NASA Messenger extends its mission life using Helium

Now orbiting the planet Mercury after over ten years in space, NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is still functioning better than expected.

Its mission will soon come to an end though, it's running out of fuel and is scheduled to crash into the planet in March.

However, mission control have come up with a novel plan that will use the helium used to pressurize the unmanned probe's engine to give it another month of life.

According to the MESSENGER team, fuel is usually the last problem that a robotic exploration team worry about because there are so many other things that can go wrong long before it runs out.

That being said, the fuel is the single most important consumable aboard an orbiter mission because it not only allows the spacecraft to maintain the correct attitude and keep its antennae pointed at Earth, it also lets it use the main engine to boost its orbit against atmospheric drag, which decays the orbit.

The upshot is that when the propellant runs out, the spacecraft starts to tumble and spirals in to burn up in the atmosphere or, in MESSENGER's case, crash into the surface at hypersonic speed.



So normally when the fuel runs out, that's it, but NASA reasoned that MESSENGER's liquid-fuel rocket engine design meant there was still a bit of thrust left even after all the propellant was expended.

The MESSENGER's engine is pressure fed, which means that it uses helium from a separate tank to push the fuel and oxidizer into the engine's combustion chamber.

Since the helium needs to work against the force of the rocket's combustion, it's under considerable pressure, and when the fuel is gone, there will be some helium left in the pressure tank.

The idea is to use the helium as a cold propellant. In other word's where the rocket engine gets its thrust by burning fuel, the helium pushes the spacecraft by simple gas pressure like a toy balloon when the open neck is let go.

Unfortunately, this is the first time a pressurant has been as an improvised thruster and MESSENGER's engine is a bit more complicated than a balloon.

According to MESSENGER Mission Systems Engineer Dan O’Shaughnessy, of the Johns Hopkins University Applied Physics Laboratory (APL), the pressure in the helium tank isn't much compared to a firing engine.

In addition, the gas passes through a number of reduction valves and nozzles that have to be taken into account, and helium is the second lightest of gases, so it doesn't provide much in the way of thrust.

If these problems can be overcome, NASA estimates that it will give MESSENGER another month of active life before impact on Mercury. MESSENGER's current closest approach to Mercury is 25 km (15 mi).

If a scheduled course correction using the helium is successful, this will rise to 80 km (50 mi). This will allow the orbiter to carry out additional low altitude observations, including collecting a new set of high-resolution images.

"During the additional period of operations, up to four weeks, MESSENGER will measure variations in Mercury’s internal magnetic field at shorter horizontal scales than ever before, scales comparable to the anticipated periapsis altitude between 7 km (4 mi) and 15 km (9 mi) above the planetary surface," says APL’s Haje Korth, the instrument scientist for the spacecraft's magnetometer.

"Combining these observations with those obtained earlier in the mission at slightly higher altitudes will allow the depths of the sources of these variations to be determined. In addition, observations by MESSENGER’s Neutron Spectrometer at the lowest altitudes of the mission will allow water ice deposits to be spatially resolved within individual impact craters at high northern latitudes."

Built and operated by John Hopkins University for NASA, the MESSENGER spacecraft was launched from Cape Canaveral on August 3, 2004 as the first mission aimed to place an orbiter around the innermost planet Mercury. 

ESA/NASA SOHO: Mercury, Antares, M4 cluster and dwarf planet 1 Ceres

Mercury, Antares, M4 cluster and dwarf planet 1 Ceres in ESA/NASA SOHO/LASCO C3 field of view.

Credit: ESA, Nasa 

NASA MESSENGER: The Van Eyck crater on Mercury

Low sun elevation creates long shadows in and around Van Eyck crater on Mercury. 

This image obtained by MESSENGER spacecraft shows how material ejected from the Caloris basin has cratered the rim of Van Eyck. 

The smooth plains making up the floor of Van Eyck also show the scars of cratering, ranging from large primary impacts to deep, linear incisions by secondary crater chains. 

Jan Van Eyck, a Flemish painter, lived from about 1390 to 1441 CE.

Credit: NASA

NASA MESSENGER Captures Images of Ice on Mercury

Nasa's MESSENGER spacecraft

NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has provided the first optical images of ice and other frozen volatile materials within permanently shadowed craters near Mercury's north pole.

The images not only reveal the morphology of the frozen volatiles, but they also provide insight into when the ices were trapped and how they've evolved, according to an article published in the journal, Geology.

Two decades ago, Earth-based radar images of Mercury revealed the polar deposits, postulated to consist of water ice.

Prokofiev, named in August 2012 for the Russian composer, is the largest crater in Mercury’s north polar region to host radar-bright material.

Credit: NASA /Johns Hopkins University Applied Physics Lab /Carnegie Iinstitution of Washington

That hypothesis was later confirmed by MESSENGER through a combination of neutron spectrometry, thermal modeling, and infrared reflectometry.

"But along with confirming the earlier idea, there is a lot new to be learned by seeing the deposits," said lead author Nancy Chabot, the Instrument Scientist for MESSENGER's Mercury Dual Imaging System (MDIS) and a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

Beginning with MESSENGER's first extended mission in 2012, scientists launched an imaging campaign with the broadband clear filter of MDIS's wide-angle camera (WAC).

Mercury Dual Imaging System (MDIS)

Although the polar deposits are in permanent shadow, through many refinements in the imaging, the WAC was able to obtain images of the surfaces of the deposits by leveraging very low levels of light scattered from illuminated crater walls. "It worked in spectacular fashion," said Chabot.

The team zeroed in on Prokofiev, the largest crater in Mercury's north polar region found to host radar-bright material.

"Those images show extensive regions with distinctive reflectance properties," Chabot said.

"A location interpreted as hosting widespread surface water ice exhibits a cratered texture indicating that the ice was emplaced more recently than any of the underlying craters."

In other areas, water ice is present, she said, "but it is covered by a thin layer of dark material inferred to consist of frozen organic-rich compounds." In the images of those areas, the dark deposits display sharp boundaries.

"This result was a little surprising, because sharp boundaries indicate that the volatile deposits at Mercury's poles are geologically young, relative to the time scale for lateral mixing by impacts," said Chabot.

"One of the big questions we've been grappling with is 'When did Mercury's water ice deposits show up?' Are they billions of years old, or were they emplaced only recently?" Chabot said.

"Understanding the age of these deposits has implications for understanding the delivery of water to all the terrestrial planets, including Earth."

Overall, the images indicate that Mercury's polar deposits either were delivered to the planet recently or are regularly restored at the surface through an ongoing process.

The images also reveal a noteworthy distinction between the Moon and Mercury, one that may shed additional light on the age of the frozen deposits.

"The polar regions of Mercury show extensive areas that host water ice, but the Moon's polar regions, which also have areas of permanent shadows and are actually colder, look different," Chabot said.

"One explanation for differences between the Moon and Mercury could be that the volatile polar deposits on Mercury were recently emplaced," according to the paper.

"If Mercury's currently substantial polar volatile inventory is the product of the most recent portion of a longer process, then a considerable mass of volatiles may have been delivered to the inner Solar System throughout its history."

"That's a key question," Chabot said. "Because if you can understand why one body looks one way and another looks different, you gain insight into the process that's behind it, which in turn is tied to the age and distribution of water ice in the Solar System. This will be a very interesting line of inquiry going forward."

NASA MESSENGER: Spacecraft finds Water Ice on Mercury

Kandinsky crater lies near Mercury's north pole, and may have hosted water ice. MESSENGER spacecraft's Wide Angle Camera broadband image appears at left, outlined in yellow, and superimposed on an MDIS polar mosaic.

The view on the right shows the same image but with the brightness and contrast adjusted to show details of the crater's shadowed floor. Image released Oct. 15, 2014.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The first-ever photos of water ice near Mercury's north pole have come down to Earth, and they have quite a story to tell.

The images, taken by NASA's MESSENGER spacecraft (short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging), suggest that the ice lurking within Mercury's polar craters was delivered recently, and may even be topped up by processes that continue today, researchers said.

More than 20 years ago, Earth-based radar imaging first spotted signs of water ice near Mercury's north and south poles, a surprise, perhaps, given that temperatures on the solar system's innermost planet can top 800 degrees Fahrenheit (427 degrees Celsius).

The left image shows a view of Berlioz crater, with the areas that contain radar-bright material marked in yellow and persistent shadows marked in red. 

The middle image, acquired a few hours later, shows details within the shadowed crater. 

A distinctively darker region sits on the crater's floor, which corresponds well with the radar-bright and shadowed regions as shown in the right image. Image released Oct. 15, 2014.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

In late 2012, MESSENGER confirmed those observations from orbit around Mercury, discovering ice in permanently shadowed craters near the planet's north pole.

MESSENGER scientists announced the find after integrating results from thermal modeling studies with data gathered by the probe's hydrogen-hunting neutron spectrometer and its laser altimeter, which measured the reflectance of the deposits.

And now the MESSENGER team has captured optical-light images of the ice for the first time, by taking advantage of small amounts of sunlight scattered off the craters' walls.

"There is a lot new to be learned by seeing the deposits," said study lead author Nancy Chabot, instrument scientist for MESSENGER’s Mercury Dual Imaging System and a researcher at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, in a statement.

For example, the texture of the ice at the bottom of Mercury's 70-mile-wide (113 kilometers) Prokofiev Crater suggests that the material was put in place relatively recently rather than billions of years ago, researchers said.

Images of other craters back up this notion. They show dark deposits, believed to be frozen organic-rich material, covering ice in some areas, with sharp boundaries between the two different types of material.

"This result was a little surprising, because sharp boundaries indicate that the volatile deposits at Mercury’s poles are geologically young, relative to the time scale for lateral mixing by impacts," Chabot said.

Earth's moon also harbors water ice inside permanently shadowed polar craters, but its deposits look different from those on Mercury, researchers said. This could be because Mercury's ice was delivered more recently.

"If you can understand why one body looks one way and another looks different, you gain insight into the process that's behind it, which in turn is tied to the age and distribution of water ice in the solar system," Chabot said. "This will be a very interesting line of inquiry going forward."

The new study was published online today (Oct. 15) in the journal Geology.

NASA Messenger: Planets with oddball orbits like Mercury could host life

On Mercury a solar day is about 176 Earth days long. 

During its first Mercury solar day in orbit the MESSENGER spacecraft imaged nearly the entire surface of Mercury to generate a global monochrome map at 250 meters per pixel resolution and a 1 kilometer per pixel resolution color map. 

Credit: NASA/JHU APL/CIW

Mercury has an oddball orbit, it takes longer for it to rotate on its axis and complete a day than it takes to orbit the sun and complete a year.

Now, researchers suggest photosynthesis could take place on an alien planet with a similarly bizarre orbit, potentially helping support complex life.

However, the scientists noted that the threat of prolonged periods of darkness and cold on these planets would present significant challenges to life, and could even potentially freeze their atmospheres.

They detailed their findings in the International Journal of Astrobiology.

Astronomers have discovered more than 1,700 alien planets in the past two decades, raising the hope that at least some might be home to extraterrestrial life.

Scientists mostly focus the search for alien life on exoplanets in the habitable zones of stars.

These are regions where worlds would be warm enough to have liquid water on their surfaces, a potential boon to life.

Although many exoplanets are potentially habitable, they may differ from Earth significantly in one or more ways.

For instance, habitable planets around dim red dwarf stars orbit much closer than Earth does to the Sun, sometimes even closer than Mercury's distance.

Red dwarfs are of interest as possible habitats for life because they are the most common stars in the universe, if life can exist around red dwarfs, then life might be very common across the cosmos.

Recent findings from NASA's Kepler Space Observatory suggest that at least half of all red dwarfs host rocky planets that are one-half to four times the mass of Earth.

Since a planet in the habitable zone of a red dwarf orbits very near its star, it experiences much stronger gravitational tidal forces than Earth does from the Sun, which slows the rate at which those worlds spin.

The most likely result of this slowdown is that the planet enters what is technically called a 1:1 spin orbit resonance, completing one rotation on its axis every time it completes one orbit around its star.

This rate of rotation means that one side of that planet will always face toward its star, while the other side will permanently face away, just as the Moon always shows the same side to Earth.

One recent study suggests that such "tidally locked" planets may develop strange lobster-shaped oceans basking in the warmth of their stars on their daysides, while the nightsides of such worlds are mostly covered in an icy shell.

More information: "Photosynthetic potential of planets in 3 : 2 spin–orbit resonances." S.P. Brown, et al. International Journal of Astrobiology DOI: dx.doi.org/10.1017/S1473550414000068

NASA MESSENGER: Seuss, a Complex Crater on Mercury

MESSENGER spacecraft in orbit around Mercury spotted Seuss, a complex crater with hollows visible on the crater's floor, at the top left of this image. 

The hollows appear brightly as blue-white features in this image which has had color enhanced.

Just outside of the crater rim, some material appears darker brown, which consists of Low Reflectance Material (LRM).

The impactor that created Seuss probably excavated this material.

Bright crater rays extending from Seuss consist of streams of ejecta thrown from the crater upon impact.

In the bottom right of the image lies an irregular, orange-yellow depression that may be a volcanic vent.

NASA Messenger: Mercury Mission - 10 Years in Space

In celebration of the 10th anniversary of its launch, the MESSENGER team released this movie showing a flyover of Mercury. The movie is sped up by a factor of seven for ease of viewing.

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory

Ten years ago, on August 3, 2004, NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft blasted off from Cape Canaveral, Florida, for a risky mission that would take the small satellite dangerously close to Mercury’s surface, paving the way for an ambitious study of the planet closest to the Sun.

The spacecraft traveled 4.9 billion miles (7.9 billion kilometers), a journey that included 15 trips around the Sun and flybys of Earth once, Venus twice, and Mercury three times, before it was inserted into orbit around its target planet in 2011.

“We have operated successfully in orbit for more than three Earth years and more than 14 Mercury years as we celebrate this amazing 10th anniversary milestone,” said MESSENGER Mission Operations Manager Andy Calloway, of the Johns Hopkins University Applied Physics Laboratory (APL).

“The MESSENGER spacecraft operates in one of the most challenging and demanding space environments in our Solar System, and we have met that challenge directly through innovation and hard work, as exemplified by the stunning discoveries and data return achievements.

Our only regret is that we have insufficient propellant to operate another 10 years, but we look forward to the incredible science returns planned for the final eight months of the mission.”

MESSENGER captured the images in the flyover movie during this flight path over Mercury's north polar region.

Image Credit: NASA

MESSENGER is only the second spacecraft sent to Mercury. Mariner 10 flew past it three times in 1974 and 1975 and gathered detailed data on less than half the surface.

MESSENGER took advantage of an ingenious trajectory design, lightweight materials, and miniaturisation of electronics, all developed in the three decades since Mariner 10 flew past Mercury.

“It was quite challenging to design and execute a trajectory that could culminate in Mercury orbit,” said Mission and Spacecraft Systems Engineer Dan O’Shaughnessy, of APL.

“Designing an attendant spacecraft that was light enough to carry the necessary propellant to execute such a trajectory with enough room left over for a payload capable of global characterisation of the planet is an impressive accomplishment.”

Additionally, he said, “the team’s concept of operations that streamlines planning while optimizing the use of our payload. despite substantial thermal and power constraints, is an amazing feat.”

MESSENGER Deputy Principal Investigator Larry Nittler, of the Carnegie Institution of Washington, said that the mission has rewritten scientists’ understanding of the planet “and given us plenty of surprises.”

“Geochemical measurements have revealed a surface poor in iron, but rich in moderately volatile elements such as sulphur and sodium,” said Nittler.

“These results rule out some long-standing theories put forward to explain Mercury’s anomalously high density compared with the other planets in the inner solar system,” he explained.

“Maps of elemental abundances show that the interior is highly chemically heterogeneous, providing important clues to the early geological history of the planet.”

“MESSENGER observations have also shown that Mercury’s surface was shaped by volcanic activity, identified unique landforms shaped by loss of volatile materials, and confirmed the presence of large amounts of water ice protected from the Sun’s heat within permanently shadowed impact craters near the planet’s poles, said Nittler

Infographic with statistics on the MESSENGER mission.

Image Credit: NASA

“We have found that the complex interplay of the interplanetary magnetic field with that of Mercury results in a remarkably dynamic electromagnetic environment surrounding the planet, including unexplained bursts of electrons and highly variable distributions of different elements in the thin exosphere,” Nittler added.

“Over the next few months, MESSENGER will observe Mercury at lower altitudes and thus smaller spatial scales than ever before, and this is sure to result both in exciting scientific discoveries and new puzzles about our solar system’s enigmatic innermost planet.”

In celebration of the 10th anniversary of its launch, the MESSENGER team has released a movie acquired during an early stage of MESSENGER’s low-altitude campaign.

Messenger narrow-angle camera (NAC)

The movie provides a bird’s-eye view of what the spacecraft sees as it flies over the planet at close range and was assembled from 214 images taken by the narrow-angle camera (NAC) on June 8, 2014.

The NAC’s field of view looked toward the horizon along the direction of MESSENGER's motion as the probe crossed the terminator into night.

Scott Murchie

“This view is what a traveller on the MESSENGER spacecraft might see during low-altitude operations in the coming year,” noted MESSENGER Co-Investigator Scott Murchie of APL.

“During the final phase of its mission, MESSENGER's science instruments will use low-altitude operations like this to explore the surface and subsurface of Mercury at unprecedented resolution.”

The image frames were taken once per second while MESSENGER was at altitudes ranging from 115 to 165 kilometers, traveling at a speed of 3.7 kilometers per second relative to the surface. The movie is sped up by a factor of six for ease of viewing.

Read the full article here

NASA's Messenger: Mercury's magnetic field reveals its interior is different from Earth's

Earth and Mercury are both rocky planets with iron cores, but Mercury's interior differs from Earth's in a way that explains why the planet has such a bizarre magnetic field, UCLA planetary physicists and colleagues report.

Measurements from NASA's Messenger spacecraft have revealed that Mercury's magnetic field is approximately three times stronger at its northern hemisphere than its southern one.

In the current research, scientists led by Hao Cao, a UCLA postdoctoral scholar working in the laboratory of Christopher T. Russell, created a model to show how the dynamics of Mercury's core contribute to this unusual phenomenon.

The magnetic fields that surround and shield many planets from the sun's energy-charged particles differ widely in strength.

While Earth's is powerful, Jupiter's is more than 12 times stronger, and Mercury has a rather weak magnetic field.

Venus likely has none at all. The magnetic fields of Earth, Jupiter and Saturn show very little difference between the planets' two hemispheres.

Within Earth's core, iron turns from a liquid to a solid at the inner boundary of the planet's liquid outer core; this results in a solid inner part and liquid outer part.

The solid inner core is growing, and this growth provides the energy that generates Earth's magnetic field. Many assumed, incorrectly, that Mercury would be similar.

"Hao's breakthrough is in understanding how Mercury is different from the Earth so we could understand Mercury's strongly hemispherical magnetic field," said Russell, a co-author of the research and a professor in the UCLA College's department of Earth, planetary and space sciences.

"We had figured out how the Earth works, and Mercury is another terrestrial, rocky planet with an iron core, so we thought it would work the same way but it's not working the same way."

Mercury's peculiar magnetic field provides evidence that iron turns from a liquid to a solid at the core's outer boundary, say the scientists, whose research currently appears online in the journal Geophysical Research Letters and will be published in an upcoming print edition.

"It's like a snow storm in which the snow formed at the top of the cloud and middle of the cloud and the bottom of the cloud too," said Russell.

"Our study of Mercury's magnetic field indicates iron is snowing throughout this fluid that is powering Mercury's magnetic field."

The research implies that planets have multiple ways of generating a magnetic field.

Hao and his colleagues conducted mathematical modeling of the processes that generate Mercury's magnetic field.

In creating the model, Hao considered many factors, including how fast Mercury rotates and the chemistry and complex motion of fluid inside the planet.

The cores of both Mercury and Earth contain light elements such as sulfur, in addition to iron; the presence of these light elements keeps the cores from being completely solid and "powers the active magnetic field–generation processes," Hao said.

Hao's model is consistent with data from Messenger and other research on Mercury and explains Mercury's asymmetric magnetic field in its hemispheres.

He said the first important step was to "abandon assumptions" that other scientists make.

"Planets are different from one another," said Hao, whose research is funded by a NASA fellowship. "They all have their individual character."

More Information: 'A dynamo explanation for Mercury's anomalous magnetic field.' Authors: Hao Cao, Christopher Russell, et al. - Article first published online: 19 JUN 2014 DOI: 10.1002/2014GL060196

Messenger Mercury: Space Weather indicates Hot Flow Anomaly

The yellow colour shows the standing bow shock in front of Mercury. 

The signature of material flowing in a vastly different direction than the solar wind, an Hot Flow Anomaly (HFA), can be seen in red at the lower left. 

Image courtesy NASA/Duberstein. 

The solar wind of particles streaming off the sun helps drive flows and swirls in space as complicated as any terrestrial weather pattern.

Scientists have now spotted at planet Mercury, for the first time, a classic space weather event called a hot flow anomaly (HFA), which has previously been spotted at Earth, Venus, Saturn and Mars.

"Planets have a bow shock the same way a supersonic jet does," said Vadim Uritsky at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"These hot flow anomalies are made of very hot solar wind deflected off the bow shock."

The results were published in the Journal of Geophysical Research: Space Physics on Jan. 15, 2014.

To identify the presence of HFAs at Mercury, the team used observations from NASA's Messenger (short for Mercury Surface, Space Environment, Geochemistry, and Ranging) to detect the presence of two HFA signatures.

The first measurement was of magnetic fields that can be used to detect giant electric current sheets that lead to HFAs.

The second was of the heating of the charged particles. The scientists then analyzed this information to quantify what kind of turbulence exists in the region, which provided the final smoking gun of an HFA.

Not only is this the first sighting of HFAs at Mercury, but the observations help round out a picture of this type of space weather in general.

HFAs come in a variety of scale sizes - from around 600 miles across at Venus to closer to 60,000 miles across at Saturn.

This study suggests that the most important factor for determining HFA size is the geometry and size of the planet's bow shock.

NASA Messenger: Mercury contracted more than previous estimates

The planet Mercury, with only a small portion of its surface illuminated. 

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Carnegie Institution of Washington

New evidence gathered by NASA's MESSENGER spacecraft at Mercury indicates the planet closest to the sun has shrunk up to 7 kilometers in radius over the past 4 billion years, much more than earlier estimates.

The new finding, published in the journal Nature Geoscience Sunday, March 16, solves an apparent enigma about Mercury's evolution.

Older images of surface features indicated that, despite cooling over its lifetime, the rocky planet had barely shrunk at all but modeling of the planet's formation and aging could not explain that finding.

Paul K. Byrne

Now, Paul K. Byrne and Christian Klimczak at the Carnegie Institution of Washington have led a team that used MESSENGER's detailed images and topographic data to build a comprehensive map of tectonic features.

That map suggests Mercury shrunk substantially as it cooled, as rock and metal that comprise its interior are expected to.

Steven A. Hauck

"With MESSENGER, we have now obtained images of the entire planet at high resolution and, crucially, at different angles to the sun that show features Mariner 10 could not in the 1970s," said Steven A. Hauck, II, a professor of planetary sciences at Case Western Reserve University and the paper's co-author.

Mariner 10, the first spacecraft sent to explore Mercury, gathered images and data over just 45% of the surface during three flybys in 1974 and 1975.

MESSENGER, which launched in 2004 and was inserted into orbit in 2011, continues collecting scientific data, completing its 2,900th orbit of Mercury later this month.

Mercury contracted more than prior estimates, evidence shows

Mercury's surface is replete with cliff-like fault scarps. 

Here, one such scarp, Fram Rupes (image centre), lies near the terminator (the divide between day and night). 

The Fram Rupes scarp is almost 1.5 km high. 

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Carnegie Institution of Washington

Mercury's surface differs from Earth's in that its outer shell, called the lithosphere, is made up of one tectonic plate instead of multiple plates.

To help gauge how the planet may have shrunk, the researchers looked at tectonic features, called lobate scarps and wrinkle ridges, which result from interior cooling and surface compression.

The features resemble long ribbons from above, ranging from 5 to more than 550 miles long.

Lobate scarps are cliffs caused by thrust faults that have broken the surface and reach up to nearly 2 miles high.

Wrinkle ridges are caused by faults that don't extend as deep and tend to have lower relief. Surface materials from one side of the fault ramp up and fold over, forming a ridge.

The scientists mapped a total of 5,934 of the tectonic features.

The scarps and ridges have much the same effect as a tailor making a series of tucks to take in the waist of a pair of pants.

With the new data, the researchers were able to see a greater number of these faults and estimate the shortening across broad sections of the surface and thus estimate the decrease in the planet's radius.

Mercury's contraction much greater than thought

This image shows a long collection of ridges and scarps on the planet Mercury called a fold-and-thrust belt. 

The belt stretches over 336 miles (540 kilometers). 

The colours correspond to elevation -- yellow-green is high and blue is low. 

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Carnegie Institution of Washington

More information: Study paper: 'Mercury’s global contraction much greater than earlier estimates dx.doi.org/10.1038/ngeo2097

NASA Messenger MDIS Image: Mercury's Caloris basin in sharp relief - Video

Mercury’s uneven surface comes into sharp relief when the sun sits low on the planet’s eastern horizon. 

The relatively smooth floor of the Caloris basin lies on the right, with the rim and exterior of the basin to the left. 

The knobby texture outside of the basin may have arisen due to blocks of material ejected by the basin-forming impact. 

MESSENGER spacecraft acquired this image as part of the Mercury Dual Imaging System (MDIS) high-incidence-angle base map. 

High incidence angles, obtained when the sun sits near the horizon, create long shadows that accentuate the small-scale topography of geologic features, as seen here.

This animation shows 89 wide-angle camera (WAC) images of Mercury’s south polar region acquired by the Mercury Dual Imaging System (MDIS) over one complete Mercury solar day (176 Earth days).

This dataset enabled the illumination conditions at Mercury’s south polar region to be quantified, producing the map seen at the end of the movie and provided as a separate image.

The map is coloured on the basis of the percentage of time that a given area is sunlit; areas appearing black in the map are regions of permanent shadow.

The movie and illumination map are shown in polar stereographic projection, extending northward to 73° S, and 0° longitude is at the top. The large crater near Mercury’s south pole, Chao Meng-Fu, has a diameter of 180 km.

NASA MESSENGER: False-Colour Image of Earth Highlights Plant Growth

On Aug. 3, 2004, NASA’s Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft began a seven-year journey, spiraling through the inner solar system to Mercury. 

One year after launch, the spacecraft zipped around Earth, getting an orbit correction from Earth’s gravity and getting a chance to test its instruments by observing its home planet.

Credit: NASA /Johns Hopkins University Applied Physics Laboratory /Carnegie Institution of Washington

This image is a view of South America and portions of North America and Africa from the Mercury Dual Imaging System’s (MDIS) wide-angle camera aboard MESSENGER.

MDIS, records light at eleven different wavelengths, including visible and infrared light. Combining blue, red, and green light results in a true-colour image from the observations.

The image substitutes infrared light for blue light in the three-band combination.

The resulting image is crisper than the natural colour version because our atmosphere scatters blue light.

Infrared light, however, passes through the atmosphere with relatively little scattering and allows a clearer view. That wavelength substitution makes plants appear red. Why?

Plants reflect near-infrared light more strongly than either red or green, and in this band combination, near-infrared is assigned to look red.

Apart from getting a clearer image, the substitution reveals more information than natural colour.

Healthy plants reflect more near-infrared light than stressed plants, so bright red indicates dense, growing foliage.

For this reason, biologists and ecologists occasionally use infrared cameras to photograph forests.

NASA MESSENGER Surpasses 200,000 Orbital Images of Mercury

This image was acquired as part of the MDIS low-altitude imaging campaign. 

During MESSENGER's second extended mission, the spacecraft makes a progressively closer approach to Mercury's surface than at any previous point in the mission, enabling the acquisition of high-spatial-resolution data. 

For spacecraft altitudes below 350 kilometers, NAC images are acquired with pixel scales ranging from 20 meters to as little as 2 meters. 

Credit: NASA

MESSENGER has now returned more than 200,000 images acquired from orbit about Mercury.

The 1996 proposal for the mission promised a return of at least 1,000 images says Robert Gold, MESSENGER's Science Payload Manager.

Robert Gold

"We expected then that we would have some data compression that would probably raise the image total to somewhere near 2,000 images," says Gold, of the Johns Hopkins University Applied Physics Laboratory (APL), but scientists did not imagine then the degree to which MESSENGER would surpass that goal.

"Returning over 200,000 images from orbit about Mercury is an impressive accomplishment for the mission, and one I've been personally counting down for the last few months," says APL's Nancy Chabot, the Instrument Scientist for the Mercury Dual Imaging System (MDIS).

"However, I'm really more excited about the many thousands of images that are still in MESSENGER's future, especially those that we plan to acquire at low altitudes and will provide the highest resolution views yet of Mercury's surface."

During MESSENGER's second extended mission, the spacecraft is making a progressively closer approach to Mercury's surface with each successive orbit.

In about two months, each closest approach will be at a lower altitude than at any previous point in the mission, enabling the acquisition of unprecedentedly high-spatial-resolution data.

For spacecraft altitudes below 350 kilometers, Narrow Angle Camera (NAC) images will be acquired with pixel scales ranging from 20 meters to as little as 2 meters.

Nancy Chabot

To commemorate the milestone, image scientists released this four-image mosaic—one of the first from the MDIS low-altitude imaging campaign—that reveals, among other features, hollows that appear to have formed in one layer in the wall of this 15-kilometer-diameter crater.

The mission marks three additional milestones today: the spacecraft concludes its 12th Mercury year in orbit, its 18th Mercury sidereal day in orbit, and its 6th Mercury solar day in orbit.

"We have come an incredible way since the first mission proposal was submitted to NASA just over 17 years ago," notes MESSENGER Project Scientist Ralph McNutt of APL.

"Getting to launch and then to Mercury, flyby by flyby, and into orbital operations were incredible accomplishments—against all sorts of odds—and yet we are now, almost routinely, noting these statistics about the mission that has literally revealed an entirely new world to humanity."

When MESSENGER was launched in August 2004, he continues, "none of the team in their wildest imagination could have foreseen the successes that we now celebrate with new data coming back week by week from the innermost planet. And we are not done.

With a little more than a year left to go, before gravity brings the end to operations, we will view the planet and its environment from altitudes lower than were ever envisioned only a few short years ago—and, as with any planetary mission providing closer and closer looks at a planetary neighbour, all we can guess is that we have not wrung all of Mercury's surprises and discoveries just yet."

NASA Messenger: Volcanoes on Mercury

NASA Messenger: Lava-flooded craters and large expanses of smooth volcanic plains on Mercury’s surface. 

Credit: NASA

Mercury has long been a mystery to scientists.

Until recently, knowledge of the planet was limited to the grey, patchy landscape revealed by the Mariner 10 probe, NASA's first mission to Mercury in the mid-1970s.

Mariner 10's photographs showed little detail of how the surface was formed. Like Venus, Earth and Mars, it was clear that Mercury's rough crust reflected millions of years of aerial bombardment by comets and meteorites.

Gaps in our understanding of the innermost planet have included some basic knowledge such as the planet's geology, how it was formed and evolved, and whether its interior was still active.

But now NASA's return mission, MESSENGER, is allowing scientists to confront the full complexity of Mercury's surface.

Amid the countless craters caused by meteor collisions, the landscape has marks that were not made by such collisions.

Using the increased resolution of MESSENGER's cameras, scientists have identified previously hidden volcanic activity, which changes what we know about the planet's formation, and even the history of our solar system.

The detailed pictures showed that Mercury seemed to have smooth, rimless depressions that were obviously not produced by meteor impacts.

They were surrounded by bright, reddish material, believed to have been left by pyroclastic flows – indicating that the depressions were volcanic vents.

The presence of pyroclastic material – which is composed of volcanic ash – showed that the eruptions had been explosive.

In some cases, the debris had been ejected more than 50km from the volcanic vents themselves. This is a remarkable distance, as it means that Mercury's volcanoes must have been much more powerful than previously thought.

Mariner 10

The force of an eruption is determined by volatile gases beneath the planet's surface.

Initially dissolved in magma, as they reach the surface, these gases rapidly swell and shred the magma into tiny shards called pyroclasts.

This means that, in general, the more volatiles there are in the magma feeding an eruption, the more explosive it will be.

To shoot debris so far, the magma in Mercury's crust would need to have been brimming with volatile gases.

The latest signs of volcanic deposits from MESSENGER suggest that nearly 1.5% of the parent magma may have been occupied by volatiles.

For gases this is a large fraction, because as they rise towards the surface their volume increases dramatically.

NASA Messenger: Mercury’s surface showing a high-reflectance area

MESSENGER spacecraft obtained this image of Mercury’s surface showing a high-reflectance area seemingly confined to a region of lower elevation bounded by linear scarp (cliff) segments. 

Such diffuse bright areas sometimes relate to the deposition of small secondary craters and ray segments by a relatively recent impact crater. 

However, regional images show no rayed craters in the immediate vicinity (except Han Kan).

So a compositional difference might account for the difference in the albedo (brightness) of the material in the low-lying area. 

Are the scarps the result of vertical movement along faults, or were they formed by secondary crater chains? 

Researchers also have yet to explain the hollows on the central peak of the crater at upper left, and the smooth impact melt on the floor of the terrace-walled crater just below center.

Mercury may biomagnify more effectively in northern regions

Mercury biomagnification rates in aquatic Arctic ecosystems are higher than in lower latitudes, says a new study from Queen's University researcher Raphael Lavoie.

Mercury is passed along through food webs in all ecosystems through a process called biomagnification. This process results in increasing concentration of substances like mercury in an organism at successively higher levels in a food chain.

But this new study expands what we know about biomagnification by showing that colder temperatures contribute to higher rates of biomagnification in Arctic food chains.

"High Arctic ecosystems are already affected by global changes. When contaminants from human activity end up in the Arctic, they tend to stay there," says Mr. Lavoie.

"Mercury will always biomagnify, but we've found that depending on the latitude, the degree of biomagnification will vary."

Low temperatures mean slower metabolism and slower growth rate for Arctic marine life.

As growth rate of organisms in this area is reduced, their bodies contain higher mercury concentrations than in areas with warmer temperatures where growth rate is accelerated.

"Our study indicates that fragile arctic ecosystems may be more at risk from mercury pollution than ecosystems in other parts of the world," says Mr. Lavoie.

"In addition, arctic food webs may be slower to respond to current efforts to reduce mercury pollution. Our study highlights the need for consistent data collection and collaboration to monitor mercury in food webs across the globe."

This colour image, taken on May 1, 2013 by the Wide Angle Camera (WAC) instrument aboard NASA's MESSENGER spacecraft orbiting Mercury, features Hovnatanian crater, named for Armenian painter Hakop Hovnatanian.

The crater's elliptical shape and the bright rays' butterfly pattern indicate that a very oblique impact produced the crater. The brightness of the rays indicate that they are relatively young features on Mercury's surface.

While mercury is produced naturally by volcanoes and forest fires, global mercury production have increased hugely because of human activities such as coal burning and artisanal gold extraction.

Data was collected from over 7000 tissue samples in 205 aquatic food webs across 31 countries and oceans.

Results from 69 different studies worldwide were collected and homogenized by Mr. Lavoie and his team to create the first comprehensive study of mercury biomagnification trends.

NASA Messenger: Sunlight on the Side of the Planet Mercury

Another day, another beautiful view of Mercury's horizon. 

In this scene, which was acquired looking from the shadows toward the sunlit side of the planet, a 120-km (75 mi.) impact crater stands out near the center.

Emanating from this unnamed crater are striking chains of secondary craters, which gouged linear tracks radially away from the crater.

While this crater is not especially fresh (its rays have faded into the background), it does appear to have more prominent secondary crater chains than many of its peers.

This image was acquired on Oct. 2, 2013 by the Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS) aboard NASA's MESSENGER spacecraft, as part of the MDIS's limb imaging campaign.

Once per week, MDIS captures images of Mercury's limb, with an emphasis on imaging the southern hemisphere limb.

These limb images provide information about Mercury's shape and complement measurements of topography made by the Mercury Laser Altimeter (MLA) of Mercury's northern hemisphere.

The MESSENGER spacecraft is the first ever to orbit the planet Mercury, and the spacecraft's seven scientific instruments and radio science investigation are unraveling the history and evolution of the solar system's innermost planet.

During the first two years of orbital operations, MESSENGER acquired over 150,000 images and extensive other data sets. MESSENGER is capable of continuing orbital operations until early 2015.

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington