|Home with a tale to tell. It had been predicted, before Nereus made it up from the seafloor, that there was a problem with one of its thrusters. But can you guess which one? Hint: this is NOT a trick question!!!|
Lat: 18° 33’N Long: 081° 43’W
Water depth: 4650m
Where to begin with a recap of the past 2 days – including two Nereus dives and two new CTD tow-yos along the ridge axis? First, the good news: using our CTD tow-yos we are confident that we have narrowed down our search area to around 4 square kilometers. That feels pretty good, since when we arrived on the evening of Sunday 11th Oct we had a mapped area of more like 1500 square kilometers to search in.
Second – more good news. Through a very happy piece of good fortune, we are confident that Nereus flew through the rising cone of a hydrothermal plume sometime on the morning of Thursday 22nd which means we should now have all the data needed to track its source down to within less than 1 kilometer. But you may ask yourself – Well, how did we get here?
You’ll remember that on Wednesday night we deployed Nereus to the seafloor. It arrived on bottom at around 8pm local time, but its main compass failed almost immediately. So while the first part of the dive seemed fine, hovering near the seafloor and adjusting ballast, it then started out its mission heading in the wrong direction! Through acoustic communications we did have the ability to try and compensate for that but, to compound things, we also had a problem with the main foil that sits amid ships on the vehicle and controls a lot of the vehicle’s vertical motion as well as forward thrust. After about 3 hours of trying to come up with strategies that could overcome all of this we decided that it would be more effective to recover the vehicle and fix it and get it ready to go again. So just before midnight we sent down the “abort” command, acoustically, and by 2am Nereus was back on the surface.
Our CTD night watch immediately leapt into action, and through the rest of Wednesday night and into Thursday we completed two more towed surveys from South to North through the area that we originally planned for Nereus to map out to prove that there were no signals there. This in itself was significant progress – it halved our remaining search area at a single stroke. Once Nereus was on deck, Dana also took the time to look at the data that it did collect while it was at depth, however briefly.
As luck would have it, since we had programmed the vehicle to start its work by driving along the ridge that we were betting was the most likely source for the vents, Nereus had indeed intercepted a patch of chemically unsual and particle laden water while hovering close above the seafloor for a few minutes. When Dana came to find me and tell me this my immediate question was “Was it warm, too?” Sure enough – it WAS. So Nereus was sat no more than ~100m above the seafloor (400m deeper than the dispersing plume we have been mapping out with our tow-yo surveys) when it passed through chemically enriched, smokey, warm water. Following the logic of: if it walks like a duck and quacks like a duck… I think we are getting very close.
Suitably encouraged, we reprogrammed Nereus for a new mission flying close above the seafloor in our now much smaller search area and sent it back down last night. I hung around until midnight by which time the vehicle was at the seafloor driving to and fro in closely spaced lines and making a map. All seemed well so I took myself off to bed. Mistake. Apparently Nereus must have known I wasn't paying due attention, because when I woke up today things had NOT continued as planned. Instead of continuing to survey at a speed of about 1m/sec, it had slowed right down. At time of writing our engineers’ best guess is that one of the thrusters might be damaged or broken. We’ll know when we get it back on deck.
So the first decision of the day, today, was whether to continue the dive or send down the abort command right away. With the vehicle moving slowly we knew we would have trouble flying close enough to the seafloor to continue to make maps OR to complete all the rest of our planned survey. But we persevered and between 8 and 10 am we figured out how to get Nereus to fly a little higher in the water column and raster backward and forward, clear of the rough topography, and sniffing for more rising plume signals across more of our targeted ridge. I think I got full value out of our AUV brains-trust today. It's really impressive when I just ask innocent questions like, can we make it go up a little higher, can we make it go further West etc etc and either Mike, James, Dana and Louis (working in shifts but it seems there are always three or more of those 4 to hand) very quickly says: yes we can - and then make it happen.
Arthur C. Clarke once wrote something along the lines that in the future, if we were to encounter some alien culture enriched in advanced technologies, the experience should be indistinguishable from watching magic. I feel like that's the kind of privileged company I have been keeping all day!
Finally, at 4pm, we called Nereus home. With the data set currently being carried up from the seabed we expect to learn two key things. First, we will know if we have found any more places where Nereus has flown through warm, smoky water (because it is warm, it must be buoyant, which means Nereus must have passed through the rising part of a plume which can only be a few hundred meters across). Second, because we have been flying close to the seafloor, we will be able to distinguish which part of the vehicle’s motion has been due to forward progress over the ground and which has been due to currents pushing the vehicle through the water column. The latter is important to calculate because that can help us deduce which way the current was blowing, and how strong, at any time when we have flown through rising columns of smokey water. And knowing that, we should finally know where to go next, tracing those columns of smoke down toward the seabed where they came from - seafloor hydrothermal vents.
But time and weather may still be against us. First, we need to get Nereus on deck and make sure we have guessed correctly what the problem is. If it is a simple matter of replacing a thruster then we are in good shape. We have a spare on deck that has already been checked out this afternoon in preparation and is ready to be installed tonight. If all goes to plan we’ll be launching Saturday, early, and getting Nereus back on deck in late afternoon ahead of a storm that is due to pass by tomorrow night. That may yet leave us enough time later on Sunday and early into Monday for one more dive before it is time to head to port, switch out science teams, and change over to ROV mode with Nereus.
But don't hold your breath - no part of the expedition has been quite that predicatable yet!