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On the next three pages we will journey to the two planets between Earth and the Sun. The closest-in is Mercury, which looks like a larger version of the Moon except for the near absence of young basaltic (mare-like) lavas. Next is Venus which normally is completely enshrouded with a very hot gas (carbon dioxide) and clouds of sulphuric acid. But landers have reached its surface (surviving only for hours). More importantly, radar, first from Earth, then from orbiting satellites, has given us remarkable views of surface features - some similar to Earth counterparts but many almost unique.

The Inner Planets: Mercury and Venus

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

Partial views of Mercury made by mosaicking individual pictures taken by Mariner 10.

The three encounters permitted 45% of the planet to be imaged. This next view shows part of Mercury as rendered in quasi-color.


Color-tinted Mariner 10 mosaic of the surface of Mercury, 1973-74.

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:


Mosaic of Mariner 10 photos that show the large Caloris Basin (left) and the cratered terrain that typifies Mercury.

19-24: How many rings can you make out for the Caloris Basin? ANSWER

The dominance of craters is evident in this view of part of the southern hemisphere:

The heavily cratered surface in the southern mercurian 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 Mercury terrain type known as Intercratered Plains (IP).

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.

A mare-like surface on Mercury with a notably lower crater density.

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.

A mare-like surface on Mercury with a notably lower crater density.

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:

Fault scarp running through craters on the mercurian surface.

In sum, the history of Mercury is twofold: volcanic events that produced plains units and impact cratering that have greatly modified the terrains dominated by flows.

19-25: Discuss major similarities and differences between the Moon and Mercury. ANSWER

Venus is another matter, being shrouded with clouds, with a complex younger surface mixed with more ancient terrain that bespeak of an intriguing history. 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 would depend either on landers or on cloud-penetrating radar. 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.

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:

T-P profiles in the venusian 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.

Venera 9 spacecraft.

The detachable lander from Venera 9.

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.

Views of the venusian surface immediately next to the Venera 9 and 10 landers (Russian); find the hamburger rock.

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.

Venera 9 panoramic view.

The first color images of the venusian surface were obtained by Venera 13 and 14.

Main image made by Venera 13.

A view in enhanced color from Venera 13 suggests an iron-rich oxidized surface:

Color Venera 13 photograph of the surface of Venus suggesting an iron-rich oxidized surface.

19-26: Ignoring the reddish iron surface stain, what does the other dark rock remind you of (in terms of rock type)? ANSWER

The next three images all were taken of the rocks around the Venera 13 landing site.

Venera 13 panorama; the fuzziness of this and other Venera longer range views is probably the result of atmospheric conditions.

Another panoramic view.

Close-up of 'pancake' slabs of venusian rock.

Venera 14 also landed successfully and operated for well over an hour. Here are its main views:

Two Venera 14 black and white images.

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).

Visible light image of Venus, taken during Mariner 10 flyby. Color Mariner 10 image taken through a UV filter of Venus, February 10, 1974.

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

Pioneer 10 image of Venus in the UV.

Two Pioneer Venus spacecraft arrived in late 1978. Venus Pioneer2 (Pioneer 13 in the series) in November 1978 deployed several probes but only one survived its transit through the atmosphere, landing and operating for about an hour. Venus Pioneer 1 (Pioneer 12), shown below, enter orbit around Venus in December 1978 with a diverse compliment of instruments, listed beneath the illustration:

The Venus Pioneer 1 space probe that orbits Venus.

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

Both Venera and Pioneer 10 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:

The chemistry of atmospheric sulfur constituents in the venusian atmosphere

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 instrument used was NIMS (Near Infrared Mapping System);

NIMS map of carbon dioxide variation in the venusian atmosphere, made as Galileo swung around Venus to get a gravity kick enroute to Jupiter.

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.

The inpenetrable cloud cover masking Venus' surface was first penetrated 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.


Earth-based radar image of part of Venus.

Venus Pioneer 1 provided 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:

Radar image of venusian surface, made from Venus Pioneer 1 data.

The next two illustrations are of parts of the venusian surface that have been colored to represent changes in elevation.

Venus Pioneer radar imagery colored to indicate topography using elevation differences as determined by altimetry.

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.

Map of Venus with named features.

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). Montain 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.

Imaging radar mosaic made by 8 cm band radar unit on Venera 15, showing the Ishtar Terra region, including Laksmi Planum and Maxwell Montes.

Closer view of Maxwell Montes, from Venera 15 observations..

19-27: What is the most conspicuous geologic feature in this scene? ANSWER

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:

Venus landforms.

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!

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Primary Author: Nicholas M. Short, Sr. email: [email protected]