Mercury (planet)

Mercury, the planet closest to the Sun, resembles Earth’s Moon. Much that was known about this planet was obtained from experiments on board and photographic images returned by the uncrewed Mariner 10 which completed three flybys of Mercury in the 1970s. A new round of investigations began in the twenty-first century with the MESSENGER and continued with BepiColombo.

Overview

Mercury completes one revolution around the Sun in 87.97 days. Mercury’s orbit has a mean distance from the Sun of only 0.387 astronomical units (one AU is the mean Earth-Sun distance), an eccentricity of 0.206, and an inclination of seven degrees concerning the ecliptic plane. Mercury rotates about an axis with no obliquity and lasts 58.65 days. The ratio of Mercury’s rotational period to its revolution period is almost precisely two to three (2:3). Mercury’s mass is 3.30 10²³ kilograms, its mean radius is 2,439 kilometers, and its mean density is 5,420 kilograms per cubic meter. Mercury’s most prevalent features are craters, scarps, and deformed terrain. Degradation of original craters has resulted from secondary impact and ballistic infilling, seismic activity resulting from impacts, lava flows, and isostatic readjustment.

Mercury’s surface appears remarkably similar to Earth’s Moon, although Mercury’s radius is about 50 percent larger than the Moon’s. Both bodies are heavily pockmarked with impact craters. Closer examination, however, reveals many essential differences between the surfaces of Mercury and the Moon. The Moon has more significant color variations across its surface than does Mercury. Mercury’s albedo, or reflectivity, is 0.12, a brightness similar to that of the lunar highlands on the Moon’s Earth-facing side. Although there are 20 percent albedo contrasts across Mercury’s surface, it lacks the dark maria and filled craters so prevalent on the Moon. On both worlds, younger craters are often higher in albedo and surrounded by prominent ejecta blankets and bright rays. On Mercury, most craters are less than two hundred kilometers across. Many larger craters are double-ringed with flat floors that are usually shallower than their lunar counterparts. Central peaks are found in intermediate-sized craters, but more prominent circular features do not have central peaks. Other lunar features are absent or rather rare on Mercury. Some regions of the planet have volcanic features and deposits. Rilles on the planet are usually straight rather than sinuous, are quite deep, and are as wide as six kilometers.

Mercury is surrounded by a highly tenuous atmosphere of helium, argon, and neon. High daytime surface temperatures coupled with a low escape velocity lead to degassing, as the average thermal kinetic energy is sufficient to permit atmospheric escape in a relatively short period of geologic time. The average planetary surface temperature is 452 kelvins. However, the maximum dayside temperature is as much as 700 kelvins at the closest approach to the Sun, and the minimum nightside temperature is ninety kelvins. Mercury exhibits the solar system's most significant equatorial temperature variation, more than 600 kelvins, of any planet.

Mercury has a magnetic field that is only 1.6 percent as strong as Earth’s. The origin of Mercury’s magnetic field remains uncertain. A metallic core composed primarily of iron would be consistent with the observed density and the magnetic field. Still, Mercury’s rotational speed could be too slow to generate currents in the core, even if it is molten. Mercury’s small magnetic field interacts with the solar wind. Mariner 10 recorded a moderately strong bow shock that traps energetic solar wind particles.

Although Mercury’s surface resembles the Moon’s, its mean density is closer to Earth's. It is believed that proportionately to Mercury's size, the planet's core contains double the amount of iron found in any other world in the solar system. High iron content would be consistent with the formation of Mercury by condensation of the solar nebula close to the Sun; heavy elements would have been more attracted to the early proto-sun than lighter elements. The existence of a magnetic field suggests that Mercury’s iron has undergone differentiation and formed a hot, convecting core. Formation of the iron core would have generated heat in addition to the original radiogenic heat, causing expansion and melting of the entire mantle. This process would have had to occur before the era of intense cratering because Mercury lacks surface expansion features and numerous preserved lava-flow formations.

According to one model of Mercury’s interior, the planet’s asthenosphere cooled quickly and thickened, possibly disappearing altogether. Mercury’s lithosphere could extend to the iron core, hundreds of kilometers below the surface. There is evidence to support such a model. After core formation, the planet would have cooled and contracted, resulting in a decrease in radius, perhaps as large as one or two kilometers. Compression of the surface would then have caused thrust faults, the result of one rock unit slipping over another. Observed thrust faults on Mercury indicate a two-kilometer contraction.

Mercury’s surface physiography can be classified into four major terrain types: heavily cratered terrain, smooth plains, intercrater plains, and hilly and lineated terrain. Intercrater plains are believed to be the oldest, predating the era of intense impact cratering. Smooth plains represent the youngest.

Smooth plains, located principally in the northern hemisphere near the large feature called Caloris Basin, are flat, lightly cratered surfaces akin to lunar maria. Craters in the smooth plains are typically sharp-rimmed and only ten kilometers across at most. Some smooth plains fill the floors of large craters. Often, plains have sinuous ridges, an aspect shared by lunar maria. Regardless of location on the surface, Mercury’s smooth plains have equal impact crater frequency, which indicates that smooth plain features were all formed at about the same time. Smooth, plain features are believed to be volcanic in origin. There are too many smooth plains for them to have resulted from a single catastrophic impact or to be ejected from the large Caloris Basin. Similarities between lunar maria and smooth plains suggest a common origin. Lunar Maria is volcanic, so smooth plains on Mercury are believed to be volcanic.

Intercrater plains, believed to be Mercury's oldest material, form the planet's largest physiographic feature. These plains have a greater crater density than smooth plains. Craters that pockmark rolling plains are typically less than ten kilometers in diameter and generally represent secondary rather than primary impacts. Intercrater plains were formed by a variety of different events occurring over long periods. These plains are probably primordial crust that has been subjected to impact cratering, but their origin is by no means clear. The variety of intercrater plains suggests several alternative origins.

Heavily cratered terrain is reminiscent of the lunar highlands, being areas of many overlapping craters. Crater diameters vary between thirty and two hundred kilometers. Ejecta deposits cannot be clearly identified with individual craters because of high degrees of overlap and disruption. This terrain was formed as the era of intense bombardment ended.

Hilly and lineated terrain is found directly opposite the Caloris Basin on Mercury and may have been formed by the Caloris impact event itself. This heavily deformed terrain, often referred to simply as Weird Terrain, covers about 250,000 square kilometers. It is made of hummocky hills five to ten kilometers wide at the base and 0.1 to 1.8 kilometers high. Seismic energy from the Caloris impact apparently underwent antipodal focusing through the planet’s core and broke or jumbled this region into hills and depressions.

The Caloris Basin is the largest single surface feature revealed by Mariner 10 photographs. More than 1,300 kilometers across, Caloris resembles the Moon’s Imbrium Basin. It may represent an important event in Mercury’s history, just as the Imbrium Basin does for the Moon. The basin is rimmed by mountains thirty to fifty kilometers wide and several kilometers high. Inside the basin are smooth plains scarred by small craters, ridges, and grooves indicative of lava flows modified by tectonic activity. Seeing the entirety of the Caloris Basin was a high-priority early objective of MESSENGER flybys before beginning prolonged orbital studies.

Methods of Study

Mercury was known to the ancients. However, Mercury reveals few of its secrets to visual observation from Earth. Because it is so close to the Sun, it is often hidden by solar glare and can be seen only briefly, visually or telescopically, at twilight or daybreak. The Hubble Space Telescope could not be used to obtain high-resolution images of Mercury’s surface because of the Sun's tremendous brightness, which would overpower and damage the orbiting observatory’s sensitive instruments. Nevertheless, Mercury studies have advanced greatly since the first days of visual observations of this innermost planet in our solar system. Astronomers once incorrectly assumed that Mercury did not rotate as it revolved around the Sun. Few surface features were known before the Mariner 10 encounters. Indeed, almost all that was known about Mercury prior to 2008 was obtained through Mariner 10's scientific investigations on its three brief flybys in the mid-1970s. That probe was equipped with seven primary experiments involving high-resolution television imaging, infrared radiometry, radio wave propagation, extreme ultraviolet spectroscopy, magnetometry, plasma detection, and charged particle flux measurements.

Mercury’s atmosphere was studied during a solar occultation using Mariner 10’s ultraviolet experiment. The instrument measured the drop in the intensity of solar ultraviolet radiation as Mercury’s disk and tenuous atmosphere obscured it. Data provided a profile of atmospheric concentration above the planet’s surface. Other atmospheric data were gathered by monitoring radio waves emitted by Mariner 10 as it passed behind Mercury and reemerged. The infrared radiometer, fixed to the spacecraft body on the sunlit side, had apertures that shielded the detectors from direct solar radiation. This experiment determined Venusian cloud temperatures and surface temperatures on Mercury. Heat-loss data obtained as Mariner 10 crossed the planet’s terminator, the line separating daylight from darkness, helped scientists infer information about the planet’s surface composition. Surface brightness temperature was measured in a pair of spectral ranges, thirty-four to fifty-five micrometers and 7.5 to 14 micrometers, which represented temperatures of 80 to 340 kelvins and 200 to 700 kelvins, respectively.

Mariner 10 measured Mercury’s magnetic field with a magnetometer package consisting of two three-axis sensors placed at different spots on a six-meter-long boom. The use of two sensors provided the capability to isolate the spacecraft’s magnetic field from the weak field of the planet. Magnetic field measurements in interplanetary space were also made.

High-resolution images of any planetary surface can provide information concerning its past and present geologic activity and surface composition. Mariner 10’s television imaging system included two vidicon cameras attached to telescopes. The assembly was mounted on a scan platform that permitted the horizontal and vertical movements necessary for precise pointing. Cassegrain telescope systems were used in the imaging system. Powerful enough to resolve ordinary print at a distance of more than four hundred meters, this system provided narrow-angle, high-resolution images. The television system also included an auxiliary optical system to obtain wide-angle, lower-resolution photography. This system was mounted on each of the television cameras. Experimenters were able to switch from narrow-angle to wide-angle imaging by moving the position of a mirror on the system’s filter wheel. The vidicon cameras had 9.8-by-12.3-millimeter apertures and could make exposures of between three milliseconds and twelve seconds. Analog signals from the vidicon camera readout were digitized for transmitting to Earth's receiving stations. An individual television image consisted of seven hundred vidicon scan lines, with each scan line consisting of 832 pixels.

The principal objectives of Mariner 10’s television imaging program included collecting data useful in studying Mercury’s planetary physiography, precisely determining Mercury’s radius and rotation rate, evaluating Mercury’s photometric properties, and categorizing the morphology of surface features. Television scans were made of the space surrounding Mercury in an attempt to locate unknown satellites, but none was found. This system was also used for studies of Venus and the comet Kohoutek before Mariner 10 even arrived near Mercury.

Data from Earth-based radar investigations of Mercury strongly suggest that at least part of the planet’s core could presently be molten. Such a molten layer would have large implications for the production of the planet’s global magnetic field and variations in Mercury’s spin rate if the liquid core is decoupled from the solid mantle. In a 1992 issue of Science, Martin A. Slade et al. presented the results of two studies using the Arecibo radio telescope, the Very Large Array, and the Goldstone tracking antenna to send radio waves to Mercury at selected frequencies and detect the reflected signals. Essentially radar-astronomy exercises, the aim of both studies was to generate a radar reflectivity map of Mercury’s surface at a resolution of about fifteen kilometers. In the process, radar-bright returns that were highly depolarized were encountered near the planet’s north and south poles. Data suggested the unexpected presence of ice on Mercury. Ice effectively reflects radar at the gigahertz frequencies used in these studies and greatly depolarizes those reflected radio waves. Some of the radar-bright areas detected in these studies coincided with crater-sized spots. This provided evidence that ice existed in crater areas that were permanently shadowed by solar radiation and, therefore, not heated tremendously, as was the rest of the planet when under daylight conditions. One of the bigger radar-bright areas was the large crater Chao Meng-Fu at Mercury’s south pole. Planetary scientists supposed that the proposed ice came from either meteoritic bombardment or planetary outgassing (or both). Confirmation of this surprising result would have to await MESSENGER's studies.

MESSENGER carried seven instruments that produced a great deal of data during the spacecraft’s January 14 and October 6, 2008 encounters—information that would take longer to analyze than the time to the next and final flyby before orbital insertion in 2011. The mission’s objectives included photographing the as-yet-unseen 50 percent of the planet’s surface, determining the composition and structure of Mercury’s crust, understanding more about the planet’s geological history, examining the planet’s thin atmosphere, measuring the planet’s quite active magnetosphere, searching for water at the poles, and providing data that would reveal the nature of Mercury’s large core. This first attempt in thirty-three years to examine Mercury from close range was designed to help answer five separate major questions: What is the elemental and mineralogical composition of the surface? What does the surface look like at a resolution of better than hundreds of meters? What is the structure and temporal variation of Mercury’s magnetic field? Does the planet’s gravitational field exhibit any anomalies that might shed light on any uneven distribution of mass within Mercury? What neutral particles and ions are found in Mercury’s magnetosphere?

MESSENGER was outfitted with wide- and narrow-angle color and black-and-white imaging systems, a laser altimeter, a radio science experiment, and four multipurpose spectrometers. The spectrometers could measure spectra of gamma rays, neutrons, energetic particles, and plasmas and reflected light from Mercury’s atmosphere and surface for compositional studies. The spacecraft’s laser altimeter was designed to determine the elevation of the planet’s surface features and to look for wobble in the planet about its rotational axis. That sort of motion could help verify the existence of a suspected liquid layer in the core. The neutron spectrometer was designed to detect water ice in the polar regions. The laser altimeter was designed to measure the topography of permanently shadowed craters that might shelter water ice deposits. The ultraviolet spectrometer was designed to look for sulfur or hydroxyl deposits atop the water ice.

In May 2008, researchers published results of laboratory modeling of Mercury’s core that included a separated molten layer surrounding a solid core. The University of Illinois and Case Western Reserve University scientists hypothesized that deep within the planet, iron snow forms and moves toward the solid core. Convection could be set up and create the planet’s magnetic field. This experiment investigated the behavior of an iron-sulfur sample under tremendous pressure and heat. The iron-sulfur sample was set up to mimic the suspected core structure of Mercury. If formed, molten iron condenses into flake-like crystals, which would fall to the core. This heavy iron “snowfall” would raise lighter liquid sulfur, establishing convection currents. Observational data for MESSENGER will be able to determine if this laboratory model actually matches Mercury’s internal structure.

In the meantime, other scientists at Virginia Tech reported results of different simulations of conditions on Mercury. This work suggests that the shrinking of the planet’s crust as Mercury cools over geological time should produce the thrust faults seen as scallop-edged cliffs and scarps on the planet. MESSENGER will also shed light on mantle convection, a process considerably different from that on Venus and Earth because of the thinness of Mercury’s crust.

Details about the prolonged analysis of MESSENGER data collected during its first flyby surfaced in science journals in early July 2008. The data confirmed that volcanic activity had played a tremendous role in the formation of Mercury’s surface, especially during a period lasting from three to four billion years ago. MESSENGER provided evidence of volcanic vents along the margins of the Caloris Basin. Other evidence demonstrated that effusion had occurred. This process sees molten material from below the crust exude upward and outward across a planet’s surface, sometimes forming features that resemble volcanic shields. Mercury had suffered lava floods that filled in fairly large craters, almost to the wrinkled scarps outlining the craters. Some layers of lava were determined to be as deep as 2.7 kilometers.

MESSENGER completed its primary mission on March 17, 2012, and its first extended mission in orbit a year later. In 2013, NASA reported that data collected by the spacecraft during its extended mission had led to several discoveries about the planet, including revelations about its surface volatiles, volcanism, topography, exosphere, solar magnetic field, and energized electrons. MESSENGER's neutron spectrometer revealed evidence that was consistent with the presence of water ice in the planet's permanently shadowed regions.

Context

Mercury is the Roman name for the Greek god Hermes, patron of trade, travel, and thieves. Timocharis is considered to have registered the first recorded observation of Mercury in 265 BCE. Little more was learned about the planet until the invention of the telescope. Observation of the phases of Mercury was first reported in 1639 CE by Italian astronomer Giovanni Battista Zupus. Telescope technology improved, and evidence of surface features was found in the early 1800s when astronomers Karl Ludwig Harding and Johann Schröter measured albedo variations.

It was not until the early 1960s that Mercury’s rotation rate was precisely measured using radar observations. Then came the launch of Mariner 10, the final spacecraft in the historic Mariner series, on November 3, 1973, at 12:45 AM Eastern time atop an Atlas-Centaur launch vehicle from Launch Complex 36B at Cape Canaveral. Photographs obtained during this flyby mission began the geologic analysis of Mercury. This spacecraft became the first to use gravity assists from large solar system bodies to redirect its trajectory to multiple photographic targets. It was recognized that the alignment of Earth, Venus, and Mercury was such that a single spacecraft could be launched between 1970 and 1973 from Earth toward Venus and then reach Mercury. Giuseppe Colombo of the Institute of Applied Mechanics in Padua, Italy, noted during an early 1970 Jet Propulsion Laboratory conference on the approved Mariner 10 mission that a 1973 launch opportunity existed in which the spacecraft could enter an orbit with a period nearly twice that of Mercury. That meant that a second Mercurian encounter was possible. Mariner 10 was indeed placed on a trajectory that permitted multiple encounters with Mercury, and this success demonstrated the feasibility of gravity-assist trajectories. The technique would prove tremendously valuable to the Voyager probes sent to the outer solar system.

Shortly after Mariner 10’s escape from Earth orbit, its planetary science experiments were activated to verify their operating condition. Mosaic photographs returned to Earth indicated that the spacecraft was in good condition to image a moon-like world with high-quality camera systems. Mariner 10 came within 5,794 kilometers of Venus on February 5, 1974. During eight days of photography, the spacecraft returned 4,165 images of Venus and a wealth of data about the Venusian atmosphere. After another forty-five days of interplanetary cruising, the spacecraft reached the mission’s principal target: the planet Mercury. Mariner 10 began taking photographs on March 23, 1974, reaching its closest approach, 5,790 kilometers, on March 29. The spacecraft then passed behind Mercury to the nightside. More than two thousand photographs were obtained on this first encounter. Mariner 10’s trajectory returned the spacecraft to Mercury on September 21, 1974. This time, it came as close as 50,000 kilometers. The probe completed a third encounter in March 1975 before running out of fuel and entering a solar orbit.

The MESSENGER orbiter was designed to continue the scientific exploration of Mercury, where Mariner 10 left off more than thirty years earlier. MESSENGER launched on August 3, 2004, and was injected into an interplanetary orbit that brought it back to Earth a year later for a gravity assist that would slow the spacecraft down to fall into the inner solar system. It was directed to encounter Venus in October 2006 and again in June 2007 for gravity assists that set it up for its first Mercury flyby. MESSENGER flew by Mercury near its equator on its first encounter on January 14, 2008. At the time, little useful information about polar ice deposits was obtained. However, a highlight of the encounter was the capture of detailed images of the remainder of the Caloris Basin, not seen during Mariner 10’s flybys. All spacecraft instruments functioned, signaling the potential for prolonged study once MESSENGER attained orbit. MESSENGER’s flight path was refined by thruster firings so that it again encountered Mercury, flying by on October 6, 2008, collecting data and images and also setting itself up by a gravity assist in such a way that it would fly by Mercury one more time in 2009 before eventually entering orbit about the planet in 2011.

Even before entering orbit, due to the three flyby encounters, MESSENGER was expected to give planetary scientists a nearly full initial map of Mercury’s globe. To achieve orbit, MESSENGER’s main propulsion system would fire to slow down by 860 meters per second. This fourteen-minute-long burn would consume 30 percent of the spacecraft’s total fuel load. The first orbit would be adjusted until MESSENGER assumed an elliptical orbit ranging from 200 kilometers to 15,193 kilometers inclined 80° to Mercury’s equator. In this nearly polar orbit, the spacecraft would orbit Mercury every twelve hours. Once in orbit around Mercury in March 2011, MESSENGER’s primary mission lasted four Mercurian years (the equivalent of one Earth year or two Mercurian solar days) and was completed on March 17, 2012. MESSENGER's extended orbital mission began on March 17, 2013, and has yielded groundbreaking discoveries around Mercury's surface volatiles, volcanism, topography, exosphere, solar magnetic field, and energized electrons. MESSENGER's mission ended on April 30, 2015, with an intended crash to the planet's surface. Its landing created a crater on Mercury's surface. MESSENGER had been hovering between five and thirty-five kilometers above the surface, collecting data on the last leg of its mission. It eventually ran out of fuel and could not escape Mercury's atmosphere. NASA scientists declared the overall mission a success.

The European Space Agency (ESA) and Japanese Aerospace Exploration Agency (JAXA) launched BepiColombo, a joint space mission to investigate Mercury using two orbiters, in 2018. The ESA orbiter is named BepiColombo after the late Italian mathematician/engineer Guiseppi (Bepi) Colombo who was the creator of gravity assist technology. BepiColombo launched on a years-long journey to Mercury and had completed several flyby observations by 2024. JAXA's Mercury Magnetospheric Orbiter (MMO or MIO) was launched with BepiColombo to study the planet's magnetic field.

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