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A study of jupiter and its moons

Rothery, Teach Yourself Planets, p. This would be a natural consequence of a decrease in temperature with distance away from the hot, young Jupiter within the disc of gas and dust from which these bodies grew.

They are described in turn in the following sections. Rothery Teach Yourself Planets, Chapter 9, pp.

Introduction

Prior to this, most people had assumed that bodies of Io's size, whether rocky like Io or icy like its companions, would be geologically dead like our own, similarly sized, Moon.

This is because their small size makes them incapable of having retained enough primordial heat or generating adequate radiogenic heat to keep their lithospheres thin and to drive mantle convection sufficiently close to the surface for melts to escape.

However, it is now realized that the orbital resonance that exists between the three innermost galilean satellites results in tidal heating. For every one orbit completed by Ganymede, Europa completes two and Io four. This means that the satellites repeatedly pass each other at the same points in their orbits, and the consequent internal stresses experienced by the satellites provide a source of heat that keeps their interiors warmer than they would otherwise be.

The effect is greatest for Io, which is closest to Jupiter and hence experiences the strongest tidal forces. There are often more than a dozen volcanoes erupting on Io at any one time. These are identified either by seeing an 'eruption plume' powered by the explosive escape of sulfur dioxide and rising 100-400 km above the surface Plate 8or by infrared detection of a hot spot.

Galileo image of lo recorded on 28 June 1997.

Learning outcomes

There are two eruption plumes visible. One is 140 km high and is seen in profile above the limb; this emanates from a volcano named Pillan Patera. The other plume, from a volcano named Prometheus, is seen from directly above, and lies near the centre of the disc. It is shown enlarged in the inset at upper left; the bluish dark ring is the outline of the plume and this casts a reddish shadow over the surface to its right.

Gravity and magnetic observations by Galileo confirm that it has a dense, presumably iron-rich core below its rocky mantle. Spectral data show that Io's surface is covered by sulfur, sulfur dioxide frost and other sulfur compounds.

However, these are no more than thin, volatile, veneers resulting from volcanic activity and the crust as a whole is some kind of silicate rock. Io has an atmosphere of sulfur dioxide, and atomic oxygen, sodium and potassium. The surface pressure is less than a millionth of the Earth's but nearly a billion times greater than the atmospheric pressure of the Moon or Mercury. Io's atmosphere continually leaks away into space, contributing to a 'cloud' of sodium and potassium that falls inwards towards Jupiter and to a magnetically confined belt of ionized sulfur that stretches right round Jupiter, concentrated around Io's orbit.

The atmosphere is replenished by a combination of volcanic activity and collisions onto Io's surface by high-speed ions channelled by Jupiter's magnetic field. When Io passes into the shadow of Jupiter its atmosphere can be seen faintly glowing in an auroral display caused by these same magnetospheric ions impinging on the atmosphere Figure 9.

There are lava flows up to several hundred km in length and vast swathes of mostly flat terrain covered by fallout from eruption plumes. Most of the lava flows are now believed to have formed from molten silicate rock, which is often discoloured by a sulfurous surface coating, but there are probably some flows that formed from molten sulfur too.

Here and there volcanoes rise above the general level of the plains, and their summits are occupied by volcanic craters described as 'calderas' up to 200 km across formed by subsidence of the roof of the volcano after magma has been erupted from within.

No impact craters are visible, because the volcanic eruptions deposit fresh materials across the globe at an average rate of something like a centimetre thickness per year. The long duration of the Galileo mission enabled many changes on Io's surface to be documented Figure 9. Galileo view of a 1200-km-wide region of Io on 4 April 1997 left and 19 September 1997 right [. Thus unlike Earth, which gets rid of heat from its interior by plate tectonics, and Venus, where heat escapes by conduction probably punctuated by orgies of resurfacing every half billion years or so, Io's heat escapes by means of hordes of volcanoes.

One factor that probably influences the difference between the Earth and Io is that, to maintain a steady state, Io has to lose heat at a rate of about 2. Possibly, the tidal heating experienced by Io is sufficient to keep a large fraction of its mantle partially molten. Europa is a transitional world, with a density almost in the terrestrial a study of jupiter and its moons league but an exterior that is icy down to a depth of about 100 km.

It is not known whether the ice is solid throughout, or whether its lower part is liquid, which raises the fascinating possibility of a global ocean sandwiched between the solid ice and the a study of jupiter and its moons rock. Gravity data from Galileo show that, like Io, Europa has a dense, presumably iron-rich core about 620 km in radius below its rocky mantle.

Europa has its own magnetic field, but it is not clear whether this is generated by convection within a liquid core or within a salty ocean beneath the ice. Europa has a highly reflective surface with an albedo of about 0. More detailed recent observations by Galileo and the Hubble Space Telescope reveal some regions where the ice appears to be salty, and also the presence of molecular oxygen O2 and ozone O3.

The oxygen and ozone are thought to result from breakdown of water molecules in the ice because of exposure to solar ultraviolet radiation and charged particles. The hydrogen so liberated would escape rapidly to space, which has been observed on Ganymede though not on Europa.

It is not known whether the oxygen and ozone detected on Europa constitute an extremely tenuous atmosphere or are mainly trapped within the ice. Voyager 2 view of part of Europa[.

  • Perhaps this process is capable of eroding the rims of small craters faster than the average rate at which such craters are forming;
  • The planet is seen in visible light, but the colourful patches on either side indicate the intensity of radiation mapped by a detector of microwaves.

This demonstrates that Europa experiences a significant amount of tidal heating, though less than Io. Images at Voyager resolution, such as Figure 9. There are several places where the pattern of these bright plains becomes blotchy, and these were dubbed 'mottled terrain' by the Voyager investigators. The high resolution images sent back by Galileo show that the bright plains are amazingly complex in detail Figure 9. The appearance of these parts of Europa has been described as resembling the surface of a ball of string; an apt description but not much help in trying to decipher a study of jupiter and its moons the surface was created.

Each grooved ridge could represent a fissure that was the site of an eruptive episode, when some kind of icy lava was erupted. Unless the ice is absolutely pure water, these properties include: Planetary scientists often use the term cryovolcanism to denote icy rather than silicate volcanism.

Although by far the most abundant component in Europa's ice is water, it is likely to be contaminated by various salts such as sulfates, carbonates and chlorides of magnesium, sodium and potassium resulting from chemical reactions between water and the underlying rock. The spectroscopic data for Europa are most consistent with the salt-rich areas of surface being rich in hydrated sulfates of magnesium or carbonates of sodium, but could also indicate the presence of frozen sulfuric acid.

Contaminants such as these could make any melt liberated from the ice behave in a much more viscous i. If erupted as a liquid this type of lava would not necessarily spread very far before congealing, especially if confined by a chilled skin of the sort likely to form upon exposure to the vacuum of space in the cold outer Solar System.

Contaminants also allow the ice to begin to melt at a much lower temperature than pure water-ice: Maybe, then, Europa's ridges are simply highly viscous cryovolcanic flows fed from their central fissures. Alternatively, the cryovolcanic lava may not have flowed across the ground at all: Irrespective of refinements such as this, it seems inescapable that each fissuring event must represent the opening of an extensional fracture in the crust.

This cannot happen across an entire planetary body unless the globe is expanding, which seems highly unlikely. Therefore there must be some regions on Europa where surface has been destroyed at a rate sufficent to match the crustal extension elsewhere. Likely candidates for this on Europa are regions described as 'chaos', and part of one of Europa's chaos regions is shown in Plate 9.

Galileo image of a 60 km wide region of Europa known as Conamara Chaos. The surface of the former a study of jupiter and its moons plains has been broken into rafts or ice floes that drifted apart before the intervening slush or water re-froze.

Bright patches on the left are splashes of recent ejecta from a 26 km diameter impact crater 1000 km to the south. The areas intervening between rafts are a jumbled mess reminiscent of re-frozen sea-ice on Earth.

Some rafts can be fitted back together, but it is apparent that many pieces of the 'jig-saw puzzle' are missing. Perhaps these missing pieces have sunk or been dragged down beneath the surface. Regions like Plate 9 appeared as mottled terrain on Voyager images, but so did the region shown in Figure 9.

Here, the surface of what was formerly normal looking bright terrain has been forced up into a number of domes up to 15 km across.

Jupiter and its moons

Presumably this is because of the rise of pods of molten or semifluid low density material described geologically as 'diapirs' toward the surface.

In some cases the upwelling pod has actually ruptured the surface, to form a small chaos region bearing raftlets of surviving crust. We cannot tell for sure how old each region of surface is, but there are abundant signs that there is, or has been, a liquid zone below the surface ice.

A salty ocean below several km of ice is not necessarily a hostile environment for life, and indeed life down there could be much richer and complex than anything that is likely to have survived on Mars. In the depths of the Earth's oceans there are whole living communities that are independent of photosynthetic plants requiring sunlight to live and depend instead on bacteria-like microbes that make a living from the chemical energy supplied by springs of hot water 'hydrothermal vents' on the ocean floor.

Given that Europa is tidally heated, we can imagine zones where water is drawn down into the rocky mantle, becomes heated, dissolves chemicals out of the rock, and emerges at hydrothermal vents surrounded by life.

This may sound far-fetched, but hydrothermal vents are now mooted as the most likely venue for life to have begun on Earth, so if it could happen here then why not on Europa too? It is the possibility of a life-bearing ocean below the ice that is the main driver for plans for the future exploration of Europa.

Cool Destination for Life? It will achieve this by gravity studies, by using an altimeter to determine the height of the tide raised on Europa by Jupiter only 1 m if the ice is solid throughout, but about 30 m for 10 km of ice overlying a global oceanand by using ice-penetrating radar to map ice thickness. High resolution conventional images will identify sites of recent eruptions.

Its waters may have been isolated from the surface for as long as 30 million years, and may host a unique ecosystem. The development of technology to drill through the ice and explore Lake Vostok, and also the adoption of protocols to avoid biological contamination of this special environment should both provide valuable experience. It is shown in comparison with its outer neighbour Callisto in Figure 9. Although these two satellites are similar in size, with bulk densities implying a roughly 60: The difference in evolution between these two bodies is probably because Ganymede was formerly subject to much more intense tidal heating than it receives today, whereas Callisto has never experienced much heating, tidal or otherwise.

Ganymede has a magnetic field with about 1 per cent the strength of the Earth, which could be generated in the core or in a study of jupiter and its moons salty ocean deep within the ice layer. Ganymede left and Callisto right seen at the same relative scales[. Spectroscopic studies show that it is dominantly water-ice, with scattered patches of carbon dioxide ice, and that the darkening is caused by silicate minerals probably in the form of clay particles and tholins.

The darkening is at least partly attributable to the much greater age of Ganymede's surface, allowing more time for the action of solar radiation to produce tholins and for silicate grains to become concentrated in the regolith by the preferential loss of ice during impacts. There are also faint traces of oxygen a study of jupiter and its moons ozone, apparently trapped within the ice as suggested for Europa, and Galileo found an extremely tenuous and continually leaking atmosphere of hydrogen that is presumably the counterpart to the oxygen produced by the breakdown of water molecules.

It is obvious even on images of the resolution of Figure 9. At higher resolution e. However, the density of impact craters on both terrain units shows that each must be very old, perhaps as much as 3 billion years. Images like Figure 9. This is not to say that the same processes were involved. One important difference between the two bodies is that on Europa there is abundant evidence of the two halves of a split tract of terrain having been moved apart to accommodate the new surface that has formed in between, whereas on Ganymede signs of lateral movement are scarce.

Ganymede's pale terrain appears to occupy sites where the older surface has been dropped down by fault movements, allowing cryovolcanic fluids to spill out.