a strong possibility that Phobos is actually pretty young, and this is just one member of a family of destroyed Martian moons. For a more in-depth analysis, check out my video The Cyclic History of the Martian Moons. But as an abridged version, it's likely that when Mars formed, it had an ancient moon at least 18 times larger than Phobos is today. This theorized ancient Martian moon doesn't have a name, but in that video I called it Anciely, which is what I'll refer to it here as as well. For an explanation of that name, check out the video. According to that paper, Anciely may have existed around 3.8 billion years ago, and orbited about three times the radius of Mars itself. It spiraled inward and was destroyed, creating a ring system around Mars that eventually formed into a new set of moons, which were then destroyed in a cycle. Phobos and Deimos may actually be the 7th generation of this cycle. So, Mars and Anciely may actually be a good example of what we may expect to happen with a planet and a moon that spirals inward. It creates a ring system that forms into new moons. Anciely is not confirmed to have actually existed, and remains hypothetical for now, but it could be a good analog for what could happen with Venus. So, let's assume Venus did have a moon in the past, maybe a large asteroid similar to Anciely, assuming that moon actually existed as well, which is again not confirmed. If that's true, then why didn't this hypothetical Venusian Anciely form rings that formed into new moons? Well, Venus's rotation may again be to blame. This is just my own thoughts and I haven't seen any research backing this up, but it could in theory be a possibility that the reason Mars formed new moons is because some debris from the previous generation ended up on wider orbits around the planet, ones that didn't spiral inward, allowing new moons to form. This is backed up by the existence of Deimos, which takes 30 hours to orbit Mars and is thus not spiraling inward. However, for Venus, there are no stable orbits that don't spiral inward, meaning any debris that would form new moons is swallowed up by the planet before it can. Maybe Venus did have an Anciely of its own, but after it was destroyed, everything just continued spiraling inward and no new moons formed, leaving modern Venus moonless. But again, this is all hypothetical. Maybe a situation like that did happen, but we have no evidence of it. Unfortunately, there's really no current way of knowing whether or not Venus had any small moons. Maybe Venus had things similar to Phobos or Deimos or Anciely, but since we've done very little research into this planet, we can't know that for sure. However, a large moon would leave a much larger footprint, something we may be able to detect today. The answer to the question of whether or not Venus had any small moons in the past is maybe, but there's no real way to tell. The answer to whether or not Venus had a large moon is a bit more complicated. One proposed explanation of why Venus rotates so slowly is a planetary collision. In this scenario, a planet hit Venus, basically canceling out its rotation and also flipping it upside down, explaining why it rotates retrograde. It's also worth noting before we continue that it is not confirmed and still remains highly speculative whether or not Venus actually underwent a planetary collision. According to a paper in the description I'm about to talk about, it's a possibility, but not strictly necessary. For Earth, a collision with Theia is required to form the moon. There's no other explanation that makes sense and also explains many other independent lines of evidence. However, for Venus, a planetary collision is one possibility of many to explain its slow rotation, meaning it's far less confirmed than the collision between the Earth and Theia. But it does remain true that a collision would explain Venus' present day rotation. So, assuming a collision did occur, which it may not have, could it have formed a large moon like Earth? According to a paper in the description, we may actually have an answer to this question, and the answer is no. This paper ran several simulations of planets hitting Venus, including a wide range of characteristics. Across their simulations, the colliding object, which I'll just call the Venus impactor, ranged in size from 0.01 to 0.1 Earth masses, or between the size of the moon to the size of Mars or Theia. They also ran several collisions with different angles, including head-on collisions and hit-and-runs, as well as slow-moving and fast-moving impacts. They found that a pretty wide range of impacts actually did produce Venus' present day rotation, providing evidence, though not confirming, that a collision may be to blame. However, importantly, the collisions that produced a slow rotation resulted in very small debris disk masses. To put it simpler, the collisions that would have resulted in the Venus that we see today did not produce enough free material to also form a large moon. And even with the disk that did form, most of the material just fell back onto Venus itself instead of remaining in orbit, further limiting opportunities for a moon to form. So, a giant impact doesn't necessarily always result in a moon, and there are plausible scenarios where even large, Theia-scale impacts don't result in a large moon around Venus. Even if a collision is responsible for Venus' rotation, which it may not be, it likely wouldn't have produced a large moon. So, the answer to if Venus ever had any small moons is that we currently don't have enough information to say for sure. However, the answer to if Venus ever had any large moons is no, if a giant impact is responsible for slowing down Venus' rotation. If there's a different explanation for the rotation, like friction with the atmosphere combined with tidal forces from the sun, which remains a very strong possibility, then the possibility of a past destroyed large moon kind of remains on the table, because different giant impacts may still be on the table for Venus. Which means the answer to whether or not it's the same as the small moons, not enough data to say, though is leaning toward no. This paper does provide evidence that if this impact did slow down Venus' rotation, it didn't produce any moons, large or small, so it actually is a possibility that Venus just never formed any, despite having plenty of opportunity to do so. Unfortunately, this does mean there currently isn't a good answer to the question in the title of this video, but there is a lot of information around it that's still worth discussing. If Venus did have a moon in the past, it was doomed by the planet's slow rotation. However, it's currently unclear of whether or not Venus could have formed a moon via a giant impact, as there's uncertainty over what exactly caused Venus' slow, retrograde rotation. If a giant impact is the cause, which it may not be, then the collision needed to produce it would not have formed a large debris disc capable of forming a moon, suggesting that Venus may have actually formed moonless. And even if this giant impact didn't happen, it's a possibility that others may have, opening up the opportunity for moons. This video is just a good example of why we need to study Venus more, because a lot of these questions could be answered with further studies. For example, studies of Venus' internal structure and its thermal environment, as explained in the paper, could provide evidence for or disfavor a collision, but more data will be needed to confirm that. So for now, that's where I'll end this video. There's not much concrete answers I can give, but Venus having a moon in the past that has since been destroyed remains a possibility, though depending on the history of this planet, either decently likely or pretty unlikely. Hopefully we can figure out which one of those options is preferred soon. Thank you for watching. If you enjoyed, check out my other videos about space and space exploration.