Venus
Overview
a
Observing from Earth
Being the closest planet to the Sun as well as the smallest planet, Mercury is very difficult to observe. To see it, you generally can only observe it very close to sunset or to sunrise (depending upon which side of the Sun it appears to be on). One helpful aspect is that, because Mercury is so close to the Sun as well as to Earth, it is fairly bright (generally -2.5 to -2.0 magnitude), and so it can easily be spotted in a telescope during a bright twilight.
Mercury, being an inferior planet (meaning that it lies inside Earth's orbit) displays phases, just like Venus and our moon. Mercury's phases coincide with where it is relative to the Sun and Earth. When Mercury is between Earth and the Sun (known as an inferior conjunction), it will display a new phase, being completely dark. When Mercuy is behind the Sun (known as opposition, or a superior conjunction), it will display a full phase, being completely lit. When Mercury is at its greatest elongation (the farthest away from the Sun it can get as seen from Earth), it will be in a half phase. It is generally difficult to actually resolve its disk and see the phases, however, because Mercury is generally ~10 arcsec in diameter as seen from Earth.
Finally, again being an inferior planet, Mercury can transit the Sun as seen from Earth. It takes over 5 hrs for Mercury's tiny disk to transit the Sun, but it can be easily photographed depsite being ~0.5% the Sun's diameter. The image at the right shows one such transit in its early stage, taken on November 8, 2006. The next transit will occur on May 9, 2016.
 |
| Geologic history of Mercury, identifying the five major epochs identified in Mercury's history. |
Global History
Mercury is believed to be geologically dead at the present time, except for those processes required to create a magnetic field. The surface of the planet is believed to be mostly ancient, dating back at least 3 billion years, much like Earth's moon. One way to talk about Mercury is in terms of geologic ages - much like the rest of the terrestrial bodies. These are shown in the figure to the right.
These boundaries are not well established in time, unlike Earth's, because they are based upon relative dating techniques as opposed to absolute dating methods. As with Mars and Venus, Mercury's epoch ages are based upon the lunar cratering record (which has been absolutely dated from Apollo sample returns). Very briefly, this works under the assumption that the older a surface, the longer it has had to accumulate craters and so the more craters will be present. We can calibrate the cratering rate from the moon and extrapolate it to other bodies in order to estimate how many craters of a certain size should be present for a surface to be a certain age.
Geologic ages on Mercury are named after major craters (like our moon). The first is simply the Pre-Tolstojan, which covers the planet's formation ~4.5 Gya (billion years ago) until ~4.0 Gya. This covers the period from the formation of the planet, the very earliest surface, and the planet's differentiation from a homogenous mixture into a core, mantle, and crust. This time also includes most of the early Heavy Bombardment of asteroids, when most of the craters seen throughout the solar system were formed. This is likely when Mercury's intercrater plains formed.
Next is the Tolstojan period, named after the crater Tolstoy. It covers ~4.0-3.9 Gya, a fairly narrow 100 million-year period. During the Heavy Bombardment mentioned above, the crater Tolstoy formed, marking the beginning of the Tolstojan epoch. During this period, the intercrater plains continued to form.
The next geologic era is Calorian, named after Mercury's largest crater, Caloris Basin. It covers ~3.9 until 3.5-3.0 Gya (the boundary with Mansurian is not well-determined). The Caloris Basin, a large multi-ring basin on the planet, formed towards the end of the Heavy Bombardment. This basin modified the surface of the entire planet (discussed below). The Calorian period marks the time when most of the smooth plains formed on the planet, likely due to the last thrusts of volcanism before the planet cooled and solidified too much to permit volcanism.
Fourth is the Mansurian era, named after the crater Mansura. It lasts from 3.5-3.0 Gya (the boundary with Calorian is not well-determined) to ~1.0 Gya. This epoch represents light cratering and a continued decrease in planetary activity and increase in cooling. Relatively fresh craters formed during this period, but they do not have rays (because the rays have eroded since formation).
Finally is the Kuiperian age, named after the crater Kuiper. This is the youngest surface age of the planet, lasting from the end of the Mansurian ~1.0 Gya to the present day. This represents a mostly geologically dead Mercury with light cratering, the craters still posessing their rays.
Interior
a
Surface
a
 |
| A pie chart illustrating the compositional breakdown of Venus' atmosphere. Data is from NASA's planetary factsheet. |
Atmosphere
In the conventional sense that Earth, Venus, or even Mars has an atmosphere, Mercury does not. It does, however, have a very tenuous envelope of gas that surrounds it. The pressure this gas exerts on the surface of the planet is only 10-15 bars. To think about this in everyday life, dropping a feather on your hand would exert a pressure of about 10-3 bars, or 1 trillion times more than Mercury's atmosphere.
The actual fractional values shown in the pie chart are not well constrained. After more data becomes available from MESSENGER, better values should be available. It should be noted that besides these five molecules shown in the chart, additional species (such as water, nitrogen, magnesium, silicon, carbon, and calcium) are present, just at much lower abundances.
It should be noted that the atomic species present - sodium, helium, oxygen, and potassium, for example - are generally ionized due to interactions between the elements and the solar wind - the ionized plasma that streams from the Sun. The atmosphere is generated due to the space weathering itself, where charged solar particles interact with Mercury's surface, knock atoms and molecules off the surface, and excite them in the near-surface environment, becoming part of the atmosphere (Zurbuchen et al., 2008). Material is lost from the atmosphere in much the same way - once it becomes excited and leaves Mercury's surface, it will generally continue to stream away from the surface, leaving the planet's gravity (McClintock et al., 2008).
Magnetosphere
Venus has no global magnetic field. It does, however, have a relatively small magnetosphere that is likely induced due to an interaction between its ionosphere and the Sun's solar wind. It is too weak to protect the planet from cosmic radiation or significantly from the solar wind.
Selected Features and Characteristics
Crater Population: a
Rotation Rate: a
Naming Conventions
The name "Venus" in English is named after the Roman goddess of love, Venus, also known by the Greek name Aphrodite. She was also associated with other aspects of love, including lust, beauty, and sexual reproduction. Because it is the only planet named after a female, all features on the planet are named for women except for the largest mountain range, Maxwell Montes, sometimes referred to as the "Only Man on Venus."
According to the USGS Gazetteer of Planetary Nomenclature, any new features are named by the following:
- Astra: goddesses.
- Chasmata: Goddesses of hunt or the moon.
- Colles: Sea goddesses.
- Coronae: Fertility and Earth goddesses.
- Craters: Deceased women who have made outstanding or fundamental contributions to their field (for craters over 20 km in diameter); or common female first names (for craters smaller thatn 20 km).
- Dorsa: Sky goddesses.
- Farra: Water goddesses.
- Fluctus: Goddesses.
- Fossae: Goddesses of war.
- Labyrinthi: Goddesses.
- Lineae: Goddeses of war.
- Montes: Goddesses.
- Paterae: Famous women.
- Planitiae: Mythological heroines.
- Plana: Goddesses of prosperity.
- Regiones: Giantesses and Titanesses (or two are Greek alphanumeric designations).
- Rupes: Goddesses of hearth and home.
- Tesserae: Goddesses of fate and fortune.
- Terrae: Goddesses of love.
- Tholi: Goddesses.
- Undae: Desert goddesses.
- Valles: Word for the planet "Venus" in various world languages (if 400 km or longer); or river goddesses (if less than 400 km long).
Past, Present, and Future Missions
The following is a list of the missions that have finished, are currently in operation, or are planned to be lanuched to explore Mercury. A brief summary is displayed, but you can click on the name of the mission to be taken to a page detailing the mission.
Sputnik 7 ~ 1961 ~ This was an attempted Venus impactor that failed.
Venera 1 ~ 1961 ~ This was an attempted Venus flyby that failed when contact was lost.
Mariner 1 ~ 1962 ~ This was an attempted Venus flyby that failed on the launchpad.
Sputnik 19 ~ 1962 ~ Attempted Venus flyby.
Mariner 2 ~ 1962 ~ Successful flyby.
Sputnik 20~ 1962 ~ Attempted Venus flyby.
Sputnik 21 ~ 1962 ~ Attempted Venus flyby.
Venera 1964A ~ 1964 ~ Attemped Venus flyby that failed due to launch failure.
Venera 1964B ~ 1964 ~ Attemped Venus flyby that failed due to launch failure.
Cosmos 27 ~ 1964 ~ Attempted Venus flyby that failed.
Zond 1 ~ 1964 ~ Attempted Venus flyby that failed when contact was lost.
Venera 2 ~ 1965 ~ Attempted Venus flyby that failed when contact was lost.
Venera 3 ~ 1965 ~ Attempted Venus lander that failed when contact was lost.
Cosmos 96 ~ 1965 ~ Possibly an attempted Venus lander.
Venera 1965A ~ 1965 ~ Attempted Venus flyby that failed when contact was lost.
Venera 4 ~ 1967 ~ Venus probe.
Mariner 5 ~ 1967 ~ Performed 1 successful flyby in 1967.
Cosmos 167 ~ 1967 ~ Attempted Venus probe.
Venera 5 ~ 1969 ~ Venus probe.
Venera 6 ~ 1969 ~ Venus probe.
Venera 7 ~ 1970 ~ Venus lander.
Cosmos 359 ~ 1970 ~ Attempted Venus probe.
Venera 8 ~ 1972 ~ Venus probe.
Cosmos 482 ~ 1972 ~ Attempted Venus probe.
Mariner 10 ~ 1974 ~ Performed 1 successful flyby in 1974.
Venera 9 ~ 1975 ~ Venus orbiter and lander.
Venera 10 ~ 1975 ~ Venus orbiter and lander.
Pioneer 12 AKA Pioneer Venus Orbiter ~ 1978-1992 ~ Successful orbiter for 15 years.
Pioneer 13 AKA Pioneer Venus Multiprobe Mission ~ 1978 ~ Orbited in 1978 and released 3 probes into the atmosphere.
Venera 11 ~ 1978 ~ Venus orbiter and lander.
Venera 12 ~ 1978 ~ Venus orbiter and lander.
Venera 13 ~ 1981 ~ Venus orbiter and lander.
Venera 14 ~ 1981 ~ Venus orbiter and lander.
Venera 15 ~ 1983 ~ Venus orbiter and lander.
Venera 16 ~ 1983 ~ Venus orbiter and lander.
Vega 1 ~ 1985 ~ Venus lander and balloon.
Vega 2 ~ 1985 ~ Venus lander and balloon.
Magellan 4 ~ 1990-1994 ~ Successfully orbited Venus for 5 years.
Venus Express ~ 2005-present ~ The European Space Agency launched this probe that reached Venus in 2006 and is currently operating today. Its main purpose is to study the planet's atmosphere, clouds, and interaction with the plasma environment of space.
Planet-C ~ 2010?-??? ~ Planned by the Japanese Space Agency (JAXA), this is currently slated to launch in 2010 with an arrival date at the end of that year and a nominal mission of 2+ years.
Data for the Planets
The following table presents selected information from NASA's planetary factsheet.
| Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | |
|---|---|---|---|---|---|---|---|---|
| Perihelion (106 km) | 46.00 | 107.5 | 147.09 | 206.62 | 740.52 | 1352.55 | 2741.30 | 4444.45 |
| Mean Orbital Distance (106 km) | 57.91 | 108.2 | 149.60 | 227.92 | 778.57 | 1433.53 | 2872.46 | 4495.06 |
| Aphelion (106 km) | 69.82 | 108.9 | 152.10 | 249.23 | 816.62 | 1514.50 | 3003.62 | 4545.67 |
| Average Orbital Velocity (km/s) | 47.87 | 35 | 29.78 | 24.13 | 13.07 | 9.69 | 6.81 | 5.43 |
| Orbital Inclination (from Earth's orbit) | 7.00° | 3.4° | 0.0° | 1.850° | 1.304° | 2.485° | 0.772° | 1.769° |
| Orbital Eccentricity | 0.2056 | 0.007 | 0.0167 | 0.0935 | 0.0489 | 0.0565 | 0.0457 | 0.0113 |
| Equatorial Radius (km) | 2439.7 | 6051.8 | 6378.1 | 3397 | 71,492 | 60,268 | 25,559 | 24,764 |
| Polar Radius (km) | 2439.7 | 6051.8 | 6356.8 | 3375 | 66,854 | 54,364 | 24,973 | 24,341 |
| Volume (1010 km3) | 6.083 | 92.843 | 108.321 | 16.318 | 143,128 | 82,713 | 6833 | 6254 |
| Ellipticity (variation from sphere) | 0.0000 | 0.000 | 0.00335 | 0.00648 | 0.06487 | 0.09796 | 0.02293 | 0.01708 |
| Axial Tilt (from Earth's geographic North) | 0.01° | 177.4° | 23.45° | 25.19° | 3.13° | 26.73° | 97.77° | 28.32° |
| Mass (1024 kg) | 0.3302 | 4.87 | 5.9736 | 0.64185 | 1898.6 | 568.46 | 86.832 | 102.43 |
| Density (water=1) | 5.427 | 5.243 | 5.515 | 3.933 | 1.326 | 0.687 | 1.27 | 1.638 |
| Escape Velocity (km/s) | 4.3 | 10.36 | 11.19 | 5.03 | 59.5 | 35.5 | 21.3 | 23.5 |
| Gravity (m/s2) | 3.70 | 8.802 | 9.78 | 3.716 | 23.1 | 9 | 8.7 | 11 |
| Surface Pressure (bars) | ≈10-15 | 92 | 1.014 | 0.000636 | N/A | N/A | N/A | N/A |
| Total Mass of Atmosphere (kg) | <1000 | 4.8·1020 | 5.1·1018 | 2.5·1016 | N/A | N/A | N/A | N/A |
| Sidereal Rotation Period (hours) | 1407.6 | -5832.5 | 23.9345 | 24.6229 | 9.9250 | 10.656 | -17.24 | 16.11 |
| Length of Day (hours) | 4222.6 | 2802 | 24 | 24.6597 | 9.9259 | 10.656 | 17.24 | 16.11 |
| Tropical Orbital Period (days) | 87.968 | 224.7 | 365.256 | 686.980 | 4330.595 | 10,746.94 | 30,588.740 | 59,799.9 |
| Bond Albedo | 0.119 | 0.750 | 0.306 | 0.250 | 0.343 | 0.342 | 0.300 | 0.290 |
| Visual Geometric Albedo | 0.106 | 0.65 | 0.367 | 0.150 | 0.52 | 0.47 | 0.51 | 0.41 |
| Visual Magnitude | -0.42 | -4.40 | -3.86 | -1.52 | -9.40 | -8.88 | -7.19 | -6.87 |
| Solar Irradiance (W/m2) | 9126.6 | 2613.9 | 1367.6 | 589.2 | 50.50 | 14.90 | 3.71 | 1.51 |
| Black-Body Temperature (K) | 442.5 | 231.7 | 254.3 | 210.1 | 110.0 | 81.1 | 58.2 | 46.6 |
| Average Surface Temperature (Celsius) | 167° | 464° | 15° | -65° | -110° | -140° | -195° | -200° |
| Number of Moons | ||||||||
| Rings? | No | No | No | No | Yes | Yes | Yes | Yes |
| Global Magnetic Field Strength (Gs) / Tilt | 0.0033 / 169° | No Field | 0.3076 / 11.4° | No Field | 4.28 / 9.6° | 0.210 / <1° | 0.228 / 58.6° | 0.142 / 46.9° |
| Discoverer | Unknown | Unknown | Unknown | Unknown | Unknown | Unknown | William Herschel | Johann Gottfried Galle |
| Discovery Date | Prehistory | Prehistory | Prehistory | Prehistory | Prehistory | Prehistory | March 13, 1781 | September 23, 1846 |