Mercury
Mercury
Mercury's eccentric orbit takes the small planet as close as 47 million km (29 million miles) and as far as 70 million km (43 million miles) from the sun. If one could stand on the scorching surface of Mercury when it is at its closest point to the sun, the sun would appear more than three times as large as it does when viewed from Earth. Temperatures on Mercury's surface can reach 800 degrees Fahrenheit (430 degrees Celsius). Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -290 degrees Fahrenheit (-180 degrees Celsius).
Because of the close proximity to the sun the instrument package on the recent Messenger probe had to be shielded.
Because Mercury is so close to the sun, it is hard to directly observe from Earth except during dawn or twilight. Mercury makes an appearance indirectly -- 13 times each century, observers on Earth can watch Mercury pass across the face of the sun, an event called a transit. These rare transits fall within several days of 8 May and 10 November.
The first transits of Mercury in the 21st century occurred 7 May 2003, 8 November 2006 and 9 May 2016. The last transit was 11 November 2019 and the next one will occur November 13, 2032 (Mark your calendars!)
Getting to Mercury and operating in its orbit is actually difficult. Mariner 10 only flew by Mercury twice in 1974 and that was the only mission to Mercury until Messenger went into orbit around Mercury in 2011. The image on the left shows Mercury as seen by Mariner 10, and the one below that shows Mercury as imaged by Messenger.The original caption for this image states:
This colorful view of Mercury was produced by using images from the color base map imaging campaign during MESSENGER's primary mission. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up Mercury's surface.
Young crater rays, extending radially from fresh impact craters, appear light blue or white. Medium- and dark-blue areas are a geologic unit of Mercury's crust known as the "low-reflectance material", thought to be rich in a dark, opaque mineral. Tan areas are plains formed by eruption of highly fluid lavas. The giant Caloris basin is the large circular tan feature located just to the upper right of center of the image.
Mercury does not have a regular atmosphere.....
Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking micrometeoroids. Because of solar radiation pressure, the atoms quickly escape into space and form a tail of neutral particles. Though Mercury's magnetic field at the surface has just one percent the strength of Earth's, it interacts with the magnetic field of the solar wind to episodically create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface. When the ions strike the surface, they knock off neutrally charged atoms and send them on a loop high into the sky.
Mercury's surface resembles that of Earth's Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. Very large impact basins, including Caloris (1,550 km, or 960 miles, in diameter) and Rachmaninoff (306 km, or 190 miles), were created by asteroid impacts on the planet's surface early in the solar system's history. While there are large areas of smooth terrain, there are also lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high, formed as the planet's interior cooled and contracted over the billions of years since Mercury formed.
Messenger mosaic of Caloris basin.
Mercury is the second densest planet after Earth, with a large metallic core having a radius of about 2,000 km (1,240 miles), about 80 percent of the planet's radius. In 2007, researchers used ground-based radars to study the core, and found evidence that it is partly molten (liquid). Mercury's outer shell, comparable to Earth's outer shell (called the mantle and crust), is only about 400 km (250 miles) thick.
The surface of Mercury is heavily cratered. As an example, this is the spectacular Rembrandt impact basin, imaged by Messenger in October 2008.
At the very end of 2012, MESSENGER obtained the final image needed to view 100% of Mercury's surface under daylight conditions. The mosaics shown here cover all of Mercury's surface and were produced by using the monochrome mosaic released by NASA's Planetary Data System (PDS) on 8 March 2013, as the base.
This is that final mosaic, which also includes the North and South poles. We will visit those poles in a module 4 again.