For part 3 of our celestial safari, we visit the second planet from the Sun, Venus - a caustic, hellish, cloud-choked nightmare whose familiar size and planetary background should give us pause in confronting runaway global warming on Earth. Brush up on your Dante, and prepare to descend into the Inferno.
The progress of our adventure so far: (current in bold)
1. The Sun
2. Mercury
3. Venus
4. Earth
5. The Moon
6. Mars
7. Asteroids
8. Jupiter
9. Io
10. Europa
11. Ganymede
12. Callisto
13. Saturn
14. Mimas
15. Enceladus
16. Tethys, Dione, and Rhea
17. Titan
18. Iapetus
19. Rings & Minor Moons of Saturn
20. Uranus
21. Neptune
22. Triton
23. The Kuiper Belt & Scattered Disk
24. Comets
25. The Stellar Neighborhood
Venus in true-color, as our eyes would see it if we were there:
I. Context
In addition to being the second planet from the Sun, Venus is also the third smallest in the solar system after Mercury and Mars, respectively. Diagrams of Venus's location and path of motion, from a receding solar Northern perspective:
As is characteristic of an inner planet, Venus is largely alone in its solar orbit. Smaller objects which may have formed in its vicinity either impacted Venus long ago, experienced orbital decay (inward spiraling) into the Sun, or were thrown into a highly-elliptical orbit with a distant far-point (aphelion). However, objects exist which do cross the orbit of Venus on closest approach to the Sun (perihelion), so there are still occasionally impacts.
Of particular note, Venus has a quasi-satellite - a co-orbital asteroid whose solar orbit makes it appear involved with the planet by coincidence despite being only secondarily influenced by it. This particular asteroid, 2002 VE68, is about 230-510 meters wide and is a member of the inner solar system Aten asteroid population. The object entered its current configuration with Venus some 7000 terrestrial years ago and is projected to leave it within about 500 terrestrial years. A general illustration of a quasi-satellite relationship:
Of course, you could not actually see the asteroid from the surface of Venus - it would be totally obscured by global cloud-cover - and even from orbit around the planet it would be a tiny pinrick of light. But it is a small loss - all such apparent relationships are purely coincidental, and usually transient: Earth, for instance, has five quasi-satellite asteroids, each of which is scheduled to exit the configuration within tens to hundreds of years (although others may enter it).
With insight from the above image, we can begin to understand the actual motion of 2002 VE68 relative to Venus, via this diagram from Mikkola et al (2004) - the frame of reference is fixed on the orbiting planet, so the objective path of the asteroid actually includes the Sun:
Many inner-solar system asteroids cross the orbits of both Venus and Earth, so the two planets share a large proportion of the same impact threats - although 2002 VE68 is not considered dangerous to Earth for the foreseeable future. Here is a complete list of Aten asteroids identified to date, some of which currently cross or will eventually cross the orbits of either planet or both. However, the thick, dense Venerean atmosphere - I use the rarer adjective Venerean rather than Venusian for purely aesthetic reasons - is known to protect the planetary surface against impacts beneath a much larger size than that permitted by Earth's.
The solar environment of Venus is not as intense as temperatures on the surface appear to indicate - about 2,614 W/m^2 (watts per square meter) compared to 1,368 W/m^2 in Earth orbit: Only about twice as much irradiance, even though the greenhouse effect causes temperatures at the surface to be between two and four times higher than Earth on the Kelvin scale. This variability in ratio is overwhelmingly due to seasonal, latitude, and weather changes on Earth, whereas temperature on Venus is largely constant due to its atmosphere. The following is a comparison of the apparent size of the Sun from the vicinity of both planets, although you can't actually see it from the Venerean surface - it is just a diffuse glow throughout the sky:
In addition, Venus's solar orbit has only about 1/40th the orbital eccentricity of Mercury, so the amount of solar energy it receives is steady throughout its year. A chart comparing solar flux densities at Mercury, Venus, and Earth:
Due to its orbit being closer to the Sun than Earth - what in observational astronomy is called an inferior planet - Venus is never seen more than about 48 degrees from the Sun as seen in the terrestrial sky. One consequence of this fact is that it is most visible in the morning and evening, when the solar glare is blocked by the Earth's horizon - hence its dual incarnation in ancient astronomy as the Morning Star and Evening Star, respectively. It is the third brightest object in the terrestrial sky, after the Sun and Moon, due to the high reflectivity of its clouds. Images of Venus from Earth (click for attribution):
As seen from Earth, Venus transits the Sun about twice per century in sets of two events occurring eight years apart, with each set separated alternately by 105.5 or 121.5 terrestrial years. The reason for these long durations is the 3.4° inclination of Venus's orbit relative to Earth's, causing it to usually appear above or below the Sun, or else be invisible in its glare. An illustration of this relationship, modified from a drawing by Theresa Knott:
Due to the rarity of Venus transits, their occurrence is a cherished event with a relatively thin (but all the more carefully documented) photographic record. Its atmosphere can be glimpsed as a thin, illuminated arc caused by sunlight refracting through the gas:
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II. History
Like the other terrestrial planets - Mercury, Earth, and Mars - Venus formed when temperatures in the inner region of the Sun's protoplanetary disk cooled sufficiently for metallic elements like iron to precipitate (i.e., form liquid droplets). These droplets flowed together, bound initially by electrical and then gravitational forces, and solidified as temperatures further declined.
Unknown cycles of collision, coalescence, ejection, and destruction passed among the solid bodies thus formed in the inner solar system, but over time the material accumulated into a decreasing number of increasingly massive objects. Due to conservation of angular momentum, the rotation of the protoplanetary disk that formed them was gradually imparted on to these objects as random collisions averaged out to a net direction of spin: Counterclockwise as seen from solar North - the same direction in which the planets orbit the Sun, and in which the Sun itself rotates. Illustration:
However, it is important to note that the conserved angular momentum of the solar system is a net effect - it is what's left over when the rotational inertia of all the matter in the system is added up, so the fewer objects there are due to collision, accounting for a larger proportion of the mass, the more consistent it would appear. But in the case of an individual object, chance encounters can radically alter the equation - and this is what happened to Venus early in its history.
A giant impact event (or two) - a familiar scenario in planetary formation - happened to hit Venus a glancing blow at an angle opposite to its early rotation, blowing off a large chunk of matter that ended up in orbit. The direction of this orbit was counter to the rotation of proto-Venus, so as it rained back down on the surface over however long a span of time, the angular momentum of the planet was bled away and even reversed. Ultimately, when the material had all either escaped or been reabsorbed, Venus was left very slowly rotating in the opposite direction. This is called retrograde rotation.
In itself, retrograde rotation has no special implications - there is no inherent difference in behavior between moving clockwise and counterclockwise. However, the relative slowness with which Venus rotates - less than a single day per Venerean year - has several, very important consequences that combine with its proximity to the Sun to create its extreme environment:
- There is much less angular momentum to drive convection in the core, so little or no dynamo effect occurs, leaving the planet without a significant magnetic field.
- The lack of convection also causes heat transfer from the core to become inefficient, increasing subsurface temperatures until there is much less temperature difference between geologic layers. This further reduces the efficiency of heat transfer, depresses convection, and inhibits magnetic field formation.
- Without a strong magnetic field, the solar wind was not deflected from the atmosphere, so a constant stream of high-energy particles were free to collide with water molecules that once existed in the air and break them down into hydrogen and oxygen. Meanwhile, in a process called photodissociation, ultraviolet (UV) rays from the Sun continuously break down carbon dioxide (CO2) and sulfur dioxide (SO2) in the upper atmosphere into compounds that react with the hydrogen and oxygen from decomposed water vapor to form sulfuric acid (H2SO4) and other corrosive substances. What remains of the hydrogen escapes the planet entirely, leaving Venus arid.
- Solar and other cosmic events that impact the upper atmosphere with full intensity are thought to trigger vast lightning discharges.
- As the planet was depleted of water, the crust lost lubrication and became brittle - plate tectonics could not be sustained. It is thought that Venus consequently lacks an asthenosphere - the "sticky" layer of mantle that in Earth geology lubricates the relationship between the hotter, more fluid mantle beneath and the lithosphere (crust) above. As a result, heat from the mantle can only break through to the surface through weak points in an otherwise solid crust, leading to rampant volcanism that continues to this day.
- At some point between 600 and 300 million years ago, the ability of the crust to release the heat of the mantle became so inefficient that volcanic activity increased radically and the entire planet was resurfaced in a period of about 100 million years. This process is thought to be cyclical, with relatively brief epochs of extreme volcanic resurfacing followed by longer eras of reduced activity with a solid crust.
- The buildup of geological heat at the surface released CO2 into the atmosphere that would otherwise be dissolved in rocks, creating thick, soupy, crushingly dense and infernally hot air at lower altitudes that barely moves (only a few kilometers per hour).
- Sulfuric acid vapor created in the upper atmosphere rains down to a certain depth before evaporating again in an endless cycle, never reaching the surface.
- Clouds of SO2 and SO4 that enshroud the planet at middle and upper altitudes are responsible for its bright yellow-white color as seen from outside the atmosphere. These clouds move at high speeds along chevron-shaped storm currents very unlike the rotating vortices of terrestrial clouds, since there is minimal Coriolis effect. Ironically, Venus's beauty and its deadliness are intimately related.
Due to a runaway greenhouse effect, a planet whose climate may once have seemed familiar is today the closest known environment to classical depictions of Hell.
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III. Properties
1. Orbital and Rotational Features
Geometrically, Venus's retrograde rotation is often expressed in theoretical terms by stating that the planet's axis is tilted 177° - in other words, by framing the planet as being upside-down relative to the rest of the solar system - but this is not literally the case. Current theory does not propose that the planet was upended in the impact(s) that reversed its rotation. Rather, this assigned axial tilt is an otherwise meaningless artifact of the right-hand rule of angular vectors: In a rotating body, the "up" direction is arbitrarily designated as the direction of a right-hand thumb when oriented so that the other fingers of the hand close along the path of rotation.
By this standard, a body that rotates in the opposite direction has a downward angular vector - 180° away, if the motion is perfectly reflected. Now, we don't know with precision what Venus's original plane of rotation was, but today it is about 3° off the plane of its solar orbit, so that is its actual axial tilt. Aside from the oddity of its spin direction, the planet's motion is remarkably regular - its 3° tilt is trivial compared to Earth's 23°, and its orbit of the Sun is the most circular of any solar system planet, so there would be little seasonal change in climate even without the tremendous insulating effects of the atmosphere.
2. Size and Mass Characteristics
The mass of Venus is roughly 82% of Earth's, its gravity about 90% of what we experience (a typical adult human would be 12-17 lbs lighter), and its radius is close to 95% of our planet's. An important lesson to glean from these facts for a non-scientist is that mass and gravity are not simply related, although the former causes the latter: Density is also a consideration, because gravity declines with the square of the distance from the center of mass. For example, the surface of a hypothetical world made entirely of solid carbon would experience lower gravity than one composed of an identical mass of iron. As a result of this fact, planetary scientists are able to determine the rough composition of a body by mapping its gravity field - a process achieved by precisely charting the motions of a space probe in a particular kind of orbit around the body.
Some size comparisons of Venus with other solar system bodies in its class - mouse over to see the title if you don't recognize the other object:
Venus is the second biggest - and also second most massive - solid object in the solar system. It bears superficial resemblance to Earth due to size and mass, and resembles the appearance of Titan because both bodies are shrouded in opaque, dense atmospheres. However, there the resemblance ends: Atmospheric pressure on Titan's surface is about one-and-a-half times that of Earth at sea level, while Venerean surface pressure is a whopping 93 times the terrestrial environment. The Titanian atmosphere (not to be confused with Uranus's airless moon Titania, which apparently has the same adjective) is overwhelmingly nitrogen (N2), while that of Venus is primarily CO2. Titan is extremely frigid, with an ice crust, hydrocarbon seas, and methane rain; Venus is a furnace, rocky and arid.
3. Surface Temperatures
The atmosphere is so efficient at trapping heat that surface temperatures don't differ substantially between night and day or between polar and equatorial regions - it is more or less a uniform temperature, changing mainly with altitude (and not very quickly). The highlands and lowlands on the planet differ only by about 20 kelvins (36 Fº), although some high-altitude regions are hotter than the lowland plains due to apparent volcanism. Average surface temperature is 736 K (865 ºF) - hot enough to vaporize phosphorus, mercury, and sulfur, and melt cadmium, lead, zinc, and various other metals. However, its highest point (11 km above median radius) is only about 653 K (716 ºF) - pleasant Mercury-like temperatures at a relatively modest 53 atmospheres of pressure.
Below is a comparison of temperature data from NASA's Magellan probe and the ESA (European Space Agency) Venus Express, with some surface features noted. The units are in Celsius, but temperature differences are the same in Celsius and Kelvin, so the lack of a significant relationship to latitude is visible:
A more comprehensive temperature map of the Southern hemisphere via Venus Express, with units in Kelvin and topographical contours:
Note the hot lowlands in yellow and red, the green plains, and the relatively cool highlands and mountain peaks indicated in shades of blue, purple, and black. However, there are significant deviations from the generally-observed altitude/temperature relationship, and these anomalous readings are considered possible indications of recent geologic activity. A closer view of the South polar region illustrates these deviations with surface features and contours noted:
This is largely the extent of detailed, visually-represented temperature data for the surface of Venus at this point: Probes must take precise, specialized readings of very narrow strips of territory, and then the data must undergo intensive processing to remove thermal interference from various layers of the atmosphere in order to expose conditions at the surface. From there, further processing is needed to combine the readings into a coherent map, so the thermal charting of Venus is still an ongoing process via the continuing mission of Venus Express.
4. Surface Composition & Features
Data on the surface composition is limited, but overwhelmingly indicates basaltic (i.e., volcanic) rocks dominated by silicates. High-temperature chemical interaction with the atmosphere may selectively promote the presence of silicates on the surface over and above their general proportion of the crust due to the carbonate-silicate cycle.
Despite the relative lack of compositional data, a lot more is known about the shape of the Venerean surface - 98% of the topography has been mapped thanks largely to the Magellan probe. Due to the lack of seas, and thus the absence of "sea level," zero altitude is set at the mean planetary radius and regions beneath that are denoted with negative altitudes. A Mercator projection map of topography from the Pioneer Venus probe:
Note the appearance of the three "continental" masses: Ishtar Terra in the North, Aphrodite Terra near the equator, and Lada Terra in the Southern polar region. Next, we see global topographic perspectives from Magellan - be aware that resolutions are not uniform within individual images, so there may be apparent discontinuities:
Without color-coding for elevation, the radar-mapped surface looks somewhat different. Note the following images are based entirely on the reflectivity of the surface in invisible spectra, and the surface wouldn't necessarily have the same light/dark relationships if you were personally seeing it:
As per standards adopted by the International Astronomical Union (IAU), Venerean surface features are all designated to be named after female historical and mythological figures. Only three features are not named for women, and all three were named prior to the adoption of the standard: Alpha Regio, Beta Regio, and the highest mountain massif on the planet, Maxwell Montes. Regio is just a vague Latin term for a distinctive region, and Montes is a plural of mons - the typical designation for a mountain on another world.
A comprehensive list of approved and rejected feature names for Venus is available here, via the IAU page on the US Geological Survey (USGS) site. Of the 2,031 listings, 59 proposals were rejected. Venerean feature types (plural denoted second, where applicable) and naming standards:
Astrum / Astra - radial features: Misc. goddesses.
Chasma / Chasmata - long, deep, steep-sided canyons: Goddesses of hunting and the Moon.
Collis / Colles - small hills: Sea goddesses.
Corona / Coronae - ovoid, concentric-ringed features: Fertility and earth goddesses.
Large craters: Famous women.
Small craters: Female first names.
Dorsum / Dorsa - ridges: Sky goddesses.
Farrum / Farra - pancake-like volcanic domes: Water goddesses.
Fluctus / Fluctūs - cooled lava flows: Misc. goddesses.
Fossa / Fossae - long, narrow, shallow depression: War goddesses.
Labyrinthus / Labyrinthi - converging networks of ridges and valleys: Misc. goddesses.
Linea / Lineae - distinctively dark or bright, thin, elongated feature: War goddesses.
Mons / Montes - mountains: Misc. goddesses and one scientist.
Patera / Paterae - irregular, complexly-shaped craters: Famous women.
Planitia / Platiniae - lowland plains: Mythological heroines.
Planum / Plana - highland plains: Goddesses of prosperity.
Regio / Regiones - vaguely distinctive regions: Giantesses and titanesses.
Rupes, Rupēs - scarps: Goddesses of hearth and home.
Terra / Terrae - "continental" land mass: Love goddesses.
Tessera / Tesserae - cracked, polygonal terrain: Goddesses of fate and fortune.
Tholus / Tholi - small, domelike hill: Misc. goddesses.
Unda / Undae - dune field: Desert goddesses.
Vallis / Valles - valleys: Words for planet Venus in various languages.
A labeled map of the largest features:
To date, there have been 15 probe landings on the surface of Venus - 11 successful, and all but one of those successes being from the USSR. Of the 10 Soviet landings, 8 were part of the Venera program, and 2 were from the balloon/lander Vega program. One success, and the only American probe to reach the surface of Venus intact, was unintentional - an atmospheric descender as part of the Pioneer Venus Multiprobe survived the entire descent and continued to transmit for eight minutes after landing, but had not been designed to gather surface data. The locations of the illustrious 11:
The landings are clustered on the left (extreme Eastern longitudes) in an archipelago that extends across Beta, Phoebe, and Themis Regiones. Four of the successful landers - Veneras 9, 10, 13, and 14 - managed to take pictures from the surface, although two (Veneras 11 & 12) failed to do so because protective lens caps failed to deploy. The remainder of the landers were not equipped with cameras.
Venera 9, from a boulder-strewn slope in Beta Regio - the first ever images from the surface of another planet:
Venera 10, from lowlands in Beta Regio:
Venera 13, near Phoebe Regio:
A relatively recent reprocessing of the original Venera 13 image data by Don P. Mitchell reveals new details:
Venera 14, on a basalt plain near Phoebe Regio:
By now, these probes are almost certainly half-melted, imploded, Dali-esque heaps of corroded slag - much like the Soviet Union that sent them. However, they gave us our first windows into a soupy, volcanic, sulfur-yellow cauldron of a world.
From a higher perspective, the surface of Venus is cartographically organized into 62 quadrangles or "quads," all of which have labeled maps viewable in PDF form here. Interestingly, it seems the USGS maintains maps of every charted body in the solar system. A sampling of the more conspicuous quads from Venus, moving from Northern to Southern latitudes - you may have to click on them and view larger sizes to read the names of smaller features:
Nearly every circular feature seen in the images above is either a volcano or the remnants of a magma dome, although they are thought to be overwhelmingly dormant. This radically differentiates the surface of Venus from that of Mercury, the Moon, and most other known solid bodies where impact craters dominate the topography, and also from bodies like Earth and Mars where erosive processes (water and wind, respectively) play a prominent role. While the temperature, pressure, and reactive chemistry of the Venerean atmosphere obliterate many features that might otherwise exist, those which do survive abide in perpetuity, preserved by those same conditions - at least until renewed lava flow resurfaces the planet again.
There are, in fact, thought to be hundreds of thousands of volcanoes on Venus - so many, occurring in such a multitude of shapes and sizes, that there has yet to be a full census. But while it is true that Venus has far more volcanoes than Earth, it must be stated that this is only because its surface is much older - Earth is far more geologically active than Venus, but that fact regularly erases the topography of past epochs through subduction. On Venus, however, the history from one resurfacing epoch to another is on full display, pristine and relatively un-eroded.
The vast majority of volcanic features on Venus occur in the form of shield volcanoes - features where lava is free to move fluidly and tends to form shallow mountains, such as those of the Hawaiian islands on Earth. This is in contrast to explosive stratovolcanoes (e.g., Mount Vesuvius), which are not a feature strongly associated with Venerean geology. However, radical swings in atmospheric abundances of particular chemicals associated with volcanic plumes are thought to indicate explosive eruptions from time to time.
Additionally, large volcanoes on Venus are much wider than those on Earth, and also much flatter - probably due to the combination of highly fluid lava flow and extreme atmospheric pressure resisting upwelling. Their bases can be several hundred kilometers across, and yet reach heights maxing out in the single-digit kilometer range (the highest is 8 km) - sometimes even just a few hundred meters tall. As a result, the surface of the planet is extremely flat and shallow, with some volcanic "peaks" being little more than imperceptible highland plateaus. About half the surface of the planet exists within only 500 meters of the median radius, and 4/5 of it within 1 km.
Many volcanoes are so flat they resemble pancakes, and form features known as pancake domes - a few of which can be seen in the quads above. These are labeled Farrum or Farra. Smaller volcanoes can occur in clusters of several hundred called shield fields, but there are approximately 150 that range in size from 100 to 600 kilometers in breadth. To appreciate the scale of this, Mauna Loa in Hawaii - the largest volcano on Earth - is only about 120 km wide.
As on Earth, the relationship between volcanoes and mountains is not always simple: Many volcanoes are mountains, but not all - and not all mountains are volcanic. Maxwell Montes, for instance - the "Himalayas" of Venus - is thought to be a secondary feature of geologic activity around it, having been thrust upward by competing forces from various directions. Due to the lack of tectonic plates, this kind of uplift appears to occur inconsistently, according to the interference patterns of surrounding volcanic pressures. A diverse sampling of volcanoes, with some impact craters as well - see if you can tell them apart:
Although the surface is dominated by volcanic features and their secondary consequences, there are still a significant number of prominent impact craters. Just a few examples:
The central features of Adivar Crater above are typical of how impacts present on Venus: Sharply-defined, rough-textured, light-colored (in radar) features that resemble flower blossoms. Complex craters are strongly represented, which are cases with uplifted central peaks - a result of inward "sloshing" by the liquefied rock of the crater. This doesn't happen both above and below a certain size threshold, because on the lower-end the rock isn't fluid enough to slosh, and on the higher end the central lava plain is too large to have any significant rebound.
However, Adivar's extended features are unusual, and apparently unique to Venus: The impact occurred at a velocity (i.e., both speed and direction) that drove ejecta above the lower atmosphere into a region of rapid winds, where they were scattered into a parabolic fan. This material will gradually migrate with the slow surface winds into the lowlands.
Impact craters on Venus have a minimum diameter of 3 km, because the atmosphere obliterates objects beneath a certain size or reduces their speed so greatly that they hit the surface like a clod of dirt rather than a solid rock - i.e., one way or another, objects that fail to meet the minimum will not leave a lasting impression. However, those that do will leave behind immaculate, relatively erosion-free craters that will sit pretty much unchanged until they are resurfaced in the next geological peak. They will additionally create something relatively lacking on Venus: Sand and dust. Three-quarters of the planetary surface is bare, solid rock because nothing has happened to change it since it cooled from lava.
Because the surface is relatively young (compared to most other solar system bodies), and so few impacts manage to reach it through the atmosphere, there is a paucity of sand and dust that removes much of the erosive power of wind - what little there would be anyway, that is, given the languid motion of air at the surface. However, there are erosive features seen in lowland plains where sand and dust from impacts have gradually been deposited by the slow-but-dense surface wind - these are the areas where dunes and other fine-grained features occur. Once dust settles into a lowland, it's not leaving again short of another major impact event.
To return to the broader context, Venus is one of the five solar system bodies (the other four being Earth, Io, Enceladus, and Triton) thus far observed to have some kind of volcanism, and one of the three (in addition to Earth and Io) whose volcanoes occur in a rocky crust: Enceladus and Triton exhibit cryovolcanism (eruptions in a crust composed of ice). Titan is suspected of cryovolcanism, but this has not been confirmed.
5. Atmosphere
The gas composition of the Venerean atmosphere is overwhelmingly CO2 (96.5%), with a small proportion of N2 (< 3.5%) and SO2 (0.015%) in addition to trace levels of noble gases, CO, and water vapor.
At the surface up to tens of kilometers, the CO2 is in a state of matter called supercritical: In this state, high temperature and pressure remove the distinction between the liquid and gaseous phases of a substance. Humans have no direct experience of supercriticality due to the extreme conditions involved, so the concept may be difficult to visualize, but we can offhandedly say that the lower atmosphere of Venus is halfway in nature between air and sea. A phase diagram illustrates the pressure/temperature relationship for carbon dioxide:
Look back at the Venera lander images, and try to realize that the roiling yellow sky you see is not exactly a gas - rather, what you are seeing bears many physical similarities to the bottom of an ocean. In fact, the pressure is so intense at this layer that the surface is beneath the crush depth for most submarines currently in service in Earth's oceans, including nuclear-powered Seawolf-class vessels of the US Navy - the deepest divers in the fleet, at a crush depth of 730 meters. Pressures on the Venerean surface are close to an equivalent depth of 910 m in terrestrial oceans. However, the current state of technology is more than adequate to survive such pressures, and specially-designed vessels have successfully reached the lowest points on Earth (11 km beneath sea level, at over 1000 atmospheres of pressure). So despite its extreme atmospheric conditions, the surface of Venus is accessible.
In some ways the pressure can even be helpful to exploration: The density of the lower atmosphere is so great that lander craft can discard their descent parachutes tens of kilometers in the air and land softly without firing thrusters. The air is so thick as to be about 6.5% the density of liquid water, and even the slight 1-3 km/h breezes that occur on the surface can easily carry away small pebbles. If it were possible to be on the surface in a space suit (it isn't, currently) you would feel every arm and leg movement as if you were walking in a pool, and the motions would cause thick, slow-moving, languid vortices in the air like the motion of smoke.
The reason for the extreme amount - and consequently density - of CO2 in the atmosphere is that the planet lacks a carbon cycle: The process that on Earth keeps most of the planetary carbon bound in rocks and living organisms. Instead, on Venus, the oxygen is all bound in rocks and acid vapors while the carbon is released. In fact, oxygen is continuously being lost into space through a process described in more detail below.
Another oddity of the surface layer is driven by temperature differences between the plains and the mountain highlands: As we have noted, temperatures at the former are hotter than the melting points of lead and various other metals, allowing for the sublimation (direct transition from solid to gas) of some metallic compounds at lower altitudes. Once these compounds circulate to higher, cooler regions, they resolidify and accumulate on the highland surfaces through deposition (direct transition from gas to solid). Two such compounds that have been identified are lead sulfide (PbS) and bismuth sulfide (Bi2S3). In other words, the mountains are coated in metal snow.
We can actually see this "snow" in quad imagery - it appears as a radar-reflective material that causes mountainous highlands to appear bright in color. Maxwell Montes presents the most striking and extensive example, so let's revisit the image of Cleopatra crater from above - some of the white coloration you are seeing is lead- and bismuth-sulfide snow above an altitude of 2.6 km, although rough textures can also contribute to radar reflectivity:
Both temperature and pressure steadily drop with increasing altitude, until at about 25-30 km conditions are suitable for sulfuric acid in the atmosphere to form diffuse hazes - a distinct layer that continues for several more kilometers until the sulfuric acid is able to form discrete droplets and cloud forms. Ascending further, the temperature actually begins to increase again as the atmosphere thins and permits more direct exposure to sunlight - remember, Venus receives about twice the intensity of solar irradiance as Earth. A diagram of the relationship between pressure, temperature, and altitude on Venus:
Sulfuric acid in the cloud layer actually precipitates and rains back down into the haze, but increases in temperature to the point of evaporating again long before it can reach the surface - a phenomenon called virga that also occurs under some conditions on Earth, although with water rather than acid. The highly reflective top of the cloud layer is what we see from Earth, and is responsible for Venus's bright appearance. However, the structure of the clouds is much more visible in UV spectra, as in this famous photo often used as a substitute for the blander-looking visible light image at the top of this diary:
Note the chevron shape of the cloud patterns as they move toward the equator, as well as the brightness and thickness of the clouds in the polar region - something also visible in the true-color photo. The chevron pattern is due to the fact that winds at the cloud layer generally increase as latitude decreases, and beneath about 50° latitude reach speeds upward of 370 km/h - an instance of zonal flow, where air moves primarily along latitude. If we didn't know how slowly the planet rotates, we might think this means the surface is dragging the atmosphere along with it, but the actual culprit is the Sun: High in the atmosphere, above most of its insulating layers, the temperature divergence between night and day and between polar and equatorial latitudes becomes significant, and the planet's slow rotation makes that fact more extreme.
This has two consequences: The first is that equatorial and mid-latitude cloud layers in daylight will drive toward the cooler night in high-speed zonal flow. And secondly, air will circulate between lower latitudes and polar regions in Hadley cells - an example of meriodional flow that moves longitudinally, with cool air sinking to lower regions near the poles and hot air rising into the cloud layer near the equator. This second process is much less powerful, but in combination with the former yields a visually-fascinating polar cloud feature: The double-vortices. At each pole are two vortex clouds connected to each other in an S-shape:
Now, it's important to note that in infrared portions of images - usually denoted in shades of red (except where noted otherwise) - the darker regions are thicker clouds: The brighter regions are breaks in the clouds through which hot lower layers are seen. In UV images - usually shown in blue or white - the opposite is true, and the thickest clouds are lighter-colored because they are able to reflect more UV light. Some vortex close-ups in UV, with the location of the pole marked by a yellow dot - note that these are evolutionary images of the same location taken hours apart, moving clockwise from upper-left:
Although they are quite large, the double-vortices look much more intimidating than their wind speeds merit - the systems only move between 130 and 180 km/h in their fastest regions, which is relatively mild compared to zonal flow at lower latitudes. A couple of videos of the Southern polar vortex in motion:
Some broader perspectives on cloud formations:
And some fascinating closeups:
Venus has no magnetosphere, but the interaction of the solar wind with the furthest reaches of its atmosphere creates a small, induced magnetic field that has nothing to do with geology. Although the field offers virtually no protection from high-energy particles, it comes with a price: The side of the field facing away from the Sun is extended outward in a magnetotail that interacts with the high-altitude nighttime atmosphere to carry away hydrogen and oxygen ions into interplanetary space.
These ions are created on the day side by high-energy solar rays, breaking apart atmospheric compounds into electrically charged atoms, and then they migrate with the zonal flow toward the cooler night. Some of them recombine and return as compounds already mentioned, but some are lost for good - and the fact that it's largely the building blocks of water that are blown away has desiccated the planet. An illustration:
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IV. Past Relevance to Humanity
The Romans originally thought that Venus was two separate objects due to its discontinuous appearances in morning and evening, and a fascinating historical symmetry followed their name for the Morning Star - Lucifer. In itself, the name holds no menace - it just means Light Bearer - but at least one of the authors of the Book of Isaiah had written an allegory about the Morning Star merely being an arrogant pretender to the throne of the Sun that soon rose to outshine it (i.e., "cast it down from heaven" because it becomes no longer visible). It was a story intended to mock a Babylonian King that was at the time oppressing the Jews, but eventually the Morning Star came to be associated with Satan, and thereby Lucifer became the devil. Ironically, the character of Lucifer came to resemble conditions on the planet even as the name ceased to be identified with it.
However, Venus played a major role in the scientific revolution due to the fact that it exhibits phases as seen from Earth while never moving beyond a certain arc of distance away from the Sun. Galileo correctly interpreted this as proof that Venus orbits the Sun, which was in turn a powerful argument for a heliocentric model of the solar system.
In the 20th century, it would be the surprising scene of humanity's first interplanetary probes, largely undertaken by the Soviet Union. Venus was preferred over Mars because the thinness of the Martian atmosphere meant that landing required precision rocket firing, whereas on Venus a craft can be designed to simply "settle" to the ground unassisted once parachutes have carried it through the upper atmosphere. But the more we learned about it, the less familiar it felt - it became a horror story rather than a world, and with the demise of the Soviet Union, the priority of exploration was permanently reset to Mars.
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V. Modern Relevance to Humanity
There is today a renewed sense of urgency to understand the global consequences of the greenhouse effect, and Venus is the perfect case-study: A world where the effect is the single, overwhelming force responsible for its current atmospheric composition, temperature profile, and sub-oceanic surface pressures.
But even as its scientific profile has rebounded, Venus has also become logistically indispensable to space exploration in general: Gravitational assist flybys of Venus are currently the only practical way to reach Mercury (see the diary on it for an explanation of the difficulties of reaching it), and are also exceedingly convenient for accessing just about anywhere else in the solar system. Both the Galileo and Cassini-Huygens probes used flybys of Venus to reach Jupiter and Saturn, respectively. An animation of the MESSENGER spacecraft's flyby en route to Mercury:
However, there remain many difficulties in trying to explore Venus itself, especially where it concerns surface landings. The laws of thermodynamics require that a refrigerator throw off heat at a much higher temperature than its environment, so the power requirements and logistical difficulties of keeping a probe cool on the surface of Venus are enormous. Apart from expelling heat, it must find a way to prevent the heat it expels from building up around it and degrading the efficiency of refrigeration to the point of failure. That is in addition to the intense pressure of the atmosphere, which demands very thick-hulled, spherical vessels to avoid being crushed. No probe has yet survived more than a few hours on Venus, and those which managed to survive that long only did so by being designed to fail in slow, planned steps: There has never been an attempt to keep a probe operational indefinitely on the Venerean surface.
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VI. Future Relevance to Humanity
Venus has the closest approach of any other planet to Earth, and there are more launch windows per unit time than for Mars. As a result, the first manned interplanetary mission could very well be a Venus flyby - such an effort would involve a small fraction of the cost of landing on Mars, a shorter round-trip, fewer risks, and yet would be nonetheless awe-inspiring. The scientific value of a manned Venus flyby would be trivial - about the same data that a probe could gather, but on a much shorter timescale - but it could very well have a major impact on global morale to see human beings looking out a window filled by another world. Such was exactly the effect of the Apollo 8 lunar flyby.
In fact, a mission like this is not only possible today, but was possible in the 1960s using Apollo hardware - a proposal was actually made at the time, but was shelved due to various factors including the lack of experience with long-term space missions. Today, there has been far more experience via the International Space Station, with continuous missions often lasting months at a time.
Further down the road, it may be easier to explore Venus by simply not landing: There is a broad swath of the atmosphere whose conditions are relatively benign, and where a human-friendly vessel would be naturally buoyant. While the concept at first seems outlandish, being in a balloon station floating around the hospitable layers of Venus's atmosphere would be much safer than being in Earth orbit, and likely healthier due to the presence of 90% Earth gravity. It would also be in a much more nurturing environment than the frigid near-vacuum of Mars or the temperature extremes of the Moon.
The surface could still be explored with probes or submersible-like "diving" vehicles, while the remainder of the crew and habitat infrastructure "sail" the skies. In dire emergencies, the crew could survive brief exposure to the environment, and normal operations could involve relatively light space suits mainly concerned with providing breathable air and protecting skin from chemical vapors. This region of Venus's atmosphere is considered the most hospitable known place off Earth:
Still further into the future, Venus's relevance to humanity would depend on which comes first: Our ability to terraform other planets - i.e., reengineer their climates to make them hospitable - or our outgrowing both the need and desire to live on planets. If the former, we would probably succeed on Mars long before doing so with Venus - it's a lot easier to heat up a frigid rock and pump out a desirable air chemistry than it is to cool down a furnace and try to get rid of 99% of its atmosphere. So if we did go the route of terraforming, then we would eventually transform Venus, but it would be Earth 3 rather than Earth 2 - and probably take much longer.
However, the value proposition of terraforming is inconclusive - over the time scales needed to reap its rewards, it may become easier to simply live anywhere in an artificial environment than to pour resources into transforming a natural environment. If that proves to be the case, then Venus would likely be ignored as an inconvenient location rather than inspiring the kind of colonial fervor that would be needed to terraform. Of course, the possibility exists that the atmosphere could be colonized, forming a kind of "membrane" of floating human civilization - although we have no basis to guess at the long-term prospects of such a thing.
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VII. Future of Venus
If we go the terraforming route, in a few thousand years Venus could look like this artistic rendering by Daein Ballard:
You'll recognize Aphrodite Terra as the big island-continent in the center of the frame, and find the cloud formations familiar. Or, of course, it could look pretty much like it does today for the rest of the life of the solar system. But whatever humanity does, Venus still has a destiny to work out with the rest of the solar system: As noted in the Mercury diary, there is a 1% chance of that planet being perturbed from its orbit into a collision with Venus at some point in their expected lifetimes. In the 99% case where that doesn't happen, it will continue at its present orbit as the Sun expands over the next few billion years, and its atmosphere will eventually be stripped off completely by the solar wind. Ultimately the Sun will engulf it, and its matter will either be blown off in one of the mass ejections of the dying star, or will become part of the white dwarf stellar remnant.
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VIII. Catalog of Exploration
1. Past and Current Probes:
Tyazhely Sputnik (USSR, 1961 - failed flyby)
Venera 1 (USSR, 1961 - failed flyby)
Mariner 1 (USA, 1962 - failed flyby)
Mariner 2 (USA, 1962 - flyby)
Zond 1 (USSR, 1964 - failed flyby + lander)
Venera 2 (USSR, 1966 - failed flyby)
Venera 3 (USSR, 1966 - failed lander)
Mariner 5 (USA, 1967 - flyby)
Venera 4 (USSR, 1967 - descender, failed lander)
Venera 5 (USSR, 1969 - descender, failed lander)
Venera 6 (USSR, 1969 - descender, failed lander)
Venera 7 (USSR, 1970 - lander)
Venera 8 (USSR, 1972 - lander)
Mariner 10 (USA, 1974 - flyby)
Venera 9 (USSR, 1975 - orbiter + lander)
Venera 10 (USSR, 1975 - orbiter + lander)
Pioneer Venus Multiprobe (USA, 1978 - 4 descenders, 1 unintentional lander)
Pioneer Venus Orbiter (USA, 1978-1992: orbiter)
Venera 11 (USSR, 1978 - flyby + lander)
Venera 12 (USSR, 1979-1980: flyby + lander)
Venera 13 (USSR, 1982 - flyby + lander)
Venera 14 (USSR, 1982 - flyby + lander)
Venera 15 (USSR, 1983-1984: orbiter)
Venera 16 (USSR, 1983-1984: orbiter)
Vega 1 (USSR, 1985 - flyby + lander + balloon)
Vega 2 (USSR, 1985 - flyby + lander + balloon)
Magellan (USA, 1990-1994: orbiter)
Galileo (USA, 1990 - gravitational assist flyby)
Cassini-Huygens (USA & Europe, 1998 & 1999 - Gravitational assist flybys)
Venus Express (Europe, 2006-Today: orbiter)
Akatsuki (Japan, 2010-Today: Failed to enter Venus orbit, currently hibernating awaiting next opportunity)
MESSENGER (USA, 2006 & 2007 - gravitational assist flybys)
2. Future Probes:
Venus In-Situ Explorer (VISE) (USA, proposed balloon/lander - launch 2013)
Venus Entry Probe (Europe, proposed balloon/landers - launch 2013)
BepiColombo (Europe, 2016 - two gravitational assist flybys)
Venera-D (Russia, proposed orbiter - launch 2016)
Solar Probe Plus (USA, beginning 2018: Seven gravitational assist flybys)
Venus rover (USA, conceptual rover)
