Flooding and Drought in Etosha Pan

Outside of the Sahara, the driest country in Africa is Namibia. Although flooding is familiar in the country’s far eastern Caprivi Strip, most of Namibia is vulnerable to prolonged drought. On average, the country receives just 258 millimeters (10 inches) of rain per year, though annual rainfall amounts can vary considerably.
One place that gives an indication of Namibia’s precipitation is Etosha Pan. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured these images on July 30, 2009; July 31, 2010; July 31, 2011; and July 31, 2012. All of the images use a combination of infrared and visible light to increase the contrast between water and land. Water varies in color from electric blue to navy, with darker shades indicating deeper water. Vegetation, even sparse vegetation, appears green. Burnt orange indicates burn scars from fires.
Today, Etosha is a saltpan, but between 2 and 10 million years ago, it was a lake fed by the Kunene (Cunene) River. Changes in land surface features re-routed the Kunene, causing it to flow directly into the Atlantic Ocean. The water gradually evaporated, and the lake hasn’t reached anything like its old capacity since. Still, generous rains can sometimes leave Etosha Pan partially filled.
Etosha Pan has two basic seasons: wet and dry. The rainy season may begin as early as November and last as late as May. In the months that follow, the lake gradually dries up. Spring begins in September. Temperatures rise and water holes shrink.
The year-to-year contrast between these images is striking. Water sprawls over most of the pan in 2009 and 2011, but only minimal amounts appear in 2010 and 2012. Water levels generally drop through July, but how much water is left in and around the pan depends on how generous rains were in the preceding wet season. The years 2009 through 2012 showed a mix of unusually wet and unusually dry conditions, including flooding in 2011.
Even by Namibia’s normally arid standards, the Etosha region was parched in July 2012, and the Namibian government pledged aid to drought-stricken regions. According to the U.S. Foreign Agricultural Service, below-normal precipitation plagued this region from early April to late June. By mid-July 2012, most of the country had not seen rain for more than 25 days.

1.      References

2. Game-Reserve.com. Etosha National Park, Namibia.
3. Kunene correspondent. (2012, July 31) Kunene welcomes govt drought relief. AllAfrica.com.
4. United Nations Environment Programme. (2008). Africa: Atlas of Our Changing Environment. Division of Early Warning and Assessment, United Nations Environment Programme, Nairobi, Kenya.
5. Warren, A. Etosha: From Sand to Sea. PBS: The Living Edens.

NASA images courtesy LANCE MODIS Rapid Response Team at NASA GSFC. Caption by Michon Scott.

Instrument: 

Aqua - MOD

Probing the Electric Space Around Earth

They were the subject of perhaps the first scientific discovery of the Space Age, and yet we still don't know much about them. The radiation belts that surround Earth are home to killer electrons, plasma waves, and intense electrical currents that can disrupt and destroy the electronics on satellites. But the behavior of the Van Allen Belts—named for James Van Allen, who led the team that discovered them in 1958—is wildly unpredictable.
This artist's conception shows the radiation belts (green), which are two doughnut-shaped (torus) regions full of high-energy particles that fill the near-space around Earth. The blue and red lines between and around the belts depict the north and south polarity of the planet’s magnetic field. The inner belt, a blend of protons and electrons, can reach down as low as 1,000 kilometers (600 miles) in altitude. The outer belt, comprised mainly of energetic electrons, can swell to as much as 60,000 kilometers (37,000 miles) above Earth’s surface. Both rings extend to roughly 65 degrees north and south latitude.
The radiation belts were discovered during the flight of the very first American satellite. Van Allen and colleagues had installed a Geiger-Müller tube on Explorer 1 to detect cosmic rays, and as the satellite made its eccentric orbit around the Earth, the readings periodically went off the top of the counter’s scale. It happened again during the flight of Explorer 3 several months later. Several followup missions proved that the space around Earth was not empty, but instead enriched with electrons, protons, and energy created by interactions between Earth's magnetic field (or magnetosphere), the solar wind, and (occasionally) cosmic rays arriving from beyond the solar system.
Fifty-four years later, NASA has embarked on a missions designed specifically to understand the space weather in the dynamic and erratic Van Allen Belts. At 4:05 a.m. Eastern Daylight Time on August 30, 2012, the Radiation Belt Storm Probes (RBSP) were launched into orbit on a United Launch Alliance Atlas V rocket that lifted off from Cape Canaveral Air Force Station in Florida. (Watch video of the launch here.) The Johns Hopkins University Applied Physics Laboratory (APL) built and will operate the twin RBSP spacecraft for NASA’s Living With a Star program.
The identical twin spacecraft will fly in separate orbits across the inner and outer Van Allen radiation belts. The mission is starting near the height of the Sun’s 11-year cycle, or solar maximum. Activity on the sun influences the behavior of the radiation belts, though scientists are puzzled by that behavior. Sometimes a solar storm can swell the belts with particles and energy, creating havoc for Earth-orbiting satellites by accelerating electrons (aka, “killer electrons”) and creating electrical currents. Other times, the radiation belts grow very calm and depleted during Sun storms. Occasionally, no change is detected at all.
The RBSP satellites are designed to observe how and when killer electrons are energized, to sample the electrical and magnetic fields in Earth’s space, to count particles, and detect plasma waves of different frequencies. The ultimate goal is to improve the prediction of space weather; that is, how solar activity can cause geomagnetic storms that upset telecommunications and electronics.

1.      Further Reading

2. Carlowicz, M., and Lopez, R. (2002) Storms from the Sun: The Emerging Science of Space Weather. The Joseph Henry Press. Accessed August 30, 2012.
3. Johns Hopkins Applied Physics Laboratory (n.d.) Radiation Belt Storm Probes Accessed August 30, 2012.
4. NASA (2012, July 18) The Electric Atmosphere: Plasma Is Next NASA Science Target. Accessed August 30, 2012.
5. NASA (n.d.) RBSP News. Accessed August 30, 2012.
6. Science@NASA (2012) ScienceCasts: The Radiation Belt Storm Probes. Accessed August 30, 2012.

Image by T. Benesch and J. Carns for the NASA Science Mission Directorate. Caption by Mike Carlowicz.

Instrument: 

Model

Horton River Delta, Arctic Canada

A river flowing down a steep slope follows a pretty straight path, with gravity exerting a tremendous pull on the water. But a river flowing over a flat landscape can meander left and right, occasionally abandoning river channels to become oxbow lakes or to take a shorter route to the sea.
In the past few centuries, the Horton River in northwestern Canada stopped wandering and assumed a more direct route to the sea. Situated about 420 kilometers (260 miles) east of the Alaska border, the modern Horton River empties into Franklin Bay; but as this image shows, it once followed a different path. The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite acquired this image on July 31, 2012.
The Horton River Delta forms a fan-shaped interruption to the otherwise straight coastline. North of the delta, lakes fill abandoned meanders. Centuries ago, the Horton River continued another 100 kilometers (60 miles) or so northward, draining into Harrowby Bay. In a 1989 study, researchers from the University of British Columbia used multiple lines of evidence to estimate a time frame for when the Horton River abandoned its old channels and adopted a shortcut to the sea.
One line of evidence comes from historical records. Dr. John Richardson of the second Franklin expedition mapped the mouth of the Horton River sometime between 1825 and 1827, and his colleague E.N. Kendall drew a sketch of it. Obviously the river had taken its shortcut to the sea by then.
Next, the researchers turned to tree-ring dating of driftwood to identify the earliest possible date for the shortcut. Near the sea, the Horton River flows through treeless tundra; but about 200 kilometers (125 miles) upstream, spruce trees grow. When pieces of wood wash into the river, they can be carried far downstream, either washing into the ocean, or getting deposited somewhere along the riverbed. That means driftwood lying in the Horton River’s abandoned channels must have been carried there before the river changed course. Analysis showed that the most recent driftwood in the old meanders was deposited before 1640.
The third line of evidence for the delta’s age comes from studies of the rock and sediment layers, including observed rates of delta growth since the area was first mapped. The researchers concluded: “If rates of delta and fan development are assumed constant, then the breakthrough on geomorphic evidence would have occurred about 1750.”
Although the land around the Horton River is flat, it is not at sea level but instead sits about about 75 meters (250 feet) above sea level, and cliffs line the coastline. So once it stopped meandering and broke through a barrier, the Horton River took a steep trip downward to the Canadian coast. The river’s shortcut formed a delta that continued growing for decades.

A comparison between Richardson’s mapping efforts and aerial photographs acquired later indicates that the Horton River Delta expanded between the early 19th century and the mid-20th century. But subsequent aerial photos showed the delta shrinking later in the 20th century. A study published in 1998 concluded that the ocean eventually began to eat away at the delta faster than river sediments could build it.

A Voyager Far From Home

On September 5, 1977, NASA’s Voyager 1 spacecraft lifted off from Cape Canaveral, Florida, aboard a Titan-Centaur rocket. Thirty-five years later, the planetary probe is now an interstellar traveler, having traveled farther from Earth than any manmade object in history. As of 21:00 Universal Time on September 4, 2012, Voyager 1 was 18.21 billion kilometers (11.31 billion miles) from home, or 121 times the distance from the Earth to the Sun. Light takes 33 hours and 44 minutes to travel the distance from Voyager 1 to Earth.

The images above were taken at a time when Voyager 1 was much closer to home. The top image of a crescent-shaped Earth and Moon was captured on September 18, 1977, when Voyager was a mere 11.66 million kilometers (7.25 million miles) from Earth and directly above Mount Everest (on the night side of the planet at 25 degrees north latitude).

The lower image and inset—often referred to as “the Pale Blue Dot” image—was acquired on February 14, 1990, when the spacecraft was 6.4 billion kilometers (4 billion miles) from Earth and 32 degrees above the ecliptic plane. Earth is a mere point of light, just 0.12 pixels (picture elements) in size when viewed from that distance. The fuzzy light in the images is scattered sunlight because Earth was very close to the Sun (from the perspective of Voyager). The image was part of a series of 60 images collected to make the first-ever mosaic portrait of our solar system.

Both images were assembled from data from the Imaging Science Subsystem on Voyager 1, a modified version of the slow-scan vidicons used in the Mariner spacecraft and similar to early television cameras. The wide-angle camera had a field of view comparable to a 200 millimeter lens with an aperture of f/3, while the narrow-angle camera had the field of view of a 1500 mm, f/8.5 lens.

Having long since passed its primary targets of Jupiter and Saturn, Voyager 1 has been cruising for decades toward the edge of the solar system. In fact, researchers have analyzed data from the probe’s particle detectors, cosmic ray detectors, and magnetometer and found evidence that they have passed the termination shock and into the heliosheath—the outer edge of influence for solar wind plasma and energy from our Sun. The probe is now in an area similar to the windless “doldrums” found in tropical seas on Earth. The solar wind has calmed, the magnetic field has piled up due to pressure from outside the solar system, and high-energy particles appear to be leaking out into interstellar space. The Voyager science team expects the spacecraft itself to pass out into that space sometime in the next year or so.

“Voyager tells us now that we're in a stagnation region in the outermost layer of the bubble around our solar system,” said Ed Stone, Voyager project scientist at the California Institute of Technology, at a December 2011 press conference. “Voyager is showing that what is outside is pushing back. We shouldn't have long to wait to find out what the space between stars is really like.”

For more information about the spacecraft, visit the Voyager web site at the Jet Propulsion Laboratory.