Mushballs

I first ran into the concept that not all planets had hard, rocky surfaces -- like Earth, and the ones I was all too familiar with from scientific documentaries like Lost in Space -- when I was about eight.

It was in one of those kids' books about astronomy, and I found the whole thing absolutely fascinating.  Mercury, Venus, Earth, and Mars were small, solid, and made mostly of silicate rocks.  Certainly, the four have their dramatic differences -- airless, scorched Mercury; Venus with its brutally hot, carbon-dioxide-rich atmosphere and clouds of sulfuric acid; temperate, lovely Earth; and chilly, windswept, dusty Mars.  But all four, at least to some extent, fit the picture I'd had of what a planet should look like.

But then the outer four -- Jupiter, Saturn, Uranus, and Neptune -- confounded that completely.

All four are gas giants, massive planets with no solid surface (or, if there is one, it's buried so deep as to be all but inaccessible).  The atmospheres are largely hydrogen, helium, carbon monoxide and dioxide, ammonia, and methane.  They rotate fast -- Jupiter, the largest planet, rotates once on its axis every ten hours -- and this, combined with some serious convection currents, creates enormous storms, the most famous of which is Jupiter's Great Red Spot, which is large enough to swallow the Earth entirely and has wind speeds over four hundred kilometers per hour.

[Image is in the Public Domain courtesy of NASA/JPL]

Even the gas giants' cores aren't like the Earth's; ours is predominantly iron and nickel, while Jupiter -- and, it is surmised, the other three -- have a core largely composed of hydrogen compressed to the point that its electrons delocalize and it begins to act like a metal.  (This metallic hydrogen core is thought to be the source of Jupiter's enormous magnetic field.)

So my picture of the outer four planets was forever changed.  They were huge, churning blobs of gas, not solid at all.  Saturn, in fact, has such a low overall density that if you could find a swimming pool big enough, it'd float.  Then, my mind was further blown when I was twenty and first saw Carl Sagan's Cosmos, where he suggested that such a planet might still host life -- floating or flying creatures that could ride the wild thermal updrafts, and somehow metabolize the anoxic stew of gases they live in.

What's coolest of all, though, is that our understanding of the gas giants is still being refined.  A study out of the University of California - Berkeley found that certain areas of Jupiter's atmosphere are strangely ammonia-depleted.  This is unexpected -- the constant turbulence, you'd think, would result in uniform mixing, just like stirring a cup of coffee distributes the cream and sugar evenly throughout.  If there are areas low in ammonia, what is keeping them that way?

The researchers found a mechanism that might be responsible.  Updrafts in low-pressure zones might, just as they do on Earth, create hailstorms.  But everything's bigger on Jupiter -- bigger than Texas, even -- and these enormous updrafts allow the formation of huge "mushballs" composed primarily of frozen ammonia and water that, once they are too heavy to keep aloft any more, fall down into the lower layers of the atmosphere, leaving upper regions depleted.

So unlike on Earth, where a three-centimeter hailstone is considered pretty huge, these would be between the size of a softball and a basketball.

"The mushball journey essentially starts about fifty to sixty kilometers below the cloud deck as water droplets," said Chris Moeckel, lead author of the paper on the phenomenon, which appeared in Science Advances this week.  "The water droplets get rapidly lofted all the way to the top of the cloud deck, where they freeze out and then fall over a hundred kilometers into the planet, where they start to evaporate and deposit material down there.  And so you have, essentially, this weird system that gets triggered far below the cloud deck, goes all the way to the top of the atmosphere and then sinks deep into the planet...  Imke [de Pater, Moeckel's advisor] and I both were like, 'There's no way in the world this is true.'  So many things have to come together to actually explain this, it seems so exotic.  I basically spent three years trying to prove this wrong.  And I couldn't prove it wrong."

So Sagan's floaters and flyers would not only have to deal with Jupiter's screaming winds and monstrous lightning storms, they'd have to dodge volleyball-sized hailstones.

Not the most hospitable place in the world.

It's pretty cool that even our own Solar System still has the capacity to amaze us.  The more we learn, the more questions we have.  It's like Neil deGrasse Tyson said; "As our knowledge grows, so too does the perimeter of our ignorance."  And sometimes it's a simple, innocuous-seeming question -- like, "why are some parts of Jupiter's atmosphere low in ammonia?" -- that leads to a huge shift in our picture of how some part of the universe works.

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