IMMEDIATE SCIENCE REPORT
K132 : Optical Properties of Sea Ice
New Zealand Antarctic Research Programme 1990/91
Antarctica New Zealand October-November 1990
A series of optical experiments were performed on sea ice in McMurdo Sound between 16 October and 8 November 1990. These included both a measurement of the spatial spreading of light introduced at a point on the surface and the temporal spreading of a brief pulse of light injected at a point. These experiments will further clarify the details of the interaction of light with this turbid natural material. A number of thermal and physical measurements were also performed on the ice in order to relate the optical behaviour to the physical structure of the ice.
2 PROPOSED PROGRAMME
The behaviour of the optical properties of sea ice is of interest, both for its impact on polar ecosystems and for the information it gives about the physical structure of the ice. It is a field which is rather little studied, principally because definitive in situ measurements are not easily performed on this inhomogeneous, anisotropic, and changing material. In the 1985 season, we tested a new measurement geometry which has permitted us to relate more fully than ever before, the optical behaviour of sea ice to its physical structure, and following experiments performed in the 1985, 1986, and 1989 spring seasons we published a number of papers on the subject1-8. The most striking result is that the surface layer of the ice is much more transparent early in the spring than it is later, which has implications for under-ice life4,6-8.
Our objectives during the 1990 spring were twofold: (i) to extend our measurements to as early in the spring as possible, and (ii) to test an entirely new optical experiment intended to complement the one we have performed to date. This second experiment is a technologically page 3 demanding one, for it involves running a very fast pulsed dye laser, fast detection equipment and a computer in a harsh environment.
3 SCIENTIFIC ENDEAVOURS AND ACHIEVEMENTS
In this section we describe the two major optical experiments performed, followed by a brief description of various auxiliary experiments intended to monitor the physical condition of the ice.
The experiment developed in 1985 consists of a monochromatic source which introduces light into a spot on the surface of the ice and detectors that measure the light emerging from the top and bottom surfaces. Since sea ice is a very turbid medium, light emerges over a large area centred on the source, and it is the intensity profile of this spot that is measured in our experiment. These profiles can be interpreted in terms of the light scattering properties at various depths. The apparatus, methods, and interpretation scheme were developed entirely by this research group. This year's use of the equipment has permitted us to extend our measurements to about two weeks earlier in the season than we had managed previously, and has confirmed that the ice is even clearer in early October than we had predicted based on our measurements of 1986 and 1989. This experiment was performed as often as convenient over the entire period in the field, 16 October - 8 November.
The second major experiment was a measurement of the distribution of path lengths taken by light as it scatters randomly through sea ice. For this purpose we introduce a very short (3 × 10−9 sec) burst of light into the surface, and measure the straggling in the emergent pulse at some distance (100-500 mm) from the source. This very demanding experiment was deployed for the first time on 22 October, and we collected many spectra then and on a page 4 number of later dates until our departure from the field. We can clearly see the effects of straggling in the pulse shape, although it still remains to be seen how well they can be interpreted. A number of items used in this experiment had to be kept above 0°C and during measurements we kept them in carefully insulated and heated boxes. At other times we had to keep them in a dry heated wannigan, and for this purpose the oil heated wannigan NZ8 was invaluable.
In addition to the two primary experiments, we continuously monitored the ice with measurements of the salinity, density, and temperature at various depths. These experiments were all operated successfully as they have been in earlier seasons. Changes in these quantities, particularly near the surface, correlate with changes in the optical behaviour.
It will take some months to analyse and interpret the data collected in the 1990 spring season. We have plans to complete that aspect of the work, in collaboration with Esther Haines (DSIR Physical Sciences) and submit a substantial paper to the international polar literature by about the middle of 1991.
5 ENVIRONMENTAL IMPACT
The entire operation was centred on a site on the sea ice about 500 m from Inaccessible island, and the disturbance to the environment was essentially nil. All waste products were returned to Scott Base.page 5
6 FUTURE RESEARCH
We have requested to return to McMurdo Sound, primarily to carry on with the time resolved (pulsed laser) optical experiment. Although this experiment ran as well as expected, we can make farther improvements in shielding the detectors from if noise generated by the laser and so intend to return for a further season.
7 MANAGEMENT OF SCIENCE IN THE ROSS DEPENDENCY
The planning and execution of the support for our research was the best we have yet experienced. We were particularly pleased to be able to get out onto the sea ice within 4 days of our arrival at Scott Base, which permitted us to get the very early measurements that were of special importance. The campsite facilities, particularly the living and laboratory wannigans NZ3 and NZ8, were excellent and contributed to the success and efficiency of the entire programme.
We acknowledge the excellent logistic support of DSIR Antarctic and particularly the assistance of John Alexander, Eric Saxby and Ron Rogers. The work received the financial support of DSIR Physical Sciences, FoRST, and Victoria University.
1 "Scattering and Absorption of Visible light by Sea Ice", R G Buckley and H J Trodahl, Nature, 326, 867 (1987).
2 "Diffusive Transport of Light in Sea Ice", H J Trodahl, R G Buckley and S Brown, Applied Optics, 26, 3005 (1987).
3 "Light Transmission in Sea Ice", H J Trodahl and R G Buckley, New Zealand Antarctic Record 7, 20 (1987).
4 "Thermally Driven Changes in the Optical Properties of Sea Ice", R G Buckley and H J Trodahl, Cold Regions Science and Technology, 14, 201 (1987).
5 "Anistotropic light Radiance in and under Sea Ice", H J Trodahl, R G Buckley and M Vignaux, Cold Regions Science and Technology, 16,305 (1989).
6 "Enhanced Ultraviolet Levels Under Sea Ice During the Antarctic Spring", H J Trodahl and R G Buckley, Science 245,194 (1989).
7 "Enhanced Ultraviolet Transmission of Antarctic sea ice during the Austral Spring", R G Buckley and H J Trodahl, accepted Geophysics Research Letters (1990).
8 "Radiation Risk", R G Buckley and H J Trodahl, Scientific Correspondence, Nature 346, 24 (1990).