This week’s satellite image was acquired over the eastern part of Mexico City.
The area pictured lies within central Mexico’s highlands plateau called the Valley of Mexico. This valley was originally covered by the waters of Lake Texcoco but over the centuries the water has been drained. The area that has not been built up is today used for hydraulic management and is made up of reservoirs and ponds such as the large, dark Nabor Carrillo lake pictured here.
The area receives more than 100,000 migratory birds each year that travel through the Central Migratory Flyway, and is a key resting, feeding and breeding ground for several species of shorebirds.
In contrast to the open space of the former Lake Texcoco, Mexico City is a densely populated metropolitan area (left and bottom).
We can see the runways of the international airport on the far left. South of the airport is the Alameda Oriente recreational park with its somewhat spiral artificial lake. North of the airport, El Bosque de San Juan de Aragón is another park and important green area.
City parks play a large role in the city’s effort to alleviate air pollution. In the early 1990s, pollution was believed to cause hundreds of deaths each year. Air quality has improved in recent decades through a series of government efforts to cut emissions.
This image, acquired by Korea’s Kompsat-2 satellite on 21 December 2012, is also featured on the Earth from Space video program.
Image credit: KARI/ESA
The magnitude of fluorescence portrayed in this visualization prompted researchers to take a closer look at the productivity of the U.S. Corn Belt. The glow represents fluorescence measured from land plants in early July, over a period from 2007 to 2011.
Data from satellite sensors show that during the Northern Hemisphere's growing season, the Midwest region of the United States boasts more photosynthetic activity than any other spot on Earth, according to NASA and university scientists.
Healthy plants convert light to energy via photosynthesis, but chlorophyll also emits a fraction of absorbed light as a fluorescent glow that is invisible to the naked eye. The magnitude of the glow is an excellent indicator of the amount of photosynthesis, or gross productivity, of plants in a given region.
Research in 2013, led by Joanna Joiner of NASA's Goddard Space Flight Center in Greenbelt, Maryland, demonstrated that fluorescence from plants could be teased out from existing data from satellites that were designed and built for other purposes. The new research, led by Luis Guanter of the Freie Universität Berlin, used the data for the first time to estimate photosynthesis from agriculture. Results were published March 25 in the Proceedings of the National Academy of Sciences.
According to co-author Christian Frankenberg of NASA's Jet Propulsion Laboratory in Pasadena, Calif., "The paper shows that fluorescence is a much better proxy for agricultural productivity than anything we've had before. This can go a long way regarding monitoring -- and maybe even predicting -- regional crop yields."
Guanter, Joiner and Frankenberg launched their collaboration at a 2012 workshop, hosted by the Keck Institute for Space Studies at the California Institute of Technology (Caltech) in Pasadena, to explore measurements of photosynthesis from space. The team noticed that on an annual basis, the tropics are the most active in photosynthesis. But during the Northern Hemisphere's growing season, the U.S. Corn Belt "really stands out," Frankenberg said. "Areas all over the world are not as productive as this area."
The researchers set out to describe the phenomenon observed by carefully interpreting data from the Global Ozone Monitoring Experiment 2 (GOME-2) on Metop-A, a European meteorological satellite. Data showed that fluorescence from the U.S. Corn Belt peaks in July at levels 40 percent greater than those observed in the Amazon.
Comparison with ground-based measurements from carbon flux towers and yield statistics confirmed the results.
"The match between ground-based measurements and satellite measurements was a pleasant surprise," said Joiner, a co-author on the paper.
Ground-based measurements have a resolution of about 0.4 square mile (1 square kilometer), while the satellite measurements currently have a resolution of more than 1,158 square miles (3,000 square kilometers). The study confirms that even with coarse resolution, the satellite method could estimate the photosynthetic activity occurring inside plants at the molecular level for areas with relatively homogenous vegetation like the Corn Belt.
Challenges remain in estimating the productivity of fragmented agricultural areas not properly sampled by current space-borne instruments. That's where missions with better resolution could help, such as NASA's Orbiting Carbon Observatory-2 (OCO-2) -- a mission planned for launch in July 2014 that will also measure solar-induced fluorescence.
The research could also help scientists improve the computer models that simulate Earth's carbon cycle, as Guanter found a strong underestimation of crop photosynthesis in models. The analysis revealed that carbon cycle models -- which scientists use to understand how carbon cycles through the ocean, land and atmosphere over time -- underestimate the productivity of the U.S. Corn Belt by 40 to 60 percent.
Unlike most vegetation, food crops are managed to maximize productivity. They usually have access to abundant nutrients and are irrigated. The U.S. Corn Belt, for example, receives water from the Mississippi River. Accounting for the region's irrigation is currently a challenge for models, which is one reason why they underestimate agricultural productivity.
"If we don't take into account irrigation and other human influences in the agricultural areas, we're not going to correctly estimate the amount of carbon taken up by vegetation, particularly corn," Joiner said. "Corn plants are very productive in terms of assimilating carbon dioxide from the atmosphere. This needs to be accounted for going forward in trying to predict how much of the atmospheric carbon dioxide will be taken up by crops in a changing climate."
According to Frankenberg, the remote sensing-based techniques now available could be a powerful monitoring tool for food security, especially data from OCO-2 in combination with data from other upcoming satellites such as NASA's Soil Moisture Active Passive (SMAP), scheduled for launch later this year.
Image credit: NASA's Goddard Space Flight Center
Mexico's active Popocatepetl volcano, located about 40 miles southeast of Mexico City, has been spewing water vapor, gas, ashes and glowing rocks since its most recent eruption period began in April 2012. Civil defense authorities have set up evacuation shelters should they become necessary. In 2000, eruptions of the nearly 18,000-foot-high (5,400 meter) volcano led to the evacuation of tens of thousands of residents. More than 30 million people live within sight of the mountain.
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument on NASA's Terra spacecraft acquired these two images on April 23, 2012. The left image is in simulated natural color; the right image combines three thermal infrared channels, displaying the eruption plume in purple and pink, blowing toward the northeast. A small hotspot can be seen in the summit crater. The image covers an area of 19.2 by 27.6 miles (30 by 44.5 kilometers) and is located at 19 degrees north latitude, 98.6 degrees west longitude.
Photo credit: NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team
