LECTURE 3: THE PLANET MERCURY
Mercury (pronounced [ˈmɝkjʊəri] ) is the innermost and smallest planet in the solar system (since Pluto was re-labelled as a dwarf planet), orbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°: It can only be seen in morning and evening twilight. Comparatively little is known about it; the first of two spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only about 45% of the planet’s surface.[6] The second was the MESSENGER spacecraft, which mapped another 30% of the planet during its flyby of January 14, 2008.[6] MESSENGER will make two more passes by Mercury, followed by orbital insertion in 2011, and will survey and map the entire planet.
Physically, Mercury is similar in appearance to the Moon. It is heavily cratered, has no natural satellites and no substantial atmosphere. It has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.[7] It is an exceptionally dense planet due to the large size of its core. The surface temperatures on Mercury range from about 90 to 700 K (−180 to 430 °C),[8] with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes.[9] The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.[10]
Mercury is one of the four terrestrial planets; rocky bodies like the Earth. It is the smallest of the four, with an equatorial radius of 2439.7 km.[2] Mercury is even smaller—albeit more massive—than the largest natural satellites in the solar system, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.[11] Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only slightly less than Earth’s density of 5.515 g/cm³.[2] If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.[12]
Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.[13] Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests Mercury has a molten core.[14][15]
Surrounding the core is a 600 km mantle. It is generally thought that early in Mercury’s history, a giant impact with a body several hundred kilometers across stripped the planet of much of its original mantle material, resulting in the relatively thin mantle compared to the sizable core.[16]
Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick.[17] One distinctive feature of Mercury’s surface are numerous narrow ridges, some extending over several hundred kilometers. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.[18]
Mercury's core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteors, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.[16] However, early in the solar system’s history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass.[16] The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.[16] A similar process has been proposed to explain the formation of Earth’s Moon (see giant impact theory).[16]
Alternatively, Mercury may have formed from the solar nebula before the Sun’s energy output had stabilized. The planet would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K (about 4,000 to 5,800 °F), and possibly even as high as 10,000 K (about 17,500 °F).[19] Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the solar wind.[19]
A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material.[20] Each of these hypotheses predicts a different surface composition, and two upcoming space missions MESSENGER and BepiColombo both aim to make observations to test them.[21][22]
Mercury’s surface is overall very similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. Since our knowledge of Mercury's geology has been based on the 1975 Mariner flyby and terrestrial observations, it is the least understood of the terrestrial planets.[15] As data from the recent MESSENGER flyby is processed this knowledge will increase. For example, an unusual crater with radiating troughs has been discovered which scientists are calling "the spider."[23]
Surface features are referred to by several names in planetary geology.Albedo features refer to areas of markedly different reflectivity, as seen by telescopic observation. Mercury is also observed to have other types of features, including Dorsa (ridges),moon-like highlands, Montes (mountains), Planitiae, or plains, Rupes (escarpments), and Valles (valleys). [24] [25]
Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the late heavy bombardment that came to an end 3.8 billion years ago.[26] During this period of intense crater formation, the planet received impacts over its entire surface[25], facilitated by the lack of any atmosphere to slow impactors down.[27] During this time the planet was volcanically active; basins such as the Caloris Basin were filled by magma from within the planet, which produced smooth plains similar to the maria found on the Moon.[28][29]
Craters on Mercury range in diameter from a few meters to hundreds of kilometers across. The largest known craters are the enormous Caloris Basin, with a diameter of 1550 km,[30] and the Skinakas Basin with a diameter of 1,600 km, but known only from low resolution Earth-based imaging of the non-Mariner-imaged hemisphere. The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for the origin of this geomorphological unit is that shock waves generated during the impact traveled around the planet, and when they converged at the basin’s antipode (180 degrees away) the high stresses were capable of fracturing the surface.[31] Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin’s antipode.[32]
The plains of Mercury have two distinct ages: the younger plains are less heavily cratered and probably formed when lava flows buried earlier terrain. One unusual feature of the planet’s surface is the numerous compression folds which crisscross the plains. It is thought that as the planet’s interior cooled, it contracted and its surface began to deform. The folds can be seen on top of other features, such as craters and smoother plains, indicating that they are more recent.[33] Mercury’s surface is also flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.[34]
The mean surface temperature of Mercury is 442.5 K (169.4 °C/336.8 °F)[2], but it ranges from 100 K (−173 °C/−280 °F) to 700 K (427 °C/800 °F)[35], due to the absence of an atmosphere. On the dark side of the planet, temperatures average 110 K (−163 °C/−262 °F).[36] The sunlight on Mercury’s surface is 6.5 times as intense as it is on Earth, with a solar constant value of 9.13 kW/m². Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including Solar wind and micrometeorite impacts.[37]
Despite the generally extremely high temperature of its surface, observations strongly suggest that ice exists on Mercury. The floors of some deep craters near the poles are never exposed to direct sunlight, and temperatures there remain far lower than the global average. Water ice strongly reflects radar, and observations by the 70m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles.[38] While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.[39]
The icy regions are believed to be covered to a depth of only a few meters, and contain about 1014–1015 kg of ice. By comparison, the Antarctic ice sheet on Earth has a mass of about 4×1018 kg, and Mars’ south polar cap contains about 1016 kg of water. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by impacts of comets.[40]
Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. It takes 88 days to complete an orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having the same semi-major axis. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet’s rotation around its axis, result in complex variations of the surface temperature.[11]
Mercury’s orbit is inclined by 7° to the plane of Earth’s orbit (the ecliptic), as shown in the diagram on the left. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between the Earth and the Sun. This occurs about every seven years on average.[49]
Functionally, Mercury’s axial tilt is nonexistent,[50][51] with measurements as low as 0.027°.[5] This is significantly smaller than that of Jupiter, which boasts the second smallest axial tilt of all planets at 3.1 degrees. This means an observer at Mercury’s equator during local noon would never see the Sun more than 1/30[52] of one degree north or south of the zenith. Conversely, at the poles the Sun never rises more than 2.1′ above the horizon.[5]
At certain points on Mercury’s surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four days prior to perihelion, Mercury’s angular orbital velocity exactly equals its angular rotational velocity so that the Sun’s apparent motion ceases; at perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.[11]
During the 19th century, French mathematician Le Verrier noticed that that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He proposed that another planet might exist in an orbit even closer to the Sun to account for this perturbation. (Other explanations considered included a slight oblateness of the Sun.) The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place great faith in this explanation, and the hypothetical planet was even named Vulcan. However, no such planet was ever found.[53]
In the early 20th century, Albert Einstein’s General Theory of Relativity provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds per century, therefore it requires a little over twelve million orbits for a full excess turn. Similar, but much smaller effects, operate for other planets, being 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.[54][55]
For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the Sun at all times, in the same way that the same side of the Moon always faces the Earth. However, radar observations in 1965 proved that the planet has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury’s orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury’s sky.[56]
The original reason astronomers thought it was synchronously locked was that whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, hence showing the same face. Due to Mercury’s 3:2 spin–orbit resonance, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days.[11] A sidereal day (the period of rotation) lasts about 58.7 Earth days.[11]
Orbital simulations indicate that the eccentricity of Mercury’s orbit varies chaotically from 0 (circular) to a very high 0.47 over millions of years.[11] This is thought to explain Mercury’s 3:2 spin-orbit resonance (rather than the more usual 1:1), since this state is more likely to arise during a period of high eccentricity.[57]
The first spacecraft to visit Mercury was NASA’s Mariner 10 (1974–75).[9] The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this gravitational “slingshot” effect and the first NASA mission to visit multiple planets.[87] Mariner 10 provided the first close-up images of Mercury’s surface, which immediately showed its heavily cratered nature, and also revealed many other types of geological features, such as the giant scarps which were later ascribed to the effect of the planet shrinking slightly as its iron core cools.[89] Unfortunately, due to the length of Mariner 10's orbital period, the same face of the planet was lit at each of Mariner 10’s close approaches. This made observation of both sides of the planet impossible[90], and resulted in the mapping of less than 45% of the planet’s surface.[91]
The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km of the surface.[92] At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury’s rotation was expected to be much too slow to generate a significant dynamo effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet’s magnetic field is much like the Earth’s, which deflects the solar wind around the planet. However, the origin of Mercury’s magnetic field is still the subject of several competing theories.[93]
Just a few days after its final close approach, Mariner 10 ran out of fuel. Since its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut itself down on March 24, 1975.[94] Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.[95]
A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004, from the Cape Canaveral Air Force Station aboard a Boeing Delta 2 rocket. The MESSENGER spacecraft will make several close approaches to planets to place it onto the correct trajectory to reach an orbit around Mercury. It made a fly-by of the Earth in August 2005, and of Venus in October 2006 and June 2007.[96] The first fly-by of Mercury occurred on January 14, 2008.[97] Two more fly-bys of Mercury are scheduled, in October 2008 and September 2009.[6] Most of the hemisphere not imaged by Mariner 10 will be mapped during the fly-bys. The probe will then enter an elliptical orbit around the planet in March 2011; the nominal mapping mission is one terrestrial year.[97]
The mission is designed to shed light on six key issues: Mercury’s high density, its geological history, the nature of its magnetic field, the structure of its core, whether it really has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe is carrying imaging devices which will gather much higher resolution images of much more of the planet than Mariner 10, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles. Detailed measurements of tiny changes in the probe’s velocity as it orbits will be used to infer details of the planet’s interior structure.[21]
READING FOR THE NEXT LECTURE
