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Here’s a Mercury diagram, showing the interior of planet Mercury.
Mercury is the closest planet to the Sun, orbiting at an average distance of 57.9 million km from the Sun. It’s also the second densest planet in the Solar System, with an average distance of 5.427 grams per cubic centimeter. Based on this density, astronomers have some estimates about the interior structure of Mercury.
The center of Mercury is its metal core, similar to the Earth’s core. But in the case of Mercury, the core occupies 42% of the volume of Mercury, while the core of Earth is only 17%. And for some reason, the metal core of Mercury doesn’t create a magnetic field with the same intensity of Earth’s magnetic field. Mercury’s magnetosphere is only 1% as strong as Earth’s field.
Surrounding the core is Mercury’s mantle. This is a 500-700 km thick layer of rock, composed of silicates. And surrounding the mantle is Mercury’s crust. Based on observations made by Mariner 10 and Earth-based telescopes, astronomers think that Mercury’s crust is 100 – 300 kilometers thick. There are many large depressions in Mercury’s crust, and scientists think these formed as Mercury slowly cooled and contracted.
Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry
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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
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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
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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.
Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry
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If you want to REALLY see Mercury up close and personal, take a look at this absolutely HUGE mosaic of the planet. It was put together by Jason Perry, who actually works with the Cassini mission but in his spare time stitched together 66 images from the MDIS narrow angle camera from the MESSENGER mission’s second flyby of Mercury in October 2008, along with some data from the Mariner 10 mission in the 1970’s. The full file is 20 MB, with a resolution of 0.6 kilometers (0.37 miles) per pixel. What fun! —for us, that is. It took Perry four days just to set up his software, according to Emily Lakdawalla at the Planetary Society Blog.
Bright spot on Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
[/caption]More new images were released today from the MESSENGER spacecraft’s third flyby of Mercury. I asked astrophysicist Dr. Jeff Goldstein (doctorjeff on Twitter), (who was on hand at the mission operations center to blog and Tweet about the flyby) which image the science team found most intriguing, and he replied that it was really hard to tell, as they were oohing and aahing at every image! But one of the most interesting was this shot of a bright spot on the planet closest to the sun. MESSENGER’s Narrow Angle Camera also saw this spot during the spacecraft’s second Mercury flyby on October 6, 2008, but the bright feature was just on the planet’s limb (edge) from the spacecraft’s vantage point. This time, however, the geometry of MESSENGER’s flyby provided a better look at this feature. Surprisingly, at the center of the bright halo is an irregular depression, which may have formed through volcanic processes. Color images from MESSENGER’s Wide Angle Camera reveal that the irregular depression and bright halo have distinctive color. This area will be of particular interest for further observation during MESSENGER’s orbital operations starting in 2011.
Craters form a paw print on Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
If you like seeing a little pareidolia, here’s a fun one: a paw print! Mercury’s surface is covered with craters in many sizes and arrangements, the result of impacts that have occurred over billions of years. In the top center of the image, outlined in a white box and shown in the enlargement at upper right, is a cluster of impact craters on Mercury that appears coincidentally to resemble a giant paw print. In the “heel” are overlapping craters, made by a series of impacts occurring on top of each other over time. The four “toes” are single craters arranged in an arc northward of the “heel.” The “toes” don’t overlap so it isn’t possible to tell their ages relative to each other. The newly identified pit-floor crater can be seen in the center of the main image as the crater containing a depression shaped like a backward and upside-down comma.
This unnamed basin was imaged as MESSENGER approached Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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The MESSENGER spacecraft went into safe mode just before its closest approach of Mercury on Sept. 29. Although the instruments were taking data as the spacecraft came near the planet during this third flyby of the mission, after going into safe mode, no further data or pictures were obtained. This means the expected science investigations from the flyby were not executed. However, as Emily Lakdawalla pointed on in the Planetary Blog, the most important purpose of this flyby was the last gravity assist that will allow MESSENGER to enter orbit in 2011, and to that end, the flyby was a complete success. Additionally, the images taken during the approach are of the 5% of Mercury that was previously unseen, as in the image above of this unnamed basin. See more images from the approach below.
A High-resolution Look over Mercury's Northern Horizon. Credit: MESSENGER team
MESSENGER skimmed just 142 miles (228 km) above Mercury at closest approach, and then whipped behind the planet for the gravity assist. During the operation, five MESSENGER “fellows” or master teachers were reporting the flyby live via Twitter. Gene Gordon (Porchdragon on Twitter) reported that unexpectedly, the signal dropped from MESSENGER before the expected signal blackout while flying on the other side of Mercury: “Suddenly room got quiet and people hovering near computers. Unexpected signal drop just occurred. Sense of nervousness seems to have happened.”
The MESSENGER team had to wait over 50 minutes until the spacecraft emerged from behind Mercury, and were relieved to be able to resume contact. As of Wednesday morning, the spacecraft was operating normally, and the reason for the signal drop was unclear. At a briefing, MESSENGER team members said the spacecraft went into safe mode when it entered Mercury’s shadow and tried to switch to battery power. The team is still looking into why this anomaly occurred.
A little less than half of the”extra” science goals for the flyby were accomplished. See our previous article on the science goals for the flyby. Following this flyby. only the polar regions of Mercury have never been seen. Previously unseen side of Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
MESSENGER made its closest approach on Tuesday at about 5:55 p.m. EDT (2155 GMT), zooming at speeds of about 12,000 mph (19,312 kph). Mercury’s gravity was expected to slow MESSENGER by about 6,000 mph (9,656 kph) during the flyby and place it on track to enter orbit of Mercury in March 2011.
Learn more about MESSENGER and the two previous flybys which occured in 2008 here.
Lead image caption: his unnamed impact basin was seen for the first time yesterday during MESSENGER’s third flyby of Mercury. The outer diameter of the basin is approximately 260 kilometers (160 miles). This basin has a double-ring structure common to basins with diameters larger than 200 kilometers (about 125 miles).
Additional information from Jeff Goldstein on Twitter (doctorjeff) was also used in this article
This enhanced-color image shows the regions targeted for MASCS and MDIS observations during Mercury flyby 3. Click the image for more information.
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Next week, on September 29, 2009 the MESSENGER spacecraft will fly by Mercury for the third and final time, looking at areas not seen before in the two previous passes. The spacecraft will pass 141.7 miles above the planet’s rocky surface, receiving an a final gravity assist that will enable it to enter orbit about Mercury in 2011. With more than 90 percent of the planet’s surface already imaged, the team will turn its instruments during this flyby to specific features to uncover more information about the planet closest to the Sun.
Determining the composition of Mercury’s surface is a major goal of the orbital phase of the mission.
“This flyby will be our last close look at the equatorial regions of Mercury, and it is our final planetary gravity assist, so it is important for the entire encounter to be executed as planned,” said Sean Solomon, principal investigator at the Carnegie Institution in Washington. “As enticing as these flybys have been for discovering some of Mercury’s secrets, they are the hors d’oeuvres to the mission’s main course — observing Mercury from orbit for an entire year.” A collage of images from the previous two flybys. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Click image for more information
As the spacecraft approaches Mercury, cameras will photograph previously unseen terrain. As the spacecraft departs, it will take high-resolution images of the southern hemisphere. Scientists expect the spacecraft’s imaging system to take more than 1,500 pictures. Those images will be used to create a mosaic to complement the high resolution, northern-hemisphere mosaic obtained during the second Mercury flyby. The first flyby took the spacecraft over the eastern hemisphere in January 2008, and the second flyby took it over western side in October 2008.
“We are going to collect high resolution, color images of scientifically interesting targets that we identified from the second flyby,” said Ralph McNutt, a project scientist at APL. “The spectrometer also will make measurements of those targets at the same time.”
The spacecraft may observe how the planet interacts with conditions in interplanetary space as a result of activity on the sun. During this encounter, high spectral- and high spatial-resolution measurements will be taken again of Mercury’s tenuous atmosphere and tail.
“Scans of the planet’s comet-like tail will provide important clues regarding the processes that maintain the atmosphere and tail,” said Noam Izenberg, the instrument’s scientist at the Johns Hopkins University Applied Physics Laboratory, or APL, in Laurel, Maryland. “The Mercury Atmospheric and Surface Composition Spectrometer will give us a snapshot of how the distribution of sodium and calcium vary with solar and planetary conditions. In addition, we will target the north and south polar regions for detailed observations and look for several new atmospheric constituents.”
For a detailed look at the MESSENGER flyby, see the MESSENGER website; additionally, Emily Lakdawalla at the Planetary Society has posted a detailed overview here.
