Mercury is a passingly strange place. It’s hot, and dense, and covered with craters. One of the most intriguing features on the diminutive planet is a vast impact basin, called Caloris Basin. This circular monstrosity is nearly 1600 kilometers across (960 miles), which is pretty frakkin’ impressive given that Mercury is just under 5000 km (3000 miles) in diameter. Whatever hit Mercury was big, and hit hard.
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| The Spider in Caloris Basin. Apollodorus is the big crater in the center. |
Caloris has been known for decades, but recent MESSENGER images have revealed new features inside it. The weirdest of them all is a series of radial cracks, dubbed The Spider (the real name is Pantheon Fossae), which appear to be centered on a small impact crater called Apollodorus, which is about 40 km (25 miles) across. The cracks radiate away for hundreds of kilometers. They appear to be associated with the crater — the fact that it sits right at the center is awfully suspicious — but no one was sure why. Most models proposed uplift in that region (from pressure buildup underneath, or possibly from the crust of the planet shrinking as it cooled, putting pressure on the region). But nothing really explained the formation well, and moreover, it’s unique: no other feature like this is seen on Mercury or its nearest analog, the Moon.
However, new results just released at the European Planetary Science Congress 2008 by MESSENGER Principal Investigator Sean Solomon may finally explain the weird structure. His team made a three-dimensional model of the region around Apollodorus. They started with the same idea as earlier — pressure from below building up stress in the crust — but then modeled what would happen if an asteroid smacked into the stressed region. What they found was that the impact relieved the stress catastrophically, and the crust fractured. The end result would be a series of radial cracks hundreds of kilometers long, centered on the impact.
Hmmmm.
There is some argument over the idea, though. Another proposal is that magma upwelling could explain the fossae, with the cracks being dikes.
But I’m not so sure. Remember, the Spider is unique, which indicates an unusual event to create it. A large impact in a region already under stress is pretty unusual, and it does seem to be the parsimonious explanation.
Either way, I’m delighted! We have bizarre things still left to explore out there, and Mercury seems to have its share of them. MESSENGER will swing by the planet again on October 6 — in just two weeks! — and we’ll get even more data. A year later it’ll make its third pass, and finally settle into orbit around Mercury in March 2011. And then we’ll get all the pictures our hearts desire.
By the way, if you want to know more about Mercury and MESSENGER, I made a video about it that is up on YouTube.
I’m really excited about this next pass in October. What new things will we see?
September 22nd, 2008 at 6:19 pm
The universe never fails to impress!
September 22nd, 2008 at 6:35 pm
If you look REAAAAL close, you can see my dog (the big crater being the nose) I KID!!!! cool pics, cant wait to see more!
September 22nd, 2008 at 6:36 pm
Forgive me BA, but that first pic of the impact crater and the spider webs looks like………….a-hole of Mercury!
September 22nd, 2008 at 6:37 pm
Oh wait, then you said this
Oy! My mind is in the scientific gutter!
September 22nd, 2008 at 6:38 pm
Where’s Nathan to doubt real research and say that they’re conductive channels caused by plasma radiating from the sun intersecting the metal-rich crust of Mercury causing splatter like an arc welder splatters metal?
Serious question… if Caloris is so large relative to the planet, does it have the effect of making the planet lopsided in some way, much like the moon is lopsided (centre of mass is shifted slightly) ?
September 22nd, 2008 at 6:40 pm
Or ES, did the impact in some way , shape, or form, effect Mercurys orbit or it’s axial rotation?
September 22nd, 2008 at 6:56 pm
@Phil: I watched the video – the first one of yours that I actually checked out. Let me make a couple of observations:
1. You are one heck of a lecturer and speaker in front of the camera. I used to do technical instruction from time to time and I absolutely loved it. One time the boss put a video camera in the classroom to capture me. It was a total disaster as I stiffened up, kept my hands clasped behind my back, talk about your wooden personality. I just couldn’t hack it. So my hat’s off to you for doing these videos.
2. Have you ever considered doing this as a sideline and a new revenue stream? There’s an outfit called The Teaching Company that sells DVDs of different courses featuring some pretty high-powered professors. They’re at. I have no affiliation with them but I’ve got a couple of their courses. Just to be clear, these courses are not transferrable or creditable on any transcript – they are for personal enjoyment and enrichment. I’m sure there are other similar outfits as well.
-Tod
September 22nd, 2008 at 7:15 pm
Little hint: It’s now 2008 and thus we are having EPSC 2008 – careful when copying URLs from a press release without checking. The one about Mercury isn’t online, by the way, but one on Jupiter’s color effects is.
September 22nd, 2008 at 7:24 pm
Shouldn’t that be “Impact *wove* Mercury’s spider webs”?
September 22nd, 2008 at 7:35 pm
1.6 megameters? Dayumn… :O.
September 22nd, 2008 at 7:41 pm
Click on my name and the link will lead you to a cool simulation of A Day On Mercury.
September 22nd, 2008 at 7:48 pm
Jasini– I originally had “wove”, but it sounded bad. I looked it up, and “weaved” is OK, though not preferred. In this context, I thought it sounded better!
And oops! The link given in the press release was wrong. I updated it.
September 22nd, 2008 at 7:48 pm
This image has always bugged me. Is Apollodorus the thing that looks like a pancake dome with a bright edge at the top? (OK, I just mentally flipped it into a crater.) Because to me, that looks quite off-center to the cracks, nearly one Apollodorus diameter off – towards the top, in this image.
September 22nd, 2008 at 9:07 pm
Phil Plait:
I would have said: Impact resulted in Mercury’s spider webs.
Phil Plait mumbles: Thank you, Mr. Smart Ass!
September 22nd, 2008 at 10:00 pm
mmm….pretty science…. much better than political crackheads (pun intended)
September 22nd, 2008 at 10:25 pm
Phil said in the video “don’t confuse size with status”.
Yep, something I live by.
There is another criteria for planetary status we could use. If you stuck the “planet” in the same orbit as mercury would the “planet” evaporate leaving only a few rocky chunks. I think we could “fail” Pluto on that too.
September 22nd, 2008 at 10:54 pm
I loves me that Messenger probe!
September 22nd, 2008 at 11:39 pm
@IVAN3MAN:
I think you missed the pun.
September 23rd, 2008 at 12:41 am
@ Thomas Siefert
Yes, well, er… that’s what happens when one works as a proofreader for too long — you take things too literally!
Come to think of it, Impact fabricated Mercury’s spider webs, sounds better.
You see what I mean!
September 23rd, 2008 at 2:18 am
Say, is that probe at 1:54 really making a sound while approaching mercury? Did I hear a faint ’swooooosh’ there? Or is it indeed the BA inhaling deeply, impressed by the massive surface features of this tiny planet?
September 23rd, 2008 at 3:15 am
800 degrees Fahrenheit or Celsius? (Most) Americans use F, scientists use C, but the BA is both… So which is it?
September 23rd, 2008 at 5:34 am
Fahrenheit it would be this time, that’s my guess .. ~400 degrees C or 700 degrees K.
September 23rd, 2008 at 6:25 am
Phil, I thought this was a political blog? You should stick to politics. If I wanted astronomy, I’d go to an astronomy blog. I’m removing you from my blogroll.
Heehee;)
September 23rd, 2008 at 7:45 am
This is so exciting!!! Being able to observe these phenomena up close and make better hypotheses about the events that shaped our universe is why astronomy’s SO FREAKIN’ COOL!!! Yay, probes!!!
September 23rd, 2008 at 8:00 am
I would say that Mercury is significantly less then half the diameter of the Earth, somewhere around 38% of the Earth’s actually.
September 23rd, 2008 at 8:43 am
looks like a giant spermatozoon hit the planet then exploded.
September 23rd, 2008 at 11:30 am
Well, I have my own pet hypothesis about this feature. Note the largest channel which extends toward the south (bottom) of the image: It’s different than the others (wider, cross-cuts the others, and tapers the farther from the crater it gets). Also, note how steep the NNW wall of the crater appears.
I speculate that this is a feature akin to Crater Lake, Oregon, USA, where a large volcanic caldera collapsed upon itself (it even has its own “Wizard Island!”). At some later time the lava-filled caldera breached the southern crater wall, forming the tapering outflow channel which overlaid the cracks (which had formed previously due to the uplift of the rising magma plume which spawned the volcano in the first place).
I won’t be proven right/wrong until MESSENGER’s in orbit and can start mapping detailed topography of this region. But I’m waiting on pins and needles (for 2.5 more years).
But what do I know? I’m not a planetary geologist.
September 23rd, 2008 at 6:53 pm
rob, I think it’s more like a giant spermatazoon impaled itself on a giant Sea Urchin. Space Urchin. Something.
An illustration of the terrible truth of Panspermia. I bet Phil didn’t include getting smacked with giant space wigglies in “Death from the Skies!”