The Solar System in Close-Up
Page 16
Probing Jupiter
The first space probe to visit Jupiter was Pioneer 10 in December 1973. This was the first probe to venture out into the solar system beyond Mars. The probe returned over 20 low-resolution images of Jupiter’s cloud system. A year later, Pioneer 11 returned 17 images during its closest approach to Jupiter; it then used Jupiter’s strong gravitational pull to propel it towards Saturn. These two probes also recorded data of Jupiter’s atmospheric temperature and pressure and took several pictures of its moons.
The probes recorded changes in Jupiter’s atmosphere, particularly around the Great Red Spot and discovered Jupiter’s huge magnetic field.
NASA’s Voyager 1 and 2 spacecraft flew by Jupiter in mid 1979 before proceeding on to Saturn. The two probes discovered that Jupiter has complicated atmospheric dynamics, lightning and auroras. These probes also found increased turbulence around the Great Red Spot. The winds to the north and south of the spot blow in opposite directions, seemingly fuelling the spot’s rotation. Three new moons were discovered as well as a ring system.
In 1989, NASA launched the Galileo space probe from a space shuttle in Earth orbit. The probe was to rendezvous with Jupiter in 1995, after a trip that used a gravity assist from Venus. In July 1994, while still 225 million km from Jupiter, Galileo was able to observe fragments of the comet Shoemaker-Levy as they hit Jupiter. The fragments hit Jupiter at a speed of about 200,000 km/h over a period lasting about a week. The collisions left visible marks in Jupiter’s atmosphere.
When Galileo reached Jupiter in 1995 it released a probe that descended into Jupiter’s atmosphere about 150 km below the cloud tops. Data from this probe indicated that there is much less water than expected. Also surprising was the high temperature and density of the uppermost parts of the atmosphere. Recent observations by the Galileo orbiter suggest the probe may have entered the atmosphere at one of the warmest and least cloudy areas on Jupiter at that time.
As the Ulysses space probe passed by Jupiter in February 1992 it gathered data, which showed that the solar wind has a much greater effect of Jupiter’s magnetic field than earlier measurements had suggested (see Table 9.2).Table 9.2Significant space probes sent to Jupiter
Probe
Country of origin
Launch year
Notes
Pioneer 10
USA
1972
Fly by in 1973
Pioneer 11
USA
1973
Fly by in 1974
Voyager 1
USA
1977
Fly by in 1979
Voyager 2
USA
1977
Fly by in 1979
Galileo
USA
1989
In orbit from 1995
Ulysses
USA/ESA
1990
Fly by 1992
New Horizons
USA
2006
Fly by in 2007
Juno
USA
2011
Orbit in 2016
In February 2007, the New Horizons probe flew by Jupiter. The reason for the fly by was to give it a gravitational boost, throwing it towards Pluto. This brief encounter was also used as a test run for both the spacecraft and its earthbound controllers in preparation for the Pluto encounter in 2015. The probe passed within 51,000 km of Jupiter. Images were taken of Jupiter’s rings, its moon Io, and the Little Red Spot on Jupiter’s surface. New Horizon’s is the fastest spacecraft ever (80,000 km/h), having bridged the gap between Earth and Jupiter in only 13 months.
NASA’s Juno spacecraft, launched in August 2011, will be the first solar-powered spacecraft to orbit Jupiter. The probe will enter a polar orbit in July 2016 and move around Jupiter observing its gravity and magnetic fields, weather and composition, and the connections between the interior, atmosphere and magnetosphere. The Europa Jupiter System Mission, due to launch around 2020, will engage in an extended study of the planet’s moon system, particularly Europa and Ganymede, and settle the long-running scientific debate over whether an ocean of liquid water exists under Europa’s icy surface.
Position and Orbit
The slightly elliptical orbit of Jupiter lies between the asteroid belt and Saturn. Jupiter has a mean distance from the Sun of just over 778 million km, placing it about 5.2 times farther from the Sun than is Earth. It travels around the Sun once every 11.86 years and it rotates on its axis with a period shorter than any other planet. The short rotational period has resulted in Jupiter becoming flattened or oblate. The equatorial diameter is 142,984 km, which is about 8000 km greater than its polar diameter. This shape suggests the interior is a liquid rather than a solid or gas. The planet’s axis that is tilted at an angle of 3.12° to the vertical.
Density and Composition
Jupiter is more than 318 times more massive than the Earth and has twice as much mass as all the other planets combined. Jupiter emits about twice as much heat as it absorbs from the Sun. Its core temperature is estimated at 20,000 °C—about four times greater than Earth’s core temperature. This heat is thought to be generated by the gravitational contraction of Jupiter by about 3 cm per year. The core pressure may be about 100 million times greater than on Earth’s surface.
The shape of the planet and its strong gravitational field suggest Jupiter must have a dense core about 10–20 times the mass of Earth. However, overall, Jupiter’s average density is lower than that of Earth—only 1.33 g/cm3 compared to Earth’s 5.52 g/cm3.
Jupiter is about 90 % hydrogen and 10 % helium with traces of methane, water, ammonia and ‘rock’. This composition is very close to the composition of the nebula that the solar system formed.
There are three main regions to Jupiter’s interior. The outer layer of Jupiter that we see from Earth is the top of the outer layer of clouds. The Galileo probe found that these clouds are mostly gaseous molecular hydrogen and helium. With increasing depth and hence pressure, the gases become more like liquids. The hydrogen becomes crushed into a liquid form called metallic hydrogen. Metallic hydrogen’s high electrical conductivity and the rapid rotation of the planet, give rise to Jupiter’s intense magnetic field and radiation belts. Most of the interior of Jupiter (its mantle) is therefore mostly liquid metallic hydrogen. Below the mantle, the third region or core of Jupiter is thought to consist of rocky material mainly iron and silicates. The core is about 20,000 km in diameter (Fig. 9.2).
Fig. 9.1The planet Jupiter and the Great Red Spot as seen by the Wide Field Camera on the Hubble Space Telescope in April 2014. The Great Red Spot (lower right) is a high-pressure anticyclone in Jupiter’s southern hemisphere (Credit: NASA and ESA teams).
The strength of gravity on Jupiter is 2.5 times that of Earth’s gravity. This means that a 75 kg person who weighs 735 N on Earth would weigh 1845 N on Jupiter.
The Surface
The outer layers of Jupiter form a shell, mostly of gaseous hydrogen. Although this layer is about 20,000 km thick, there is no solid surface. The gas just gets thicker and thicker, until the pressure is three million times the air pressure at sea level on Earth. At this point, hydrogen becomes crushed into liquid metallic hydrogen.
Seen from Earth, Jupiter is one of the brightest planets in the sky. Viewed through a telescope, its disc is crossed by numerous belts or zones of various colours including red, orange, brown and yellow. The brighter zones are regions where fluids from within the planet are rising to the surface to cool, while the darker belts are regions where material is descending.
Also visible on the surface of Jupiter is the Great Red Spot first observed by Giovanni Cassini through his telescope in 1665. This spot is a huge, oval-shaped atmospheric feature located in the southern hemisphere. The size of the spot varies but is roughly 30,000 km in length and 12,000 km in width. The Pioneer and Voyager missions suggested that the spot is a hurricane-like storm whose red colour may be caused by the presence of red phosphorus and ye
llow sulfur in the ammonia crystals. Infrared observations show the spot is a high-pressure region whose cloud tops are much higher and colder than the surrounding regions. It is not fully understood how the spot lasts for so long, as it must be absorbing a lot of energy to keep surviving. The edge of the spot rotates at a speed of about 360 km/h. The spot moves east to west around Jupiter but stays about the same distance from the equator (see Fig. 9.1).
Fig. 9.2Interior structure of Jupiter.
The Atmosphere
Spectra analysis of Jupiter’s atmosphere shows it is 86 % by mass hydrogen and 13 % helium. The remainder consists of small amounts of simple compounds such as methane, ammonia, and water vapour.
Data from the Galileo probe released into Jupiter’s atmosphere only goes down to about 150 km below the cloud tops. It is thought that the high pressure and radiation on Jupiter destroyed the Galileo probes sensors.
Three distinct layers of clouds are thought to exist on Jupiter. The upper layer is the coldest (−153 °C) and contains mainly ammonia ice crystals. The middle layer contains crystals of ammonium hydrosulfide and a mixture of ammonia and hydrogen sulfide. The lower layer contains water ice. The vivid colours seen in Jupiter’s clouds are probably due to chemical reactions between the elements in the atmosphere. The colours seem to correlate with altitude: blue the lowest clouds, followed by browns and whites, with reds highest.
Clouds in Jupiter’s turbulent atmosphere move at high velocity in east-west belts parallel to the equator. The winds blow in opposite directions in adjacent belts. Data from the Galileo probe indicate the winds travel at about 600 km/h and extend down thousands of kilometres into the interior. The winds are mainly driven by Jupiter’s internal heat and the planet’s rapid rotation.
Pictures from the Pioneer and Voyager probes showed changes often occur in Jupiter’s atmosphere. The most notable of these was in the region around the Great Red Spot. At the time of the Pioneer probes (1973–1974) the spot was surrounded by a white zone. By 1979 when Voyager visited Jupiter, a dark belt had crossed the spot, and there was increased turbulence around the area. Measurements showed the spot rotates anti-clockwise over a period of about 6 days. The winds north of the spot blow in the opposite direction to those south of the spot. Lightning 10,000 times more powerful than any seen on Earth has also been detected in Jupiter’s atmosphere.
Other oval shaped features called eddies or circular winds can be seen in the atmosphere of Jupiter. These eddies move about within the zones in which they are trapped by opposing winds. They usually appear white in colour. The Great Red Spot is a huge eddy (see Fig. 9.3).
Fig. 9.3Close up image of Jupiter and the Great Red Spot as seen by the Hubble Space telescope. The moon Ganymede is also shown (lower right) (Credit: NASA/ESA).
Jupiter’s Ring System
Jupiter also has a system of four rings surrounding its atmosphere in an equatorial plane. These rings are much fainter and lighter than those around Saturn and can’t be seen from Earth through normal telescopes. The rings were discovered by the Voyager probes in 1979, but they have since been imaged in the infrared from ground based telescopes, the Hubble Space Telescope (HST) and by the Galileo probe. Observations of the rings require the largest available telescopes.
The ring system’s four main components are: a thick inner torus of particles known as the “halo ring”; a relatively bright, exceptionally thin “main ring”; and two wide, thick and faint outer “gossamer rings”, named for the moons of whose material they are composed: Amalthea and Thebe. The main ring is about 6500 km wide and 30 km thick. Its inner edge is about 123,000 km above Jupiter’s cloud tops (see Fig. 9.4).
Fig. 9.4The Long Range Reconnaissance Imager on the New Horizons probe snapped this photo of Jupiter’s ring system as it flew by in 2007. This image shows a narrow ring, about 1000 km wide, with a fainter sheet of material inside it (caused by fine dust that diffuses in toward Jupiter) (Credit: NASA, GSFC, NSSDC).
The rings are dark and are composed of very fine-grained dust particles and rock fragments, and unlike Saturn’s rings, they do not contain any ice. Galileo found evidence that the particles are continuously being kicked out of orbit by radiation from Jupiter and the Sun. High-resolution images obtained in February and March 2007 by the New Horizons spacecraft revealed a rich fine structure in the main ring. The rings are probably re-supplied with material by dust particles formed by micrometer impacts on the four inner moons. The New Horizons spacecraft conducted a deep search for new small moons inside the main ring. While no satellites larger than 0.5 km were found, the cameras of the spacecraft detected seven small clumps of ring particles. Spectra of the main ring obtained by the HST, Keck, Galileo and Cassini have shown that particles forming it are red in colour.
Temperature and Seasons
The temperature near the cloud tops of Jupiter measures about −153 °C. Temperatures increase with depth below the clouds, reaching 20 °C at a level where the atmospheric pressure is about ten times as great as it is on Earth. At a depth of about 20,000 km below the cloud tops the temperature is about 10,000 °C. Below this depth, the pressure and temperature are high enough to transform liquid hydrogen into liquid metallic hydrogen. The pressure at Jupiter’s centre is about 80 million atmospheres and the temperature about 24,000 °C, which is hotter than the surface of the Sun.
Because Jupiter takes 11.86 years to orbit the Sun and it has a small axial tilt, there are not any real seasons on Jupiter.
Magnetic Field
Jupiter has a huge magnetic field, about 20 times stronger than Earth’s magnetic field. Its magnetosphere extends only a few million kilometres towards the Sun, but away from the Sun it extends to a distance of more than 650 million km (past the orbit of Saturn). Many of the moons of Jupiter lie within its magnetosphere. The magnetic field contains high levels of radiation and energetic particles that would be fatal to space travellers. The Galileo probe discovered a new intense radiation belt between Jupiter’s rings and the upper most atmospheric layers. This belt is about ten times stronger as Earth’s Van Allen radiation belts, and contains high-energy helium ions (see Fig. 9.5).
Fig. 9.5Jupiter’s magnetic poles are offset from the rotational axis by nearly 10°.
Jupiter emits radio waves strong enough to be picked up by radio telescopes on Earth. Scientists use these waves to calculate the rotational speed of Jupiter. The strength of the waves varies as the planet rotates and is influenced by Jupiter’s magnetic field. The radio waves come in two forms. The first is a continuous emission from Jupiter’s surface; the second is a strong burst that occurs when the moon Io passes through certain regions of the magnetic field and radiation belt.
X-ray telescopes and the Hubble Space Telescope regularly detect auroras on Jupiter. These auroras however are thousands of times more powerful than those on Earth. On Earth, the most intense auroras are caused by outbursts of charged particles from the Sun interacting with the polar magnetic field of Earth. On Jupiter, however, the particles seem to come from the moon Io that has volcanoes that spew out oxygen and sulfur ions. Jupiter’s strong magnetic field produces about 10 million volts around its poles, and this field captures the charged particles and slams them into the planet’s atmosphere. The particles interact with molecules in the atmosphere and the result is intense X-ray auroras, virtually all the time (see Fig. 9.6).
Fig. 9.6An aurora around Jupiter’s north pole. Imaged by the Hubble Space Telescope in UV light (Credit: NASA/HST).
Moons of Jupiter
At least 63 natural satellites or moons orbit Jupiter. Galileo was the first to observe the four largest moons of Jupiter through his telescope in 1610. These moons, now known as the Galilean moons, orbit Jupiter in an equatorial plane, creating the appearance of a mini-solar system. In order of distance from Jupiter, the four Galilean moons are Io, Europa, Ganymede, and Callisto (Table 9.3).Table 9.3Details of the Galilean moons of Jupiter
Name of moon
Distance from Jupiter (km)
Period (days)
Diameter (km)
Discovery year
Io
420,700
1.77
3630
1610
Europa
671,000
3.55
3138
1610
Ganymede
1,070,000
7.16
5262
1610
Callisto
1,883,000
16.69
4800
1610
The first three Galilean moons, are locked together into a 1:2:4 orbital resonance by tidal forces from Jupiter. In a few hundred million years, Callisto will be locked in too, orbiting at exactly twice the period of Ganymede and eight times the period of Io (Fig. 9.7).
Fig. 9.7The four Galilean moons of Jupiter (Credit: NASA).
Io is about the same size as Earth’s Moon and it orbits around Jupiter once every 1.77 days. This moon is the most volcanically active body in the solar system. Images from Voyager 1 showed Io has nine giant erupting volcanoes on its surface and up to 200 smaller volcanoes. These sulfurous eruptions give Io a white-, yellow- and orange-coloured surface.
Io’s volcanoes are all relatively flat. The largest volcano, called Pele, is about 1400 km across. There are also mountains up to 10 km high, but these are not volcanic. Voyager also discovered numerous black spots scattered across Io, which are thought to be volcanic vents through which eruptions occur. The volcanic eruptions change rapidly. In the 4 months between the arrivals of Voyager 1 and Voyager 2 some of the eruptions had stopped, while others had begun, and deposits around the vents also changed visibly.