Mercury is also the smallest planet in the solar system, measuring just 3, miles wide at its equator. Because Mercury is so small and so close to the sun, it is the most elusive of the five planets that are visible to the naked eye. You can only catch Mercury at dawn and dusk, and it usually does not rise far above the horizon. Because of its position in the solar system, Mercury passes between Earth and the sun 13 times each century in an event known as a transit. Since ancient times, people observing the heavens have noticed that the naked-eye planets sometimes appear to move backward, briefly shifting from their usual eastward motion to a westward path across the sky.
This retrograde motion is an illusion created when one planet moves faster in its orbit than another, so that it catches up to and passes the slower world.
For observers on Earth, Mercury is in retrograde about three to four times a year. If you could watch the skies from the surface of Mercury, you would sometimes catch the rare sight of the sun in retrograde. As Mercury makes its closest approach to our smoldering star, its orbital speed exceeds its rate of rotation on its axis. That means someone standing on Mercury would see the sun start to rise, then briefly set, then rise again all within the same day. Like the other planets in the solar system, Mercury was born about 4. However, the small planet cooled very quickly , contracting enough within the first billion or so years to prevent magma from escaping through the outer crust and ending geologic activity such as volcanism on the surface.
Despite its proximity to our star, Mercury is not the hottest planet in the solar system. With no atmosphere to trap heat, surface temperatures on Mercury can swing from degrees Fahrenheit during the day to degrees Fahrenheit at night. Mercury may even have reservoirs of ice sitting deep inside permanently shadowed craters at its poles.
By contrast, the surface of hazy Venus sits at a sweltering degrees Fahrenheit year-round, making it the hottest planet in our solar system. The process crumpled the surface, creating lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high, as well as Mercury's " Great Valley ," which at about miles long, miles wide and 2 miles deep 1, by by 3.
Indeed, a study of cliffs on Mercury's surface suggested the planet may still rumble with earthquakes, or "Mercuryquakes. In addition, in the past, Mercury's surface was constantly reshaped by volcanic activity. However, another study suggested Mercury's volcano eruptions likely ended about 3. Mercury is the second densest planet after Earth, with a huge metallic core roughly 2, to 2, miles 3, to 3, km wide, or about 75 percent of the planet's diameter. In comparison, Mercury's outer shell is only to miles to km thick. The combination of its massive core and abundance of volatile elements has left scientists puzzled for years.
A completely unexpected discovery made by Mariner 10 was that Mercury possessed a magnetic field. Planets theoretically generate magnetic fields only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small — just roughly one-third Earth's size — that its core should have cooled off long ago. An unusual interior could help to explain the differences in Mercury's magnetic field when compared to Earth. Russell co-authored a model that suggests that Mercury's iron core may be turning from liquid to solid at the core's outer boundary rather than the inner.
The discovery in by Earth-based radar observations that Mercury's core may still be molten could help explain its magnetism, though the solar wind may play a role in dampening the planet's magnetic field. Although Mercury's magnetic field is just 1 percent the strength of Earth's, it is very active. The magnetic field in the solar wind — the charged particles streaming off the sun — periodically touches upon Mercury's field, creating powerful magnetic tornadoes that channel the fast, hot plasma of the solar wind down to the planet's surface.
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Instead of a substantial atmosphere, Mercury possesses an ultra-thin "exosphere" made up of atoms blasted off its surface by solar radiation, the solar wind and micrometeoroid impacts. These quickly escape into space, forming a tail of particles.
- Surface Features of Mercury!
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One study suggested that Mercury's surface features can generally be divided into two groups — one consisting of older material that melted at higher pressures at the core-mantle boundary, and the other of newer material that formed closer to Mercury's surface. Another study found that the dark hue of Mercury's surface is due to carbon. This carbon wasn't deposited by impacting comets, as some researchers suspected — instead, it may be a remnant of the planet's primordial crust. Its oval-shaped orbit is highly elliptical, taking Mercury as close as 29 million miles 47 million km and as far as 43 million miles 70 million km from the sun.
If one could stand on Mercury when it is nearest to the sun, it would appear more than three times as large as it does when viewed from Earth.
Oddly, due to Mercury's highly elliptical orbit and the 59 Earth-days or so it takes to rotate on its axis, when on the scorching surface of the planet, the sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the sun appears to set, rise again briefly, and then set again. In , a rare transit of Mercury happened , where the planet crossed the face of the sun. Mercury's transit may have yielded secrets about its thin atmosphere, assisted in the hunt for worlds around other stars, and helped NASA hone some of its instruments.
Internal structure : Iron core roughly 2, to 2, miles 3, to 3, km wide.
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Outer silicate shell about to miles to km thick. Average distance from the sun : 35,, miles 57,, km. By comparison: 0.