Earth, by Juno

On October 9, NASA's Juno spacecraft flew past Earth, using our home planet's gravity to get the final boost it needed to reach Jupiter. The JunoCam instrument captured this monochrome view of Earth, and other instruments were tested to ensure they work as designed during a close planetary encounter.

The Juno spacecraft was launched from NASA's Kennedy Space Center in Florida on August 5, 2011. Juno's rocket, the Atlas 551, was only capable of giving Juno enough energy or speed to reach the asteroid belt, at which point the Sun's gravity pulled Juno back toward the inner solar system. The Earth flyby gravity assist put Juno on course for arrival at Jupiter on July 4, 2016.

Photo credit: NASA/JPL-Caltech/Malin Space Science Systems

Note: For more information, see PIA17516: Juno's Earth Flyby (Artist's Rendering).

Meteor Smoke and Noctilucent Clouds

Anyone who's ever seen a noctilucent cloud or “NLC” would agree: They look alien. The electric-blue ripples and pale tendrils of NLCs reaching across the night sky resemble something from another world.

Researchers say that's not far off. A key ingredient for the mysterious clouds comes from outer space.

"We've detected bits of 'meteor smoke' embedded in noctilucent clouds," reports James Russell of Hampton University, principal investigator of NASA's AIM mission to study the phenomenon. "This discovery supports the theory that meteor dust is the nucleating agent around which NLCs form."

Noctilucent clouds are a mystery dating back to the late 19th century. Northern sky watchers first noticed them in 1885 about two years after the eruption of Krakatoa. Ash from the Indonesian volcano caused such splendid sunsets that evening sky watching became a worldwide pastime. One observer in particular, a German named T.W. Backhouse who is often credited with the discovery of NLCs, noticed something odd. He stayed outside longer than most people, long enough for the twilight to fully darken, and on some nights he saw wispy filaments glowing electric blue against the black sky. Scientists of the day figured they were some manifestation of volcanic dust.

Eventually Krakatoa’s ash settled and the sunsets faded, but strangely the noctilucent clouds didn’t go away. They’re still present today, stronger than ever. Researchers aren’t sure what role Krakatoa’s ash played in those early sightings. One thing is clear, however: The dust behind the clouds we see now is space dust.

Mark Hervig of the company GATS, Inc, led the team that found the extraterrestrial connection.

"Using AIM's Solar Occultation for Ice Experiment (SOFIE), we found that about 3% of each ice crystal in a noctilucent cloud is meteoritic," says Hervig.

The inner solar system is littered with meteoroids of all shapes and sizes--from asteroid-sized chunks of rock to microscopic specks of dust. Every day Earth scoops up tons of the material, mostly the small stuff. When meteoroids hit our atmosphere and burn up, they leave behind a haze of tiny particles suspended 70 km to 100 km above Earth's surface.

It's no coincidence that NLCs form 83 km high, squarely inside the meteor smoke zone.

Specks of meteor smoke act as gathering points where water molecules can assemble themselves into ice crystals. The process is called "nucleation."

Nucleation happens all the time in the lower atmosphere. In ordinary clouds, airborne specks of dust and even living microbes can serve as nucleation sites. Tiny ice crystals, drops of water, and snowflakes grow around these particles, falling to Earth if and when they become heavy enough.

Nucleating agents are especially important in the ethereal realm of NLCs. The clouds form at the edge of space where the air pressure is little more than vacuum. The odds of two water molecules meeting is slim, and of sticking together slimmer still.

Meteor smoke helps beat the odds. According AIM data, ice crystals can grow around meteoritic dust to sizes ranging from 20 to 70 nanometers. For comparison, cirrus clouds in the lower atmosphere where water is abundant contain crystals 10 to 100 times larger.

The small size of the ice crystals explains the clouds' blue color. Small particles tend to scatter short wavelengths of light (blue) more strongly than long wavelengths (red). So when a beam of sunlight hits an NLC, blue is the color that gets scattered down to Earth.

Meteor smoke explains much about NLCs, but a key mystery remains: Why are the clouds brightening and spreading?

In the 19th century, NLCs were confined to high latitudes—places like Canada and Scandinavia. In recent times, however, they have been spotted as far south as Colorado, Utah and Nebraska. The reason, Russell believes, is climate change. One of the greenhouse gases that has become more abundant in Earth's atmosphere since the 19th century is methane. It comes from landfills, natural gas and petroleum systems, agricultural activities, and coal mining.

It turns out that methane boosts NLCs.

Russell explains: "When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor. This extra water vapor is then available to grow ice crystals for NLCs."

If this idea is correct, noctilucent clouds are a sort of "canary in a coal mine" for one of the most important greenhouse gases.

And that, says Russell, is a great reason to study them. "Noctilucent clouds might look alien, but they're telling us something very important about our own planet."

Photo credit: NASA; video credit: NASA

Earth

MSG-3 first image of Earth, acquired on 7 August 2012 by its Spinning Enhanced Visible and Infrared Imager (SEVIRI).

Photo credit: Eumetsat

Earth's Gravilty Field Map, Before and After GRACE

Before the launch of NASA's Gravity Recovery and Climate Experiment (GRACE) in 2002, scientists' knowledge of Earth's gravity field was based upon decades of data from geodetic satellites (left image). GRACE has allowed monthly gravity measurements that are more than 100 times more accurate than these previous models, permitting scientists to characterize how Earth's gravity field varies over time and space, as illustrated by the map on the right, produced after just 13 months of GRACE data.

Image credit: NASA/University of Texas Center for Space Research

Note: For more information, see At 10, GRACE Continues Defying, and Defining, Gravity.

Blue Marble 2012 - Africa, the Middle East and India

Responding to public demand, NASA scientists created a companion image to the wildly popular 'Blue Marble' released last week (January 25, 2012).

The new image is a composite of six separate orbits taken on January 23, 2012 by the Suomi National Polar-orbiting Partnership satellite. Both of these new 'Blue Marble' images are images taken by a new instrument flying aboard Suomi NPP, the Visible Infrared Imaging Radiometer Suite (VIIRS).

Compiled by NASA Goddard scientist Norman Kuring, this image has the perspective of a viewer looking down from 7,918 miles (about 12,742 kilometers) above the Earth's surface from a viewpoint of 10 degrees South by 45 degrees East. The four vertical lines of 'haze' visible in this image shows the reflection of sunlight off the ocean, or 'glint,' that VIIRS captured as it orbited the globe. Suomi NPP is the result of a partnership between NASA, NOAA and the Department of Defense.

Photo credit: NASA/NOAA

Note: For more information, see VIIRS Eastern Hemisphere Image - Behind the Scenes.

Blue Marble 2012

A 'Blue Marble' image of the Earth taken from the VIIRS instrument aboard NASA's most recently launched Earth-observing satellite - Suomi NPP. This composite image uses a number of swaths of the Earth's surface taken on January 4, 2012. The NPP satellite was renamed 'Suomi NPP' on January 24, 2012 to honor the late Verner E. Suomi of the University of Wisconsin.

Suomi NPP is NASA's next Earth-observing research satellite. It is the first of a new generation of satellites that will observe many facets of our changing Earth.

Suomi NPP is carrying five instruments on board. The biggest and most important instrument is The Visible/Infrared Imager Radiometer Suite or VIIRS.

Photo credit: NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring