Mercury orbits at an average distance of 0.387 A.U. relative to Earth but its high eccentricity (0.205) results in a perihelion of 0.307 A.U. and aphelion of 0.467 A.U. Its diameter is 4880 km (3030 miles) compared with 3478 km (2160 miles) for the Moon and 12756 km (7921 miles) for Earth; volumewise Mercury is 1/50th that of Earth and three times larger than the Moon. Mercury's density (5.44 gm/cc) is close to Earth's (5.53 gm/cc), suggesting it too has differentiated into a crust, mantle and large iron core but presence of a weak magnetic field implies that, while the core has maybe largely solidified, it has a molten component. Without an atmosphere, Mercury is heated directly by the Sun, to which it is the closest planet, to a mean surface temperature of 180° C (on the perisolar side, this temperature rises to as high as 425° C; on the nightside, it can drop to - 170° C). This innermost planet has periods of revolution and rotation of 88 and 59 days respectively, in a ratio of 3 to 2; this relation is said to be spin-orbit coupled.
The surface of Mercury bears no resemblance to Earth but to the casual eye is almost a twin of the Moon. This is immediately evident in this view which shows the eastern and western hemispheres of the mercurian surface imaged by TV vidicon cameras onboard the Mariner 10 spacecraft launched on November 3, 1973 first to Venus (see below) and then past Mercury on March 29, 1974 and around the Sun twice to return for additional Mercury flybys in September 1974. and March 1975
The three encounters permitted 45% of the planet to be imaged. This next view shows part of Mercury as rendered in quasi-color.
Craters, once again, are the prevalent geomorphic feature, with some impact basins exceeding 200 km (124 miles) (largest is the multi-ringed Caloris Basin at 1300 km [807 miles] diameter, part of which is seen in this next image):
The dominance of craters is evident in this view of part of the southern hemisphere:
Unlike the Moon, distinctly basalt-filled maria are sparse, although some small darker patches have been seen. This implies that the general second melting that occurred on the Moon about 3.8 - 3.9 billion years ago did not happen on Mercury whose surface probably is even older and preserves the same period of impact devastation associated with the lunar highlands.
However, the bulk of the Mercurian surface is described as being a relatively flat volcanic plains made up of iron-rich lavas. Some plains units may be original crust. The most common terrain type is Intercratered Plains as seen here:
A second low relief unit has been termed Smooth Plains (although likely volcanic in origin, its mode of emplacement is debatable). It resembles visually the lunar maria and also shows a notably reduced population of craters, suggesting this Plains unit is younger than most of the mercurian surface and partly fills many older craters.
This reduction in crater numbers is further evidenced in this image:
Some regions on Mercury are rugged, with large hills and lineations, as seen here. They may be a mix of ejecta units and volcanic structures.
Mercury, in some differences from the Moon, displays occasional structural features indicative of compression. One example is this fault, interpreted to be a thrust in nature, that forms a scarp 3 km high:
The history of Mercury is twofold: volcanic events that produced plains units and impact cratering that have greatly modified the terrains dominated by flows.
NASA has renewed the Mercury Exploration program with the August 3, 2004 launch of MESSENGER, which will make several passes near Venus and the Sun (for gravity boosts of orbital velocity) and then reach orbit around Mercury in 2011. This sophisticated satellite looks like this (artist's rendition)
Among its proposed instruments are 1) MDIS = Mercury Dual Imaging System; 2) GRNS = Gamma Ray and Neutron Spectrometer; 3)XRS = X-ray Spectrometer; 4) MAG = Magnetometer; 5) MLA = Mercury Laser Altimeter; 6)MASCS = Mercury Atmospheric and Surface Composition Spectrometer; 7) EPPS = Energetic Particle and Plasma Spectrometer. Included in its scientific goals are learning more about Mercury's crust, core, and magnetic field. You can follow this mission by tracking its home page
Messenger made its first fly-by pass on January 14, 2008. The MDIS produced images of the side of Mercury that was not seen during the Mariner 10 mission. Here are three of the first series of panoramas and close-ups:
The impression given by these images is that the previously unseen mercurian landscape is much like that envisioned decades earlier after the Mariner flyby.
Messenger is sending back improved images of mercurian craters. This one is comparable to the Gosses Bluff crater in Australia inasmuch as it has a prominent inner ring analogous to the central peak that forms from rebound as the impact shock waves force up material from below. Note the smooth areas in and out of this ring, which could be volcanic material extruded after impact.
Two other similar mutliringed craters appear in these Messenger images:
One crater imaged during this first flyby is almost certainly volcanic in origin. Note the radial grooves: these are depressions, a feature associated with tensional stresses resulting in an updoming by magma. The crater itself (below) is typical of the morphology of a volcanic caldera. However, the association with the grooves, which appear to be grabens, does not have a comparable counterpart on Earth, so this feature may be unique. A plausible explanation: shallow magma could have domed the crust, causing tension that led to the grabens, followed by volcanic eruption and caldera collapse.
The image below shows surface features that may have a volcanic origin. Two craters near the bottom each have narrow dark rims. Melt glass from an impact is an alternate hypothesis. Possibly, the craters have exposed a pre-formation dark layer in the mercurian near-surface.
Messenger afforded a better look at the Caloris Basin. It actually is about 100 km wider than earlier estimates, confirming it to still be the largest megacrater in the Solar System. As such, it dug deep into the crust, probably into the mantle, and has brought this material to the surface. Chemical analysis of the ejecta will establish the composition of the upper interior of Mercury.The Messenger images can be in quasi-natural color. Here is one example:
Messenger confirmed that the planet's core is large, extending to about 60% of the radius. This makes the mercurian interior different from Earth and Venus. The outsized core needs to be explained. One hypothesis postulates that Mercury was once larger and has lost some of its outer layers, by process(es) still speculative.
The second Mercury flyby took place on October 6, 2008. Messenger imaged surfaces never seen in previous passes (but examined by earth-based radar). This view shows several prominent craters which appear fresher (younger), from which ejecta rays emanate:
This flyby discovered another, previously unknown large (715 km [425 miles] diameter) impact basin, given the name "Rembrandt":
Another flyby occurred in September of 2009. In this color image, a bright (colored yellow) apron around a crater is most likely impact ejecta but possibly could be ash from a volcanic eruption. In the central image is a dark blue area which shows faint traces of a rim of a crater 180 km in diameter.
Both Messenger and Mariner provided evidence of Mercury's interior. Its density of ~5.4 gm/cc indicates a large core made up of a heavy metal - most likely iron. Compared with Earth, Mercury's core is larger, its mantle is less thick, and its crust is at least 100 km thick on average. This is a reasonable model of the interior:
The planet Venus is nearly the size of Earth. Unlike the Earth, it does not disclose its surface features, as these are perpetually shrouded with clouds. Its high reflectivity (albedo of 0.71; it is evidenced in the night sky as the bright "Evening Star" easily seen from Earth) implies a dense atmosphere, so that knowledge of its surface has depended either on landers or on cloud-penetrating radar.
Often the brightest "star" in the sky, Venus is Earth's closest neighbor. It was considered special to the ancients, some of whom thought it to be a planet. Along with the four moons of Jupiter, the phases of Venus led Galileo (who observed it with his remarkable telescope) to use these discoveries to argue for the heliocentric concept of the Solar System, i.e., the Earth and the other planets all revolve around the Sun. The phases could be best explained with the fact that, like our Moon, to an Earth-based observer the progressive lighting of the venusian disc results from the changing position of Venus as it too revolves around the Sun. This image, made as a montage of successive observations of the venusian phase stages, demonstrates the progressively greater illumination of the venusian surface as seen from Earth:
Venus, which lies at 0.72 A.U. from the Sun (and comes as close as 44 million km to Earth), has a period of revolution around the Sun of 225 Earth days and a very slow 243 day rotation which is retrograde (spins clockwise as seen from the north pole instead of the counterclockwise motion of Earth and most other solar planets), so that a sunrise would appear to begin on the western sky to an observer on the planet. Being slightly smaller (diameter: 12,100 km) than Earth, and 88% of Earth's volume, Venus has sometimes been called Earth's twin, but on close examination of its atmosphere and its surface, both quite different from Earth, the similarity in size is coincidental. Venus' interior is constructed of a solid iron core (thus no magnetism), a thick mantle, and an outer crust whose major features are relatively young.
The inpenetrable cloud cover masking Venus's surface was first breached by Earth-based imaging radar beams sent from antennae at the Arecibo Observatory in Puerto Rico, the Goldstone Tracking Station in California, Haystack in Massachusetts, and others. Wavelengths vary from 3.8 to 70 centimeters. Interference techniques using Doppler shifts process the reflected signals which offer some information on dielectric constants, surface roughness, slopes and rather crude estimates of elevation differences. Surface resolutions (areal) can be as low as 100 km (62 miles) or can be better than 3 km (2 miles). Here is an image showing variations in intensity of a backscattered radar beam transmitted to Venus at 12.5 cm from the Jet Propulsion Laboratory's Goldstone Tracking Station in the Mojave desert.
A series of missions by both the U.S. and the Soviet Union have unlocked some of its mysteries. Exploration of Venus by flyby probes was part of NASA's Mariner program which also included trips to Mars and the above-mentioned Mercury passes. Mariner 2, with its infrared and microwave radiometers, was the first American interplanetary probe, launched on August 27, 1962. Passing Venus as close as 41,000 km (25460 miles), it determined an approximate temperature for the outer cloud deck of ~500° C. Mariner 5, in 1967, came within 10,150 km (6300 miles), using these and a UV sensor to add more to the database.
About this time, the first Soviet probe, Venera 4, descended by parachute through the atmosphere in an attempt to touch down on the surface. It apparently was crushed by the dense atmosphere (~90 atm) and high temperatures but did return information confirming that CO2 makes up about 97% of all gases present (very little water) and detecting droplets of sulphuric acid in the outer cloud deck. Venera data (refined by later Pioneer data) also lead to a general profile of temperature and pressure distributions in the atmosphere:
After two more failures, Venera 7 reached the surface and survived for 23 minutes in 1970. It gave the first specific surface temperature (750°K) and pressure (90 to 100 atm.). Venera 8 also succeeded in 1972, adding chemical composition data on radioactive U, Th, and K from analysis by a gamma ray spectrometer that suggests local rocks are potassium-rich (4%) basalts containing 200 ppm Uranium and 650 ppm Thorium (both major heat sources). Measured surface temperatures were ~470° C. Four more Veneras reached the surface between 1975 and 1982; each carried a photographic system that returned pictures of the immediate surroundings. Venera 9 is shown in these photos, first of the entire spacecraft and second of the lander.
Two views, taken from Venera 9 and 10, disclose a rocky surface; note in the upper image a distinctive rock that reminds some viewers of a MacDonald's hamburger.
This is a close-up view of some of the Venera 9 rocks.
Here is another Venera 9 image, a panoramic view. Note its darkness, the result of much less sunlight reaching the venusian surface.
The first color images of the venusian surface were obtained by Venera 13 and 14.
A view in enhanced color from Venera 13 suggests an iron-rich oxidized surface:
There is some debate as to the actual color of the landscape if one could stand at the venusian surface. This view made by Venus Express emphasizes the color based on the Venera results:
Not only the surface appears reddish, but also the sky (blue and green light is filtered out).
The next pair of images were taken of the rocks around the Venera 13 landing site.
Venera 14 also landed successfully and operated for well over an hour. Here are its main views:
The next U.S. probe to Venus was Mariner 10, arriving in February 1974. Using a special UV filter, its imaging camera was able to penetrate the CO2-dominated atmosphere to detect cloud swirls that emphasized concentrations of excited carbon monoxide, suitable as markers of the general circulation patterns (winds up to 370 km/hr [230 mph] within the gas envelope).
This rendition, using blue instead of the near true color seen above, helps to define the cloud-rich from the cloud-poor parts of the atmosphere
Mariner 10 found a circulating pattern of clouds - a vortex - developed over the southern polar region:
The Mariner 10 view of the north polar vortex was even more dramatic:
Two Pioneer Venus spacecraft - the first an orbiter; the second a multiprobe - arrived in late 1978. Pioneer Venus 1 (Pioneer 12 in the multi-planet series), shown below, entered orbit around Venus with a diverse compliment of instruments, listed beneath the illustration:
Cloud photopolarimeter - measured the vertical distribution of the clouds
Surface radar mapper - mapped planetary topography and surface characteristics
Infrared radiometer - monitored IR emissions from the Venusian atmosphere
Airglow ultraviolet spectrometer - measured scattered and emitted UV radiation
Neutral mass spectrometer - evaluated the composition of the upper atmosphere
Solar wind plasma analyzer - measured properties of the solar wind
Magnetometer - examined Venus' magnetic field
Electric field detector - studied the solar wind and its interactions with the Venusian atmosphere
Electron temperature probe - examined the thermal properties of Venus' ionosphere
Ion mass spectrometer - measured the ionospheric ion population
Charged particle retarding potential analyzer - Studied ionospheric particles
2 radio science experiments - mapped Venus' gravity field
Radio occultation experiment - helped characterize the atmosphere
Atmospheric drag experiment - upper atmosphere density measurements
Radio science atmospheric and solar wind turbulence experiment
Gamma ray burst detector - monitored gamma ray burst events
After a six-month journey, Pioneer Venus 1 entered an elliptical orbit around Venus in December 1978 and began a lengthy reconnaissance of the planet. The spacecraft returned global maps of the Venusian clouds, atmosphere, and ionosphere, measurements of the interaction between the atmosphere and the solar wind, and radar maps of 93% of the planet�s surface. In 1991 the Radar Mapper was reactivated to investigate previously inaccessible southern portions of the planet. In May 1992 Pioneer Venus began the final phase of its mission, in which the periapsis (orbital low point) was held at 150-250 km until the fuel ran out and atmospheric entry destroyed the spacecraft. Despite a planned primary mission duration of only eight months, the probe remained in operation until Oct. 8, 1992.
Pioneer Venus 2 consisted of a bus which carried one large and three small atmospheric probes. The large probe was released on Nov. 16, 1978, and the three small probes on Nov. 20. All four entered the Venusian atmosphere on Dec. 9, followed by the bus. The small probes were each targeted at different parts of the planet and were named accordingly. The North probe entered the atmosphere at about 60� latitude on the day side. The Night probe entered on the night side. The Day probe entered well into the day side, and was the only one of the four probes that continued to send radio signals back after impact, for over an hour. With no heat-shield or parachute, the bus survived and made measurements only to about 110 km altitude before burning up. It afforded the only direct view of the upper atmosphere of Venus, as the probes did not begin making direct measurements until they had decelerated lower in the atmosphere.
Among the Pioneer Venus results of atmospheric measurements were: 1. The atmosphere was found to circulate in large planetwide systems, much simpler than the circulation patterns on the Earth; 2. The atmosphere, which has winds moving at speeds in excess of 300 km/hr, appears to be decoupled from the rotation of the planet itself, which is much slower on a daily basis (it takes 244 Earth days to make one complete rotation); at these speeds it would take about 4 Earth days for an object inserted into the atmosphere to be driven completely around the planet by atmospheric circulation; 3. A collar of polar clouds was discovered, which may be part of a large atmospheric circulation vortex; 4. At least four distinct cloud and haze layers were found at different altitudes; 5. The haze layers contain small aerosol particles, including sulfuric acid droplets.
Both Venera and Pioneer Venus data confirmed the small (1.5%) but important role of sulphur in the venusian clouds. These clouds are composed of droplets of sulphuric acid, sulfur particles, and SO2 (this last constituent accounts for absorption in the UV that explains the darker bands. The sulphuric acid is derived from reactions of water vapor in the atmosphere with sulphur compounds released from volcanoes (no evidence yet of active ones today). One model, by Keven McGouldrick, shows a possible chemical scenario:
Although this suphuric acid has a strong corrosive effect on the surface and on manmade probes that pass through the atmosphere, possibly to land, it is the CO2 that most influences the planet. The gas came largely from volcanoes. Over time it built up concentrations in the atmosphere much higher than on Earth. This has led to a classic example of the "Runaway Greenhouse" effect (see Section 16). The result is that heat from both the surface and the Sun has been trapped in the gaseous envelope causing the high observed temperatures. Variations in CO2 levels within the atmosphere were measured during a flyby by the Galileo satellite enroute to Jupiter. The Galileo instrument used was NIMS (Near Infrared Mapping System);
Various probes through the atmosphere have revealed why the observed cloud patterns seem to be "bent" in a V profile. The upper atmosphere conditions cause winds near the equator to move faster than those in polar regions, causing a drag effect that is almost independent of the planetary surface retrograde motion. Technically, the phenomenon of differential cloud movement is the result of combined 4 (earth) day equatorial and 5 day Rosslyn wave interactions.
Pioneer Venus 1 provided good quality radar imagery of most of the venusian surface. A radar altimeter was used. The image below is of a part of that surface, rendered in strong contrast to emphasize slopes:
The next two Pioneer Venus 1 illustrations are of parts of the venusian surface that have been colored to represent changes in elevation.
Venus Pioneer 1 data were eventually organized into a map the covers almost the entire surface of Venus (some polar data are missing). This provided the first map of this planet. Science teams had "fun" in giving names to various features that were then officially adopted. The later Magellan maps (next page) have added more names.
Even these earlier radar images pointed to a relatively flat Venus but with some highlands exceeding 6 km (3.7 miles). Two continental-sized areas of higher elevation are Ishtar Terra and Aphrodite Terra. These contain most of the mountains that may not be primarily volcanic in origin (are more beltlike). Mountain terrain comprises 10% of the venusian surface; 70% are upland plains; 20% are lowland plains.
The Soviet Venera 15 and 16 orbiting spacecraft (1983) carried imaging radar (8 cm wavelength) capable of 1-2 km (0.6-1.2 miles) ground resolution that gathered coverage over about 25% of the planet. The scene below is in the general Ishtar Terra region of northern Venus and shows the eastern part of Laksmi Planum, the wrinkled Maxwell Montes, and the large crater Cleopatra, a scene well over 2000 km (1240 miles) wide at the base. Beneath it is an enlargement of part of the image, showing Maxwell Montes in more detail.
Starting with these first views of the venusian surface, and amplified by the Magellan observations, a nomenclature for landforms and features on Venus evolved. Here is a table which summarizes these:
The Category column indicates how individual names, based mainly on Greek mythology, are chosen.
Now on to Magellan - one of the most successful NASA missions ever!