You may be wondering why two planets have no natural satellites. Moons are simply extraterrestrial objects that orbit around other planets. The two innermost planets Mercury and Venus are the only known planets without moons. The outermost planets such as Jupiter and Saturn have many moons. Dwarf planets such as Pluto Ceres and Neptune have moons orbiting them.
Phobos
The planets Mars and Venus have two natural satellites. Phobos has a radius of seven miles and Deimos has a radius of 1.1 pounds. Phobos is seven times more massive than Deimos and both moons orbit at a distance of approximately nine million kilometers. The two moons orbit at an average distance of 6.2 kilometers from each other. Phobos was named after the Greek god Phobos who was the son of Ares (Mars) and Aphrodite (Venus). In addition to being the personification of Horror the name Phobos is pronounced ‘[email protected]’ like the Greek ‘phobos’.
Mercury’s natural satellites are not known but it is possible that the planets may have had moons in the past. Scientists believe that Phobos is in an unstable orbit and may crash into Mars within 10 million years. It is also possible that Venus had moons in the past but it is unlikely that they exist today because the planet is so close to the Sun that solar tides would be too powerful for moons.
Mercury Mars and Pluto all have moons. Earth’s moon was formed by a collision with a Mars-sized planetoid and the moons were formed through a process called asteroid formation. Mars has two moons Phobos and Demos and it is expected that these two planets will collide with Mars in the future. Because of their low altitude and tidal forces Phobos and Demos will crash into Mars one day.
Despite their proximity to the sun Mercury and Venus do not have natural satellites. Their gravitational pull would tug on any moon they may have once had. This is because Venus and Mercury have a double planet system where the moon is a moon of the larger planet. Although Venus is one of the largest planets in the solar system Phobos and Venus do not have natural satellites. This makes it difficult to predict their orbital motions.
The other moons of Mercury and Venus are called ‘secondary satellites.’ Although they are not known as such they are still considered natural satellites of their planets. Besides being the closest planets to the sun Venus has no natural moon. They do have moons but these are very small and erratic. The moons of Mercury and Venus may have been created during the evolution of the solar system.
Deimos
While they are both relatively small both Deimos and Jupiter have no natural satellite. However this might change one day if their orbits are sufficiently close together. Although Deimos is far away from Jupiter it may be close enough for a human to jump from Deimos to Jupiter. And if that day ever comes we might be able to use this information to find out more about how these planets formed.
The moons of Jupiter are very small. They are only 6000 km from Mars. They orbit their parent planet at a distance of about 48 million miles. Unlike the inner terrestrial planets the outer solar system planets have more natural satellites. This is because the planets formed in a colder environment. Deimos for example was influenced by meteorite impacts so the moon’s gravity is low enough to allow ejecta to escape into space.
Phobos and Deimos were discovered in 1877. Both moons are cratered irregular in shape and their names derive from Greek words. Scientists believe that they were once asteroids captured by Mars but this has not been proven. However they have the potential to become Earth’s first natural satellites. But it may take decades for the first humans to visit the moons so we need to be patient.
The larger planet captures a natural satellite from another body. This happens if the planets share the same mass. That’s how Deimos and Jupiter acquired their moons. The moons of Jupiter and Mars are also natural satellites. Jupiter and Mars have several natural satellites including the largest moon Titan. And Saturn has four moons – Deimos and Iota.
Mercury and Venus have no natural satellites. Mercury takes 88 days to orbit the Sun. Mercury is the smallest planet in our solar system and is fourth from the Sun. Mars’ moon Phobos is a cratered rocky moon about 14 miles across. Similarly Phobos is 7 miles across and is named after the Roman god of panic. The smaller moon Phobos is named Phobos and is twice as far away as Mars’s largest moon Phobos. They orbit Mars three times a day and take 30 hours to complete an orbit around Mars.
Io
The tenuous atmosphere of Io and Pluto has been a puzzle for astronomers for years. While their sizes masses and densities are fairly well constrained we still do not know the surface morphology. But the recent evidence of a trace atmosphere on Pluto is impressive. The gaseous atmosphere on Pluto is either condensing or precipitating. Moreover it is not known whether either of these bodies are linked to the Neptune system.
However it is possible that they have natural satellites which may contain ices. Charon may have liquid water which is a possibility. This could explain the red color of Charon. Scientists believe that it is not entirely natural that these two objects have no natural satellites because they are so different in size and composition. Io and Pluto are both relatively small and may not have a natural atmosphere.
When first discovered scientists thought that Pluto was the escaped satellite of Neptune. However recent calculations suggest that Pluto could not have acquired angular momentum during its ejection. It is therefore likely to be a large planetesimal from the early stages of the solar system. Its axial inclination is 122 degrees and its rotation period is 6.34 days. And its rocky surface was discovered only recently.
Io and Pluto have no natural satellites but it has a subsurface ocean which is a result of the tidal heating from Jupiter. Ganymede and Io have similar orbital periods but their gravitational pulls stretch their orbits into elliptical shapes. Europa is the most icy moon of all – with an icy surface and a rocky interior – but it may have a salty ocean beneath the ice.
The upcoming New Horizons mission is destined to visit the Pluto-Charon system. The mission will include a FUV imaging spectrograph that operates in the ultraviolet range (520-1870 A). This spectrograph will study the surfaces and atmospheres of the planets. And like Pluto Charon has no natural satellites. However they have a lot of potential as satellites.
Ganymede
In the solar system Ganymede is the largest Galilean moon of Jupiter and is composed of almost equal parts of rock and water-ice. The icy exterior of Ganymede has a surface resembling that of the moons Mercury and Moon. From a distance the moon’s surface shows large contrasts in color and brightness. The surface of Ganymede has ridges and valleys that were striated by tectonic activity. Dark patches show fewer craters while bright areas have many.
The asymmetry in crater density suggests that these two bodies did not have any natural satellites before their formation. This observation is supported by the observations of fragments of comet Shoemaker-Levy 9 near Jupiter. The fragments would have separated before impact and formed the crater-like surfaces of Ganymede and Callisto. In a recent study scientists concluded that the two bodies share a common origin.
During Galileo spacecraft flybys scientists discovered a magnetosphere on Ganymede’s surface. The magnetosphere is a magnetic field generated by motion of conductive material inside the planet. Although it is not known how the magnetosphere is formed astronomers suspect a molten iron or sulfur core and an icy crust. On Ganymede’s surface scientists have detected ozone.
Besides impact craters the moons also have various other types. The craters on Ganymede have a variety of shapes and sizes. The craters on Callisto are the largest and they are characterized by central peaks. Callisto also shows an anomalous dome crater and a penepalimpsest Memphis Facula. The moons of Jupiter’s other moons including Europa have no natural satellites.
Although the moons of Jupiter and Ganymede do not have any natural satellites their surfaces show different features. The icy surfaces of both are marked by impact craters and the decay of ice. The dark dust on both moons suggests that they are ancient though they are different in their composition. They also experienced heavy bombardment. Ancient impact basins were reduced by viscous flow in the icy lithosphere. This may have led to the emergence of bright volcanic terrain and extensional faults on Ganymede’s surface.
