ISS-EMIT (Earth Surface Mineral Dust Source Investigation)References
Blown by wind across continents and oceans, dust does more than make skies hazy, congest lungs, and leave a film on windshields. Also known as mineral dust or desert dust, it can influence weather, hasten snowmelt, and fertilize plants on land and in the ocean. Particles from North Africa can travel thousands of miles around the globe, sparking phytoplankton blooms, seeding Amazonian rainforests with nutrients, and blanketing some American cities in a veil of grit while also absorbing and scattering sunlight. 1) 2) 3)
To help researchers model climate effects, the Earth Surface Mineral Dust Source Investigation mission will measure the composition of minerals that become airborne dust.
Figure 1: Soon to be operating from the International Space Station, NASA's Earth Surface Mineral Dust Source Investigation (EMIT) mission will comprehensively measure the mineral composition of Earth's mineral dust source regions to help scientists understand how dust particles carried by wind heat or cool our planet as they move through the atmosphere. In addition to potentially influencing warming on regional and global scales, dust can affect cloud formation, air quality, and human health. When deposited in the ocean, dust can also trigger blooms of microscopic algae (video credits: NASA/JPL-Caltech)
NASA's Earth Surface Mineral Dust Source Investigation (EMIT) mission, set for launch in June 2022, aims to deepen researchers' understanding of these fine particles of soil, silt, and clay from Earth's deserts and, ultimately, how they affect climate.
Figure 2: EMIT will collect color and composition information of surface minerals in the world's dry regions, highlighted on this map. The data will help climate scientists better understand how airborne dust influences air temperatures, weather, and climate (image credit:: NASA/JPL-Caltech)
Darker, iron-rich dust absorbs the Sun's heat and warms the surrounding air, while lighter-colored particles, rich in clay, do the opposite. "Different kinds of dust have different properties – they're acidic, they're basic, they're light-colored, they're dark – that determine how the particles interact with Earth's atmosphere, as well as its land, water, and organisms," said Robert O. Green, EMIT's principal investigator and a longtime researcher at NASA's Jet Propulsion Laboratory in Southern California. With the EMIT data, he added, "we'll be on track to map the world's dust-source regions and understand how dust heats and cools the planet, as well as how that might change under future climate scenarios."
Researchers at NASA and elsewhere have long focused on dust's flight – a journey that can span hours or weeks, depending on particle sizes. Its atmospheric impacts are included in climate models, but it remains unclear whether dust has a net warming or cooling effect on the planet, and how this is changing over time.
Figure 3: JPL engineers and technicians put together components of EMIT, including its telescope, imaging spectrometer, and baseplate, which holds its electronics (image credits: NASA/JPL-Caltech)
The uncertainty comes from lack of data on dust composition, said Natalie Mahowald, EMIT's deputy principal investigator and an Earth system scientist at Cornell University in Ithaca, New York. What knowledge researchers do have comes from fewer than 5,000 sampling sites that are mostly in farming areas, where detailed soil information can serve agricultural or commercial purposes. Because few crops grow in deserts, the world's dust-producing regions tend to be undersampled, so scientists must assume dust composition in their computer simulations, which combine land, water, and air data to model climate changes.
"Normally in climate models, we model dust as yellow – the average color of all types of dust – but if you've ever gone to a desert region, you'll know that sand is not all one color," Mahowald said. "So this assumption that it's uniform across the globe doesn't reflect what's happening in reality."
Mapping Dust's Origins
EMIT should improve that scenario. From its perch aboard the International Space Station, the state-of-the-art imaging spectrometer will map the world's mineral-dust sources, gathering information about particle color and composition as the instrument orbits over dry, sparsely vegetated regions.
EMIT will focus on 10 important dust varieties, including those containing iron oxides, whose dark-red hues can cause strong warming of the atmosphere. Knowing which kinds of dust prevail on the surface in each region will provide new information about the composition of particles lifted and transported through the air. With these insights, climate scientists can hone their understanding of mineral dust's regional and global climate effects.
"There is a lot of variability in the dust emissions – every second there's some variability due to shifts in wind or rain, and there is seasonal, annual, and longer-term variability," Mahowald said. "EMIT will provide information about the source regions of dust, which we combine with other atmospheric and climate information to evaluate the changes in emissions and better understand what has been going on in the past and what will happen in the future."
More Than a Billion Measurements
EMIT's spectrometer receives sunlight reflected from Earth, then divides it into hundreds of distinct colors and records it on a grid of light detectors. The grid has 1,280 columns, each with 480 elements, and every column is effectively its own spectrometer, reading the colors of a soccer-field-size patch of Earth's surface. Together, the instrument's detectors can scan a strip of land 50 miles (80 km) wide, at a rate of more than 4.4 miles (7 km) each second.
"In the beginning, scientists worked with single spectrometers," Green said. "Now we're going to be effectively flying 1,280 spectrometers over the surface of the Earth, each collecting hundreds of measurements per second."
EMIT will deliver more than 1 billion new measurements during its mission. Because each dust type has a unique light-reflecting signature, researchers will be able to determine the mineral and chemical composition of substances on the surface.
The precision of those observations will make EMIT's instrument one of the most sophisticated Earth-facing imaging spectrometers ever put in space.
EMIT uses an advanced imaging spectrometer instrument that measures a spectrum for every point in the image. Sunlight reflected from minerals on the Earth's surface is imaged by a telescope and spectrometer system onto a detector area array that is sensitive from the visible to short wavelength infrared portion of the electromagnetic spectrum. The EMIT dust source minerals have distinct spectral signatures in this wavelength range. Each column of the detector array records the spectrum for a sample of 1240 cross-track EMIT instrument swath. The along-track dimension of the image is built up with the forward motion of the ISS. The imaging spectrometer measured image cubes are calibrated to spectral radiance, then atmospherically corrected and analyzed to determine the mineral composition that is required to achieve the EMIT science objectives.
Figure 4: The Compact Wide-swath Imaging Spectrometer (CWIS) is prototype for the EMIT imaging spectrometer. CWIS demonstrated a full spectral range Dyson design imaging spectrometer operation from the visible to the short wavelength infrared portion of the spectrum (image credit: NASA/JPL-Caltech)
Figure 5: EMIT imaging spectrometer instrument approach (image credit: NASA/JPL-Caltech)
EMIT uses an advanced two mirror telescope and high throughput F/1.8 Dyson imaging spectrometer. The telescope focuses light entering EMIT on the spectrometer slit where it passed through calcium fluoride crystal refractive element to the grating. The concave grating has a structured blaze written by electron beam lithography to optimize the diffraction efficiency over the full spectral range. After being dispersed into the spectrum by the grating, light passes back through CaF2 block to the order sorting filter and detector array.
Table 1: Key parameters of EMIT
Figure 6: EMIT instrument configuration (image credit: NASA/JPL-Caltech)
This is the EMIT instrument configuration for operation on the ISS. The optical elements of the telescope and spectrometer are contained within the Optical Bench Assembly (OBA). The electronics are contained within the Electronics Baseplate (EB). The electronics receive, amplify, and digitize the weak analog signals from the detector array. These high rate data are compressed and stored on a digital recorder for replay to the ISS for transmission to the ground. EMIT also includes a sophisticated thermal control system that is required to measure the signals in the short wavelength (infrared) region of the spectrum and for optomechanical stability. Heat generated within EMIT is radiated to space to maintain thermal balance.
Launch: The EMIT team plans to launch the instrument on a SpaceX resupply mission in 2022. Once mounted on the ExPRESS Logistics Carrier 1 (ELC1) — one of the station's four primary hubs for externally-mounted instruments — EMIT will begin to map the surface composition of Earth's arid land mineral dust source regions. From there, the instrument will study our planet's mineral dust source regions for 12 months.
Figure 7: Dust from northwest Africa blows over the Canary Islands in this image captured by the NOAA-20 satellite on Jan. 14. An upcoming NASA mission, the Earth Surface Mineral Dust Source Investigation (EMIT), will help scientists better understand the role of airborne dust in heating and cooling the atmosphere (image credits: NASA Earth Observatory)
1) Andrew Wang, "NASA's EMIT Will Map Tiny Dust Particles to Study Big Climate Impacts," NASA/JPL Feature, 2 May 2022, URL: https://www.nasa.gov/feature/jpl/nasa-s-emit-will-map-tiny-dust-particles-to-study-big-climate-impacts
2) Robert O. Green, Natalie Mahowald, Charlene Ung, David R. Thompson, Lori Bator, Matthew Bennet, Michael Bernas, et al., "The Earth Surface Mineral Dust Source Investigation: An Earth Science Imaging Spectroscopy Mission," IEEE Big Sky Conference, Big Sky MT, USA, 7-14 March 2020, https://doi.org/10.1109/AERO47225.2020.9172731
3) Robert O. Green, Natalie Mahowald, David R. Thompson, Roger Clark, Bethany Ehlmann, Paul Ginoux, Olga Kalashnikova, Ron Miller, Greg Okin, Thomas H. Painter, Carlos Perez Garcia-Pando, Vincent Realmuto, Gregg Swayze, Eyal Ben Dor, Philip G. Brodrick, Longlei Li, Nimrod Camron, Benjamin Phillips, and Kevin Reath,"The NASA Earth Surface Mineral Dust Source Investigation (EMIT)," 12 Oct. 2021, URL: https://aero-sat.org/sites/wdc.dlr.de.esa-aerosol-cci/files/sites/wdc.dlr.de.esa-aerosol-cci/AA2021_O4.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).
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