## 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

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.

## Sunday, 17 February 2008

### Hari Lahir Jagat Raya, Astro Fisika Luar Tata Surya (Cosmology)

Hari Lahir Jagat Raya
(How Old Our Universe? )

By:
Arip Nurahman
Department of Physics
Faculty of Sciences and Mathematics, Indonesia University of Education

and

Follower Open Course Ware at Massachusetts Institute of Technology
Cambridge, USA
Department of Physics
http://web.mit.edu/physics/
http://ocw.mit.edu/OcwWeb/Physics/index.htm
&
Aeronautics and Astronautics Engineering
http://web.mit.edu/aeroastro/www/
http://ocw.mit.edu/OcwWeb/Aeronautics-and-Astronautics/index.htm

"Ketika riwayat sang kala ditentukan maka terpancarlah cahaya penciptaan menerobos ruang berdimensi 0 tercabik oleh kekuatan tunggal yang menggenggam Alam Raya, telah lahir suatu sonata kehidupan yang akan diarungi oleh berbagai karya agung Sang Pencipta."
-(H2O)-

Introduction

Ada pertanyaan yang sangat 'menggoda' tentang alam semesta, yaitu: berapa usia jagat raya saat ini.? Menggoda karena jawaban pertanyaan ini tidak mudah, padahal pertanyaan ini telah ada sejak menusia ada. Kalaupun ada jawaban, tentu akan bertentangan dengan jawaban yang telah tersedia pada Kitab-Kitab Suci, tapi kalaulah kita tahu hari kapan, jam berapa, dimana? Kita pasti bisa memperingati hari kelahirannya yang agung, dan mengucap segenap rasa kasih kepada Sang Arsitek ulung Jagat Raya.

Namun, ada baiknya kita berjalan pada ilmu pengetahuan saja karena ini memang telaah kita. Perbedaan jawaban ilmu pengetahuan dengan jawaban agama marilah kita pandang sebagai salah satu bukti bahwa jagat raya ini masih berproses. Dalam berproses inilah terjadi perbedaan-perbedaan yang makin lama makin sedikit hingga pada suatu saat kelak tidak ada lagi perbedaan antara jawaban ilmu pengetahuan dan jawaban agama. Jawaban ilmu pengetahuan dan jawaban agama sama-sama menunjukkan suatu kebenaran. Kebenaran itu berasal dari Yang Mahabenar. Yang Mahabenar hanya satu. "The One and Only"

Contents

Kembali ke pertanyaan semula, yakni: berapa usia jagat raya saat ini?. Banyak cara yang dapat ditempuh para ahli untuk menjawab pertanyaan ini. Pada umumnya dapat dikelompokkan menjadi dua jenis, secara kimiawi dan secara fisik. Secara kimiawi pada umumnya didasarkan sifat-sifat radioaktivitas. Unsur radioaktif akan meluruh menjadi unsur lain. Dengan cara menghitung bagian yang telah meluruh dapat diperkirakan waktu yang diperlukan untuk meluruh semua. Unsur-unsur tertua pada batuan bumi, meteor atau benda-benda langit lainnya diamati. Secara umum diperoleh umur alam semesta ini berkisar antara 11-20 milyar tahun. Pengukuran-pengukuran paling sering menghasilkan nilai untara 13-15 milyar tahun.

Dengan menggunakan alat yang disebut spektrometer berkas cahaya dapat diuraikan menjadi seperti pelangi yang menyebar dari warna merah hingga warna biru/ungu. Spektrum-spektrum yang dihasilkan dari cahaya bintang di langit ini oleh para ilmuwan diperlakukan sebagai semacam sidik jari bintang. Karena, tiap bintang mengahsilkan spektrum yang khas.

Berdasarkan prinsip efek Doppler, pergeseran posisi spektrum dari seberkas cahaya yang dipancarkan oleh sebuah bintang digunakan untuk memperkirakan arah gerak dan kecepatannya saat itu. Jika spektrum yang terjadi bergeser mendekati warna merah maka bitang tersebut bergerak menjauhi bumi dan sebaliknya jika bergeser ke arah biru, bintang yang bersangkutan sedang mendekati bumi.

Pergeseran posisi spektrum ini mencerminkan pergeseran frekuensi dan tentunya pergeseran panjang gelombang dari posisi yang baku. Fraksi selisih panjang gelombang yang tergeser dan panjang gelombang yang baku menyatakan perbandingan kecepatannya relatif terhadap kecepatan cahaya. Fraksi ini digunakan untuk menetapkan kecepatan gerak bintang relatif terhadap tata surya kita, berarti juga relatif terhadap bumi.

Dengan perkakas yang lain, misalnya paralax, kita dapat memperkiran jarak bintang terhadap kita. Jarak itu, karena sangat panjang maka dinyatakan dalam tahun cahaya. Satu tahun cahaya adalah jarak yang ditempuh oleh seberkas cahaya setelah merambat selama satu tahun. Satu tahun cahaya itu sama dengan 9,5 x 10 pangkat 15 meter.

Dengan mengukur kecepatan gerak relatif terhadap tata surya kita dan jaraknya terhadap tata surya kita beberapa bintang dapatlah diperkirakan usia alam semesta ini. Lihat Gambar utama. Titik-titik itu merupakan representasi beberapa bintang yang diamati. Usia alam semesta ditetapkan sebagai berikut. Kita tarik sepenggal garis lurus vertikal ke atas melalui titik 1.0 pada sumbu X (kecepatan bintang) hingga memotong garis lurus (g) yang melewati titik-titik representasi bintang-bintang. Kemudian ditarik penggal garis lurus mendatar ke kiri hingga memotong sumbu Y (Jarak bintang). Titik potong itu menunjukkan usia alam semesta. Sekitar 16 milyar tahun.

Mencermati umur alam semesta yang telah mencapai puluhan milyar tahun itu, maka terasa bahwa umur kita (rata-rata sekitar 60 tahun) sangat pendek sekali. Apa lagi kalau kita bandingkan dengan waktu hidup yang abadi. Kita sangatlah kecil. Umur kita sangatlah pendek. Namun demikian, betapa pun pendeknya usia kita, kita sebagai manusia seorang demi seorang tetap unik. Tidak ada duanya di dunia ini. Karena itu, tentu Sang Pemilik Alam Semesta mempunyai penugasan khusus bagi kita masing-masing. Sudahkah Anda mempunyai jawaban tentang apa tugas Anda di dunia ini? Silahkan mencarinya di antara bintang-bintang di angkasa raya. Di sana ada jawabnya. Semoga berhasil!.

"Kalaulah hari ini hari lahir jagat raya dimana sang kala mulai berdetak maka selamat dan selamat (Happy Brith Universe)semoga keindahan alam semesta selalu terpacar ke segala arah penjuru Cosmos."

English Version

The age of the Universe has been a subject of religious, mythological and scientific importance. On the scientific side, Sir Isaac Newton's guess for the age of the Universe was only a few thousand years. Einstein, the developer of the General Theory of Relativity, preferred to believe that the Universe was ageless and eternal. However, in 1929, observational evidence proved his fantasy was not to be fulfilled by Nature.

n order to understand this evidence, let's think about how a train sounds to a person standing on the platform. An arriving train makes a noise that starts low and gets higher pitched as the train approaches the listener, sounding like oooooohEEEEEEEE. A departing train makes a noise that gets lower pitched as the train goes away from the listener, sounding like EEEEEEEEoooooooh. This change in the sound of the pitch of the train noise depending on whether it is arriving or departing the listener is called the Doppler shift.

The Doppler shift

. The Doppler shift happens with light as well as with sound. A source of light that is approaching the viewer will seem to the viewer to have a higher frequency than a source of light that is receding from that viewer. In 1929, observations of distant galaxies showed that the light from those galaxies behaved as if they were going away from us. If all the distant galaxies are all receding from us on the average, that means that the Universe as a whole could be expanding. It could be blowing up like a balloon.
. If the Universe is expanding, then what did it expand from?
. This is what tells us that the Universe probably does have a finite age, it probably is not eternal and ageless as Einstein wanted to believe.
. But then, okay, how old is the Universe?
. We know from studies of radioactivity of the Earth and Sun that our solar system probably formed about 4.5 billions years ago, which means that the Universe must be at least twice that old, because before our solar system formed, our Milky Way galaxy had to form, and that probably took several billions years by itself.
. It would be reasonable to guess that the Universe is at least twice as old as our Sun and Earth. However, we can't do radioactive dating on distant stars and galaxies. The best we can do is balance a lot of different measurements of the brightness and distance of stars and the red shifting of their light to come up with some ballpark figure. The oldest star clusters whose age we can estimate are about 12 to 15 billions years old.
. So it seems safe to estimate that the age of the Universe is at least 15 billion years old, but probably not more than 20 billion years old.
. This matter is far from being settled by astrophysicists and cosmologists, so stay tuned. There could be radical new developments in the future.

There are at least 3 ways that the age of the Universe can be estimated. I will describe

* The age of the chemical elements.
* The age of the oldest star clusters.
* The age of the oldest white dwarf stars.

The age of the Universe can also be estimated from a cosmological model based on the Hubble constant and the densities of matter and dark energy. This model-based age is currently 13.7 +/- 0.2 Gyr. But this Web page will only deal with actual age measurements, not estimates from cosmological models. The actual age measurements are consistent with the model-based age which increases our confidence in the Big Bang model.

The Age of the Elements

The age of the chemical elements can be estimated using radioactive decay to determine how old a given mixture of atoms is. The most definite ages that can be determined this way are ages since the solidification of rock samples. When a rock solidifies, the chemical elements often get separated into different crystalline grains in the rock. For example, sodium and calcium are both common elements, but their chemical behaviours are quite different, so one usually finds sodium and calcium in different grains in a differentiated rock. Rubidium and strontium are heavier elements that behave chemically much like sodium and calcium. Thus rubidium and strontium are usually found in different grains in a rock. But Rb-87 decays into Sr-87 with a half-life of 47 billion years. And there is another isotope of strontium, Sr-86, which is not produced by any rubidium decay. The isotope Sr-87 is called radiogenic, because it can be produced by radioactive decay, while Sr-86 is non-radiogenic. The Sr-86 is used to determine what fraction of the Sr-87 was produced by radioactive decay. This is done by plotting the Sr-87/Sr-86 ratio versus the Rb-87/Sr-86 ratio. When a rock is first formed, the different grains have a wide range of Rb-87/Sr-86 ratios, but the Sr-87/Sr-86 ratio is the same in all grains because the chemical processes leading to differentiated grains do not separate isotopes. After the rock has been solid for several billion years, a fraction of the Rb-87 will have decayed into Sr-87. Then the Sr-87/Sr-86 ratio will be larger in grains with a large Rb-87/Sr-86 ratio. Do a linear fit of

Sr-87/Sr-86 = a + b*(Rb-87/Sr-86)

and then the slope term is given by

b = 2x - 1

with x being the number of half-lives that the rock has been solid. See the talk.origins isochrone FAQ for more on radioactive dating.

When applied to rocks on the surface of the Earth, the oldest rocks are about 3.8 billion years old. When applied to meteorites, the oldest are 4.56 billion years old. This very well determined age is the age of the Solar System. See the talk.origins age of the Earth FAQ for more on the age of the solar system.

When applied to a mixed together and evolving system like the gas in the Milky Way, no great precision is possible. One problem is that there is no chemical separation into grains of different crystals, so the absolute values of the isotope ratios have to be used instead of the slopes of a linear fit. This requires that we know precisely how much of each isotope was originally present, so an accurate model for element production is needed. One isotope pair that has been used is rhenium and osmium: in particular Re-187 which decays into Os-187 with a half-life of 40 billion years. It looks like 15% of the original Re-187 has decayed, which leads to an age of 8-11 billion years. But this is just the mean formation age of the stuff in the Solar System, and no rhenium or osmium has been made for the last 4.56 billion years. Thus to use this age to determine the age of the Universe, a model of when the elements were made is needed. If all the elements were made in a burst soon after the Big Bang, then the age of the Universe would be to = 8-11 billion years. But if the elements are made continuously at a constant rate, then the mean age of stuff in the Solar System is

(to + tSS)/2 = 8-11 Gyr

which we can solve for the age of the Universe giving

to = 11.5-17.5 Gyr

238U and 232Th are both radioactive with half-lives of 4.468 and 14.05 Gyrs, but the uranium is underabundant in the Solar System compared to the expected production ratio in supernovae. This is not surprising since the 238U has a shorter half-life, and the magnitude of the difference gives an estimate for the age of the Universe. Dauphas (2005, Nature, 435, 1203) combines the Solar System 238U:232Th ratio with the ratio observed in very old, metal poor stars to solve simultaneous equations for both the production ratio and the age of the Universe, obtaining 14.5+2.8-2.2 Gyr.

Radioactive Dating of an Old Star

A very interesting paper by Cowan et al. (1997, ApJ, 480, 246) discusses the thorium abundance in an old halo star. Normally it is not possible to measure the abundance of radioactive isotopes in other stars because the lines are too weak. But in CS 22892-052 the thorium lines can be seen because the iron lines are very weak. The Th/Eu (Europium) ratio in this star is 0.219 compared to 0.369 in the Solar System now. Thorium decays with a half-life of 14.05 Gyr, so the Solar System formed with Th/Eu = 24.6/14.05*0.369 = 0.463. If CS 22892-052 formed with the same Th/Eu ratio it is then 15.2 +/- 3.5 Gyr old. It is actually probably slightly older because some of the thorium that would have gone into the Solar System decayed before the Sun formed, and this correction depends on the nucleosynthesis history of the Milky Way. Nonetheless, this is still an interesting measure of the age of the oldest stars that is independent of the main-sequence lifetime method.

A later paper by Cowan et al. (1999, ApJ, 521, 194) gives 15.6 +/- 4.6 Gyr for the age based on two stars: CS 22892-052 and HD 115444.

A another star, CS 31082-001, shows an age of 12.5 +/- 3 Gyr based on the decay of U-238 [Cayrel, et al. 2001, Nature, 409, 691-692]. Wanajo et al. refine the predicted U/Th production ratio and get 14.1 +/- 2.5 Gyr for the age of this star.

The Age of the Oldest Star Clusters

When stars are burning hydrogen to helium in their cores, they fall on a single curve in the luminosity-temperature plot known as the H-R diagram after its inventors, Hertzsprung and Russell. This track is known as the main sequence, since most stars are found there. Since the luminosity of a star varies like M3 or M4, the lifetime of a star on the main sequence varies like t=const*M/L=k/L0.7. Thus if you measure the luminosity of the most luminous star on the main sequence, you get an upper limit for the age of the cluster:

Age <> 11.5 Gyr.

Hansen et al. have used the HST to measure the ages of white dwarfs in the globular cluster M4, obtaining 12.7 +/- 0.7 Gyr. In 2004 Hansen et al. updated their analysis to give an age for M4 of 12.1 +/- 0.9 Gyr, which is very consistent with the age of globular clusters from the main sequence turnoff. Allowing allowing for the time between the Big Bang and the formation of globular clusters (and its uncertainty) implies an age for the Universe of 12.8 +/- 1.1 Gyr.

Until recently, astronomers estimated that the Big Bang occurred between 12 and 14 billion years ago. To put this in perspective, the Solar System is thought to be 4.5 billion years old and humans have existed as a species for a few million years. Astronomers estimate the age of the universe in two ways: 1) by looking for the oldest stars; and 2) by measuring the rate of expansion of the universe and extrapolating back to the Big Bang; just as crime detectives can trace the origin of a bullet from the holes in a wall.
Older Than the Oldest Stars?

Astronomers can place a lower limit to the age of the universe by studying globular clusters. Globular clusters are a dense collection of roughly a million stars. Stellar densities near the center of the globular cluster are enormous. If we lived near the center of one, there would be several hundred thousand stars closer to us than Proxima Centauri, the star nearest to the Sun.

The life cycle of a star depends upon its mass. High mass stars are much brighter than low mass stars, thus they rapidly burn through their supply of hydrogen fuel. A star like the Sun has enough fuel in its core to burn at its current brightness for approximately 9 billion years. A star that is twice as massive as the Sun will burn through its fuel supply in only 800 million years. A 10 solar mass star, a star that is 10 times more massive than the Sun, burns nearly a thousand times brighter and has only a 20 million year fuel supply. Conversely, a star that is half as massive as the Sun burns slowly enough for its fuel to last more than 20 billion years.

All of the stars in a globular cluster formed at roughly the same time, thus they can serve as cosmic clocks. If a globular cluster is more than 20 million years old, then all of its hydrogen burning stars will be less massive than 10 solar masses. This implies that no individual hydrogen burning star will be more than 1000 times brighter than the Sun. If a globular cluster is more than 2 billion years old, then there will be no hydrogen-burning star more massive than 2 solar masses.

The oldest globular clusters contain only stars less massive than 0.7 solar masses. These low mass stars are much dimmer than the Sun. This observation suggests that the oldest globular clusters are between 11 and 18 billion years old. The uncertainty in this estimate is due to the difficulty in determining the exact distance to a globular cluster (hence, an uncertainty in the brightness (and mass) of the stars in the cluster). Another source of uncertainty in this estimate lies in our ignorance of some of the finer details of stellar evolution. Presumably, the universe itself is at least as old as the oldest globular clusters that reside in it.
Extrapolating Back to the Big Bang

An alternative approach to estimating is the age of the universe is to measure the “Hubble constant”. The Hubble constant is a measure of the current expansion rate of the universe. Cosmologists use this measurement to extrapolate back to the Big Bang. This extrapolation depends on the history of the expansion rate which in turn depends on the current density of the universe and on the composition of the universe.

If the universe is flat and composed mostly of matter, then the age of the universe is
2/(3 Ho)

where Ho is the value of the Hubble constant.

If the universe has a very low density of matter, then its extrapolated age is larger:
1/Ho

If the universe contains a form of matter similar to the cosmological constant, then the inferred age can be even larger.

Many astronomers are working hard to measure the Hubble constant using a variety of different techniques. Until recently, the best estimates ranged from 65 km/sec/Megaparsec to 80 km/sec/Megaparsec, with the best value being about 72 km/sec/Megaparsec. In more familiar units, astronomers believe that 1/Ho is between 12 and 14 billion years.
An Age Crisis?

If we compare the two age determinations, there is a potential crisis. If the universe is flat, and dominated by ordinary or dark matter, the age of the universe as inferred from the Hubble constant would be about 9 billion years. The age of the universe would be shorter than the age of oldest stars. This contradiction implies that either 1) our measurement of the Hubble constant is incorrect, 2) the Big Bang theory is incorrect or 3) that we need a form of matter like a cosmological constant that implies an older age for a given observed expansion rate.

Some astronomers believe that this crisis will pass as soon as measurements improve. If the astronomers who have measured the smaller values of the Hubble constant are correct, and if the smaller estimates of globular cluster ages are also correct, then all is well for the Big Bang theory, even without a cosmological constant.
WMAP Can Measure the Age of the Universe

Measurements by the WMAP satellite can help resolve this crisis. If current ideas about the origin of large-scale structure are correct, then the detailed structure of the cosmic microwave background fluctuations will depend on the current density of the universe, the composition of the universe and its expansion rate. WMAP has been able to determine these parameters with an accuracy of better than 5%. Thus, we can estimate the expansion age of the universe to better than 5%. When we combine the WMAP data with complimentary observations from other CMB experiments (ACBAR and CBI), we are able to determine an age for the universe closer to an accuracy of 1%.

The expansion age measured by WMAP is larger than the oldest globular clusters, so the Big Bang theory has passed an important test. If the expansion age measured by WMAP had been smaller than the oldest globular clusters, then there would have been something fundamentally wrong about either the Big Bang theory or the theory of stellar evolution. Either way, astronomers would have needed to rethink many of their cherished ideas. But our current estimate of age fits well with what we know from other kinds of measurements: the Universe is about 13.7 billion years old!

Closing

Memang banyak sekali pendapat berbeda mengenai usia jagat raya itu, tapi yang terpenting adalah:"read in the name of lord who was created" karena ujung dari pencarian adalah Dia di Atas Segalanya.

sumber:
http:// astrophysicblogs.blogspot.com,
Pontianak post
Another Source in Book and The Internet Web-Site .

## Saturday, 16 February 2008

### Apakah Kita Sendiri?

Oleh:
Arip Nurahman
Department of Physics
Faculty of Sciences and Mathematics, Indonesia University of Education

and

Follower Open Course Ware at Massachusetts Institute of Technology
Cambridge, USA
Department of Physics
http://web.mit.edu/physics/
http://ocw.mit.edu/OcwWeb/Physics/index.htm
&
Aeronautics and Astronautics Engineering
http://web.mit.edu/aeroastro/www/
http://ocw.mit.edu/OcwWeb/Aeronautics-and-Astronautics/index.htm

Sepi, sendiri mengarungi lautan sang kala diatas butir biru kehampaan, terbentang dihorizon langit malam, tidak diragukan kesendirian mengajak kita berderai air mata, melantunkan sajak penderitaan karena terpasung masa yang silam. ada yang datang dan pergi dari palung kalbu, ada yang berdegup ketika kenangan datang, masihkah ada...harapan..ataukah aku memang sendiri disini...Agaknya kesepian masih senantiasa memayungiku. Kesepian ini kini merasuk hingga ke tulang-tulangku...
(180888)

# Extraterrestrial life

A 1967 Soviet Union 16 kopeks postage stamp, with a satellite from an imagined extraterrestrial civilization.
Extraterrestrial life is life originating outside of the Earth. It is the subject of astrobiology, and its existence remains hypothetical. There is no credible evidence of extraterrestrial life that has been widely accepted by the scientific community. There are several hypotheses regarding the origin of extraterrestrial life if it exists. One proposes that it may have emerged, independently, in different places in the universe. An alternative hypothesis is panspermia, which holds that life emerging in one location then spreads between habitable planets. These two hypotheses are not mutually exclusive. The study and theorization of extraterrestrial life is known as astrobiology, exobiology or xenobiology. Speculative forms of extraterrestrial life range from sapient or sentient beings to life at the scale of bacteria.
Suggested locations that might have once developed or continue to host life include the planets Venus[1] and Mars, moons of Jupiter and Saturn (e.g. Europa,[2] Enceladus and Titan). Gliese 581 c and d, recently discovered to be near Earth-mass extrasolar planets apparently located in their star's habitable zone, and having the potential to have liquid water.[3]

## Scientific search for extraterrestrial life

The scientific search for extraterrestrial life is being carried out in two different ways, directly and indirectly.

### Direct search

The planned NASA Kepler mission for the search of extrasolar planets.
Scientists are directly searching for evidence of unicellular life within the solar system, carrying out studies on the surface of Mars and examining meteors that have fallen to Earth. A mission is also proposed to Europa, one of Jupiter's moons with a possible liquid water layer under its surface, which might contain life.
There is some limited evidence that microbial life might possibly exist or have existed on Mars.[16] An experiment on the Viking Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes. However, the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. Recently, Circadian rhythms have been allegedly discovered in Viking data. The interpretation is controversial, see Viking biological experiments. Independently in 1996 structures resembling nanobacteria were reportedly discovered in a meteorite, ALH84001, thought to be formed of rock ejected from Mars. This report is also controversial and scientific debate continues.
In February 2005, NASA scientists reported that they had found strong evidence of present life on Mars.[17] The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claims on methane signatures found in Mars' atmosphere that resemble the methane production of some forms of primitive life on Earth, as well as their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon denied the scientists' claims, and Stoker herself backed off from her initial assertions.[18]
Though such findings are still very much in debate, support among scientists for the belief in the existence of life on Mars seems to be growing. In an informal survey conducted at the conference in which the European Space Agency presented its findings, 75 percent of the scientists in attendance reported to believe that life once existed on Mars; 25 percent reported a belief that life currently exists there.[19]
The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere that is not in chemical equilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near to their star are necessary before this can be used to analyze extrasolar planets

### Indirect search

Terrestrial Planet Finder - A planned Infrared interferometer for finding Earth-like extrasolar planets (as of 2007 , it has not received the funding from NASA it needs — that funding is going towards the Kepler mission).
It is theorised that any technological society in space will be transmitting information. Projects such as SETI are conducting an astronomical search for radio activity that would confirm the presence of intelligent life. A related suggestion is that aliens might broadcast pulsed and continuous laser signals in the optical as well as infrared spectrum;[20] laser signals have the advantage of not "smearing" in the interstellar medium and may prove more conducive to communication between the stars. And while other communication techniques including laser transmission and interstellar spaceflight have been discussed seriously and may not be infeasible, the measure of effectiveness is the amount of information communicated per unit cost, resulting with the radio as method of choice.

### Extrasolar planets

Astronomers also search for extrasolar planets that they believe would be conducive to life, such as Gliese 581 c and OGLE-2005-BLG-390Lb, which have been found to have Earth-like qualities.[21][22] Current radiodetection methods have been inadequate for such a search, as the resolution afforded by recent technology is inadequate for detailed study of extrasolar planetary objects. Future telescopes should be able to image planets around nearby stars, which may reveal the presence of life (either directly or through spectrography which would reveal key information such as the presence of free oxygen in a planet's atmosphere):

Artist's Impression of Gliese 581 c, the first extrasolar planet discovered within its star's habitable zone.
• Darwin is an ESA mission designed to find Earth-like planets, and analyze their atmosphere.
• The COROT mission, initiated by the French Space Agency, was launched in 2006 and is currently looking for extrasolar planets -- it is the first of its kind
• The Terrestrial Planet Finder was supposed to be launched by NASA, but as of 2007, budget cuts have caused it to be delayed indefinitely
• The Kepler Mission, largely replacing the Terrestrial Planet Finder, to be launched in November 2008
It has been argued that Alpha Centauri, the closest star system to Earth, may contain planets which could be capable of sustaining life.[23]
On April 24, 2007, scientists at the European Southern Observatory in La Silla, Chile said they had found the first Earth-like planet. The planet, known as Gliese 581 c, orbits within the habitable zone of its star Gliese 581, a red dwarf star which is a scant 20.5 light years (194 trillion km) from Earth. It was initially thought that this planet could contain liquid water. However, recent computer simulations of the climate on Gliese 581c by Werner von Bloh and his team at Germany's Institute for Climate Impact Research suggest carbon dioxide and methane in the atmosphere would create a runaway greenhouse effect. This would warm the planet well above the boiling point of water (100 degrees Celsius/212 degrees Fahrenheit), thus dimming the hopes of finding life. As a result of greenhouse models, scientists are now turning their attention to Gliese 581 d, which lies just outside of the star's traditional habitable zone.[24]
On May 29, 2007, the Associated Press released a report stating that scientists identified twenty-eight new extra-solar planetary bodies. One of these newly discovered planets is said to have many similarities with Neptune.[25]
To date, 313 extrasolar planets have been discovered (with 29 multi-planet systems), with new discoveries occuring monthly.[26]

## Drake equation

Main article: Drake equation
In 1961, University of California, Santa Cruz astronomer and astrophysicist Dr. Frank Drake devised the Drake equation. This controversial equation multiplied estimates of the following terms together:
• The rate of formation of suitable stars.
• The fraction of those stars which contain planets.
• The number of Earth-like worlds per planetary system.
• The fraction of planets where intelligent life develops.
• The fraction of possible communicative planets.
• The “lifetime” of possible communicative civilizations.
Drake used the equation to scientifically state that there are an estimated 10,000 planets containing intelligent life with the possible capability of communicating with Earth in the Milky Way galaxy.[27]
Based on observations from the Hubble Space Telescope, there are at least 125 billion galaxies in the universe. It is estimated that at least ten percent of all sun-like stars have a system of planets[28], thus if a thousandth of a percent of all stars are sun-like, and there are roughly (estimates may vary) 500 billion stars on average in each galaxy, then there are 6.25*1018 stars with planets orbiting them in the universe. If only a billionth of these stars have planets that support life, there are 6.25 billion life-supporting solar systems in the universe.

Introduction

Pertanyaan ini telah dicoba untuk dirumuskan secara matematis oleh Prof. Frank Drake di tahun 1960-an. Prof. Frank Drake menyusun rumus ini pada pertemuan mengenai SETI (Search of Extra Terrestrial Intelligence /Pencarian Kehidupan yang cerdas di luar Bumi) di Green Bank, Virginia Barat.

Contents

Mari kita sedikit bermain hitung-hitungan, menggunakan persamaan yang dirumuskan oleh Prof. Drake, sbb:

N = R* Ãƒ— fp Ãƒ— fe Ãƒ— fl Ãƒ— fi Ãƒ— fc Ãƒ— ft

N = jumlah peradaban di dalam galaksi kita, yang memungkinkan kita bisa melakukan kontak.;

R* = rasio pembentukan bintang di dalam galaksi kita

fp = fraksi dari bintang-bintang yang mempunyai planet

fe = rata-rata jumlah planet yang berpotensi punya penunjang hidup per-bintang (yang mempunyai planet)

fl = fraksi dari yang di atas dimana ada kehidupannya berkembang

fi = fraksi dari yang di atas dimana kehidupannya mengembangkan kecerdasan

fc = fraksi dari peradabaan yang mengembangkan teknologi, yang bisa mengirimkan sinyal tentang keberadaannya, ke luar angkasa

ft = rentang waktu peradaban tersebut untuk mengirimkan sinyal ke luar angkasa

Tentu saja angka-angka yang berada pada sisi kanan tidaklah selalu disepakati oleh banyak pihak. Tetapi, tidaklah salah untuk mencobakan satu atau dua angka tertentu sebagai pendekatan awal, sehingga bisa menggambarkan kemungkinannya. Bahkan kita bisa mencoba asumsi kita sendiri.

Pada tahun 1961, Prof. Drake mempergunakan pendekatan angka-angka sebagai berikut:

R* = 10/tahun (10 bintang terbentuk dalam setahun)

fp = 0.5 (setengah dari setiap bintang terbentuk punya planet)

fe = 2 (2 planet per bintang memungkinkan adanya kehidupan)

fl = 1 (100% dari setiap planet mengembangkan kehidupan)

fi = 0.01 (1% dari setiap kehidupan mengembangkan kecerdasan)

fc = 0.01 (1% yang bisa berkomunikasi)

ft = 10000 tahun (hanya bisa terjadi komunikasi setelah 10000 tahun, setelah sinyal dikirimkan)

Maka N = 10 Ãƒ— 0.5 Ãƒ— 2 Ãƒ— 1 Ãƒ— 0.01 Ãƒ— 0.01 Ãƒ— 10,000 = 10. Baiklah, ada 10 kemungkinan peradaban lain di luar sana yang mungkin berkomunikasi dengan kita. Tapi apakah benar demikian adanya?

Nilai R* bisa diterima karena memang banyak ditemukan di alam dari pengamatan-pengamatan; demikian juga dengan nilai fp tidak terlalu diperdebatkan. Tetapi angka-angka yang lain masih harus di uji lagi, dan disesuaikan dengan pengamatan-pengamatan terkini.

Penemuan terkini tentang planet-planet gas di dekat orbit bintang menyebabkan nilai fe semakin tidak pasti, karena memberikan keragu-raguan apakah planet yang mempunyai penunjang-hidup, dapat bertahan di dalam sistem bintang? Sebagai tambahan, kebanyakan bintang di dalam galaksi merupakan raksasa merah, dengan radiasi UV yang sangat kecil, menambah kecil kemungkinan adanya planet yang bisa ditinggali. Alih-alih terjadi semburan energi bintang pada UV oleh bintang, semburan terjadi pada sinar-X, yang malah menyebabkan adanya erosi pada atmosfer planet. Jadi apakah mungkin suatu planet dengan penunjang kehidupan bertahan dalam kondisi seperti itu?

Selain itu, adanya kemungkinan jika planet-planet gas tersebut mempunyai bulan yang mempunyai kemungkinan adanya kehidupan
(seperti contoh kasus satelit Jupiter Europa) menambah ketidak-pastian-nya menjadi semakin besar.

Angka-angka untuk fl, fi dan fc, sampai sekarang masih menjadi perdebatan.

Perdebatannya melibatkan Geologi, Biologi dan semua ilmu yang berkaitan dengan bagaimana asal muasal planet Bumi serta kehidupannya. Penemuan adanya kehidupan di Mars menambah keruwetan dalam menentukan angka-angka tersebut, jika kehidupan di Mars berasal dari proses pembentukan yang berbeda dengan di Bumi, maka angka fl harus lebih lagi. Itu pun, jika ada kecerdasan lain, apakah mereka akan berpartisipasi dalam mencari kehidupan di luar planet-nya? Apakah mereka peduli mereka sendirian atau tidak di alam semesta? Mempunyai kecerdasan, berarti mempunyai kebebasan untuk memilih, sekalipun memiliki teknologinya, tetapi jika tidak berminat mencari tahu, maka kita di Bumi tidak akan pernah tahu. Atau, jangan-jangan, malah mereka sudah datang kesini, sudah mencoba berkomunikasi dengan kita?

Nilai ft, hanya berlaku untuk daerah di sekitar Matahari, sementara berapa jauh alam semesta ini? Luas sekali. Jika saja ada suatu peradaban yang mengirimkan pesan ke Bumi setelah 10ribu tahun dan baru kita terima sekarang,

Sampai saat ini, angka-angka yang diterapkan dalam perumusan tersebut masih dianggap spekulatif, tetapi, spekulatif atau bukan; rumus yang dibuat oleh Prof. Drake memberikan harapan, bahwa, secara statistik, masih ada kemungkinan kehidupan lain di luar Bumi, dan ini menjadi pijakan untuk kita terus menerus melakukan pengamatan, studi astronomi dan mencari tahu tentang banyak hal yang sampai saat ini belum kita ketahui tentang alam semesta kita.

Penutup

"Akan tiba masanya ketika sang kala mempertemukan kita dan kita telah benar2 siap memulai awal dan mengarungi akhir dibawah payung kebesaran sang Maha cerdas semoga tetap lestari kenangan ini serta harapan yang dikandungnya"

-(for u all)-.

Events and objects
Searches for extraterrestrial life
Subjects
Theories

## References

1. ^ "Venus clouds 'might harbour life'". BBC News (2004-05-25). Retrieved on 2007-12-05.
2. ^ a b c http://www.planetary.org/programs/projects/explore_europa/update_12142005.html"
3. ^ "The Habitability of Super-Earths in Gliese 581". Retrieved on 2007-12-01.
4. ^ "Ammonia based life". daviddarling.info.
5. ^ Plants on Other Planets
6. ^ "Variety of extraterrestrial life". daviddarling.info.
7. ^ "Star Struck, a letter to a Rabbi". ohr.edu.
8. ^ Kaplan, Rabbi Aryeh. "Extraterrestrial life". torah.org.
9. ^ Revelation, Rationality, Knowledge & Truth, by Mirza Tahir Ahmad. Chapter; The Quran and Extraterrestrial Life
10. ^ Wiker, Benjamin D.. "Christianity and the Search for Extraterrestrial Life". crisismagazine.com.
11. ^ "Rheita.htm". cosmovisions.com.
12. ^ An intelligent design : Controlled hominization in cosmic apartheid
13. ^ Crichton, Michael (January 17, 2003). "Aliens Cause Global Warming". crichton-official.com.
14. ^ SETI: Search For Extra-Terrestial Intelligence
15. ^ Amazon.com: Rare Earth: Why Complex Life is Uncommon in the Universe: Books: Peter Ward,Donald Brownlee
16. ^ Spherix: Makers of Naturlose (tagatose), a natural, low-calorie sugar made from whey that may be useful as a treatment for Type 2 diabetes
17. ^ Berger, Brian (2005). "Exclusive: NASA Researchers Claim Evidence of Present Life on Mars".
18. ^ "NASA denies Mars life reports", spacetoday.net (2005).
19. ^ Spotts, Peter N. (2005-02-28). "Sea boosts hope of finding signs of life on Mars", The Christian Science Monitor. Retrieved on 2006-12-18.
20. ^ "The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum". The Columbus Optical SETI Observatory.
21. ^ "http://planet.iap.fr/OB05390.news.html" .
22. ^ SPACE.com - Major Discovery: New Planet Could Harbor Water and Life
23. ^ 1997AJ 113.1445W Page 1445
24. ^ Hopes dim for life on distant planet - USATODAY.com
25. ^ BBC NEWS | Science/Nature | Planet hunters spy distant haul
26. ^ Extrasolar Planets Encyclopedia
27. ^ Boyd, Padi. "The Drake Equation". Imagine the Universe. NASA. Retrieved on 2008-02-05. "Frank Drake's own current estimate puts the number of communicating civilizations in the galaxy at 10,000"
28. ^ Marcy, G.; Butler, R.; Fischer, D.; et.al. (2005). "Observed Properties of Exoplanets: Masses, Orbits and Metallicities". Progress of Theoretical Physics Supplement 158: 24 – 42. doi:10.1143/PTPS.158.24.
29. ^ Possibility of Life on Europa
30. ^ BBC NEWS | Science/Nature | Water 'flowed recently' on Mars
31. ^ SPACE.com - Scientists Reconsider Habitability of Saturn's Moon
32. ^ SPACE.com - Lakes Found on Saturn's Moon Titan
33. ^ "Lakes on Titan, Full-Res: PIA08630" (2006-07-24).
34. ^ Venusian Cloud Colonies :: Astrobiology Magazine - earth science - evolution distribution Origin of life universe - life beyond :: Astrobiology is study of earth science evolution distribution Origin of life in universe terrestrial