This is actually a very good question that is not as simple to answer as it may seem. First, let me tell you that the marshmallow will not implode. I am not certain whether it will "explode" or not, but I think that it probably will break apart to some extent. Out in space, there is no atmosphere, so things are a lot different than on earth. Animals, plants, and objects that are stable (do not explode or implode) on earth are stable because they either have an internal pressure that balances the pressure of the atmosphere pushing in on them or they have a very strong "shell" that keeps the object intact despite the pressure difference between inside and outside. Out in space, the absence of an atmosphere means that there is no pressure pushing in on an object, and if the object has a high pressure inside of it and does not have a strong shell, it will explode.

A good example of this is a water balloon. If you filled a balloon with water and put it out in space it would likely explode since the pressure inside has nothing outside of it to hold it together. This is not entirely correct, since the balloon material is holding the water in place. If the material is strong enough, the balloon will stay together. Otherwise, it will explode.
So, back to the Marshmallow. There probably will be pockets of air within the Marshmallow that will explode out in space, much like the water balloon. Whether these will cause the marshmallow to explode violently, I am not sure. The answer probably will be different for different marshmallows.

One answer:Marshmallows have quite a bit of air whipped into them during the manufacturing process. As the pressure decreases as you go into space, the gas inside the marshmallow would expand, sort of like a balloon being blown up. At some point, the marshmallow would explode, just like the balloon. You can do the same thing with a balloon as it goes up into the atmosphere - the balloon expands and eventually pops if it gets high enough.

It would do neither. In fact, the marshmallow would freeze and become very brittle like a piece of rigid plastic. You could then shatter it into many small pieces !! The pressure above the atmosphere is essentially zero. The pressure at the earths surface is 100000 pascals or what we call 1 atm pressure. The marshmallow would not expand very much due to this difference in pressure because marshmallows, like most solids, are pretty incompressible. Look this word up!! Incompressible means that even when you squeeeze an object, it doesnt densify too much. A GAS, on the otherhand, is an example of a COMPRESSIBLE material...if the pressure changes, the density of the gas WILL change a lot. This is what happens when one climbs a mountain and goes upwards above sea level--the density of the air decreases towards zero in the vacuum of space. Of course, a marshmallow, unlike a solid object, has some porosity (tiny holes filled with air).
Do an experiment: is the density of a marshmallow greater than or smaller than that of ordinary tap water at room temp and 1 atmosphere pressure???

Wow, that's a doozy of a question. First, you have to think about why something would explode or implode. When things explode, the pressure on the inside is higher than the outside. When things implode the pressure on the outside is higher than the inside. What happens if you shake a can of soda and then open it really fast? It sprays all over the place. That's because by shaking it, you've released a bunch of gas and that increases the pressure on the _inside_ of the can. So, when you open the can, it squirts out. Also, try filling your cheeks with air and puckering your lips to hold the air in. Then have someone slowly push on your cheecks. Can you feel the pressure build up? Then, when that pressure is high enough, the air gets squeezed out of your mouth.

So...back to your question. What is a marshmellow? It's a bunch of sugar with little air pockets in it. Is there any air in space? Nope. So, which is going to have the higher pressure, space or the marshmellow? The answer is the marshmellow. So that means the marshmellow would explode if you sent it into space.

Here's a bonus question: What happens if you send a marshmellow to the bottom of the ocean?

Good question! This actually involves a pretty complicated combination of effects. One is that all the air inside the marshmallow tries to escape since it is now in vacuum. It is not clear to me whether the air would escape explosively or not. It would depend on the marshmallow molecular structure. Another effect is that the escaping air and radiation will cause the marshmallow to cool quickly. Perhaps this would cause the marshmallow to shrink up. Of course, there could be a big difference between whether the marshmallow is in the sun or in the shade when put into space. We've agreed here in the lab that the best thing to do is to try to rig up a test. I'll let you know when I have some results.
....
Two weeks later...

Results of tests I've run:

1) Marshmallow in liquid nitrogen (-196 degrees Celsius)

I dipped the marshmallow in liquid nitrogen for a few minutes until it seemed like the marshmallow was at liquid nitrogen temperature. It got brittle and hard and when I crushed it, little pieces broke off. It never really shattered though.

2) Marshmallow in a vacuum chamber

We put a couple marshmallows in a bell jar (clear glass vacuum chamber). When pumping down with a vacuum pump the marshmallow expanded to about double it's size and then, after a minute or so, shrank back down a little bit. When we opened up the bell jar again the marshmallow immediately contracted to about half its original size.

3) Marshmallow in liquid nitrogen and then quickly placed into vacuum chamber

The marshmallow didn't expand this time the way it did before. When I took the marshmallow out of the chamber it stayed the same size as it was before. The marshmallow had cracks in it and it was pretty easy to break into pieces (it was still cold).

These three tests seem to me to be the easiest way to mimic the possible effects of putting a marshmallow into space. It is probably the case that the marshmallow would lose all its air before it has cooled down very much. So with these tests in mind what do you think would happen if you put a marshmallow into space? How exactly would you put a marshmallow in space and how might the way you do it
affect the results?

I should say right out front that I am not a marshmallow expert. I haven't studied the science of marshmellow-ology, neither am I accredited by any national marshmallow research agencies. That said, here is my answer. As I understand it a marshmallow is pretty much the same as a sponge or a rice crispy (not in the way they taste, but in the way they are put together). The structure of all of these things is also like a foam. If you look at your bath when you have a lot of soap in the water you will notice that huge mounds of suds form. Those mounds, if you look at them closely, are made up of thousands of tiny bubbles.

Now, with marshmellows, rice crispies, or sponges the same thing is happening except that the wall of the bubble is made out of marshmellow, or rice crispy, or sponge. If you look very closely at any of these things you will see that they are made up of thousands of tiny bubbles of material all jammed together."That's great," you are probably saying by now, "but what does that have to do with whether a marshmallow explodes or not?" To answer that I'll need to give two examples. The first is something that certainly _would_ explode in outer space, a soap bubble. Now a soap bubble doesn't burst down here in the atmosphere because the air pressure inside the bubble is the same as the air pressure outside. As you go higher and higher there is less and less air, therefore lower and lower air pressure. Once you get to outer space there is practically no air at all so the air pressure outside of the bubble is almost zero. If we were to take a soap bubble from your bath and raise it up higher and higher it would expand, because the air pushing from the inside would be pushing harder than the air from the outside. Once it had stretched as far as the soap could stretch it would burst.

My other example is a whiffle ball. You might at first glance say that a whiffle ball is a lot like a soap bubble, they are both round at least. But as I take the whiffle ball higher and higher the air inside it, instead of pushing on the sides of the whiffle ball and making it expand, just rushes out the holes. If I take it all the way into space the whiffle ball would be just fine, only it wouldn't have any air in it any more.

Now back to marshmellows. When the marshmellow is first formed it is made out of some white gooey liquid. Just after it is formed into the marshmellow shape it is a lot like a collection of soap bubbles. If you were to take it into space at that point you better be prepared to make interstellar rice crispy treats because you'd have marshmellow everywhere. You may have noticed, however, that is not what the marshmellow looks like when you take it out of the bag. It's basically just a soft, fluffy ball. That is because it has dried, and as it dried all of the little bubbles that make up the marshmellow go from having liquid walls to squishy semi-solid walls. In its dried form a marshmellow looks a lot more like a collection of thousands of tiny, soft, whiffle balls than tiny soap bubbles. So my answer is no, I don't think it will explode. But then again I haven't tried it, so who knows?

*When I'm talking about sponges in my answer above I mean sponges that are made synthetically. There are animals called sponges that live in the ocean and they grow by a totally different process. If you want to know more about "aquatic" (meaning water-living) sponges the encyclopedia is a good place to start.

A marshmallow in outer space would explode for sure! The physical science instructor at Allan Hancock College in Santa Maria does a demo for his class in which he takes a glob of whipped cream and places it in a glass bubble that he can suck the air out of, making it nearly a vacuum, like in outer space. Wow, you should see that whipped cream expand!

What would happen if you took a marshmallow down to the bottom of the ocean in a submarine? (OK, let's say you put it in a plastic bag to protect it from the water so it won't dissolve!)

Actually, I have never thought in sending a marshmellow into space...
(1)The more you go up in altitude, the less pressure you have,
(2) In space, there is the vacuum.
So any object sent to space should explode.