The "Fried" Planet:
Mercury is the smallest of the major planets with a radii of only 2440 km (1590 miles). Since it is the closest planet to the Sun its greatest elongation is only 28 degrees (i.e. its only visible right after sunset or right before sunrise). Since it is located near the Sun at sunrise or sunset, the ancients thought that Mercury was two different planets, Lucifer and Hermes.
From its radius (i.e. volume) and mass we calculate it has a mean density of 5.4 g/cm3 which implies a dense iron (Fe) core. All planets form as molten balls, then cool and solidify. The cooling rate is proportional to the amount of material (the planet's mass) and its surface area (its radius). Since Mercury is low in mass (less heat stored from formation), its core is probably solid rather than liquid.
The rotation period of Mercury is 58.6 days and its orbital period (year) is 87.9 days. Notice that 2/3 times 87.9 is 58.6; thus, Mercury suffers from spin-orbit coupling where the tidal forces from the Sun has locked Mercury's rotation into a resonance number (1/2, 2/3, 4/5, 5/6, etc...).
Daytime temperatures on the surface of Mercury hover around 700 degrees Kelvin (enough to melt lead). Whereas, at night the ground temperature plunge to a mere 100 Kelvins (air turns to liquid at 77 Kelvins). Dawn is ten times more brilliant than on the Earth, since the Sun is ten times larger. The lack of any significant atmosphere means that before dawn you can see the Sun's corona spreading over the horizon.
The above image is of Mercury from the Messenger mission (2008) The north pole is at the top and the equator extends from left to right about two-thirds down from the top. Bright rayed craters are prominent in this view of Mercury. One such ray seems to join in both east-west and north-south directions.
The above mosaic shows the Caloris Basin (located half-way in shadow on the morning terminator). Caloris is Latin for heat and the basin is named this because it is near the subsolar point (the point closest to the sun) when Mercury is at aphelion. Caloris Basin is 1,300 kilometers (800 miles) in diameter and is the largest know structure on Mercury. It was formed from an impact of a projectile with asteroid dimensions. The interior floor of the basin contains smooth plains but is highly ridged and fractured. North is towards the top of this image.
The above "weird terrain" best describes this hilly region of Mercury. This area is at the antipodal point from the large Caloris basin. The shock wave produced by the Caloris impact was reflected and focused to this antipodal point, thus jumbling the crust and breaking it into a series of complex blocks. The area covered is about 100 kilometers (62 miles) on a side.
The above picture is a Mariner 10 image shows Santa Maria Rupes, the sinuous dark feature running through the crater at the center of this image (note how is passes through the crater rim indicating that it was created after the impact crater). Many such features were discovered in the Mariner images of Mercury and are interpreted to be enormous thrust faults where part of the mercurian crust was pushed slightly over an adjacent part by compressional forces. The abundance and length of the thrust faults indicate that the radius of Mercury decreased by 1-2 kilometers (0.6 - 1.2 miles) after the solidification and impact cratering of the surface. This volume change probably was due to the cooling of the planet, following the formation of a metallic core three-fourths the size of the planet. North is towards the top and is 200 kilometers (120 miles) across.
In general, the surface of Mercury is similar to the Moon (i.e. heavily cratered due to a lack of a heavy atmosphere to erode away primordial impacts). However, there are some key differences:
- There are few maria on Mercury and they are small. No large impact era like the Moon. Therefore,
Mercury must have cooled faster.
- Cratering is less heavy, more plain region between craters. Due to the higher surface gravity on
Mercury (you weight more than on Mercury than the Moon). This means impacts did not throw debris as far,
fewer secondary craters and more concentrated around primary crater.
- Long scarps or wrinkles are found on the crust and the tops of craters (i.e. after cratering epoch). After Mercury cooled, its crust solidified first. Mercury was still rotating quickly back then and had an equatorial bulge. As Mercury slowed in its rotation, due to tidal forces with the Sun, gravity pulled Mercury into a more spherical shape and the crust had to fold producing long scarps.
Mercury's Magnetic Field:
Although Mercury has a high density, implying it is rich in iron (Fe), almost no Fe is detected by spectroscopy of its surface. Thus, most of the Fe must has sunk into the core while Mercury was young and molten. And, that it must have stayed in a molten state for much longer than the other planets formed at the same time.
These facts are confirmed by the fact that Mercury has a strong magnetic field, even though it rotates slowly. This implies a liquid core for Mercury, which is in contradiction with the theory that the core is solid from cooling arguments. One possible solution is that the core is a mixture of iron and some other material such as sulfur.
Mercury's strong magnetic field also gives it a very weak atmosphere. Planet's as hot as Mercury quickly lose their atmosphere's as the high temperatures heat the atmosphere molecules to escape velocity. However, Mercury is very close to the Sun and is able to capture some of the Sun's solar wind (composed of protons and electrons) in its magnetic field, giving it a very thin and tenuous atmosphere.
The principal Mercury's characteristcs are:
- Surface heavily cratered—in some ways very similar to Earth's Moon.
- Very dark—only reflects a small amount of Sunlight (it has a low albedo).
- Temperature on the surface varies from 100K to almost 700K.
- In a 3:2 synchronous orbit with the Sun—three Mercury days (59 Earth days each) = two Mercury years (88 Earth days each).
- The iron core of Mercury is very large, taking up most of the interior of the planet and creating a relatively strong magnetic field for a planet of its size.
- No moon.
- Less than the size of the Earth.
An unexpected feature of Mercury was its magnetism, apparently due to a magnetic core like Earth's. The field is weak, too weak to create a symmetric sheltered "magnetosphere" around the planet where charged particles may be trapped ("radiation belt"). However, its field lines are "dragged out" by the solar wind to create a long magnetic tail somewhat like the Earth's, and when Mariner 10 passed that region it unexpectedly observed a sudden impulsive acceleration event of charged particles.
The above image was taken on 14 January 2008 by the Messenger spacecraft operated for NASA by the Applied Physics Lab of the Johns Hopkins University in Baltimore.
On 05 October 2011, after its first Mercury solar day (176 Earth days) in orbit, MESSENGER has nearly completed two of its main global imaging campaigns: a monochrome map at 250 m/pixel and an eight-color, 1-km/pixel color map. Apart from small gaps, which will be filled in during the next solar day, global maps now provide uniform lighting conditions ideal for assessing the form of Mercury’s surface features as well as the color and compositional variations across the planet. The orthographic views seen here, centered at 75° E longitude, are each mosaics of thousands of individual images. At right, images taken through the wide-angle camera filters at 1000, 750, and 430 nm wavelength are displayed in red, green, and blue, respectively.