This article was originally posted on Mike Brown's Planets blog and is reposted here with permission.
It didn’t snow much in northern Alabama where I grew up, so, when I went to college further north, I was at a serious disadvantage when the first blizzard came through and everyone streamed out of the dorms to engage in an all night snowball fight. After my first rounds of fusillades ended up splintering to little wispy bits in midair I quickly got the hang of compaction, looking for wetter snow, and doing what I could to increase the density of the snowballs. I broke a window, confessed, and escaped punishment with the lame but true excuse that I had no idea snowballs could break windows. Friends with more snowball experience and more delinquent childhoods told me about burying a stone or two inside of the snowball to increase its destructive power.
I don’t get much snow in southern California, but I do spend a lot of my time thinking about those early snowball experiences and about the snowball fights that have made the objects of the outer solar system.
Before I explain, a few bits of background:
- The solid objects in the outer solar system are made, essentially, of nothing but water ice and rock. There are other minor components – and I spend a lot of my time trying to identify those minor components to learn more about the history of these objects – but at the biggest scale you can just think of water ice and rock and ignore everything else.
- The general idea of how to get large solid objects in the solar system is that you start with tiny solid objects, they coagulate, the newer larger objects coagulate, and the process continues until something stops it (Neptune was probably the culprit in the outer solar system). Big things are simply collections of smaller things. As trivial as this statement sounds, it is not true for all things in the universe. Big stars are not – in general – collections of smaller stars. Big people are not made of large numbers of small people. As far as I know.
- Because ice is light and rock is heavy (by which I really mean ice has a low density and rock has a high density), we can figure out how much rock vs. ice an object in the Kuiper belt has by measuring its density. Density is mass divided by volume, so we need both of these, but if we can measure them we get density and a measure of the rock to ice ratio.
- Every single object in the Kuiper belt with a diameter below 350 km that has a well-measured density has a density lower than ice.
- Every single object in the Kuiper belt with a diameter above 800 km that has a well-measured density has a density higher than ice, and often high enough to imply that the object is mostly rock.
If you’ve been paying close attention, you might have realized that something fishy is going on. If it is true that the big objects in the Kuiper belt were simply made by collecting many of the small objects, how did the big objects get so rocky when the small objects appear not to be? It would be just as strange if I took all of the snowballs I made that first blizzard night, stuck them all together to make a really big snowball, and found that someone had slipped stones into the mix when I wasn’t looking. Even in space, ice doesn’t magically change into rock. So what is going on?
For the last few years now, the only plausible explanation (and I think “plausible” is being a bit generous here) is that the small objects have just as much rock as the large objects, but they are fluffy, rather than compacted, much like those first snowballs I tried to throw. They have to be really fluffy if they have all of that rock inside of them and are still light (where again, by “light” I really mean “have a density smaller than that of ice”): more than half of their insides needs to be empty space. It’s not impossible, though. We have pretty good measurements of the fluffiness of small asteroids, which are mostly rock with little to no ice, and for objects smaller than about 350 km in diameter, asteroids can indeed be quite fluffy (the mathematical measure of fluffiness is porosity, which can be thought of as the fraction of empty space inside of the body).
For objects bigger than about 350 km, however, fluffiness goes away quickly, because it is squeezed out by the force of gravity. Small asteroids might be able to hide empty space inside, but when the mass increases, the gravitational pull increases, the pressure on the inside increases, and all of that rock gets crushed down into the formed empty spaces. For objects of this size and larger, the density of the object is nearly just the average density of the material inside.
What about the Kuiper belt? We don’t really know anything about porosity in the Kuiper belt except that the objects that have densities below that of water ice must be fluffy even if they have no rock. We do know that, in general, it is easier to crush the porosity out of ice than rock. Imagine, for example, taking a collection of ice cubes and squeezing until there is not space between the cubes. Now imagine doing the same thing except for gravel instead of ice cubes. It’s pretty easy to imagine that squeezing space out of ice is going to be easier. So even if there is significant porosity in the Kuiper belt, objects that are larger than 350 km probably have little and, like asteroids, are in the what-you-see-if-what-you-get range of density.
Reviewing the facts, all Kuiper belt objects with sizes below 350 km have densities less than that of ice, but they also have the possibility of being fluff balls. All objects 800 km and larger have densities that require the inclusion of rock, and it seems unlikely that they have much porosity.
So, did the small objects make the big objects, like we have assumed for the past 50 years, or not? The key lies in measuring the density of something in that middle range between 350 and 800 km in diameter -- an large enough to have little porosity, but small enough to show the true density of the smaller objects. One object is perfect for figuring this all out. The Kuiper belt object 2002 UX25 was discovered 11 years ago by Spacewatch on Kitt Peak. Its size was recently measured by a large team using the Herschel Space Telescope and found to be about 650 km in diameter; perfectly placed midway between the large and small objects. And, most critically, I discovered a satellite around 2002 UX25 a few years back. If an object has a satellite, we can use that the measure – quite precisely – the mass of the object. As you can imagine, a heavy object will cause the satellite to whip around quickly, while something light in the middle will have a satellite which circles in a more leisurely fashion. Determination of the orbit of the satellite and measuring its speed takes a while – we measured satellite positions from the Hubble Space Telescope a few years back and finally from the Keck Observatory last December. But, finally, now, we have a mass. And a mass, combined with the Herschel density measurement gives the density. And a 650 km diameter object in the Kuiper belt should have essentially no porosity, so, finally, we should be able to see if the smaller objects are really just super fluffy balls of rock and ice.
I gave a presentation of these results at the 2013 meeting of the Division for Planetary Sciences of the American Astronomical Society in October. At this point in the talk I stopped and had the ~100 people in the audience guess what the density of 2002 UX25 was going to be. I gave them the option of (1) lower than water ice like the small objects (2) midway between the small and large objects (3) very rocky like the big objects. The votes were nearly evenly split. Evenly split! Often a group of experts kind of knows what the answer is going to be before you give it. Here no one really had a clue. Including me, I must say. Though, for the record, I voted (2). What’s your vote?
And the answer is….. (1). The Kuiper belt object 2002 UX25 has a density smaller than that of water ice. In fact, 2002 UX25 is the largest solid object in the solar system which could float in water. If you could find a big enough body of water to float it in. As I explained to the audience at the time, this is such a startling result that everyone should currently be gasping.
This answer begs an important question: WHAT IS GOING ON????
I can’t answer that for you. But I can give you options. First: perhaps 2002 UX25 is fluffy, after all. I find this extremely unlikely, not just because of the analogy with the asteroids but also because of what I learned early on in my snowball fight days: wet snow compacts easily. 2002 UX25 is large enough that it should have gotten warm during its formation. If its temperature raised any substantial amount, melting would have quickly destroyed porosity. Second: perhaps 2002 UX25 is an anomaly. It is the only object with a measured density in this range. If it is not actually representative all bets are off. We’ll only know by finding more objects of this size and measuring their density.
The third possibility is the one that I find most intriguing: perhaps the large objects aren’t made up of the small objects. Then where did the large objects come from? I have no idea. OK, that’s not really true, I have too many ideas, none of which make much sense, but all of which would require some pretty major revisions to what we think we know about how to build bodies in the planetary systems. And that’s where science gets fun. Theories that have been around for 50 years deserve respect, as they wouldn’t have survived 50 years if they didn’t explain a lot of what we see around us, but when they fail, when they require that snowballs magically grow stones inside of them when you stick them together, you might have a hint that you are onto something even more interesting than the giant snowball
The technical paper describing this work is being published soon in the Astrophysical Journal Letters, but you can read the whole thing right here in the mean time.