Clues from measuring radioactivity in lunar rocks and meteorites

Jupsat Pro Astronomy Software

Secrets of the Deep Sky

Get Instant Access

What means have we of dating solar system objects? The method used by astronomers was developed during the second half of the twentieth century. It is based on measuring the age of certain radioactive elements which disintegrate very slowly over a period comparable to the lifetime of the Sun. These 'parent' elements are often isotopes* of a stable element, decaying into a different 'daughter' element of the same atomic mass, as a function of time, according to an exponential law. Through measurements of the amounts of the stable initial element and the parent and daughter elements, we can estimate the epoch at which the decay commenced, and thereby the age of the sample in question (for

A lunar crater. Like those on Mars and Mercury they are evidence of the period of intense meteoritic bombardment which occurred during the first billion years of the history of the solar system.

A fragment of a meteorite -

possibly part of the asteroid Vesta -which fell in western Australia in 1960.

* Most chemical elements have one or more isotopes; that is, elements with the same number of protons and electrons but a different number of neutrons, giving them a different atomic mass.

4.2 How old is the solar system? 69

Dating a planet according to the degree of cratering on its surface. The horizontal axis shows crater diameters D, and the vertical axis the number of craters with diameters greater than D. (After Hartmann and Neukum, SSRV, 96, 165 (2001).)

example, an Earth rock, a lunar sample or a meteorite). This method is known as isotopic dating. Often, the elements involved are the pairs argon-40/potas-sium-40 (40Ar/40K), ura-nium-238/lead-238 (238U/238Pb) or rubi-dium-87/strontium-87 (87Rb/87Sr). In the case of this last pair, the amounts of the elements 87Rb and 87Sr are compared with that of (stable) strontium-86 (86Sr), providing an indication of the initial abundance of strontium in the sample tested.

These analyses can lead to some spectacular conclusions. They have shown us that the age of the solar system, as measured in the most ancient parent bodies of meteorites, is 4.56 billion years, +1%. It is thought that the various bodies comprising the solar system - planets, asteroids, comets and so on - formed over a relatively short period of some 100 million years at most. As well as chemical analysis of extraterrestrial samples there is another way of dating planetary surfaces. This involves examining craters upon planets and satellites without dense atmospheres, such as Mercury, the Moon, Mars, the asteroids and the Galilean satellites of Jupiter. The more heavily cratered a surface, the older it must be. By comparing the ages of lunar rocks with the number of craters we can examine the relationship between the age of the surface and the degree of cratering, which can then be applied to other planetary

surfaces. It becomes apparent that during the first few hundred million years of the lifetime of the solar system there was an extremely heavy bombardment, which reached its zenith about 3.8 billion years ago. It will be seen that this bombardment was due to a particularly active episode in the life of the young Sun, when it drove the debris of its protoplanetary disk outwards. So, by dating surfaces and analysing extraterrestrial samples we can discover precious information about our very origins.

4.3 LOOKING AT NEARBY STARS

The Sun is halfway though its life: in mass and luminosity, a very ordinary star

Since the Sun is a very unremarkable star in our galaxy, a look at its neighbours may help us to understand its history. Let us concentrate upon very young stars and star-forming regions. These regions are dense, rapidly rotating molecular clouds which will in time collapse, giving birth to a new star. During recent decades the study of the early phases of a star's life has proceeded in leaps and bounds, due mainly to infrared and millimetric astronomy, which allows us to penetrate cold environments opaque to visible light. It has also been possible - especially with the Hubble Space Telescope - to secure images of protoplanetary disks around infant stars.

The phases of star formation. A

rotating cloud of material collapses into a disk; and at its centre, material is concentrated into a protostar which becomes a star when nucleosynthesis is triggered at a temperature of a few million degrees. This protostar is surrounded by a disk within which grains agglomerate to form planetesimals.

4.3 Looking at nearby stars 71

wF

f*

*

Protoplanetary disks in the Orion Nebula, imaged by the Hubble Space Telescope's wide-field planetary camera (WFPC2).

Protoplanetary disks in the Orion Nebula, imaged by the Hubble Space Telescope's wide-field planetary camera (WFPC2).

Certain very young stars are particularly intriguing to astronomers. These stars too are surrounded by disks, and are characterised by intense activity involving the outward impulsion of very large quantities of matter. This active period is known as the 'T Tauri phase' (from the name of the first star around which such activity was observed), which occurs quite soon after the birth of a star, during the first ten million years of its lifetime.

It is reasonable to suppose that the Sun underwent a largely similar process to that observed in the case of nearby stars of about the same mass. The most telling evidence is the existence of the ecliptic plane, a vestige of the original protoplanetary disk, and the fact that the planets orbit in near-circles, all proceeding in the same direction. So, early on a rotating cloud of matter collapsed into a disk; and at its heart, matter was concentrated into a protostar.

When the object's internal temperature reached a few million degrees, nucleosynthesis was triggered, and the object became a star. Within the disk, grains of matter collided to form planetesimals, which later, by accretion, became planets. When the Sun's T Tauri phase intervened, the gas and dust of the protoplanetary disk were expelled. All that remained were protoplanets, or the largest embryonic planets. Later came the era of bombardment, lasting for several hundred million years - ample evidence of which is presented by the crater-strewn surfaces of Mercury and the Moon.

4.4 THE PROTOPLANETARY DISK: THE CURRENT PICTURE

Hydrogen: dominant element of the protoplanetary disk

Let us try to picture the protosolar disk shortly after its collapse. Given the positions of the resulting planets it must have extended for several tens of AU. Its total mass was a small fraction (perhaps a tenth to a hundredth) of that of the protostar, and its density and temperature decreased from the centre outwards. Near the Sun, temperatures might have approached 2,000 K

A curve showing the abundance of chemical elements in the Universe.

Only elements lighter than zinc are included. These values are measured principally in the Sun (with the exception of deuterium) in the case of relatively light elements - C and O - and in meteorites in the case of the heavier elements. Li, B and Be are very rare, while Fe (in red) is very abundant. (After C. Allègre, 1985 (modified).)

log of relative abundance log of relative abundance

4.4 The protoplanetary disk: the current picture 73

An artist's impression of a brown dwarf surrounded by a protoplanetary disk.

NASA's Spitzer Space Telescope has found a disk like this around the very-low-mass brown dwarf star OTS 44, which is about fifteen times larger than Jupiter.

(we shall later see how this is known), while at distances between 30 and 50 AU they would have been only 100 K at most. The whole disk would slowly cool down as time passed.

What did this protoplanetary disk contain? All the elements present in the cosmos were there, in the relative abundances with which we are familiar. Hydrogen and then helium were the most abundant. Both of these were in gaseous form, since their condensation temperatures are so low. Then, in decreasing order of abundance, there were oxygen, carbon and nitrogen - the first elements to be formed after helium during stellar nucleosynthesis. We shall see how these can be found in solid or gaseous form according to the ambient temperature within the disk; that is, according to their distance from the Sun at a given time. Then come the heavier elements, also synthesised in stars, but in smaller quantities: sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, calcium and so on. Iron, with its very stable configuration, is particularly abundant, given its high atomic mass. All these 'refractory' elements were present in solid form within the protosolar disk. They can be found in meteorites and lunar samples.

Note that other heavy elements, likewise made inside stars, are also present in the disk in gaseous form. These are the rare or noble gases which, together with helium, are neon, argon, krypton and xenon. Because they are chemically inert they are precious indicators of the evolution of the atmospheres containing them.

4.5 THE ICE LINE

The formation of water ice marks the boundary between terrestrial planets and giants

It was solid particles which, within the protosolar disk, came together in multiple collisions to form embryonic planets or planetesimals. Later, these gave rise to the planets and their attendants, to asteroids and to comets. To understand the mechanism of their formation we must first identify the solid constituents of the protosolar disk.

The phase diagram of water.

The curves of the diagram represent boundaries between the different states of water, and function of temperature and pressure. Roman numerals refer to the different types of crystalline ice. Note the change of scale on the vertical axis at 2 atmospheres.

critical point

The phase diagram of water.

The curves of the diagram represent boundaries between the different states of water, and function of temperature and pressure. Roman numerals refer to the different types of crystalline ice. Note the change of scale on the vertical axis at 2 atmospheres.

critical point

4.5 The ice line 75

Phase diagrams (saturated vapour pressure/temperature) of some condensable gases. The inset shows the saturated vapour pressure of water, as a function of temperature. Water vapour is the gas which, at a given pressure, condenses at the highest temperature. (After S.K. Atreya, Atmospheres and Ionospheres of the Outer Planets and Their Satellites, Springer-Verlag, 1986.)

As we have seen, elements of atomic mass above approximately 20 - with the exception of the rare gases - are refractory. What of the lighter and therefore more abundant elements carbon, nitrogen and oxygen? They enter into association with hydrogen to form methane (CH4), ammonia (NH3) and water (H20). They can also form CO, C02, HCN, H2CO and other compounds, and atoms of hydrogen combine to form the H2 molecule.

In what form are these molecules found? Near the Sun they are all in gaseous form, and cannot have contributed to the composition of the solid embryonic bodies which became planets. However, at a few AU from the Sun the temperature was low enough for all of them, except molecular hydrogen, to exist as ice. As a function of the distance from the Sun, the first molecule to condense out is H20, and, with increasing distance, as the temperature falls, there follow NH3, HCN, C02, H2CO, CH4 and so on (see diagram above).

There is therefore a critical distance from the Sun beyond which the matter contained within the protoplanetary disk will occur mainly in solid form: the 'ice line'. At present it is situated between 1 and 2 AU from the Sun, although during the early history of the disk, before cooling set in, it lay further out.

How did planets form close to the Sun? Remember that the mass contained within the refractory elements, which are heavy and scarce in the Universe, is small compared with that of the ices containing carbon, nitrogen and oxygen,

temperature (k)

Condensation sequence for refractory elements for a gas of solar-type composition.

(After Grossmann and Larimer, Rev. of Geophysics and Space Physics, 12, 71 (1974).)

the most abundant elements after hydrogen. So, the mass of material available for planet formation was limited. Models predict the emergence of only a few protoplanets, of masses lower than or similar to that of Earth, and with high densities (3.9-5.5 g/cm3). This is precisely what we find in the inner solar system.

Beyond the ice line the amount of available solid matter within the disk was sufficient for the formation of large icy nuclei, perhaps a dozen times more massive than the Earth. Theory shows that their gravitational fields would have been strong enough to draw in surrounding protosolar gases, essentially hydrogen and helium. New planets were born. They were very massive and extremely large, with low densities (0.7-1.7 g/cm3) - today's giant planets.

4.6 TERRESTRfAL AND GIANT PLANETS

Can the differentiation between terrestrial and giant planets be found in extrasolar systems?

The model of planetary formation by accretion around solid particles leads naturally to the existence of two distinct classes of planet: terrestrials and giants. What is the composition of their atmospheres? In order to investigate this

Terrestrial and giant planets 77

heliocentric distance [A.U.)

1

5

10 SO

asteroid belt

Sun

terrestrial pi a nets

giant planets

Temperature (K)

300

100 40

condensed elements

silicate metals

silicate m eta is + ices[H20,NH3,CH4,C02)

mass of nucleus

M„

-10 Me

accretion from surrounding nebula

no i

yes 1

final object

solid planets visible surfaces

[terrestrial planets]

accretion of primordial gas (H j, He]

giant planets

The two formation processes of the solar planets (terrestrial and giant).

question, let us return to the chemical composition of the protoplanetary disk. Thermochemical models predict that within giant planets - where the pressure is relatively high but temperatures are low - carbon and nitrogen occur preferentially in the form of methane and ammonia. Indeed, this is what we observe in the case of the giants. Within the inner solar system, thermochemical equilibria involving CH4> H20 and NH3 evolve to form CO, C02, H20, N2 and H2. Molecular hydrogen is too light to remain trapped in the gravitational fields of the terrestrial planets, and escapes. This is the initial global composition of the atmospheres of the inner planets, and all of them have undergone considerable evolution. We shall return to them later.

The giant planets fall into two distinct categories. Jupiter and Saturn - with masses respectively 318 and 95 times the mass of the Earth - are essentially composed of protosolar gases, and are rightly called 'gas giants'. However, Uranus and Neptune (15 and 17 Earth masses) consist mostly of their initial ice cores, and are 'ice giants'. The chemical composition of the giant planets offers clear confirmation of the model of planetary formation by accretion around a central nucleus.

The planets of the solar system fall into two main categories: terrestrials (within 2 AU from the Sun) and giants (beyond 5 AU). The terrestrials (Mercury, Venus, Earth and Mars) are rocky, small, very dense, possess few satellites and, with the exception of Mercury, are enveloped in stable atmospheres. The giant planets are massive but not dense, and have rings and many satellites. The two largest - Jupiter and Saturn - are the nearest of the giants to the Sun and are composed mainly of gas, while the other two - Uranus and Neptune - consist mainly of ice. The last object, Pluto, beyond the orbit of Neptune, and previously considered as a planet, is now seen as one of the largest members of a recently discovered class of bodies: the trans-Neptunian or Kuiper Belt objects.

The planets of the solar system fall into two main categories: terrestrials (within 2 AU from the Sun) and giants (beyond 5 AU). The terrestrials (Mercury, Venus, Earth and Mars) are rocky, small, very dense, possess few satellites and, with the exception of Mercury, are enveloped in stable atmospheres. The giant planets are massive but not dense, and have rings and many satellites. The two largest - Jupiter and Saturn - are the nearest of the giants to the Sun and are composed mainly of gas, while the other two - Uranus and Neptune - consist mainly of ice. The last object, Pluto, beyond the orbit of Neptune, and previously considered as a planet, is now seen as one of the largest members of a recently discovered class of bodies: the trans-Neptunian or Kuiper Belt objects.

Now, 2% of the protosolar gas consists of heavy elements - a standard value in the Universe - but in initial nuclei they make up 100% of the material. In accordance with theoretical models, let us take an initial nucleus of 12 Earth masses. Suppose that all the heavy elements are fixed in ices, and that the mixture within giant planets is homogeneous, after the collapse of the protosolar gas. For these planets we can now calculate the excess of the heavy elements visa-vis hydrogen, compared to cosmic values: 3 in the case of Jupiter, 7 for Saturn, and 30-50 for Uranus and Neptune. These excesses have indeed been measured in these planets, thereby confirming unambiguously the nucleation model.

Returning to the terrestrial planets, we now ask: how did their atmospheres originate? As already stated, protosolar gases do not figure here. Hydrogen and helium are too light to be held by the terrestrials' gravitational fields, which doubtless acquired their atmospheres partly through outgassing and partly through meteoritic bombardment, from the gaseous elements contained within asteroids and comets. The Earth's water in particular is certainly, at least in part, of cometary and asteroidal origin, as may be the prebiotic molecules which were the precursors of life on Earth.

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment