| | Funafuti Atoll, Tuvalu
|  | Tuvalu natural colour image | |
From an astronaut's point of view
An astronaut inspecting the Pacific Ocean with a telescope would suddenly spot this scene: the immense blueness of water with countless white flakes of clouds, and some light blue and greenish rounded jewels – the atolls. Atolls are circular coal reef or string of coral islands surrounding a lagoon. Envisat took this image on 4 February 2004 from a height of approximately 700 km. In the centre of the image, we can see the Funafuti Atoll, at 8°31' South and 179°13' East.
There are three more atolls in the image. At least two of them you can surely spot!
What are their distances to Funafuti, the main island?
Download here larger version of Tuvalu natural colour image (0.5 Mb).
Use LEOWorks to measure this. The size of each pixel of this image is 300 m.
| | | Algal concentration |
Algal concentration
The MERIS sensor onboard Envisat enables us to see much more. The 15 spectral bands can be modelled to estimate the chlorophyll production and other 'quality parameters' of our oceans. Chlorophyll production is an indicator of the amount of nutrients and hence of plankton and fish.
Atolls are famous for their treasures of marine life. Envisat can reveal this, and can also quantify it. To make things clearer we have used false colours in the image.
Clouds and all landmasses are detected by the model and excluded from computation; they are shown in black. The other colours are organised as follows:
| Colour |
Chlorophyll content of the water
(mg/m3) |
| Orange |
>0,6 |
| Red |
0,6 - 0,4 |
| Brown |
0,4 - 0,3 |
| Green |
0,3 - 0,2 |
| Light blue |
0,2 - 0,09 |
| Blue |
<0,09 |
| | Tuvalu, names of all the atolls | |
Download a larger version of the Algal concentration image (0.7 Mb).
Thanks to the false colours, you can now probably spot all four atolls on the image. The northern island Vaitupu is the smallest one, and forms a triangle with a right angle with Funafuti and Nukufetau. The large, black, oblong feature with some red points around it is merely a cloud. A further atoll, Nukulaelae, is situated southeast of Funafuti.
Click here for an image with atolls names.
Which of the islands is richest in nutrients and hence in corals and fish?
Describe other differences between the atolls concerning the basal platforms of the islands. The extension of the coral reef will help you to assess this.
What is the origin of all these atolls? Bear in mind that the surrounding ocean is several thousand metres deep!
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