By Dr. Charles Ichoku, NASA Scientist for GLOBE Student Research Campaign on Climate
The recent oil slick in the Gulf of Mexico is an example of a large environmental event that can have multiple ramifications. The oil slick resulted from an explosion that occurred on April 20, 2010, on the Deepwater Horizon rig, causing it to sink to the ocean floor and breaking a pipe, from which millions of gallons of oil have leaked.
This true color satellite image obtained from the NASA Earth Observatory website was acquired on April 29, 2010 by the Moderate-resolution Imaging Spectro-radiometer (MODIS) sensor aboard the NASA Terra satellite flying around the earth at a 705 km high orbit. The greenish and brownish land surfaces in the upper half of the image are contrasted with the darker ocean waters in the lower half. The whitish features of different shapes and sizes that look like paint over the land and ocean surfaces are clouds. The oil slick is shown enclosed in a rectangular white box near the center of this image. The scale at the lower right part of the image allows us to visualize how extensive the oil slick was on April 29; well over 100 km in diameter.
In fact, calculations of the approximate amount, area coverage, and thickness of the slick have been provided at the NASA Space Math website, click on Problem 339), which shows many excellent examples of useful mathematical applications in earth and space sciences for students. About 10 days after the incident, it was estimated that about 2 million gallons of oil had already leaked, and covered an area of over 6,000 km2 of the ocean surface.
Since the oil spill occurred, different emergency response teams and other agencies have made intensive efforts to try to contain the oil leak as it has spread across both the surface and floor of the ocean. Such efforts are geared toward stopping the leak and limiting the spread of the oil. As we can imagine, if such measures are not taken urgently, the oil could have many unpleasant consequences for the affected ocean and land ecosystems (the physical and biological components of these environments) including the plants and animals that inhabit them, and the humans who depend on ocean and coastal ecosystems for food and water.
How do satellite sensors acquire information from space?
One may wonder, how it is possible for a satellite sensor to see the oil slick from as far away as 705 km above the earth’s surface, even though it may not be easy to see it with the human eyes from a few kilometers away. You may also wonder why the oil slick appears light colored on this image even though oil slicks are typically very dark in color. This demonstrates the power of ‘remote sensing’, which is the science of making measurements from far away, without physical contact with the object being measured. Satellite remote sensing is one of the most powerful and efficient ways of monitoring Earth in modern times and documenting different events. This is achieved by making instruments or sensors that measure the intensities of different types of electromagnetic radiation, which includes visible light, as well as other types of radiation that are invisible to the human eye, such as X-rays, ultraviolet (UV), and infrared (IR). These different electromagnetic waves travel with different wavelengths (which is the distance between the midlines of two consecutive crests or troughs of the wave). Detailed discussion of the electromagnetic radiation is beyond the scope of this blog. However, although the radiation measured in remote sensing can be from different sources, in the case of the above image, it is the reflection of sunlight from the various features and objects on the earth and ocean surfaces, as well as the molecules and particles in the atmosphere, such as air, aerosols (described in my previous blog), and clouds. The intensities of sunlight reflected at different wavelengths are measured by the satellite sensors, and transmitted to ground receiving stations, where they are recorded and forwarded to processing centers. By combining the measurements at different wavelengths on the computer according to logical scientific principles, a variety of images of the scene can be created. Depending on how the data are processed, different objects on the scene can be made to appear more prominently than some others. This is made possible because the surfaces and objects in the scene reflect and/or absorb the Sun’s radiation differently at different wavelengths. A more detailed discussion of how remote sensing works will be presented in a future blog.
Even with the capability to display satellite data as images, in such a way that certain objects and surfaces could appear more prominently, absolute confirmation of what an object really is can be achieved by matching the characteristics of these objects on the image with related observations made at close range on the ground. Such close range observations that are known with absolute certainty are referred to as “ground truth”. Whereas ground truth can only be obtained over limited areas, when combined with satellite observations, very large areas can be monitored considerably well. For instance, in the case of the satellite image shown above, it has been possible to visualize, practically in an instant, an area of ocean surface over 6,000 km2 covered by the oil slick, by linking the testimony of human observers, who have seen the oil in a small area on the ocean surface, to the satellite image. Without the knowledge from the ground that this is surely an oil slick, just by seeing it on the satellite image alone, it might have been mistaken for something else, such as ocean sediments or even clouds. On the other hand, without this satellite observation capability, such a large area over the ocean could have taken months to measure, even by the most advanced ship-based mapping technique. Now that the oil slick has been recognized and detected, it can be mapped accurately and its spread monitored from several satellites that pass over the area daily, until the slick breaks up so much so that it is no longer visible from satellite. For example, the image below was acquired on May 4, 2010 (one week after the previous image) by another MODIS sensor aboard a different NASA satellite called Aqua, and obtained from the MODIS Rapid Response website. This time, the image is displayed on an online mapping system called GoogleEarth, so that the oil slick’s location could be seen relative to accurate map features, such as the land/sea boundaries in yellow, and the position of the city of New Orleans. The oil slick is shown enclosed in a white box. I leave you to compare the position, shape, and size of the oil slick on the two images in this blog acquired one week apart. Bearing in mind that the oil is gradually breaking up and spreading: What are your observations or conclusions?
An important lesson to learn from this blog is that many environmental events and situations that can affect our lives and even our climate can be monitored from satellite, but observations and measurements on the ground are equally important for accurate identification of such events and situations. Students everywhere can help in acquiring the ground-truth information that can contribute toward solving important problems related to our environment and climate by active participation in various programs coordinated by GLOBE, which has very close relationships with agencies responsible for environmental and climate monitoring from satellites.
