In September 2017, on a research cruise off northeastern Greenland, the crew of the German icebreaker Polarstern found themselves with an awkward recovery problem: a CTD they needed to retrieve was still on the sea floor, but sea ice had closed over the water above it. The instrument, short for conductivity, temperature and depth, was anchored below and used to measure how properties such as salinity and temperature vary with depth. To retrieve it, Polarstern had to skirt carefully around the equipment, breaking the ice above it.
The scene was captured by Richard Jones, now a polar-ice researcher at Monash University in Melbourne, whose photograph later won Nature’s 2024 Working Scientist photography competition. In the winning image, the ship’s orange equipment stands out against the blue-white ice as the crew works over the opening.
What a CTD actually does, and why one ends up under ice
A CTD is one of the least glamorous, most useful instruments in oceanography. It measures conductivity, which is used to calculate salinity, along with temperature and depth. Those measurements allow oceanographers to distinguish water masses, track stratification and identify features such as meltwater or warmer water entering a region.
In the Arctic, CTD sensors can form part of moored observing systems that remain in place for months. Instruments are deployed, left to record, and later recovered. Between those two visits, the ice above them can drift, fracture and close again.
The 2017 Polarstern cruise was working off northeastern Greenland, close to one of the routes through which Arctic sea ice is carried toward Fram Strait. Conditions around a recovery site can therefore change substantially between deployment and retrieval.
Jones, describing the trip to Nature, remembered the constant background of the work. “You’re crashing into ice and breaking through it. So it wasn’t particularly calm sailing for the majority of the trip.”
Why the ice can close over a sensor
Sea ice is not a fixed lid. It is a fractured surface moved by wind and ocean currents, and open leads can narrow or close as the pack shifts. For a ship, that affects navigation. For equipment waiting below the surface, it can turn an otherwise routine recovery into a much slower operation.
The contrast was especially striking during the 2020 MOSAiC expedition, also aboard Polarstern. Near the North Pole, Captain Thomas Wunderlich described ice that had become thin, porous and unusually easy for the ship to traverse. “Even after passing 88° North we mostly maintained a speed of 5-7 knots,” he told The Barents Observer. “I’ve never seen that so far north.”
For recovery crews, the important point is that a patch of workable water is temporary. The pack keeps moving, and a route that is open during one operation may not stay that way.
The awkward physics of getting it back
Recovering a moored instrument beneath ice requires control. The crew has to locate the equipment, create access through the surface without damaging the hardware or its line, and then haul it up. Some moorings use an acoustic release that lets a buoyant component rise toward the surface, a straightforward idea in open water but a more complicated one when solid ice is overhead.
A 2019 paper in Frontiers in Marine Science describes a related challenge: researchers manually recovered an ice-tethered ocean profiler from beneath roughly 1.5 meters of sea ice using rope techniques and no powered lifting equipment. The details differ from the Polarstern operation, but the practical problem is familiar: the instrument may be accessible from below while the surface above it is anything but convenient.
The 2017 Polarstern retrieval was the ship-based version of that problem. Find the instrument. Open a safe route through the ice. Bring the equipment back aboard.
Why anyone bothers
The data these sensors return is why research teams accept the risk and difficulty of deploying them. CTDs, moored profilers and ice-tethered buoys make it possible to observe water beneath an ice cover that is otherwise difficult to sample continuously.
That matters because the Arctic Ocean is changing from below as well as above. A team led by physical oceanographers at the Scripps Institution of Oceanography documented plumes of warm Pacific-origin water entering the Arctic and accelerating sea-ice melt from below. Their work combined in-water profiling instruments with other observations to resolve processes occurring beneath the surface.
Sea ice also participates in exchanges between the ocean and atmosphere rather than acting as a perfectly sealed cap. A 2026 Nature Communications study compiled more than 6,000 chamber measurements from the Arctic and Southern Oceans and found seasonal air-ice carbon dioxide exchange, with winter release and summer uptake. Measurements made directly in and around the ice are what allow researchers to resolve processes that broad remote observations alone cannot capture.
The same logic applies at the other pole. Profiling floats deployed in the Ross Sea near the Ross Ice Shelf recorded temperature and salinity through the water column. Those observations helped show that sunlight warming surface waters after seasonal sea ice disappeared was an important source of heat driving summer melting near the ice-shelf front.
More instruments are going into the ice
Oceanographers now use a mix of sea-floor moorings, autonomous profilers and instruments deliberately frozen into sea ice and left to drift. Each solves a slightly different observational problem, and each creates its own recovery or survival challenge.
In August 2026, a U.S. Coast Guard icebreaker and a Royal Canadian Navy patrol vessel spent nearly a month monitoring the Chinese icebreaker Xue Long in the Arctic. According to gCaptain, the Chinese vessel completed 12 ice-station operations north of 82 degrees north and deployed more than 30 ice-based buoys of various types during the expedition.
Some Arctic instruments are recovered. Others are designed to continue reporting after the ship leaves, drifting with the ice and transmitting observations from places that would otherwise go largely unmeasured.
An old habit of patient science
Polar research has always required patience with instruments operating on timescales humans cannot control. We have written before about the University of Queensland’s pitch drop experiment, created in 1927 and opened to begin dripping in 1930. Nine drops have been recorded, yet no one has witnessed one fall.
The polar version is less whimsical. A sensor left in the ocean does not care whether the ice above it has shifted into a convenient position when the ship comes back. It records until its mission ends, its batteries fail, or someone manages to retrieve it.
Nature’s 2024 photography feature included another image from Polarstern, taken in August 2023 by Emiliano Cimoli. It showed University of Magdeburg mathematicians Carolin Mehlmann and Thomas Richter measuring snow depth across a drifting ice floe during a two-month Alfred Wegener Institute voyage. Different instruments, same underlying problem: making precise measurements on a surface that refuses to stay still.
Jones, describing the visual world of his 2017 trip, kept coming back to the color scheme. “All you really see is blue and white. And sometimes that might feel pretty monotonous, but the colours from the CTD instrument and the orange of the crane contrast the scene and also complement it quite nicely.”
The photograph caught the moment when that hidden instrument was finally being brought back through the ice. Around it, the pack remained what it had been throughout the voyage: moving, closing and forcing the scientists aboard Polarstern to work on its terms.