Monday, 25 February 2008

Sekitar Tata Surya

How we can Measure Age of the Moon According the Moon Rock Ages?

Bagaiman Kita Dapat Menetukan Usia Bulan dengan Mengacu Kepada Usia Batuan Bulan?

By: Mitsunobu Tatsumoto,
Dr Andrew A. Snelling and David E. Rush

Add and Edited by:
Arip Nurahman
Department of Physics, Faculty of Sciences and Mathematics
Indonesia University of Education

"Human desire always guide to the new question about the Universe and new question always follow by another question, how we can end this question?"
(H2O)
Abstract

The idea that the age of the Earth can be determined from the age of the Moon depends upon a known relationship between the age of the Earth and the age of the Moon. If one knows that the Earth and the Moon are the same age, then knowing the age of one tells the age of the other. If one knows the Moon is younger than the Earth, then knowing the age of the Moon establishes a minimum age of the Earth. If one knows the Moon is older than the Earth, then the age of the Moon establishes a maximum age of the Earth.

Introduction

Some scientists have made guesses about how the Moon was formed. Some think that lots of rocks were attracted by gravity to form the Earth while fewer rocks were attracted by gravity to form the Moon. They say the Moon formed in orbit around the Earth at the same time as the Earth was formed. Other scientists believe the Moon was belched out of the place where the Pacific Ocean is now by an enormous eruption. That would make the Moon younger than the Earth. Still others believe the Moon is a big rock that had been drifting around the universe for a long time before it got captured by Earth's gravitational field. If that is true, the Moon could be much older than the Earth. All we know for sure is that scientists don't know for sure. Therefore, the age of the Moon has absolutely no bearing on the age of the Earth.

Contents

But let's assume, just for the sake of discussion, that the age of the Earth is the same as the age of the Moon. Can we really tell the age of the Moon using a mass spectrometer? Scientists can smash a piece of moon rock down to individual atoms. Each kind of atom (each isotope) has a different atomic weight (mass). A mass spectrometer separates atoms by weight and tells you how much of each isotope is in the rock. What does knowing how much of each isotope is in the rock tell you? It just tells how much of each isotope is in the rock—nothing more.

Certain radioactive isotopes turn into more stable isotopes at a known rate. For example, uranium turns into lead very slowly. Scientists can measure the amount of uranium and lead in a moon rock and figure out how much uranium and lead will be in the rock a billion years from now. (Of course, by that time people will have forgotten about the experiment and lost the rock, so it isn't a very useful experiment to do.)

If we know the amount of uranium and lead in a rock, can we tell how old it is? We can if we know how much uranium and lead were in the rock to begin with. We can make one of two assumptions. (1) There was some lead in the rock when it was formed. (2) There wasn't any lead in the rock when it was formed.

If we make the first assumption, then we have to figure out how much there was. Since scientists don't know what process formed the rock in the first place, we can't possible know how much uranium and how much lead that process created. Therefore, the accuracy of the computed date depends entirely upon how well we guess the initial concentrations of uranium and lead. There is no more reason to believe that the rock initially contained 20% uranium and 80% lead than there is to believe that the rock initially contained 80% uranium and 20% lead. If you assume an initial concentration of each kind of material, the calculations will yield an age determined entirely by whatever wild guess you make.

If we make the second assumption, the calculation will yield the oldest possible age. This assumption is attractive to people who want to try to justify their belief in an old age of the Earth. The second assumption is a tough one to swallow, though, because one must postulate a natural process that turns hydrogen and helium into iron, oxygen, nickel, carbon, gold, copper and uranium, but not lead. What is there about lead which would make it harder to produce than nickel or copper? Nothing. So the imaginary process that creates uranium must not produce lead for some unexplained reason. This hardly seems like solid, scientific reasoning.

If one uses three different dating techniques on two different rocks from the same rock formation, it is quite possible that one will get six different dates. If one uses Potassium/Argon and Lead/Lead on the same rock, the Potassium/Argon date will probably be millions of years while the Lead/Lead date will probably be billions of years. Geologists know this, so they never bother to do Lead/Lead dating on recent lava flow, nor do they do Potassium/Argon on "ancient" gneiss. Whenever a radioactive date calculation does not agree with the preconceived notion of how old the rock is, that date is declared "discordant" and is ignored.

We'll bet that Mitsunobu Tatsumoto didn't do any Potassium/Argon dating tests on the moon rocks. If he had, he would have come up with ages tens or hundreds of million years old because the Potassium/Argon method simply can't produce dates that are billions of years old. All the original potassium would have decayed into argon in a few billion years, so there isn't anything left to date. If there is any potassium at all, the computed date will be in the tens or hundreds of millions of years.
Given the current prejudice today, few scientists would admit to believing any moon rock age that is expressed in millions of years. Any "young" age calculation would be blamed on potassium contamination during the trip back to Earth, no matter how carefully the rocks were shielded from outside contamination.

But suppose that tomorrow someone comes up with a popular theory that an asteroid struck the Earth 65 millions years ago, causing molten rock from the center of the Earth to squirt out into space, where surface tension shaped it into the ball that we call, "the Moon". Then some scientist would certainly do Potassium/Argon tests on moon rocks until he finds one 62 ± 4 million years old, and would offer that as definitive proof of the theory. Mitsunobu Tatsumoto's 4.5 billion year age calculations would be declared invalid for one reason or another.
We hate to sound too cynical, but this sort of thing happens all the time in geology and paleontology. You just have to read the scientific literature about all the controversy of the dating of fossils like skull KNM-ER 1470 and certain Grand Canyon rocks.

They say that someone who has respect for the law and loves to eat sausage has never seen how either one is made. One might also say that anyone who believes in radioactive dates doesn't understand the radioactive dating process.

Only two other micrometeoroid and meteor influx measuring techniques appear to have been tried. One of these was the Apollo 17 Lunar Ejecta and Micrometeorite Experiment, a device deployed by the Apollo 17 crew which was specifically designed to detect micrometeorites. It consisted of a box containing monitoring equipment with its outside cover being sensitive to impacting dust particles. Evidently, it was capable not only of counting dust particles, but also of measuring their masses and velocities, the objective being to establish some firm limits on the numbers of micro particles in a given size range which strike the lunar surface every year. However, the results do not seem to have added to the large database already established by micro crater investigations.

The other direct measurement technique used was the Passive Seismic Experiment in which a seismograph was deployed by the Apollo astronauts and left to register subsequent impact events. In this case, however, the particle sizes and masses were in the gram to kilogram range of meteorites that impacted the moon’s surface with sufficient force to cause the vibrations to be recorded by the seismograph. Between 70 and 150 meteorite impacts per year were recorded, with masses in the range 100g to 1,000 kg, implying a flux rate of

log N = -1.62 -1.16 log m,

where N is the number of bodies that impact the lunar surface per square kilometer per year, with masses greater than m grams.115 This flux works out to be about one order of magnitude less than the average integrated flux from micro crater data. However, the data collected by this experiment have been used to cover that particle mass range in the development of cumulative flux curves and the resultant cumulative mass flux estimates.

Moon Dust and the Moon’s Age
The final question to be resolved is, now that we know how much meteoritic dust falls to the moon’s surface each year, then what does our current knowledge of the lunar surface layer tell us about the moon’s age? For example, what period of time is represented by the actual layer of dust found on the moon? On the one hand creationists have been using the earlier large dust influx figures to support a young age of the moon, and on the other hand evolutionists are satisfied that the small amount of dust on the moon supports their billions-of-years moon age.

‘Age’ Considerations
So how much dust is there on the lunar surface? Because of their apparent negligible or non-existent contribution, it may be safe to ignore thermal, sputter and radiation erosion. This leaves the meteoritic dust influx itself and the dust it generates when it hits bare rock on the lunar surface (impact erosion). However, our primary objective is to determine whether the amount of meteoritic dust in the lunar regolith and surface dust layer, when compared to the current meteoritic dust influx rate, is an accurate indication of the age of the moon itself, and by implication the earth and the solar system also.

Now we concluded earlier that the consensus from all the available evidence, and estimate techniques employed by different scientists, is that the meteoritic dust influx to the lunar surface is about 10,000 tons per year or 2x10-9g cm-2yr-1. Estimates of the density of micrometeorites vary widely, but an average value of 19/cm3 is commonly used. Thus at this apparent rate of dust influx it would take about a billion years for a dust layer a mere 2cm thick to accumulate over the lunar surface. Now the Apollo astronauts apparently reported a surface dust layer of between less than 1/8 inch (3mm)and 3 inches (7.6cm). Thus, if this surface dust layer were composed only of meteoritic dust, then at the current rate of dust influx this surface dust layer would have accumulated over a period of between 150 million years (3mm) and 3.8 billion years (7.6cm). Obviously, this line of reasoning cannot be used as an argument for a young age for the moon and therefore the solar system.

However, as we have already seen, below the thin surface dust layer is the lunar regolith, which is up to 5 metres thick across the lunar maria and averages 10 metres thick in the lunar highlands. Evidently, the thin surface dust layer is very loose due to stirring by impacting meteoritic dust (micrometeorites), but the regolith beneath which consists of rock rubble of all sizes down to fines (that are referred to as lunar soil) is strongly compacted. Nevertheless, the regolith appears to be continuously ‘gardened’ by large and small meteorites and micrometeorites, particles now at the surface potentially being buried deeply by future impacts. This of course means then that as the regolith is turned over meteoritic dust particles in the thin surface layer will after some time end up being mixed into the lunar soil in the regolith below. Therefore, also, it cannot be assumed that the thin loose surface layer is entirely composed of meteoritic dust, since lunar soil is also brought up into this loose surface layer by impacts.

However, attempts have been made to estimate the proportion of meteoritic material mixed into the regolith. Taylor198 reported that the meteoritic compositions recognised in the maria soils turn out to be surprisingly uniform at about 1.5% and that the abundance patterns are close to those for primitive unfractionated Type I carbonaceous chondrites. As described earlier, this meteoritic component was identified by analysing for trace elements in the broken-down rocks and soils in the regolith and then assuming that any trace element differences represented the meteoritic material added to the soils. Taylor also adds that the compositions of other meteorites, the ordinary chondrites, the iron meteorites and the stony-irons, do not appear to be present in the lunar regolith, which may have some significance as to the origin of this meteoritic material, most of which is attributed to the influx of micrometeorites. It is unknown what the large crater-forming meteorites contribute to the regolith, but Taylor suggests possibly as much as 10% of the total regolith. Additionally, a further source of exotic elements is the solar wind, which is estimated to contribute between 3% and 4% to the soil. This means that the total contribution to the regolith from extra-lunar sources is around 15%. Thus in a five metre thick regolith over the maria, the thickness of the meteoritic component would be close to 60cm, which at the current estimated meteoritic influx rate would have taken almost 30 billion years to accumulate, a timespan six times the claimed evolutionary age of the moon.

The lunar surface is heavily cratered, the largest crater having a diameter of 295kms. The highland areas are much more heavily cratered than the maria, which suggested to early investigators that the lunar highland areas might represent the oldest exposed rocks on the lunar surface. This has been confirmed by radiometric dating of rock samples brought back by the Apollo astronauts, so that a detailed lunar stratigraphy and evolutionary geochronological framework has been constructed. This has led to the conclusion that early in its history the moon suffered intense bombardment from scores of meteorites, so that all highland areas presumed to be older than 3.9 billion years have been found to be saturated with craters 50-100 km in diameter, and beneath the 10 metre-thick regolith is a zone of breccia and fractured bedrock estimated in places to be more than 1 km thick.

ClosingSemoga tulisan ini dapat bermanfaaat bagi kita semua, dengan mengetahui berapakah usia bulan dan bagaimana metode-metode perhitungan penentuan usia bulan (Penentuan melalui perhitungan Umur kawah bulan dan batuan bulan), diharapkan kita semakin memahami arti penting sebuah penciptaan dan keteraturan yang sangat seimbang dari alam semesta sehingga mengantarkan kita pada suatu Kearifan Universal terhadap jagat raya, bahwasannya dibalik layar dan pangung sandiwara cosmos ada suatu Tangan Maha Kreatif di dalamnya "Our God"

Ucapan trimakasih kepada semua sumber yang telah mengizinkan tulisannya saya sadur dan saya edit, kepada semua teman-teman dan semua orang yang saya cintai kalianlah cahaya hidup semangat ku.

References :
# Pettersson, H., 1960. Cosmic spherules and meteoric dust. Scientific American, 202(2):123-132.
# Morris, H. M. (ed.), 1974. Scientific Creationism, Creation-Life Publishers, San Diego, pp. 151-152.