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Scientists studying the Earth as a system have a staggering
array of questions. What role do clouds play in global warming?
How much of the world is being deforested, and what effect
will that have on our climate? Is the hole in the ozone getting
bigger or smaller? What will happen to the billions of tons
of carbon dioxide we generate with our cars and our power
plants? Scientists are asking a lot of complicated questions,
and they’re hoping that MODIS can provide some answers.
While scientists have numerous, complex questions, MODIS itself
really has only one—one question that it asks over and
over as it orbits the Earth: How much light (electromagnetic
radiation) is there?
As MODIS orbits the Earth, its scan mirror rotates round and
round, continually diverting electromagnetic signals from
Earth onto its photon, or light particle, detectors. Like
a small child on a long car ride asking every few minutes,
“How far it is now? How far is it now?”, MODIS continuously
looks down at the Earth’s surface and asks, “How
bright is it now? How bright is it now?”
How much radiation MODIS sees depends on how much light, or
solar radiation, the Earth is reflecting back into space and
on how much heat, or thermal radiation, the Earth is giving
off. Different features of the Earth’s surface and atmosphere
reflect or emit radiation in different ways, namely at different
wavelengths. MODIS has detectors that allow it to distinguish
36 spectral bands, or groups of wavelengths. These bands represent
a portion of the electromagnetic spectrum, including visible
light and infrared radiation. When light of a certain wavelength
hits a detector that is specially designed to recognize that
particular wavelength, the detector registers that it has
been hit by creating either an electric current or voltage.
These analog signals are translated into digital signals,
which a satellite relays down to Earth.
Scientists call the readings from the detectors radiance counts
because the voltages and currents that are the basis for the
digital signals are directly proportional to the number, or
count, of photons of radiation that hit the detectors. Radiance
counts are the first level of data scientists on Earth receive
from MODIS. These data come down from MODIS along with other
important information: where the spacecraft was in its orbit
when the readings were made; whether some data points, called
pixels, are missing; and information from onboard calibration
devices.
How much radiation is coming from Earth? MODIS answers that
question nearly 500 million times a day. What does it all
mean for our planet’s future? Scientists will be working
on the answer to that question for years to come.
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