The End Is Near For Our Intrepid Iceberg A23a

Credit: NASA

I bet you weren't expecting to hear the name A23a again, but the iceberg has once again made the news. It's been a long journey for the utterly massive block of ice, but at long last A23a's time has come. Satellite images shows blue meltwater all over its surface, and it also seems as if A23a has sprung a leak, with the weight of the water on top punching a hole through the ice to the ocean below. It is not expected to last through the Southern Hemisphere summer, which means it's only got a couple months at most.

Since I've covered A23a twice before (here and here), I won't go into too many details into its history, but it is nice that it was able to reach its 40th year. Considering how the planet is continuing to warm, I don't imagine we'll see future icebergs get so old.

You Spin My Iceberg Right 'Round

(For full effect, here's a link to the song for some appropriate mood setting.)

The world's largest iceberg is spinning right 'round, like a record. And it'll be doing so for a long time.

You'd probably like some context to that statement.

Way back in 1986, an enormous iceberg, even larger than Rhode Island, split off from Antarctica's Filchner-Ronne ice shelf. Dubbed A23a (not a particularly catchy name), it almost immediately hit the bottom of the sea, sitting in place for over 30 years until 2020. At that point, A23a melted enough to drift free, resuming a long journey north to join the Antarctic Circumpolar Current. And while A23a is certainly much larger than your average iceberg, its fate would eventually be the same, drifting along until finally the last ice melted.

But before all that could happen, something rather amusing happened: A23a passed directly over a seamount. And not just any seamount, but the Pirie Bank Seamount, which juts out about a kilometer above the surrounding abyssal plain. Crucially, this seamount is slightly larger than A23a (roughly 60 by 60 kilometers), which made the iceberg the perfect size to get caught in something called a Taylor column – a vortex formed by ocean currents hitting and flowing around the seamount. 

The end result is something scientists say they've never seen before, as this enormous iceberg spins round and round (completing one rotation in roughly 25 days), trapped by the currents flowing around it. Not only did the A23a have to hit the bulls-eye with quite literally the entire Southern Ocean to play with, the conditions around the seamount also had to be correct (the water had to be flowing at certain speed, not too fast or to slow) in order to capture the enormous block of ice trundling overhead.

While the conditions keeping A23a in place will eventually end, releasing it from its 80s pop music prison, it will likely take a few years before the iceberg can continue on its merry way. Until then, we need to find the world's largest needle ... what? It's spinning like a record, maybe we ought to treat it like one. I'm all for guitar solos longer than your average geologic epoch.

Could the Death Star Moon Have an Ocean?

NASA/Jet Propulsion Laboratory

People know the Saturnian moon Mimas for one thing: The enormous crater that makes it look like the Death Star. In every other regard, Mimas appears to be a cold, dead, icy rock with pretty much nothing going on. The surface has been dead for billions of years. It's a far cry from moons like Europa or its neighbor Enceladus, both of which sport young, smooth surfaces courtesy of subsurface oceans. We've even seen the cryovolcanism on Enceladus. 

However, there is evidence that Mimas also harbors a subsurface ocean, despite all appearances to the contrary. It all comes back to that massive crater, Herschel. If Mimas had a completely icy shell around its core, an impact of that size would have completely shattered the crust. Obviously, that's not the case. However, the physics do work out if Mimas had a subsurface ocean when the impact occurred. Not only that, but the icy shell around the ocean may be thinning, shrinking from 34 miles thick during the Herschel impact to 19 miles today.

If we are able to confirm a subsurface ocean on Mimas, that would excellent news for the prospects of finding life in our solar system. Beyond the obvious of "hey, here's one more place we could look," there are a lot of moons like Mimas floating around, especially around Neptune. If Mimas has an ocean, there's no reason not to believe other moons like it wouldn't have them as well. There could be a dozen moons with secret oceans in the solar system, which is just that many more places we could find life. Let's just hope that, if Mimas does have life floating around, they don't convert the moon into a fully operational battle station. 

Earth Now Has A Fifth Ocean

Don't panic, Earth hasn't suddenly changed, all the continents are right where we left them. This is a change in terminology only. We all know the four oceans: the Pacific, Atlantic, Indian, and Arctic, but back in elementary school geography you may have come across the name "Southern Ocean," referring to the body of water surrounding Antarctica. In terms of geographic separation there's really nothing delineating the Southern Ocean from the southern Pacific, Indian, and Atlantic, but there's a big difference in the water itself, thanks to the Antarctic Circumpolar Current, which makes the water of the Southern Ocean much colder and less salty than the oceans surrounding it.

The Southern Ocean has always exists, but it's always been an unofficial term, but now National Geographic has recognized it as the fifth ocean. I'm not sure how much that changes the reality of the world, and I didn't know National Geographic had the authority to declare this, but here we are. Earth now has five oceans.

Does Pluto Have a Subsurface Ocean?

As it turns out, apparently you can't go five feet in the outer solar system without hitting a subsurface ocean. Europa's got one, Enceladus has one, and Titan, Ganymede, and Callisto probably have them. That's not even mentioning the list of objects that may have them, according to models of radioactive decay. Here's a list of those.

If you're observant (or read the title of this post), you'll notice Pluto on there. I've written about Pluto on several occasions now, as data from New Horizons slowly filtered through. The data return is done, but we still have a lot of things to learn from it. Like this, for example. Based on research of Sputnik Planitia, otherwise known as Pluto's "heart", it is very likely that Pluto possesses a subsurface ocean.

It is believed Sputnik Planitia was once a vast impact basin filled with a layer of nitrogen ice about 6 miles thick. That's fine, but what's interesting is that Sputnik Planitia is oriented almost directly opposite to Charon, the smaller partner in the Pluto-Charon double planet system. This position on Pluto's tidal axis is probably not a coincidence. After the impact, the basin filled up with nitrogen ice. This happened slowly, but as this gigantic mass of ice formed, it changed the shape of the planet, causing the basin to slowly wander towards its current, more natural position on the tidal axis. And as it moved, a series of faults and fractures would have appeared across the surface. And Pluto has these faults and fractures.

Where does the subsurface ocean come in? The sort of movement that Sputnik Planitia has undergone would be impossible without a subsurface, the model depends on it, and observations seem to back up the model. The model predicts faults and fractures, and there are faults and fractures.

Further proof comes from a second study. Those scientists calculated the odds of Sputnik Planitia forming in it's current spot at about 5%, and so also believe the feature has moved over time. They found that the impact which formed the basin would have weakened the crust, bringing the subsurface ocean close to the surface. This along with the accumulation of nitrogen ice would essentially roll the (dwarf) planet around. Without the liquid water, the ice of Sputnik Planitia would have to be several times thicker than currently believed, and that doesn't match up with our observations.

So there we have it. Pluto: likely possessor of a subsurface ocean. It's still not a planet though.

Earth's Ocean Mapped in Greatest Detail Yet

When talking about the ocean, you often hear that we know the surface of Mars better than we know our own oceans. A new map changes that a bit. It's twice as accurate overall and more than four times more accurate in some areas. It adds a lot of information to our admittedly sketchy knowledge of the ocean floor.

About 15,000 new seamounts have been identified, and while most of them run in chains, there are plenty that are more isolated, and studying these may help scientists just what cause volcanoes to form away from fault lines. Mantle plumes are a topic of debate in the geologic community, apparently. Hey, I don't know, I'm not much a geologist.

What I find more interesting is the mapping of abyssal hills. I, just like pretty much everybody else in the world, have probably never heard of these before, and yet, they're the most common surface feature on Earth. These hills aren't a new discovery, but they've never been mapped before, and they must be everywhere. It really is surprising to learn how little we know about our own oceans, every new bit of research brings back so much information, and raises so many more questions.

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