Exploring the Solar System. Lecture 10: Exploration of the Jovian Planets and Uranus and Neptune

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Exploring the Solar System Lecture 10:

Exploration of the Jovian Planets and

Uranus and Neptune

Professor Paul Sellin

Department of Physics University of Surrey

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Overview

Exploration of Jupiter, Saturn and Pluto: Galileo and the probe descent on Jupiter Cassini-Huygen imaging of Saturn

New Horizons mission to Pluto and the Kuiper Belt Physical data of Uranus, Neptune and Pluto

The rotation of Uranus

Atmospheric storms on Uranus and Neptune Internal structure and magnetic fields Rings, moons and satellites

Pluto and its moon Charon: The re-classification of Pluto Latest exploration data:

New Horizon’s first glimpse of Pluto Huygens – the descent to Titan’s surface

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Exploration of Jupiter: the Galileo probe

Galileo was NASA’s mission to Jupiter, launched 1989, and ended in 2003 Venusflyby in February 1990

Earthflybys in December 1990 and December 1992 Arrival at JupiterDecember 1995

Total number of orbits around Jupiter: 35

Galileo returned 14,000 images during its mission, which ended after 8 years orbiting Jupiter The Galileo Probewas released in July 1995:

the probe descended through 150 km of atmosphere, sending data for 58 minutes

at the end of the measurement period winds of up to 450 mph were measured

the Jovian weather data from the probe showed a dry atmosphere with few clouds – this was later discovered to be a ‘hot spot’ region:

- the probe did not detect the expected three-tiered cloud structure - the amount of helium measured was about one-half of what was expected

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Exploration: Cassini-Huygens

Four NASA spacecraft have been sent to explore Saturn. Pioneer 11 was first to fly past Saturn in 1979. Voyager 1 and 2 in 1980, 1981, and Cassini-Huygens in 2004

launch in October 1997

Venusflybys in April 1998 and June 1999 Earthflyby in August 1999

Jupiter flyby in December 2000 arrival at Saturn July 2004

The Cassini spacecraft carried the ESA Huygens probe, which separated from Cassini in December 2004:

20 days travel to Titan

Parachutes deployed upon entering Titan’s upper atmosphere, landed on Titan’s surface 2 hrs 27 minutes later

the first spacecraft to land on a moon in the outer solar system

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Titan

The largest Saturnian satellite, Titan, is a terrestrial world with a dense nitrogen atmosphere A variety of hydrocarbons are produced there by the interaction of sunlight with methane These compounds form an aerosol layer in Titan’s atmosphere and possibly cover some of its surface with lakes of ethane

The Voyager images (left, centre) show how Titan’s atmosphere gives it a featureless appearance – the colour enhanced left picture shows a haze on the edge of the atmosphere HST image (far right) shows the shadow of Titan on Saturn’s clouds.

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Lakes on Titan

The existence of oceans or lakes of liquid methaneon Saturn's moon Titanwas predicted more than 20 years ago. But with a dense haze preventing a closer look it has not been possible to confirm their presence.

Radar images taken during a Cassini fly-by of Titan provide convincing evidence for large bodies of liquid on Saturn’s largest moon

The image shows a false-colour radar view from Cassini, which provides definitive evidence of the presence of liquid-filled lakes

Dark/Blue regions are areas of low radar reflectivity, consistent with water ice or liquid organics

Scientists believe that these are lakes of liquid methane, replenished by methane rainfall - published in Nature, January 2007

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Dunes on Titan

This radar image, obtained by Cassini's radar instrument on 22 February 2007, shows dunessurrounding a bright feature on Saturn's moon Titan.

Dunes have been previously seen on Titan, so far concentrated near the equator. They are thought to be composed of small hydrocarbon or water ice particles -probably about 250 microns in diameter, similar to sand grains on Earth. These are formed into dunes by the prevailing west-to-east surface winds, and are probably ‘longitudinal’ (lying in the same direction as the average wind) rather than transverse dunes, more common on Earth. There are several kinds of interaction between the dunes and the brighter features in this image. At the left, the dunes seem to be covering the bright material, while at the centre and right, they seem to be terminated against it. At the lower centre and lower right, they flow around it.

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Relative scales of the outer planets

The Earth and the 3 outer worlds to scale – imaged in true colour visible light

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The featureless atmosphere of Uranus

Uranus imaged from Voyager 2. This image looks nearly straight down onto Uranus’ south pole which was pointing almost directly at the Sun as Voyager flew past in 1968

None of the Voyager 2 images of Uranus shows any pronounced cloud patterns The colour us due to methane in the planet’s atmosphere, which absorbs red but reflects green and blue

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The perpendicular rotation axis of Uranus

For most planets the rotation axis is roughly perpendicular to the plane of the planet’s orbit However for Uranus the rotation axis is tilted by 98° from the perpendicular which causes severely exaggerated seasons.

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Uranian storms

When HST took this false-colour IR image

of Uranus in 1998, sunlight was returning to the planet’s northern hemisphere

Solar heating of the northern hemisphere triggered a series of storms each more than 1000 km across

This image also shows Uranus’ rings and two of its satellites

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Neptune’s Great Dark spot

Voyager 2 took this image of Neptune’s

southern hemisphere in 1989 – the Great Dark Spot measured 12,000 by 8,000 km, similar in size to the Earth

A smaller storm appear lower left

The white clouds are supposed to be made of crystals of methane ice (the colour contrast of the image has been exaggerated)

No white ammonia clouds are seen on Uranus or Neptune. Presumably the low temperatures have caused almost all the ammonia to precipitate into the interiors of the planets All of these planets’ clouds are composed of methane. Much more cloud activity is seen on Neptune than on Uranus.

This is because Uranus lacks a substantial internal heat source.

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Cirrus clouds over Neptune

Voyager 2 recorded this image of clouds near Neptune’s terminator

Like wispy high-altitude cirrus clouds in the Earth’s atmosphere, these clouds are thought to be made of ice crystals – methane ice not water ice

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Neptune’s banded structure

This enhanced-colour HST image

shows Neptune’s belts and zones: white areas denote high-altitude clouds

the very highest clouds are shown as yellow-red

the atmosphere absorbs blue light in the green belt near the south pole, indicating a different chemical composition

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Internal structure

Uranus and Neptune contain a higher proportion of heavy elements than Jupiter and Saturn Both Uranus and Neptune may have a rocky core surrounded by a mantle of water with ammonia dissolved in it, and an outer layer of liquid molecular hydrogen and helium Electric currents in the mantles may generate the magnetic fields of the planets

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Magnetic fields

The magnetic fields of 5 planets, showing the rotation of the magnetic axis relative to the rotation axis

For both Uranus and Neptune the magnetic fields are offset from the planet’s centre, and steeply inclined to the rotation axis

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The 5 large moons of Uranus

A ‘family portrait’ shows a composite of images of Uranus’ 5 largest moons to the same scale, and correctly shows the relative reflectivity.

Note that the darkest satellite Umbriel is actually more reflective than our Moon. All 5 satellites are greyish in colour.

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Uranus’ small satellites

The image shows 8 smaller

satellites, shown as false-colour IR from HST

All lie within 86,000 km of the planet’s centre (one fifth the distance from Earth to our Moon) and all are less than 160 km in diameter

The arcs show how far each satellite moves around its orbit in 90 minutes

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Triton – the largest moon of Neptune

Neptune has 13 satellites, one of which (Triton) is comparable in size to our Moon or the Galilean satellites of Jupiter

Triton has a young, icy surface indicative of tectonic activity

The energy for this activity may have been provided by tidal heating that occurred when Triton was captured by Neptune’s gravity into a retrograde orbit

Triton has a tenuous nitrogen atmosphere

Composite image created from several Voyager 2 photographs

The pinkish material surrounding Triton’s south pole is probably nitrogen frost – some of this evaporates in the summer period

The northward flow of the evaporated gas may cause the dark surface markings

Farther north is a brown area of “cantaloupe terrain” which is supposed to resemble the skin of a melon

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Pluto – viewed from Earth

Images of Pluto’s surface:

the bright regions at the top and bottom of each hemisphere of Pluto may be polar ice caps

The bright regions nearer the equator may be impact basins where more reflective subsurface ice has been exposed

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Pluto and Charon viewed from Earth

Pluto and Charon resemble each other in mass and size more than any other planet-satellite pair in the solar system

Their separation is also the smallest between planet and satellite

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The Kuiper Belt

The Kuiper Belt lies close to the plane of the ecliptic and extends from 30-500 AU from the Sun About 1000 objects from the Kuiper Belt have been directly observed, and it is estimated that ~100,000 objects exist with diameters > 100km

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Pluto’s reclassification

It’s official – there are only 8 planets, as of August 2006:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune

The part of "IAU Resolution 5 for GA-XXVI" that describes the new planet definition, reads: A planet is a celestial body that:

(a)has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(b)is in orbit around a star, and is neither a star nor a satellite of a planet

The IAU also defined a new classification of Plutonwhich are distinguished from classical planets by:

they reside in orbits around the Sun that take longer than 200 years to complete, ie. they orbit beyond Neptune

they typically have orbits that are highly tilted with respect to the classical planets (a high orbital inclination). They also typically have a large orbital eccentricity.

Consequently the 4 new plutonsare: Plutoand Charonare re-classified as plutons, plus Ceresand 2003 UB313(popularly called Xena, which will be awarded an official new name at a later time).

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New Horizon’s mission to Pluto

Expected closest approach to Pluto: July 2015

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New Horizons sees Pluto

Pluto was first observed from New Horizons on Sept. 21, 2006, at a distance of 4.2 billion kilometers from the spacecraft

Pluto is little more than a faint point of light among a dense field of stars

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And Finally…

We finish by returning to Titan and the

Huygens probe landing.

Probably the most remarkable landing in mankind’s history, and certainly the furthest occasion from Earth of a controlled descent to an planet’s surface

The image shows a series of high resolution panoramic views from the Huygens probe, taken by the Descent Imager/Spectral Radiometer (DISR) on 14 January 2005, during the 147-minute descent through Titan's thick orange-brown atmosphere to a soft sandy riverbed, with a surface

temperature of -180°C