Using Light to Study Planets

In this activity, students will build a spectrometer using basic materials to observe the light emitted and absorbed by several sources. This will be used as a model for how NASA uses spectroscopy to determine the nature of elements found on Earth and other planets. For higher grades, this activity can also be used to discuss advanced spectroscopic topics, such as how NASA research is advancing spectroscopic techniques to teach us more about plant life on Earth.

• Provide one set of the materials listed above per group.
• Safety Note: Break the CDs in half for students. This can be done safely by placing the disc between a book and the edge of a table and breaking the CD along the straight line to ensure a clean break. Wear gardening gloves for additional safety.

NASA scientists often use a technique called “remote sensing” to study the composition of different elements and structures on planets. Remote sensing refers to making measurements without directly touching the object being measured. These images are one kind of remote-sensing measurement. But what we can see with the naked eye only tells us part of the story.

When we look out into the solar system and beyond, we use remote sensing to see things far beyond what we can see with our naked eye. For example, using remote sensing we’re able to detect the jets of water shooting out from Saturn’s moon Enceladus nearly 900 million miles away.

In space exploration, NASA uses remote-sensing tools that are capable of seeing deeper into the electromagnetic spectrum than we can with our eyes. Because wavelengths in the electromagnetic spectrum are so fine, these tools allow us to quantify the amount of light as it moves. These movements are called reflection, absorption and re-emission spectra and they happen at very specific wavelengths. Further, because reflectance spectra come in myriad colors and at a finer resolution than what we can see, we can use these spectra to learn much more about how our planet is changing and the composition of other planets.

Upon its arrival at the Jovian system, the Europa Clipper spacecraft will use remote sensing as it studies Jupiter’s icy moon Europa. It will collect ultraviolet light in the onboard UV spectrograph to determine the composition of Europa’s atmospheric gases and surface materials. It will also gather infrared light with an IR spectrometer to map the composition and distribution of ices, salts, organic compounds, and the warmest hotspots on Europa.

Another way scientists are interested in using spectra is to develop techniques to measure how life on Earth is changing as a result of climate change. For example, one major player in climate change is carbon dioxide. We know that plant photosynthesis drives the global carbon cycle and ultimately provides energy for life on Earth. Because of a changing climate and increased human impact on our planet’s ecosystems, it is critical that we improve our capability to make highly resolved measurements related to photosynthesis and ecosystem health. Over the past few decades, NASA scientists and engineers have aimed to develop new – and validate current – remote sensing approaches that are responsive to vegetation changes around the world. Ultimately, we want to understand, for example, how the rainforest is responding to deforestation, or how the Arctic tundra might be greening as a result of melting permafrost and increasing temperatures. In general, two remote sensing techniques can be used to measure the changing pulse of the planet (i.e., our terrestrial ecosystems).

The first and most widely used set of information that comes from satellite imagery is measuring the colors of plants on the planet, most commonly in near-infrared (which is highly reflected by plants) and in red (which is highly absorbed by plants for photosynthesis). The difference between near-infrared and red parts of the spectrum can give us an idea of our planet’s “greenness” and tell us where and how much plant life there is. This technique is commonly done using NASA’s Landsat satellites and Moderate Resolution Imaging Spectrometer (MODIS) instrument.

Another technique that effectively measures a leaf’s “pulse” uses solar-induced chlorophyll fluorescence, or SIF. SIF is re-radiated light energy emitted at longer wavelengths of the electromagnetic spectrum than were initially absorbed. It is measured using portions of the light in the electromagnetic spectrum that are absorbed by the sun’s atmosphere, then re-emitted, creating a very slight glow. Because the fluorescent glow is so faint, it cannot be measured using traditional reflectance. Until recently, measuring a plant’s glow using SIF had been too difficult simply because the light from cities was so much greater than this tiny amount of light. As we get more advanced at detecting this glow, NASA hopes to match data from older satellites with newer ones, such as the Orbiting Carbon Observatory, or OCO-2, to map this glow as a measure of actual photosynthesis. This would ultimately reduce uncertainties in not only the when and where, but also the why and how of land-atmosphere CO₂ exchange. By coupling physics and biology in technology, we can measure the health of plants on Earth.

• Students should discuss what impact our atmosphere has on the colors we observe.
• Would we see the same colors emitted and observed on Mars where the atmosphere is mostly carbon dioxide? What about the spectra of the sun as observed from Earth versus space? When observing the sun from space, you are are not looking through an oxygen and nitrogen atmosphere.
• From what we learned about NASA’s OCO-2 satellite, what solar-induced chlorophyll fluorescence (SIF) observations would you expect when observing photosynthesis as deforestation continues and carbon dioxide levels rise?
• If we can see certain lines representative of elements, how can we use this technique to determine the composition of other worlds? Which spectral lines are being absorbed in your samples? What could these correspond to (what exists between our eyes and the spectrometer)?

Have students explore several elements or chemicals. Simple chemicals available at home or in most chemistry labs have unique spectra and flame ionization spectra that will provide visibly different patterns on the CD. Using a candle and wooden splint, have one student take a scoop of salt and place it under the candle. It will burn yellow. Potassium chloride burns purple. Strontium chloride burns red.