WELCOME TO THE ILLDARK
We Are Always Connected To The Rest Of The World
We Are Always Connected To The Rest Of The World
--Planets In The Solar System--
1. Mercury
Mercury is the innermost planet in the Solar System. It is also the smallest, and its orbit is the most eccentric (that is, the least perfectly circular) of the eight planets. It orbits the Sun once in about 88 Earth days, completing three rotations about its axis for every two orbits. The planet is named after the Roman god Mercury, the messenger to the gods.
Mercury's surface is heavily cratered and similar in appearance to Earth's Moon,
indicating that it has been geologically inactive for billions of
years. Due to its near lack of an atmosphere to retain heat, Mercury's
surface experiences the steepest temperature gradient of all the
planets, ranging from a very cold 100 K at night to a very hot 700 K during the day. Mercury's axis has the smallest tilt of any of the Solar System's planets, but Mercury's orbital eccentricity
is the largest. The seasons on the planet's surface are caused by the
variation of its distance from the Sun rather than by the axial tilt,
which is the main cause of seasons on Earth and other planets. At perihelion, the intensity of sunlight on Mercury's surface is more than twice the intensity at aphelion. Mercury and Venus can each make appearances in Earth's sky both as a morning star and an evening star (because they are closer to the Sun than the Earth),
and at times Mercury can technically be regarded as a very bright
object when viewed from Earth; however, its proximity in the sky to the
Sun makes it more difficult to see than Venus .
Internal structure
 |
| Terrestrial planets: Mercury, Venus, Earth, and Mars (to scale) |
Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 km. Mercury is even smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm3, only slightly less than Earth’s density of 5.515 g/cm3. If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm3 versus Earth’s 4.4 g/cm3.
Mercury’s density can be used to infer details of its inner
structure. While the Earth’s high density results appreciably from
gravitational compression, particularly at the core,
Mercury is much smaller and its inner regions are not nearly as
strongly compressed. Therefore, for it to have such a high density, its
core must be large and rich in iron.
Geologists estimate that Mercury’s core occupies about 42% of its
volume; for Earth this proportion is 17%. Recent research strongly
suggests that Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick.
One distinctive feature of Mercury’s surface is the presence of
numerous narrow ridges, extending up to several hundred kilometers in
length. It is believed that these were formed as Mercury’s core and
mantle cooled and contracted at a time when the crust had already
solidified.
Mercury's core has a higher iron content than that of any other major
planet in the Solar System, and several theories have been proposed to
explain this. The most widely accepted theory is that Mercury originally
had a metal-silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass. Early in the Solar System’s history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and several hundred kilometers across.
The impact would have stripped away much of the original crust and
mantle, leaving the core behind as a relatively major component. A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of Earth’s Moon.
Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. The planet would initially have had twice its present mass, but as the protosun
contracted, temperatures near Mercury could have been between 2,500 and
3,500 K (Celsius equivalents about 273 degrees less), and possibly even
as high as 10,000 K.
Much of Mercury’s surface rock could have been vaporized at such
temperatures, forming an atmosphere of "rock vapor" which could have
been carried away by the solar wind.
A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material. Each hypothesis predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to make observations to test them. MESSENGER
has found higher-than-expected potassium and sulfur levels on the
surface, suggesting that the giant impact hypothesis and vaporization of
the crust and mantle did not occur since potassium and sulfur would
have been driven off by the extreme heat of these events. The findings
seem to favor the third hypothesis in which many lighter planetary
materials were driven off leaving behind higher metal concentrations.
Orbit and Rotation
Mercury has the most eccentric
orbit of all the planets; its eccentricity is 0.21 with its distance
from the Sun ranging from 46,000,000 to 70,000,000 km (29,000,000 to
43,000,000 mi). It takes 87.969 earth days to complete an orbit. The
diagram on the right illustrates the effects of the eccentricity,
showing Mercury's orbit overlaid with a circular orbit having the same semi-major axis.
The higher velocity of the planet when it is near perihelion is clear
from the greater distance it covers in each 5-day interval. The size of
the spheres, inversely proportional to their distance from the Sun, is
used to illustrate the varying heliocentric distance. This varying
distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet's rotation around its axis, result in complex variations of the surface temperature.This resonance makes a single day on Mercury last exactly two Mercury years, or about 176 Earth days.
Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic), as shown in the diagram on the right. As a result, transits
of Mercury across the face of the Sun can only occur when the planet is
crossing the plane of the ecliptic at the time it lies between the
Earth and the Sun. This occurs about every seven years on average.
Mercury's axial tilt is almost zero, with the best measured value as low as 0.027 degrees. This is significantly smaller than that of Jupiter,
which has the second smallest axial tilt of all planets at 3.1 degrees.
This means that to an observer at Mercury's poles, the center of the
Sun never rises more than 2.1 arcminutes above the horizon.
At certain points on Mercury's surface, an observer would be able to
see the Sun rise about halfway, then reverse and set before rising
again, all within the same Mercurian day. This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity exactly equals its angular rotational velocity so that the Sun's apparent motion
ceases; at perihelion, Mercury's angular orbital velocity then exceeds
the angular rotational velocity. Thus, to a hypothetical observer on
Mercury, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.
Mercury attains inferior conjunction (near approach to the Earth) every 116 Earth days on average,
but this interval can range from 105 days to 129 days due to the
planet’s eccentric orbit. Mercury can come as close as 77.3 million km
to the Earth,
but it will not be closer to Earth than 80 Gm until AD 28,622. The next
approach to within 82.1 Gm is in 2679, and to within 82 Gm in 4487. Its period of retrograde motion
as seen from Earth can vary from 8 to 15 days on either side of
inferior conjunction. This large range arises from the planet's high
orbital eccentricity.
Impact basins and craters
Impact basins and craters
Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins
hundreds of kilometers across. They appear in all states of
degradation, from relatively fresh rayed craters to highly degraded
crater remnants. Mercurian craters differ subtly from lunar craters in
that the area blanketed by their ejecta is much smaller, a consequence
of Mercury's stronger surface gravity.
The largest known crater is Caloris Basin, with a diameter of 1,550 km. The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode
of the Caloris Basin is a large region of unusual, hilly terrain known
as the "Weird Terrain". One hypothesis for its origin is that shock
waves generated during the Caloris impact traveled around the planet,
converging at the basin’s antipode (180 degrees away). The resulting
high stresses fractured the surface.
Alternatively, it has been suggested that this terrain formed as a
result of the convergence of ejecta at this basin’s antipode.
Overall, about 15 impact basins have been identified on the imaged
part of Mercury. A notable basin is the 400 km wide, multi-ring Tolstoj Basin which has an ejecta blanket extending up to 500 km from its rim and a floor that has been filled by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a 625 km diameter rim. Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including Solar wind and micrometeorite impacts.
2 . Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows. Because Venus is an inferior planet from Earth, it never appears to venture far from the Sun: its elongation
reaches a maximum of 47.8°. Venus reaches its maximum brightness
shortly before sunrise or shortly after sunset, for which reason it has
been referred to by ancient cultures as the Morning Star or Evening
Star.
Venus is classified as a terrestrial planet
and is sometimes called Earth's "sister planet" owing to their similar
size, gravity, and bulk composition (Venus is both the closest planet to
Earth and the planet closest in size to Earth). However, it has been
shown to be very different from Earth in other respects. Venus is
shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. It has the densest atmosphere of the four terrestrial planets, consisting mostly of carbon dioxide. The atmospheric pressure at the planet's surface is 92 times that of Earth's. With a mean surface temperature of 735 K (462 °C; 863 °F), Venus is by far the hottest planet in the Solar System. It has no carbon cycle
to lock carbon back into rocks and surface features, nor does it seem
to have any organic life to absorb it in biomass. Venus is believed to
have previously possessed oceans, but these vaporized as the temperature rose due to the runaway greenhouse effect. The water has most probably photodissociated, and, because of the lack of a planetary magnetic field, the free hydrogen has been swept into interplanetary space by the solar wind. Venus's surface is a dry desertscape interspersed with slab-like rocks and periodically refreshed by volcanism.
Physical characteristics
Venus is one of the four solar terrestrial planets, meaning that, like the Earth, it is a rocky body. In size and mass, it is similar to the Earth, and is often described as Earth's "sister" or "twin". The diameter of Venus is 12,092 km (only 650 km less than the Earth's) and its mass is 81.5% of the Earth's. Conditions on the Venusian surface differ radically from those on Earth, owing to its dense carbon dioxide atmosphere. The mass of the atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% being nitrogen.
Geography
The Venusian surface was a subject of speculation until some of its secrets were revealed by planetary science in the 20th century. It was finally mapped in detail by Project Magellan in 1990–91. The ground shows evidence of extensive volcanism, and the sulfur in the atmosphere may indicate there have been some recent eruptions.About 80% of the Venusian surface is covered by smooth, volcanic plains, consisting of 70% plains with wrinkle ridges and 10% smooth or lobate plains. Two highland "continents" make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra, after Ishtar, the Babylonian goddess of love, and is about the size of Australia. Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km above the Venusian average surface elevation. The southern continent is called Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland regions at roughly the size of South America. A network of fractures and faults covers much of this area.
The absence of evidence of lava flow accompanying any of the visible caldera remains an enigma. The planet has few impact craters, demonstrating the surface is relatively young, approximately 300–600 million years old. In addition to the impact craters, mountains, and valleys commonly found on rocky planets, Venus has a number of unique surface features. Among these are flat-topped volcanic features called "farra", which look somewhat like pancakes and range in size from 20–50 km across, and 100–1,000 m high; radial, star-like fracture systems called "novae"; features with both radial and concentric fractures resembling spider webs, known as "arachnoids"; and "coronae", circular rings of fractures sometimes surrounded by a depression. These features are volcanic in origin.
Most Venusian surface features are named after historical and mythological women. Exceptions are Maxwell Montes, named after James Clerk Maxwell, and highland regions Alpha Regio, Beta Regio and Ovda Regio. The former three features were named before the current system was adopted by the International Astronomical Union, the body that oversees planetary nomenclature.
The longitudes of physical features on Venus are expressed relative to its prime meridian. The original prime meridian passed through the radar-bright spot at the center of the oval feature Eve, located south of Alpha Regio. After the Venera missions were completed, the prime meridian was redefined to pass through the central peak in the crater Ariadne.
Surface geology
Main article: Geology of Venus
Much of the Venusian surface appears to have been shaped by volcanic
activity. Venus has several times as many volcanoes as Earth, and it
possesses some 167 large volcanoes that are over 100 km across. The only
volcanic complex of this size on Earth is the Big Island of Hawaii.[25] This is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years,[31] while the Venusian surface is estimated to be 300–600 million years old.[23][25]
Venusian craters range from 3 km to 280 km in diameter. No craters are smaller than 3 km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain kinetic energy are slowed down so much by the atmosphere, they do not create an impact crater.[35] Incoming projectiles less than 50 meters in diameter will fragment and burn up in the atmosphere before reaching the ground.[36]
Internal structure
Without seismic data or knowledge of its moment of inertia, little direct information is available about the internal structure and geochemistry of Venus.[37] The similarity in size and density between Venus and Earth suggests they share a similar internal structure: a core, mantle, and crust. Like that of Earth, the Venusian core is at least partially liquid because the two planets have been cooling at about the same rate.[38] The slightly smaller size of Venus suggests pressures are significantly lower in its deep interior than Earth. The principal difference between the two planets is the lack of evidence for plate tectonics on Venus, possibly because its crust is too strong to subduct without water to make it less viscous. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field.[39] Instead, Venus may lose its internal heat in periodic major resurfacing events.[23]
Atmosphere and climate
Main article: Atmosphere of Venus
Synthetic stick absorption spectrum of a simple gas mixture corresponding to the Earth's atmosphereStudies have suggested that billions of years ago, the Venusian atmosphere was much more like Earth's than it is now, and that there were probably substantial quantities of liquid water on the surface, but, after a period of 600 million to several billion years,[45] a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere.[46] Although the surface conditions on the planet are no longer hospitable to any Earthlike life that may have formed prior to this event, the possibility that a habitable niche still exists in the lower and middle cloud layers of Venus can not yet be excluded.[47]
Thermal inertia and the transfer of heat by winds in the lower atmosphere mean that the temperature of the Venusian surface does not vary significantly between the night and day sides, despite the planet's extremely slow rotation. Winds at the surface are slow, moving at a few kilometers per hour, but because of the high density of the atmosphere at the Venusian surface, they exert a significant amount of force against obstructions, and transport dust and small stones across the surface. This alone would make it difficult for a human to walk through, even if the heat, pressure and lack of oxygen were not a problem.[48]
Above the dense CO2 layer are thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets.[49][50] These clouds reflect and scatter about 90% of the sunlight that falls on them back into space, and prevent visual observation of the Venusian surface. The permanent cloud cover means that although Venus is closer than Earth to the Sun, the Venusian surface is not as well lit. Strong 300 km/h winds at the cloud tops circle the planet about every four to five earth days.[51] Venusian winds move at up to 60 times the speed of the planet's rotation, while Earth's fastest winds are only 10% to 20% rotation speed.[52]
The surface of Venus is effectively isothermal; it retains a constant temperature not only between day and night but between the equator and the poles.[2][53] The planet's minute axial tilt (less than three degrees, compared with 23 degrees for Earth), also minimizes seasonal temperature variation.[54] The only appreciable variation in temperature occurs with altitude. In 1995, the Magellan probe imaged a highly reflective substance at the tops of the highest mountain peaks that bore a strong resemblance to terrestrial snow. This substance arguably formed from a similar process to snow, albeit at a far higher temperature. Too volatile to condense on the surface, it rose in gas form to cooler higher elevations, where it then fell as precipitation. The identity of this substance is not known with certainty, but speculation has ranged from elemental tellurium to lead sulfide (galena).[55]
The clouds of Venus are capable of producing lightning much like the clouds on Earth.[56] The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet Venera probes. In 2006–07 Venus Express clearly detected whistler mode waves, the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.[56] In 2007 the Venus Express probe discovered that a huge double atmospheric vortex exists at the south pole of the planet.[57][58]
Another discovery made by the Venus Express probe in 2011 is that an ozone layer exists high in the atmosphere of Venus.[59]
Synthetic stick absorption spectrum of a simple gas mixture corresponding to the Earth's atmosphere
Synthetic stick absorption spectrum of a simple gas mixture corresponding to the Earth's atmosphere
Orbit and Rotation
Venus orbits the Sun at an average distance of about 0.72 AU (108,000,000 km; 67,000,000 mi), and completes an orbit every 224.65 days. Although all planetary orbits are elliptical, Venus's orbit is the closest to circular, with an eccentricity of less than 0.01.[2] When Venus lies between the Earth and the Sun, a position known as inferior conjunction, it makes the closest approach to Earth of any planet at an average distance of 41 million km.[2] The planet reaches inferior conjunction every 584 days, on average.[2] Owing to the decreasing eccentricity of Earth's orbit,
the minimum distances will become greater over tens of thousands of
years. From the year 1 to 5383, there are 526 approaches less than
40 million km; then there are none for about 60,158 years.[68] During periods of greater eccentricity, Venus can come as close as 38.2 million km.[2]All the planets of the Solar System orbit the Sun in a counter-clockwise direction as viewed from above the Sun's north pole. Most planets also rotate on their axis in a counter-clockwise direction, but Venus rotates clockwise (called "retrograde" rotation) once every 243 Earth days—by far the slowest rotation period of any major planet. The equator of the Venusian surface rotates at 6.5 km/h, while on Earth rotation speed at the equator is about 1,670 km/h.[69] Venus's rotation has slowed down by 6.5 minutes per day since the Magellan spacecraft visited it 16 years ago.[70] A Venusian sidereal day thus lasts longer than a Venusian year (243 versus 224.7 Earth days). Because of the retrograde rotation, the length of a solar day on Venus is significantly shorter than the sidereal day. As a result of Venus's relatively long solar day, one Venusian year is about 1.92 Venusian days long.[12] To an observer on the surface of Venus, the Sun would appear to rise in the west and set in the east, and the time from one sunrise to the next would be 116.75 Earth days (making the Venusian solar day shorter than Mercury's 176 Earth days).[12]
Venus may have formed from the solar nebula with a different rotation period and obliquity, reaching to its current state because of chaotic spin changes caused by planetary perturbations and tidal effects on its dense atmosphere, a change that would have occurred over the course of billions of years. The rotation period of Venus may represent an equilibrium state between tidal locking to the Sun's gravitation, which tends to slow rotation, and an atmospheric tide created by solar heating of the thick Venusian atmosphere.[71][72] A curious aspect of the Venusian orbit and rotation periods is the 584-day average interval between successive close approaches to the Earth is almost exactly equal to five Venusian solar days.[73] However, the hypothesis of a spin–orbit resonance with Earth has been discounted.[74]
Venus currently has no natural satellite,[75] though the asteroid 2002 VE68 presently maintains a quasi-orbital relationship with it.[76] In the 17th century, Giovanni Cassini reported a moon orbiting Venus, which was named Neith and numerous sightings were reported over the following 200 years, but most were determined to be stars in the vicinity. Alex Alemi's and David Stevenson's 2006 study of models of the early Solar System at the California Institute of Technology shows Venus likely had at least one moon created by a huge impact event billions of years ago.[77][78] About 10 million years later, according to the study, another impact reversed the planet's spin direction and caused the Venusian moon gradually to spiral inward[79] until it collided and merged with Venus. If later impacts created moons, these also were absorbed in the same way. An alternative explanation for the lack of satellites is the effect of strong solar tides, which can destabilize large satellites orbiting the inner terrestrial planets.[75]
