MESSENGER's new solar system portrait, from the inside out
[/caption]
Say cheese! The MESSENGER spacecraft has captured the first portrait of our Solar System from the inside looking out. The images, captured Nov. 3 and 16, 2010, were snapped with the Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) of MESSENGER’s Mercury Dual Imaging System (MDIS).
All of the planets are visible except for Uranus and Neptune, which at distances of 3.0 and 4.4 billion kilometers were too faint to detect with even the longest camera exposure time of 10 seconds. Their positions are indicated. The dwarf-planet Pluto, smaller and farther away, would have been even more difficult to observe.
Earth’s Moon and Jupiter’s Galilean satellites (Callisto, Ganymede, Europa, and Io) can be seen in the NAC image insets. Our Solar System’s perch on a spiral arm provided a beautiful view of part of the Milky Way galaxy, bottom center.
The following is a graphic showing the positions of the planets when the graphic was acquired:
The new mosaic provides a complement to the Solar System portrait – that one from the outside looking in – taken by Voyager 1 in 1990.
These six narrow-angle color images were made from the first ever 'portrait' of the solar system taken by Voyager 1, which was more than 4 billion miles from Earth and about 32 degrees above the ecliptic. The spacecraft acquired a total of 60 frames for a mosaic of the solar system which shows six of the planets. Mercury is too close to the sun to be seen. Mars was not detectable by the Voyager cameras due to scattered sunlight in the optics, and Pluto was not included in the mosaic because of its small size and distance from the sun. These blown-up images, left to right and top to bottom are Venus, Earth, Jupiter, and Saturn, Uranus, Neptune. The background features in the images are artifacts resulting from the magnification. The images were taken through three color filters -- violet, blue and green -- and recombined to produce the color images. Jupiter and Saturn were resolved by the camera but Uranus and Neptune appear larger than they really are because of image smear due to spacecraft motion during the long (15 second) exposure times. Earth appears to be in a band of light because it coincidentally lies right in the center of the scattered light rays resulting from taking the image so close to the sun. Earth was a crescent only 0.12 pixels in size. Venus was 0.11 pixel in diameter. The planetary images were taken with the narrow-angle camera (1500 mm focal length). Credit: NASA/JPL
“Obtaining this portrait was a terrific feat by the MESSENGER team,” says Sean Solomon, MESSENGER principal investigator and a researcher at the Carnegie Institution. “This snapshot of our neighborhood also reminds us that Earth is a member of a planetary family that was formed by common processes four and a half billion years ago. Our spacecraft is soon to orbit the innermost member of the family, one that holds many new answers to how Earth-like planets are assembled and evolve.”
BepiColombo's Mercury Magnetospheric Orbiter (MMO) in the Large Space Simulator at ESTEC, The Netherlands. The octagonal spacecraft is Japan’s contribution to BepiColombo and will explore Mercury's magnetic field. Credits: ESA/JAXA
[/caption]
Most of us have heard the expression “hot enough to cook eggs on the sidewalk”, but have we really thought about what kind of technology it would take to send a probe to Mercury? Just what kind of tests would we need to do to ensure a spacecraft could endure the kind of temperatures present while in orbit of the inner planet? It’s going to take more than a microwave set on high to find out…
According to ESA’s press release, the key components of the ESA-led Mercury mapper BepiColombo have been tested in a specially upgraded European space simulator. ESA’s Large Space Simulator is now the most powerful in the world and the only facility capable of reproducing Mercury’s hellish environment for a full-scale spacecraft. The Mercury Magnetospheric Orbiter (MMO) has survived a simulated voyage to the innermost planet. The octagonal spacecraft, which is Japan’s contribution to BepiColombo, and its ESA sunshield withstood temperatures higher than 350 degrees C. Worse than a Ohio August day!
The Mercury Magnetospheric Orbiter (MMO) is tested inside ESA's Large Space Simulator. The octagonal spacecraft is Japan’s contribution to BepiColombo, and must survive temperatures of 350°C. Credits: ESA/JAXAThis is a taste of things to come for the spacecraft. BepiColombo will encounter fully ten times the radiation power received by a satellite in orbit around Earth and, to simulate this, the Large Space Simulator (LSS) at ESA’s ESTEC center in the Netherlands had to be specially adapted. Engineers talk about the power of the Sun in units called the solar constant. This is how much energy is received every second through a square meter of space at the distance of Earth’s orbit. “Previously, the LSS was capable of simulating a solar constant or two. Now it has been upgraded to produce ten solar constants,” says Jan van Casteren, ESA BepiColombo project manager.
The improvements have been achieved in two ways: the lamps from the simulators are being used at their maximum power and the mirrors that focus the beam have been adjusted. (Think magnifying glass focusing the Sun. We’ve all done it!) Instead of producing a parallel beam of light 6 m across, they now concentrate the light into a cone just 2.7 m in diameter when it reaches the spacecraft. This creates a beam so fierce that a new shroud with a larger cooling capacity had to be installed to ‘catch’ the light that missed the spacecraft and prevent the chamber walls from heating up. BepiColombo consists of separate modules. The MMO will investigate the magnetic environment of Mercury. It is kept cool during its six-year cruise to Mercury by the sunshield. These are the two modules that have now completed their thermal tests. “The sunshield test was successful. Its function to protect the MMO spacecraft during the cruise phase was demonstrated,” says Jan.
BepiColombo consists of two spacecraft that will orbit Mercury. The Mercury Magnetospheric Orbiter (MMO) follows a larger orbit and investigates the planet's magnetic field. The Mercury Planetary Orbiter (MPO) traces a lower orbit and is designed to study the planet itself. Credits: ESA, C. Carreau
Once at Mercury, most of the Sun’s fearsome heat will be prevented from entering BepiColombo by special thermal blankets. They consist of multiple layers including a white ceramic outer layer and several metallic layers to reflect as much heat as possible back into space. “The tests allowed us to measure the thermal blanket’s performance. The results allow us to prepare some adjustments for the tests of the Mercury Planetary Orbiter next year,” says Jan.
In addition to enduring temperatures of 350 degrees C, ESA’s Mercury Planetary Orbiter (MPO) will go where no spacecraft has gone before: down into a low elliptical orbit around Mercury, of between just 400 km and 1500 km above the planet’s scorching surface. At that proximity, Mercury is worse than a hot plate on a cooker, releasing floods of infrared radiation into space. So, the MPO will have to deal with this as well as the solar heat. The MPO begins its tests in the LSS in the summer.
An image of Mercury’s tail obtained from combining a full day of data from a camera aboard the STEREO-A spacecraft. The reflected sunlight off the planet's surface results in a type of over-exposure that causes Mercury to appear much larger than its actual size. The tail-like structure extending anti-sunward from the planet is visible over several days and spans an angular size exceeding that of a full Moon in the night sky. Credit: Boston University
The STEREO mission to study the Sun also has observed some unusual comet-like features exhibited by the planet Mercury, with a coma of tenuous gas surrounding the planet and a very long tail extending away from the sun. These types of features had been seen before from telescopes on Earth, but the STEREO observations are helping scientists to understand the nature of the emissions coming from Mercury, which might be different from what was previously thought.
Another note of interest: the tail in the STEREO data was actually discovered by a fellow blogger, Ian Musgrave, who writes Astroblog. He is a medical researcher in Australia who has a strong interest in astronomy. Viewing the on-line data base of STEREO images and movies, Dr. Musgrave recognized the tail and sent news of it to a team of astronomers from Boston University to compare it with their observations.
[/caption]
The STEREO mission has two satellites placed in the same orbit around the Sun that the Earth has, but at locations ahead and behind it. This configuration offers multi-directional views of the electrons and ions that make up the escaping solar wind. On occasion, the planet Mercury appears in the field of view of one or both satellites. In addition to its appearance as a bright disk of reflected sunlight, a ‘tail’ of emission can be seen in some of the images.
From Earth-based telescopes, astronomers have seen how the Sun’s radiation pressure pushes sodium atoms from Mercury’s surface away from the planet – and away from the Sun – creating a tail that extends many hundreds of times the physical size of Mercury.
Much closer to Mercury, several smaller tails composed of other gases, both neutral and ionized, have been found by NASA’s MESSENGER satellite as it flew by Mercury in its long approach to entering into a stable orbit there.
“We have observed this extended sodium tail to great distances using our telescope at the McDonald Observatory in Texas,” Boston University graduate student Carl Schmidt explained, “and now the tail can also be seen from satellites near Earth.”
“What makes the STEREO detections so interesting is that the brightness levels seem to be too strong to be from sodium,” said Boston University graduate student Carl Schmidt, lead author on a paper that was presented at European Planetary Science Congress in Rome this week.
Now, the Boston University scientists are working with the STEREO scientists to try and sort everything out.
The current focus of the team is to sort out all of the possibilities for the gases that make up the tail. Dr. Christopher Davis from the Rutherford Appleton Laboratory in Chilton, England, a member of the STEREO team is working closely with the Boston University group on refining the brightness calibration methods, and determining the precise wavelengths of light that would get through the cameras’ filters.
“The combination of our ground-based data with the new STEREO data is an exciting way to learn as much as possible about the sources and fates of gases escaping from Mercury,” said Michael Mendillo, Professor of Astronomy at Boston University and director of the Imaging Science Lab where the work is being done.
“This is precisely the type of research that makes for a terrific Ph.D. dissertation,” Mendillo added.
Earth and Moon from 114 Million Miles.Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
[/caption]
A new image to add to the family photo album! The MESSENGER spacecraft is working its way to enter orbit around Mercury in March of 2011, and while wending its way, took this image of the Earth and Moon, visible in the lower left. When the image was taken in May 2010, MESSENGER was 183 million kilometers (114 million miles) away from Earth. For context, the average separation between the Earth and the Sun is about 150 million kilometers (93 million miles). It’s a thought provoking image (every one of us is in that image!), just like other Earth-Moon photos — Fraser put together a gallery of Earth-Moon images from other worlds, and this one will have to be added. But this image was taken not just for the aesthetics.
This image was taken as part of MESSENGER’s campaign to search for vulcanoids, small rocky objects hypothesized to exist in orbits between Mercury and the Sun. Though no vulcanoids have yet been detected, the MESSENGER spacecraft is in a unique position to look for smaller and fainter vulcanoids than has ever before been possible. MESSENGER’s vulcanoid searches occur near perihelion passages, when the spacecraft’s orbit brings it closest to the Sun. August 17, 2010 was another such perihelion, so if MESSENGER was successful in finding any tiny asteroids lurking close to the Sun, we may hear about it soon.
A double-ring basin named Rachmaninoff reveals young volcanism on Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
[/caption]
Younger volcanoes, stronger magnetic storms and a more intriguing exosphere: three new papers from data gathered during the MESSENGER spacecraft’s third flyby of Mercury in September of last year provide new insights into the planet closest to our Sun. The new findings make the science teams even more anxious for getting the spacecraft into orbit around Mercury. “Every time we’ve encountered Mercury, we’ve discovered new phenomena,” said principal investigator Sean Solomon. “We’re learning that Mercury is an extremely dynamic planet, and it has been so throughout its history. Once MESSENGER has been safely inserted into orbit about Mercury next March, we’ll be in for a terrific show.”
The closest look ever at some of Mercury’s plains suggests the planet’s volcanic activity lasted much longer than previously thought. From new images, researchers identified a 290-kilometer-diameter peak-ring impact basin, among the youngest to be observed on the planet. Named Rachmininoff, the region is characterized by exceptionally smooth, sparsely cratered plains, which formed later than the basin itself, likely from volcanic flow.
“We interpret these plains to be the youngest volcanic deposits yet found on Mercury,” said lead author Louise Prockter, from Johns Hopkins University Applied Physics Laboratory, one of MESSENGER’s deputy project scientists. “Moreover, an irregular depression surrounded by a diffuse halo of bright material northeast of the basin marks a candidate explosive volcanic vent larger than any previously identified on Mercury.
These observations suggest that volcanism on the planet spanned a much greater duration than previously thought, perhaps extending well into the second half of solar system history.”
A depression northeast of the basin is surrounded by a halo of bright mineral deposits, which Prockter and her team propose to be the largest volcanic vent identified on Mercury so far. Both of these findings mean that volcanism continued well into the second half of our Solar System’s history.
A double-ring basin named Rachmaninoff reveals young volcanism on Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
During the third fly-by, the team was able to take measurements of Mercury’s magnetic field, and this happened to occur during a time when the planet was being hit by a strong solar wind. MESSENGER’s Magnetometer documented for the first time the substorm-like build-up, or “loading,” of magnetic energy in Mercury’s magnetic tail. The tail’s magnetic field increased and decreased by factors ranging from two to 3.5 during very brief periods of just two to three minutes.
“The extreme tail loading and unloading observed at Mercury implies that the relative intensity of substorms must be much larger than at Earth,” said lead author James A. Slavin, a space physicist at NASA’s Goddard Space Flight Center and a member of MESSENGER’s Science Team. “However, what is even more exciting is the correspondence between the duration of tail field enhancements and the Dungey cycle time, which describes plasma circulation through a magnetosphere.”
Substorms on Earth are powered by similar processes—except that the loading of our planet’s magnetosphere is ten times weaker and occurs over the course of a full hour. Therefore, the team said, Mercury’s substorms must release more energy than terrestrial ones.
A third paper analyzed data from specialized instruments on-board the spacecraft to gain a clearer picture of Mercury’s neutral and ionic exospheres. Mercury’s exosphere is a tenuous atmosphere of atoms and ions derived from the planet’s surface and from the solar wind. Notable in the new observations were the differences in altitude of elements like magnesium, calcium, and sodium above the planet’s north and south poles. The team said this indicates that several processes are at work and that a given process may affect each element quite differently
“A striking feature in the near-planet tail ward region is the emission from neutral calcium atoms, which exhibits an equatorial peak in the dawn direction that has been consistent in both location and intensity through all three flybys,” said lead author Ron Vervack, also at the Applied Physics Laboratory. “The exosphere of Mercury is highly variable owing to Mercury’s eccentric orbit and the effects of a constantly changing space environment. That this observed calcium distribution has remained relatively unchanged is a complete surprise.”
Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry
[/caption]
The length of year on Mercury is 87.969 days. In other words, it takes almost 88 Earth days for Mercury to complete one orbit around the Sun. Mercury completes just over 4 orbits for each year on Earth.
Mercury has the most eccentric of all the orbits of the planets. Its distance from the Sun varies between 46 million and 70 million kilometers. This means that the speed of its orbit varies dramatically depending on the point of its orbit. If you could stand on the surface of Mercury and watch the Sun, you would see the Sun rise in the morning go part way up in to the sky and then go backwards in the sky, and set again. And then it would rise again and this time it would go across the sky and set. Four days before the fastest point of its orbit around the Sun, Mercury’s orbital speed matches its rotational velocity so that the Sun appears to stop in the sky. Then it’s orbiting faster than it’s rotating for about 8 days and so the Sun appears to move backwards.
Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry
[/caption]
In scientific terms, an orbital revolution is the amount of time it takes for one object to orbit completely around another. So a Mercury revolution is the amount of time it takes for Mercury to completely orbit the Sun and then come back to its initial position. Here on Earth, we call that a year.
Mercury’s revolution around the Sun takes 87.969 days. So, you could say that Mercury’s year lasts almost 88 days.
But if you were standing on the surface of Mercury, you wouldn’t experience that many days. That’s because Mercury rotates on its axis very slowly, taking almost 59 days to rotate once. The strange thing is that if you were standing on the surface of Mercury, you would experience something very different. You would see the sun rise halfway, and then go back down again, and then rise up again before setting. The whole process would take about 2 of Mercury’s years.
Remember, the revolution of Mercury is how long the planet takes to travel around the Sun. The rotation of Mercury is how long it takes to turn once on its axis.
We’ve written many articles about Mercury for Universe Today. Here’s an article about the gravity on Mercury, and here’s an article about the composition of Mercury.
The first-ever global mosaic map of Mercury was released today, which will be a critical tool for the MESSENGER mission’s observations of the planet when it enters orbit in 2011. The map was created from images taken during MESSENGER’s three flybys of Mercury – the most recent of which took place in September 2009 — and those of Mariner 10 in the 1970s. “The production of this global mosaic represents a major milestone for everyone on the MESSENGER imaging team,” said Sean Solomon, MESSENGER Principal Investiagor. “Beyond its extremely important use as a planning tool, this global map signifies that MESSENGER is no longer a flyby mission but instead will soon become an in-depth, non-stop global observatory of the Solar System’s innermost planet.”
The map was created by the MESSENGER mission team and cartographic experts from the U. S. Geological Survey. It will help scientists pinpoint craters, faults, and other features for observation.
While creating a mosaic map may seem straightforward – just stitch together multiple images taken by the spacecraft — it was actually quite a challenge to create cartographically accurate maps from images with varying resolution (from about 100 to 900 meters per pixel) and lighting conditions (from noontime high Sun to dawn and dusk) taken from a spacecraft traveling at speeds greater than 2 kilometers per second (2,237 miles per hour).
Small uncertainties in camera pointing and changes in image scale can introduce small errors between frames.
“With lots of images, small errors add up and lead to large mismatches between features in the final mosaic,” said MESSENGER team member Mark Robinson. “By picking control points—the same features in two or more images—the camera pointing can be adjusted until the image boundaries match.”
This operation is known as a bundle-block adjustment and requires highly specialized software. Cartographic experts at the USGS Astrogeology Science Center in Flagstaff, Ariz., picked the control points to solve the bundle-block adjustment to construct the final mosaic using the Integrated Software for Imagers and Spectrometers (ISIS). For the MESSENGER mosaic, 5,301 control points were selected, and each control point on average was found in more than three images (18,834 measurements) from a total of 917 images.
“This mosaic represents the best geodetic map of Mercury’s surface. We want to provide the most accurate map for planning imaging sequences once MESSENGER achieves orbit around Mercury”, said Kris Becker of the USGS. “As the systematic mapping of Mercury’s surface progresses, we will continually add new images to the control point network, thus refining the map”, he says. “It has already provided us with a start in the process of naming newly identified features on the surface.”
In the final bundle-block adjustment the average error was about two-tenths of a pixel or only about 100 meters—which is an excellent match from image-to-image, the team said. Absolute positional errors in the new mosaic are about two kilometers, according to the MESSENGER team. Once the spacecraft enters orbit around Mercury, the team will be able to make even more refinements and the entire planet will be imaged at even higher resolution. The global mosaic is available for download on the USGS Map-a-Planet web site. It is also available at the MESSENGER site.
A presentation on the new global mosaic was given today at the Fall Meeting of the American Geophysical Union in San Francisco.
Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry
[/caption]
Mercury is the closest planet to the Sun, and so it’s the fastest to orbit the Sun. In fact, Mercury only takes 88 days to orbit the Sun. In other words, Mercury’s orbit only takes 24% as long as Earth’s orbit.
If you were born on Mercury, you would have celebrated 4 times as many birthdays as you do on Earth. In other words, if you’re 10 here on Earth, you’d be 40 in Mercury years. Now that’s a possible way to grow up more quickly.
Mercury orbits the Sun at an average distance of only 57.9 million km. Compare this with Earth’s average orbital distance of 150 million km.
Unlike the other planets in the Solar System, Mercury doesn’t really experience any seasons. This is because Mercury has no atmosphere to trap heat from the Sun. Whichever side of Mercury is currently facing the Sun experience temperatures of up to 700 Kelvin. And then the side of the planet that’s in the shade dips down to only 100 Kelvin; that’s well below freezing. Even though Mercury is close, you would experience incredibly cold temperatures if you lived on the surface.
The orbit of Mercury was actually a great puzzle to astronomers until the 20th century. They couldn’t explain why the point of Mercury’s furthest orbit of the Sun was slowly drifting at a rate of 43 arcseconds per century. But this strange motion was finally explained perfectly by predictions made by Albert Einstein with his Theory of Relativity.
Even though the MESSENGER spacecraft experienced a “hiccup” during its third and final flyby of Mercury on Sept. 29, scientists are still pleased and surprised by the data garnered. The spacecraft went into safe mode, shutting down temporarily because of a power system switchover during a solar eclipse as it approached the planet and only half of the expected observations were carried out. But the new data – combined with observations from the two previous flybys — provide an almost complete view of Mercury’s surface and offer new, unexpected scientific findings. “Although the area viewed for the first time by spacecraft was less than 350 miles across at the equator, the new images reminded us that Mercury continues to hold surprises,” said principal investigator Sean Solomon.
The most important aspect of the flyby was a critical gravity assist to remain on course to enter into orbit around Mercury in 2011. Additionally, the spacecraft’s cameras and instruments collected high-resolution and color images unveiling another 6 percent of the planet’s surface never before seen at close range. Image coverage map of Mercury after the third MESSENGER flyby. Credit: NASA, Applied Physics Lab
Solomon said at today’s press conference that all the data gathered on Mercury so far are like first few chapters of a novel; we’ve learned much, but much more of the story remains. Approximately 98 percent of Mercury’s surface now has been imaged by NASA spacecraft. After MESSENGER goes into orbit around Mercury, it will see the polar regions, which are the only unobserved areas of the planet.
Many new features were revealed during the third flyby, including a region with a bright area surrounding an irregular depression, suspected to be volcanic in origin. Other images revealed a double-ring impact basin approximately 180 miles across. The basin is similar to a feature scientists call the Raditladi basin, which was viewed during the probe’s first flyby of Mercury in January 2008.
This spectacular 290-km-diameter double-ring basin seen in detail for the first time during MESSENGER’s third flyby of Mercury bears a striking resemblance to the Raditladi basin, observed during the first flyby.
“This double-ring basin, seen in detail for the first time, is remarkably well preserved,” said Brett Denevi, a member of the probe’s imaging team and a postdoctoral researcher at Arizona State University in Tempe. “One similarity to Raditladi is its age, which has been estimated to be approximately one billion years old. Such an age is quite young for an impact basin, because most basins are about four times older. The inner floor of this basin is even younger than the basin itself and differs in color from its surroundings. We may have found the youngest volcanic material on Mercury.”
One of the spacecraft’s instruments conducted its most extensive observations to date of Mercury’s exosphere, or thin atmosphere, during this encounter. The flyby allowed for the first detailed scans over Mercury’s north and south poles. The probe also has begun to reveal how Mercury’s atmosphere varies with its distance from the sun. Comparison of neutral sodium observed during MESSENGER’s second and third Mercury flybys
“A striking illustration of what we call ‘seasonal’ effects in Mercury’s exosphere is that the neutral sodium tail, so prominent in the first two flybys, is 10 to 20 times less intense in emission and significantly reduced in extent,” says participating scientist Ron Vervack, of the Johns Hopkins University Applied Physics Laboratory, or APL, in Laurel, Md. “This difference is related to expected variations in solar radiation pressure as Mercury moves in its orbit and demonstrates why Mercury’s exosphere is one of the most dynamic in the solar system.”
The observations also show that calcium and magnesium exhibit different seasonal changes than sodium. Studying the seasonal changes in all exospheric constituents during the mission orbital phase will provide key information on the relative importance of the processes that generate, sustain, and modify Mercury’s atmosphere. Schematic view of Mercury’s interior showing its large, iron-rich core, which constitutes at least ~60% of the planet’s mass.
The third flyby also revealed new information on the abundances of iron and titanium in Mercury’s surface materials. Earlier Earth and spacecraft-based observations showed that Mercury’s surface has a very low concentration of iron in silicate minerals, a result that led to the view that the planet’s crust is generally low in iron.
“Now we know Mercury’s surface has an average iron and titanium abundance that is higher than most of us expected, similar to some lunar mare basalts,” says David Lawrence, an APL participating mission scientist.
The spacecraft has completed nearly three-quarters of its 4.9-billion-mile journey to enter orbit around Mercury. The full trip will include more than 15 trips around the sun. In addition to flying by Mercury, the spacecraft flew past Earth in August 2005 and Venus in October 2006 and June 2007.
