# Could a cave-in or avalanche in low gravity be dangerous?

I have a character on the surface of Enceladus, one of the moons of Saturn. It has an icy surface on top of what is believed to be a liquid ocean. There is no atmosphere, though near its south polar region there are what appear to be canyons filled with long lines of geysers that eject mostly water vapor into space.

I need to have a way for this character to be trapped and buried on Enceladus (don't worry about his fate), either as a result of the ground caving in beneath him causing him to fall below the surface, or in something like an avalanche of ice down the side of the one of the canyons. The trouble is, the surface gravity on Enceladus is only 1.13% of Earth's. In that case, it seems to me most scenarios would be easily escapable, both because a cave-in or avalanche would occur very slowly and because the character could jump so high he should be able to simply leap away from the danger. I thought of having him hit by a large piece of ice and knocked out, thus preventing him from escaping; but could a person really be knocked out by a large mass that is slow-moving, or would he just be crushed?

So my question is: is there a realistic way for someone to get trapped an buried under these conditions?

• If the ground opens up below you, then there's nothing for you to push of of for a jump. Thus, you'll fall. – RonJohn Apr 9 '18 at 14:26
• If he's wearing a spacesuit I'm not sure how he can be "knocked out" without suffering decompression – SilverCookies Apr 9 '18 at 15:15
• If a pound of feathers lands on you... does it weigh you down more or less than a pound of rocks... – WernerCD Apr 9 '18 at 16:25
• Your astronaut won't be able to hear the avalanche. There is less sunlight at Saturn compared to Earth and you have to ask how far electric lights will illuminate the surroundings. There's probably very little warning before the avalanche hits. – CJ Dennis Apr 10 '18 at 1:36
• A couple of comments not worthy of an answer: 1) I think you (the author) can contrive an artificial scenario where the character drops something valuable in a crevasse, has to retrieve it, and then gets trapped by a mini cave-in or similar. 2) I might suggest the New Yorker article on Antarctic expeditioner Henry Worsley for some insight on visible and hidden dangers (crevasses!) in a similarly harsh environment newyorker.com/magazine/2018/02/12/the-white-darkness – BurnsBA Apr 10 '18 at 13:26

Yes, though Enceladus is probably much safer than Earth for these sorts of things.

It all depends on how high the avalanche starts from and how much material is involved. A ton of rock or ice hitting you at 50 mph is going to hurt regardless of whether it's on Earth or Enceladus -- if anything, Enceladus would be worse because of the likelihood of damage to your spacesuit.

It's certainly true that an avalanche starting from the same height will do less damage on Enceladus, but if it's high enough, it will still kill. (Also, there's the distinct possibility that Enceladus's low gravity may make much greater elevation differences more common.)

Likewise with getting buried. The same volume avalanche will weigh less on Enceladus, and for that reason will be easier to get out of (pressure suit damage aside). But a big enough avalanche will still bury you under too much overlaying material for you to dig your way out even if you survived.

Next there's the question of escape. Once more, it's probably easier to escape on Enceladus -- though just how athletic and controlled you can be in a spacesuit is an open question -- but escape is far from guaranteed. Further, in a vacuum, will you always be aware of an oncoming mass of ice? And if you're in a confined space, will you be able to escape? Consider a deep valley and an avalanche which starts far above you. By the time you're aware of it, it's moving 30 mph and is quite inexorable, with the same momentum it would have on Earth. Can you escape? Probably not.

The lack of atmosphere had a negligible effect, as air resistance doesn't play a large role in the dynamics. (Its major impact is that the lack of air on Enceladus forces people into space suits and this makes them more vulnerable.)

So, assuming your space suit is reasonably rugged, you're most likely safer on Enceladus, but a large avalanche can still trap you and kill you.

• @JanDoggen But it has something to do with the weight... – wizzwizz4 Apr 9 '18 at 15:47
• @JanDoggen And that's to do with inertia(l mass) – momentum – kinetic energy. None of those have anything to do with weight. – wizzwizz4 Apr 9 '18 at 15:50
• In H. Beam Piper's Cosmic Computer, he has the lines: "Yves Jacquemont began posting signs in conspicuous places: WEIGHT IS WHAT YOU LIFT, MASS IS WHAT HURTS WHEN IT HITS YOU. WEIGHT DEPENDS ON GRAVITY; MASS IS ALWAYS CONSTANT." – Mark Olson Apr 9 '18 at 15:55
• @MarkOlson I'm not convinced by your answer. Lack of air resistance means terminal velocities are higher and things like a cave collapse can happen without needing to push the air out of the way and also less warning. (You won't hear an avalanche coming although you may feel a rumble in the ground). On the other hand though fast-moving events often ride on or mix with air in order to reduce friction so without air the avalanche may behave differently. I don't know the answer to all of this but I doubt the net effect is "nothing".. – Tim B Apr 9 '18 at 16:03
• @Tim B: It's not "nothing", it's "not much, negligible". One way to look at it is to consider how much effect air resistance has on Earth -- it will be less on Enceladus. And when you're dealing with materials like ice and rock, it has very little effect on Earth. (Snow's another matter, of course, but that's not what the OP was asking about.) – Mark Olson Apr 9 '18 at 16:14

The surface gravity of Enceladus is 0.113m/s2. At such a low gravity, you cannot run, for the force you would use in a step will send you on a very long jump that may last more than a minute (if you don't hit anything along the way before you touch ground again). This may be quite dangerous. If you don't have the means to fly, like a jetpack, you may end up landing on a sharp shard of ice that will rip your spacesuit open. Alternatively, you may accidentally jump from a high place to a lower one. And while lower gravity means smaller acceleration, the fact that you can jump dozens to hundreds of meters upwards, to fall on a hole/crater/depression that might be dozens to hundreds of meters lower than your starting point, means that you can land with enough speed on hardened ice to break bones and equipment.

If you want to see how walking on such a gravity might look like, I can recommend you a simulator. Like any simulator, this one does not model reality with 100% accuracy, but it is close enough to reality to give you a general idea. Get yourself a copy of Kerbal Space Program and go take a walk on Gilly (surface gravity = 0.049m/s2) or Pol (surface gravity = 0.373m/s2), which are the bodies with gravity that is closest to Enceladus.

That said, unless your astronaut has a jetpack, even walking may be suicidal. But if he does have a jetpack, he would never be in trouble in the first hand.

As for whether the snow can crush him... the density of snow on Earth is 0.1 to 0.8g/cm3. Let us assume that the density of snow on Enceladus is around the lowest range, 0.1g/cm3 so as to be nice with your astronaut. Now let's say that he gets 100 meters of snow on him. Let's do some calculations.

Under 100 meters of snow, the mass of snow above a section of one square meter is:

$$10^2m \times 1m^2 \times 10^{-1}g/cm^3 = \frac{10m^3g}{cm^3} = \frac{10^6 cm^3g}{cm^3} = 10^6 g = 1 \space metric \space ton$$

Impressive, right? But at 1.13% the gravity of the Earth, that metric ton would do for a pressure of 11.3 kilograms per square meter.

The average surface of an adult humans is around 2m2. This means that, laying down, your astrounaut is exposing about one square meter to the snow. We can then infer that under ten meters of snow, he would be facing 11.3 kilograms of pressure. That is a laughable fraction of an atmposphere.

So is he out of the hook? No.

Don't forget that the astronaut is considerably denser than the snow around him. If he were naked, he could be ten times as dense as that snow - I figure the equipment in his spacesuit might be denser yet.

In other words, he will sink in the snow. The snow will behave like a very viscous liquid, and it should feel like sinking in quicksand for the astrounaut. In the end, he is in for a very slow death in the dark and cold bottom of the avalanche.

• What goes up must come down. Get in the way of one the geysers and that's going to be a bad day. – Mazura Apr 9 '18 at 23:13
• Reminds me of A Fall Of Moondust by Clarke – tox123 Apr 10 '18 at 2:25
• For the KSP surface gravities, I'm assuming you meant 0.049\0.373 meters per second per second, rather than 0.049\0.373 square meters? – Sean Apr 10 '18 at 16:55
• @Sean thanks, I will fix the metrics in the post. – The Square-Cube Law Apr 10 '18 at 17:10
• What about some sort of "gecko tape" like substance on the boots to help maintain traction? You'd still be able to "peel" your boot away from most surfaces just through the normal heel-to-toe roll of a footfall, but if you walk such that your other foot makes contact (and thus adheres) before your first foot's toe departs from the surface, you won't be launching yourself into space in the course of ordinary walking. Incidentally, it would make jumping almost impossible, thus preventing that means of escape! – Doktor J Apr 10 '18 at 18:50

Yes, it's definitely feasible for either a cave in or an avalanche to trap this character, and ice is heavy enough that chunks the size of two sedans would be very difficult for the average person to move even under Enceladus' gravity.

Material Required To Trap a Person: Since the gravity is ~1% of Earth's (rounded for easier math), 100kg on Earth would be only 1kg on Enceladus. Assuming that the character did get trapped under some amount of ice, let's see how much is needed to prevent the character from just pushing their way out once they've been buried.

Benchpress world records are around 485kg, so if your character is a world record body builder they could theoretically lift 48,500kg, or about 40 Toyota Corollas. Let's assume a more modest 100kg to make the math easy.

This site claims the volume of their truck trailers are 82 cubic meters, and this site claims that 82 cubic meters of ice is about 75,000kg. A Toyota Corolla is about 12 cubic meters, which is about 7,300kg of ice. So, a chunk of ice the size of 1 and a half sedans could trap, but not completely crush, someone on Enceladus, and presumably your disaster would involve much more than that.

Avoiding a Cave In: This is trivially easy to avoid if the character is next to a stable wall to grab on to since they'd fall slowly, so let's assume the entire area around the character is collapsing.

This question covers the idea of climbing up falling debris, however the answer's best case scenario involves large pieces of rubble that you were already about to jump off of. If the character is just standing, then they will fall at the same speed as the ground below them so they would not be able to push off of anything. Therefore, the character could not jump to safety if the ground below them caves in and they had no solid ground to grab onto.

Avoiding an Avalanche: Although it would be moving slow, it would actually be pretty hard to avoid being buried in an avalanche on Enceladus. I don't have enough physics degrees to understand the math, however I'd imagine that since the avalanche would behave much like a liquid, trying to stay on top of it would be like try to walk through a flood of quicksand or molasses. This, coupled with a cloud of powdery ice blocking attempts to find a safe route, could definitely lead to the character sinking and getting buried.

• You would probably not be able to bench the Toyota. Mass is the same. It would take you minutes of pushing at your full stength to accelerate the mass and get it moving. Once it finally moves it would blast off taking you with them if you made the mistake of holding on. – Andrey Apr 9 '18 at 15:42
• @Andrey: Unless there's no gravity then you don't lift mass, you lift weight. Lifting 'Thing A' that weighs 100kg on Earth would take the same effort as lifting 'Thing B' that weighs 100kg on Enceladus. However, Thing A would weigh 1kg on Enceladus, and Thing B would weigh 10,000kg on Earth. – Giter Apr 9 '18 at 15:50
• not exactly. V=F/M So even at 0 gravity, it takes a huge amount of force to put any useful velocity on an object. Heavy object in low G are extremely dangerous. They soak energy like a sponge and then become freight trains slowly moving forward crushing you – Andrey Apr 9 '18 at 16:07
• @Andrey I happen to have pushed vehicles as heavy as a Toyota Landcruiser, and I can verify that it does NOT take minutes worth of output from a human to get meaningful velocities in multi-ton objects. Think accelerations on the order of ~0.1 M/s^2. Absolutely doable and worthwhile, although one thing that I haven't seen mentioned is that the posture of the person trapped may prevent the same kind of leverage one applies with the proper form. Anyone who has benched can tell you proper form is everything. – wedstrom Apr 10 '18 at 17:44
• @wedstrom that's a good point. Just try pushing at car on a 25 degree incline, and you will have 10% gravity perfectly simulated. See if you can still move it. On a flat surface you are converting 95% of your energy into acceleration, just a little loss to friction, on a lift most of it is being lost to fighting gravity – Andrey Apr 10 '18 at 18:59

Despite the lower gravity, a cave-in in a sufficiently deep crevasse or cave could still easily happen quickly enough to block the escape.

And once the only entrance is blocked by several tons of ice (and remember, the ice still has the same mass as on earth, so you can't just push it away), your explorer is truly trapped without advanced mining equipment.

In fact, the use of such equipment might even pose another hazard, if it melts the ice or causes tremors, which could instabilize the rest of the ice.

Being knocked out is also a possibility: Even if the actual collapse is far slower than on earth, the large masses colliding can cause shards and boulders of ice to be ejected at dangerous velocities.

Finally, ice can be quite sharp, so if your character falls on or is hit by an icycle, they could end up pinned in place, with the ice stuck in their pressure suite being the only thing between them and decompression.

I think the other answers have sufficiently covered how dangerous a cave-in or avalanche might be, but I want to point out that the chance and severity of them will be far higher.

The lower gravity will create a far steeper angle of repose as the cohesiveness of snow, rock, etc. will be much greater relative to gravity than we are used to on Earth. That means you can have far more material build up into very steep, even over-hanging and exotic structures. Add to that the lower atmospheric disturbance (no wind) and no critters or humans to disturb this moon's surface and you will probably have large, critically balanced structures that are ready to be knocked over at any moment.

Whether or not cave-ins or avalanches would be as dangerous as on Earth, you will have them occur far more often in virgin territory, and the mass of the material involved will likely be much greater.

What I miss in other answers is that the morphology of mountains and avalanche material will be completely different at 1.13% of gravity. The amount of material stacking up before surfaces get crushed to the degree of starting a conversion from sticking to moving friction will be quite higher. So when finally things start getting ugly, the amount of ugliness unleashed will be quite different from that on Earth and the amount of potential energy leading to a chain reaction will be comparable, making the involved masses quite larger. Avalanches will be quite slower at taking up speed, but they will be just as deadly in their effects and the height colloding material will take on will have similar relations and densities compared to the jumping height of a human as on Earth. It's not just the human energy and time frame getting better payoff.

• You apparently didn't read my answer because I addressed that. – BlackThorn Apr 11 '18 at 15:18

Absolutely yes. Even though the force of gravity is 1.3% of the Earth's, the planetary weight of a landslide or cave-in could still be fatal.

Weight is the force of gravity on a mass. Newton's second law formula (F = m•a) shows the relationship between mass, acceleration and force. The following weight formula uses Newton's second law:

w = m • g

where:

Using this weight calculator with a g value of 0.1274 m/s^2 (1.13% of 9.8 m/s^2), you can make some simple calculations. If 1,000 pounds of material would crush you on Earth (453.59 kg), that's a fourth of the weight of a VW Beetle. On your planet, ~34,800 kg would have the same effect, and that could be 21 cubic meters of stone, i.e. not much.