1. Introduction
  2. The Surface of Mercury
  3. Motion and Temperature
  4. Exosphere
  5. Geology and Planet Evolution
  6. Thermal History
  7. Geologic History
  8. Origin
 
 
 
Animation of Mercury‘s and Earth‘s revolution around the Sun
 
 

Introduction

Mercury, the planet closest to the Sun, has almost no atmosphere, and its dusty surface of craters resembles the Moon. Mercury is only slightly larger that Earth’s Moon. The planet was named for the Roman god Mercury, a winged messenger, and it travels around the Sun faster than any other planet. Mercury is difficult to see from Earth—in fact, the famous astronomer Nicolaus Copernicus, for all his years of research and observation, never once was able to see Mercury.

Mercury’s dayside is super-heated by the sun, but at night temperatures drop hundreds of degrees below freezing. Ice may even exist in craters. Mercury’s egg-shaped orbit takes it around the sun every 88 days.

Mercury spins on its axis very slowly, so one side of the planet can be in complete darkness for weeks. Mercury is geologically dead. The surface shows no sign of current volcanic activity or any other form of geological activity. It is for this reason that astronomers believe Mercury’s core to be extremely cold.

Mercury is one of only two planets in the Solar System not to have a moon. The other moonless planet is Venus. Mercury has been visited a couple of times by spacecraft. Its first visit was Mariner 10 which launched in 1973. Mariner 10 took in the sights and sounds of Venus first before going on to visit Mercury in early 1974. Its only other visitor is Messenger which launched in 2004 and made its first flyby of Mercury in January 2008.
   
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The Surface of Mercury

Mercury’s surface resembles that of Earth’s Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. While there are 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 by contraction of the crust.


The Caloris Basin, one of the largest features on Mercury, is about 1,550 km in diameter. It was the result of an asteroid impact on the planet’s surface early in the solar system’s history.

Over the next several billion years, Mercury shrank in radius about 1 to 2 km (0.6 to 1.2 miles) as the planet cooled after its formation. The outer crust contracted and grew strong enough to prevent magma from reaching the surface, ending the period of volcanic activity.

 

Mercury’s diameter is only 4878 km. The manner in which it reflects light is very similar to the way light is reflected by the Moon. The brightness (albedo) of certain terrains is greater than comparable terrains on the Moon. Mercury’s surface is heavily cratered with smooth plains that fill and surround large impact basins.

Long lobate scarps traverse the surface and large expanses of intercrater plainsfill regions between clusters of craters in the highlands. Also a peculiar terrain consisting of a jumble of large blocks and linear troughs occurs antipodal to the Caloris Basin

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Motion and Temperature

Mercury has the most eccentric and inclined orbit of any planet. Its average distance from the Sun is 0.3871 AU (5.79 × 107 km). The distance varies from 0.3075 AU (4.6 × 107 km) at perihelion to 0.4667 AU (6.98 × 107 km) at aphelion.

At perihelion the Sun’s apparent diameter is over three times larger than its apparent diameter as seen from Earth. Mercury’s rotation period is 58.646 Earth days, and its orbital period is 87.969 Earth days. It makes exactly three rotations on its axis for every two orbits around the Sun. As a consequence of this, a solar day (sunrise to sunrise) lasts two Mercurian years or 176 Earth days.

The green mark on Mercury represents the position of a large crater called the Caloris Basin. The animation shows that Mercury spins on its axis one and a half times during each orbit of the Sun.

The obliquity of Mercury is close to 0°, therefore, it does not experience seasons as do Earth and Mars. The subsolar points at the 90° and 270° longitudes are called warm poles because they occur at aphelion. Yet another consequence of the 3:2 resonance and the large eccentricity is that an observer on Mercury (depending on location) would witness a double sunrise, or a double sunset, or the Sun would backtrack in the sky at noon during perihelion passage. Near perihelion Mercury’s orbital velocity is so great compared to its rotation rate that it overcomes the Sun’s apparent motion in the sky as viewed from Mercury.

Although Mercury is closest to the Sun, it is not the hottest planet. The surface of Venus is hotter because of its atmospheric greenhouse effect. However, Mercury experiences the greatest range (day to night) in surface temperatures (650°C) of any planet or satellite in the solar system because of its close proximity to the Sun. Its maximum surface temperature is about 467°C at perihelion on the equator; hot enough to melt zinc. At night just before dawn, the surface temperature plunges to about ­183°C.
   
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Exosphere

The exosphere is a thin, atmosphere-like volume surrounding a planetary body. In the case of bodies with substantial atmospheres, such as the Earth’s atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with interplanetary space.

During MESSENGER’s third flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer detected emission from ionized calcium concentrated 1 to 2 Mercury radii tailward of the planet. This measurement provides evidence for tailward magnetospheric convection of photoions produced inside the magnetosphere.

Observations of neutral sodium, calcium, and magnesium above the planet’s north and south poles reveal altitude distributions that are distinct for each species. A two-component sodium distribution and markedly different magnesium distributions above the two poles are direct indications that multiple processes control the distribution of even single species in Mercury’s exosphere.
Mercury’s Main Exospheric Constituents
 
Constituent Vertical Column Abundance(atoms/cm²)
 
Hydrogen (H) ~5 × 1010
Helium (He) ~2 × 1013
Oxygen (O) ~7 × 1012
Sodium (Na) ~2 × 1012
Potassium (K) ~1 × 1010
Calcium (Ca) ~1 × 107
 
*The Earth‘s atmosphere has ~2 x 1018 molecules/cm²
Although both sodium and potassium are probably derived from the surface of Mercury, the mechanism by which they are supplied is not well understood.  Both sodium and potassium show day-to-day changes in their global distribution. If surface minerals are important sources for the exosphere, then a possible explanation is that their sodium/potassium ratio varies with location on Mercury.

A possible explanation for some of the K and Na variations is that Na and K ion implantation into regolith grains during the long Mercurian night (88 Earth days), and subsequent diffusion to the exosphere when the enriched surface rotates into the intense sunlight. At least one area of enhanced exospheric potassium emission apparently coincides with the Caloris Basin whose floor is highly fractured.

This exospheric enhancement has been attributed to increased diffusion and degassing in the surface and subsurface through fractures on the basin floor, although other explanations may be possible.

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Geology and Planet Evolution

Mercury has heavily cratered upland regions and large areas of younger smooth plains that surround and fill impact basins. Thermal infrared measurements from Mariner 10 indicate that the surface is a good insulator and, therefore, consists of a porous cover of fine-grained regolith.

Earth-based microwave measurements indicate that this layer is a few centimeters thick. Mercury’s heavily cratered terrain contains large areas of gently rolling intercrater plains, the major terrain type on the planet. Mercury’s surface is also traversed by a unique system of contractional thrust faults called lobate scarps.

The largest well-preserved structure viewed by Mariner 10 is the Caloris impact basin some 1300 km in diameter. Antipodal to this basin is a large region of brokenup terrain called the hilly and lineated terrain, probably caused by focused seismic waves from the Caloris impact.

 
  1. Geologic Surface Units
  2. Surface Composition
  3. Tectonic Framework
  4. Thermal History
  5. Geologic History
 

Geologic Surface Units

   
The origin of some of the major terrains and their inferred geologic history are somewhat uncertain because of the limited photographic coverage and resolution and the poor quality or lack of other remotely sensed data. In general, the surface of Mercury can be divided into four major terrains:

  (1) Heavily Cratered Regions
  (2) Hilly and Lineated Terrain
  (3) Intercrater Plains
  (4) Smooth Plains
   
Other relatively minor units have been identified, such as ejecta deposits exterior to the Caloris and other basins.
   

Heavily Cratered Regions

   
The heavily cratered uplands probably record the period of late heavy meteoroid bombardment that ended about 3.8 billion years ago on the Moon, and presumably at about the same time on Mercury. Small craters are bowl-shaped, but with increasing size they develop central peaks, flat floors, and terraces on their inner walls. The freshest craters have extensive ray systems, some of which extend for distances over 1000 km.

The images reveal giant craters on the surface of Mercury seen in unprecedented detail.
This basin‘s outer rim is about 306 km in diameter and the inner (peak) ring is about 140 km in diameter (Courtesy NASA.)

The 1300-km-diameter Caloris impact basin is the largest well-preserved impact, although the much more degraded Borealis Basin is larger (1530 km). The floor structure of the Caloris Basin is like no other basin floor structure in the solar system. The fractures get progressively deeper and wider toward the center of the basin. Near the edge of the basin there are very few fractures. This pattern may have been caused by subsidence and subsequent uplift of the basin floor.
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Hilly and Lineated Terrain

   
Directly opposite the Caloris Basin on the other side of is the unusual hilly and lineated terrain that disrupts preexisting landforms, particularly crater rims ( A & B ). The hills are 5–10 km wide and about 0.1–1.8 km high. Geologic relationships suggest that the age of this terrain is the same as that of the Caloris Basin.

A B
(a) A portion of the hilly and lineated terrain antipodal to the Caloris impact basin. The image is 543 km across.
(b) Detail of the hilly and lineated terrain. The largest crater in (b) is 31 km in diameter. (Courtesy NASA.)

The hilly and lineated terrain is thought to be the result of shock waves generated by the Caloris (Fig. 10). Computer simulations of shock wave propagation indicate that focused shock waves from an impact of this size can cause vertical ground motions of about 1 km or more and tensile failure to depths of tens of kilometers below the antipode.

Diagrammatic representation of the formation of the hilly and lineated terrain by focused seismic waves from the Caloris impact

Although the lunar Imbrium Basin (1400 km diameter) is larger than the Caloris Basin, the disrupted terrain at its antipode is much smaller than that at the Caloris antipode. The larger disrupted terrain on Mercury may be the result of enhanced shock wave focusing due to the large iron core.
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Intercrater Plains

   
Mercury’s two plains units have been interpreted as either impact basin ejecta or as lava plains. The older intercrater plains are the most extensive terrain on Mercury. They both partially fill and are superimposed by craters in the heavily cratered uplands.

Smooth Plains: Sometime shortly after the Caloris impact, flood lavas formed smooth plains. These younger plains cover abut 40% of the surface and are observed within and around the Caloris basin and can be seen filling craters. (Image credit: NASA/John Hopkins University Applied Physics Laboritory/Carnegie Institute of Washington)

Because intercrater plains were emplaced during the period of late heavy bombardment, they are probably extensively fragmented and do not retain any signature of their original surface morphology. Although no landforms diagnostic of volcanic activity have been discovered, there are also no obvious source basins to provide ballistically emplaced ejecta. The Intercrater plains are probably about 3.9 billions years old.
  Back to Geologic Surface Units

Smooth Plains

   
The younger smooth plains cover almost 40% of the total area imaged by Mariner 10. The largest occurrence of smooth plains fill and surround the Caloris Basin, and occupy a large circular area in the north polar region that is probably an old impact basin (Borealis Basin).

Craters within the Borealis, Goethe, Tolstoy, and other basins have been flooded by smooth plains. This indicates the plains are younger than the basins they occupy. Furthermore, several irregular rimless depressions that are probably of volcanic origin occur in smooth plains on the floors of the Caloris and the Tolstoy basins.

Photomosaic of the Borealis Basin showing numerous craters that have been flooded by smooth plains. (Courtesy NASA.)

Mariner 10 enhanced color images show the boundary of smooth plains within the Tolstoy Basin is also a color boundary, further strengthening the volcanic interpretation for the smooth plains. Based on the shape and density of the size/frequency distribution of superimposed craters, the smooth plains probably formed near the end of late heavy bombardment. They may have an average age of about 3.8 billion years as indicated by crater densities.

Water-rich comets or asteroids responsible for one or more of these craters could be the source of the polar water-ice deposits.

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Surface Composition

Little is known about the surface composition of Mercury. If the plains units are lava flows, then they must have been very fluid similar to fluid flood basalts on the Moon, Mars, Venus, and Earth. The way in which light is reflected from the surface is very similar to that of the Moon.

Scientists have attempted to deduce the makeup of Mercury’s surface from studies of the sunlight reflected from different regions. One of the differences noted between Mercury and the Moon, beyond the fact that Mercury is on average somewhat darker than the Moon, is that the range of surface brightnesses is narrower on Mercury.

In particular, Mercury’s rocks may be low in oxidized iron (FeO), and this leads to speculation that the planet was formed in conditions much more reducing than other terrestrial planets.

Determination of the composition of Mercury’s surface from such remote-sensing data involving reflected sunlight and the spectrum of Mercury’s emitted thermal radiation is fraught with difficulties.

Messenger is equipped with several instruments, which were not aboard Mariner 10, that can measure chemical and mineral compositions directly. These instruments need to observe Mercury for long periods of time while the spacecraft remains near Mercury, so there were no definitive results from Messenger’s three early and brief flybys of the planet.

During Messenger’s mission in orbit around Mercury there will be abundant new information about the composition of the planet’s surface.
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Tectonic Framework

No other planet or satellite in the solar system has a tectonic framework like Mercury’s. It consists of a system of contractional thrust faults called lobate scarps. They have a random spatial and azimuthal distribution over the imaged half of the planet and presumably occur on a global scale.

Mercury was subjected to global contractional stresses. The only occurrences of features indicative of extensional stresses are localized fractures associated with the floor of the Caloris Basin and at its antipode, both of which are the direct or indirect result of the Caloris impact.

Evidence of tectonic movwmwnta on the surface of Mercury (Courtesy NASA.)

This tectonic framework was probably caused by crustal shortening resulting from a decrease in the planet radius due to cooling of the planet. The amount of radius decrease is estimated to have been anywhere between 0.5 and 2 km.

Also there is apparently a system of structural lineaments consisting of ridges, troughs, and linear crater rims that have at least three preferred orientations trending in northeast, northwest, and north–south directions.
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Thermal History

All thermal history models of planets depend on compositional assumptions. Since our knowledge of the composition of Mercury is so poor, these models can only provide a general idea of the thermal history.

Starting from initially molten conditions for Mercury, the planetary radius decrease due to cooling. About 6 km of this contraction is solely due to mantle cooling during about the first 700 million years before the start of inner core formation.

Thermal models suggest that inner core formation may have begun about 3 billion years ago. The surface record of the period of intense contraction caused by mantle cooling has probably been erased by the period of cataclysmic bombardment and intercrater plains formation that occurred from about 3.9 to 3.8 billion years ago. That would explain why there is no evidence for old compressive structures.

If the smooth and intercrater plains are volcanic flows, then they must have had some way to easily reach the surface to form such extensive deposits. Early lithospheric compressive stresses would make it difficult for lavas to reach the surface, but the lithosphere may have been relatively thin at this time (<50 km). Large impacts would be expected to strongly fracture it, possibly providing egress for lavas to reach the surface and bury compressive structures.

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Geologic History

Mercury’s earliest history is very uncertain.

If a portion of the mantle was stripped away then Mercury’s earliest recorded surface history began after core formation. The earliest events are the formation of intercrater during the period of late heavy bombardment. These plains may have been erupted through fractures caused by large impacts in a thin lithosphere.

Near the end of late heavy bombardment, the Caloris Basin was formed by a large impact that caused the hilly and lineated terrain from seismic waves focused at the antipodal region. Further eruption of lava within and surrounding the Caloris and other large basins formed the smooth plains about 3.8 billion years ago.

The system of thrust faults formed after the intercrater plains, but how soon after is not known. If the observed thrust faults resulted only from core cooling, then they may have begun after smooth plains formation and resulted in a decrease in Mercury’s radius.

As the core continued to cool and the lithosphere thickened, compressive stresses closed off the magma sources, and volcanism ceased near the end of late heavy bombardment. All of Mercury’s volcanic events probably took place very early in its history.

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Origin

The origin of Mercury and how it acquired such a large fraction of iron compared to the other terrestrial planets is not well determined. Although these early models are probably inaccurate, revised models that take into account material supplied from feeding zones in more distant regions of the inner solar system. At Mercury’s present distance, the models predict the almost complete absence of sulfur, which is apparently required to account for the presently molten outer core.
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Mercury
Discovery
Discovered by Sumerians - Nabu
Discovery date 2BC

Designations
Pronunciation mɜːrkjəri

Orbital Characteristics
Aphelion 0.466 697 AU 69,816,900 km
Perihelion 0.307 499 AU 46,001,200 km
Orbital period 87.969 1 d 0.5 Mercury solar day
Aerage Orbital Speed 47.362 km/s
Satellites None

Physical Characteristics
Mean radius 2,439.7±1.0 km 0.3829 Earths
Surface area 7.48×107 km2 0.147 Earths
Mass 3.3011×1023kg
0.055 Earths
Volume 6.083×1010km 0.056 Earths
Axial tilt .04′ ± 0.08′ (to orbit)

Surface temp.
min mean max
100K 340K
700K

Atmosphere Composition
  42% Molecular Oxygen
  29.0% Sodium
  22.0% Hydrogen
  6.0% Helium
  0.5% Potassium
   
  Trace amounts of argon
  nitrogen, carbon dioxide, water
  vapor, xenon, krypton, and neon