Sun, Earth, Moon, Earthrise, “Terra”

4.56 billion years ago

The Hadean eon begins with the origin of the Earth 4.56 Gya.

Take a look at the Moon tonight. It’s a waning crescent, a few days from the new moon; you can cover it with your thumb. 4.56 billion years ago the new moon was ruddy with volcanic activity even on its far side. The Moon seen from Earth was 16 times wider, covering 250 times more sky, and 250 times brighter when full. That is what you would have seen just after Earth acquired a surface you could stand on, although you would have needed an oxygen mask. And watch out for massive meteorites, still falling frequently, and volcanism.

Chance events late in the history of planet formation played a huge role in shaping the solar system, including the collision with the planet Theia (named after the Greek goddess of the Moon) that gave Earth her outsized satellite. We’ve known about Theia for a while; the latest theory is that the collision resulted in the formation of a synestia, a donut of vaporized rock, which condensed to form Moon and Earth. Life might have developed very differently – there might be no intelligent life — without the Moon’s influence on tides and on Earth’s axis.

Here’s a movie from NASA showing the whole moon, including her far side, as seen by the Lunar Reconnaisance Orbiter. And here’s the famous picture of Earthrise, taken December 24, 1968, by William Anders abroad Apollo 8.earthrise copy

In 1969, the Brazilian singer Caetano Veloso was imprisoned by Brazil’s military dictatorship. He was expelled from the country and lived in exile until 1972. In prison he saw a picture of the Earth from space and wrote this song, “Terra” (Earth).

TerraQuando eu me encontrava preso, na cela de uma cadeia 
Foi que eu vi pela primeira vez, as tais fotografias 
Em que apareces inteira, porém lá não estava nua 
E sim coberta de nuvens
Terra, terra, Por mais distante o errante navegante Quem jamais te esqueceria?

Ninguém supõe a morena, dentro da estrela azulada
. Na vertigem do cinema, mando um abraço pra ti 
Pequenina como se eu fosse o saudoso poeta 
E fosses a Paraíba
Terra, terra, 
Por mais distânte o errante navegante Quem jamais te esqueceria

Eu estou apaixonado, por uma menina terra,
 Signo de elemento terra. Do mar se diz terra à vista 
Terra para o pé firmeza, terra para a mão carícia
 Outros astros lhe são guia
Terra, terra,
 Por mais distânte o errante navegante Quem jamais te esqueceria

Eu sou um leão de fogo, sem ti me consumiria
 A mim mesmo eternamente, e de nada valeria 
Acontecer de eu ser gente. e gente é outra alegria 
Diferente das estrelas
Terra, terra,
 Por mais distânte o errante navegante Quem jamais te esqueceria

De onde nem tempo e nem espaço, que a força te de coragem
 Pra gente te dar carinho, durante toda a viagem 
Que realizas do nada, através do qual carregas 
O nome da tua carne
Terra, terra, 
Por mais distânte o errante navegante Quem jamais te esqueceria
Terra, terra, 
Por mais distânte o errante navegante Quem jamais te esqueceria
Terra, terra,
 Por mais distânte o errante navegante Quem jamais te esqueceria?

Na sacadas do sobrado, Da eterna São Salvador 
Há lembranças de donzelas, do tempo do Imperador
 Tudo, tudo na Bahia faz a gente querer bem
A Bahia tem um jeito
Terra, terra,
 Por mais distante o errante navegante 
Quem jamais te esqueceria. Terra

EarthWhen I found myself arrested
 In a prison cell
, That’s when I first saw
 Those famous pictures
 In which you appear entire, 
However you were not naked 
But covered by clouds.
Earth! Earth!
 However distant 
The wandering navigator 
Who could ever forget you?

Nobody thinks of the brunette
 Inside the bluish star. 
In the vertigo of the movie
 I send you an embrace, 
Little one – as if I were
 the homesick poet 
And you were the Paraíba
Earth! Earth!
 However distant 
The wandering navigator 
Who could ever forget you?

I’m just in love 
With an earth girl, 
Sign of the element “Earth.” 
From the sea is said “Land in sight.” 
Earth to the foot: solidity. 
Earth to the hand: a caress.
 Other stars are guides for you
Earth! Earth! 
However distant 
The wandering navigator 
Who could ever forget you?

I am a lion of fire
 Without you 
I would burn myself up eternally 
And it would be worth nothing, 
The fact of my being human. 
And human is another joy 
Different than the stars’
Earth! Earth! 
However distant 
The wandering navigator 
Who could ever forget you?

From where there’s neither time nor space 
May the force send courage 
For us to treat you tenderly
 During all the journey 
That you carry out through nothing
 Through which you bear
 The name of your flesh
Earth! Earth!
 However distant 
The wandering navigator 
Who could ever forget you?
Earth! Earth! 
However distant
 The wandering navigator
 Who could ever forget you?
Earth! Earth! 
However distant
 The wandering navigator
Who could ever forget you?

In the townhouses’ terraces 
Of eternal Salvador 
There are reminders of maidens 
From the time of the Emperor 
Everything, everything in Bahia
 Makes us fond 
Bahia has such a way.
Earth! Earth! 
However distant
 The wandering navigator
 Who could ever forget you? 

Rare Earth

4.76-4.51 billion years ago

A big day on Logarithmic History, the biggest since the beginning of the year: the origin of our Solar System including planet Earth. First a note on what’s odd about our planetary system.

Two preceding posts wrestled with the Fermi Paradox: If the universe is full of advanced civilizations, why haven’t we seen any sign of them so far? One answer to the paradox might be that our solar system is wildly unusual, so that abodes for the evolution of complex life are rare. We can finally start to address this matter with some real evidence. According to the NASA exoplanet archive, we’ve now discovered 4,108 exoplanets (planets outside our solar system; up from 3885 last year at this date), with many more unconfirmed candidates. This is enough to do some statistics, and indications are that our solar system might indeed be out of the ordinary.


Exoplanets smaller than Jupiter are overwhelmingly closer, mostly a lot closer, to their primary stars than Earth is to the Sun. And the same models of planet formation that have done a pretty good job predicting some of the wild variation we see in other systems – “Hot Jupiters” orbiting closer to their primaries than Mercury, “Super Earths” in between Earth and gas giants in size – don’t readily generate systems that look much like ours. One model that does seem to do a good job with our solar system, the Nice Model, involves something special, a Grand Tack, where Jupiter and Saturn are caught in an orbital resonance that carries them into the inner solar system and back out, shaking up inner-system planet formation in the process. Wild stuff, but another variation on the Nice model is even wilder: at the beginning of planet formation, there may have been a generation of Super Earths in the inner solar system. The Grand Tack of Jupiter and Saturn would have sent these planets colliding into one another. The Super-Earths and most of the debris of these collisions would have fallen into the sun, but what the debris left would then have condensed into the unusual inner planets we know, Mercury, Venus, Earth, Mars. And Theia. (Theia? you ask. See the next post). Remarkably, we may have recently found evidence here on Earth of minerals that formed deep inside a lost planet or protoplanet from the earliest days of the solar system.

If this model holds up, the formation of our solar system, like other major events in our history, takes on some of the flavor of mythology. This isn’t quite the old story about Chronos slaying Ouranos, and Zeus slaying Chronos. Instead, in the new story, two giants, Jupiter and Saturn, travel closer to the sun and set a generation of Titans – their like will not be there again – to fighting and destroying one another. Jupiter and Saturn depart, and a new generation is spawned from the wreckage. An unlikely sequence of events, but then our planet could be a very unlikely place. And all the more special for that.

Where is everybody? Maybe we’re (some of) the first

A followup to the previous post on the Fermi Paradox, some reasons the Universe could have been less suitable for the evolution of complex life until recently, making us one of the first intelligent species to evolve.

1) Metallicity. Chemical elements heavier than helium are formed inside stars, after the Big Bang. Elements heavier than iron are formed in exploding supernovas. These elements have been building up over time. Maybe they had to reach a threshold abundance to make complex life possible. Consider that in the “family tree” for the Sun, based on the concentrations of different elements, the Sun is the oldest member of its subfamily. Maybe it is only planetary systems associated with this subfamily that are well-suited for the evolution of intelligent life. And recent work suggests that phosphorus in particular may be a limiting and cosmically limited resource for the evolution of life.

2) Gamma Ray Bursts (GRBs). GRBs are bursts of gamma rays (high frequency radiation) lasting from milliseconds to minutes, like GRB 080319B. (Check out this tweet from January 11.) GRBs are probably supernovas or even larger explosions where one pole of the exploding star is pointed at the Earth. A major GRB could irradiate one side of the planet, and also affect the other side by destroying the ozone layer, causing mass extinctions. GRBs may have swept the Milky Way frequently in the past. The good news is they’re probably getting less frequent. This could be the first time in the history of the Milky Way that enough time has passed without a major GRB for intelligent life to evolve. If true, we should think about how to protect ourselves from the next one – lots of sunblock recommended.

If GRBs are such a threat, we might expect to find evidence that they have caused mass extinctions in the past (not wiping out all life obviously). For more on this, check out upcoming blog posts and tweets for the end-Ordovician, March 3.

3) Panspermia (life from elsewhere). Pretty much as soon as Earth could support life, we see evidence of single-celled organisms. Then life evolves slowly for a long time. The usual story about this is that the origin of life is easy, and it happens as soon as possible. But there is another possibility (illustrated below). It may be that the transition from simple replicating chemical systems to bacteria with genomes of tens of thousands of DNA base pairs is a slow process that happened over many billions of years somewhere other than Earth. Then newly forming planets in the nebula that gave rise to Earth were “infected” by this source, by meteorites carrying early cells. (It would have been easier for meteorites to carry life from star system to star system when the Earth was first formed than it would be today.) Back when our hypothetical “Urth” was forming, a billion years before Earth, there might not have been any planets with cellular life on them as potential sources of life-bearing meteorites.


Where is everybody?

5.04 – 4.77 billion years ago

Tomorrow is a big day on Logarithmic History, the origin of our solar system, of the Sun and planet Earth. But is this really such a big deal in a cosmic perspective? After all, stars and planetary systems have been forming a fast clip in the Milky Way and other spiral galaxies since long before this date. So let’s suppose … Suppose there was a star like the Sun, but older by a billion years. And suppose this star had a planet like Earth orbiting around it – call it Urth. And suppose life originated on Urth more or less as on Earth and followed more or less the same evolutionary path. With this head start, intelligent life could have evolved a billion years ago, and today there could be intelligent Urthians (or their self-replicating robot descendants) a billion years ahead of us.

There’s an urban legend that says that Einstein called compound interest the strongest force in the universe. Einstein didn’t actually quite say this, but it’s not a crazy thing to say. For example, consider how compound interest works, backward, on our Logarithmic History calendar. December 30 covers a period 5.46% longer than December 31, December 29 is 11.2% longer (because 1.0546 * 1.0546 = 1.112), and so on. At this rate of compounding (and allowing for 2020 being a leap year) we wind up with January 1 covering 751 million years. The same math implies that if we invested 1 dollar at 5.46% interest, compounded annually, then after 365 years we’d have 751 million dollars.

With even the slightest compound rate of increase, a billion year old Elder Race would have plenty of time to fill up a galaxy, and undertake huge projects like dismantling planets to capture more of their suns’ energy. Indeed, they could arguably colonize the whole reachable universe.

Traveling between galaxies – indeed launching a colonization project for the entire reachable universe – is a relatively simple task for a star-spanning civilization, requiring modest amounts of energy and resources. … There are millions of galaxies that could have reached us by now.

Which raises the question, posed by Enrico Fermi in 1950: “Where is everybody?” There are more than 100 billion stars in our galaxies, more than 100 billion galaxies in the visible universe (actually, according to recent estimates, the number is more than  1 trillion). If there are huge numbers of billion year old Elder Races around, why hasn’t at least one of them taken the exponential road and made themselves conspicuous? Yet a recent survey of more than 100,000 galaxies found no evidence of any really advanced civilizations harnessing the power of stars on a large scale.

A possible resolution of the paradox has been suggested recently. The argument goes like this: the easiest route to estimating the number of advanced civilizations in the universe is to multiply point estimates of the probabilities of events like the formation of an Earthlike planet around an appropriate star, the origin of life on such a planet, the origin of human-level intelligence, and so on. But there are enormous uncertainties in these estimates. Properly speaking, instead of just multiplying mean estimates, we should be doing a convolution of the range of estimates. The general point is that if you’re multiplying a whole lot of probabilities, and you’re not certain what those probabilities are, there a strong likelihood that at least one of those probabilities is close to 0, so their product is also close to 0. The authors therefore suggest that there is a strong possibility that there are few or no advanced civilizations, or even other human-level civilizations in the Milky Way or even the observable universe.

In short, if this is true, tomorrow on Logarithmic History may mark a truly momentous occasion, not just in the history of the Earth, but in the history of the universe.

A family tree for the sun

7.45-7.06 billion years ago.

No big news in the universe today. Some evolutionary thoughts: Species evolve. Do planets? stars? galaxies?

Charles Darwin didn’t use the word “evolution” often. But he did write a lot about “descent with modification,” which is pretty much what biologists mean by evolution. For example, the usual definition of genetic evolution is “change in gene frequency,” i.e. descent with (genetic) modification. And Darwin argued that all living things belong to one or a few family trees linked by recent or remote common descent.

Recently, some astronomers teamed up with some evolutionary biologists to produce a “family tree” of our Sun and some of its neighbors. The tree is based on the abundances of different chemical elements; these abundances don’t change much over the lifetime of a star, and can be thought of as a kind of inherited trait, something like DNA. The tree groups stars roughly according to their ages, with younger stars having more “metals” (elements other than hydrogen and helium), but only roughly, since other processes affect stellar chemistry.

sun family tree

Drawing a family tree for stars might seem like an odd thing to do. In what sense are stars related as parent and offspring? Evolutionary biologists face a similar situation where group selection is concerned. Suppose you have a population of organisms. Those organisms form groups that last for a time and disband, with their members “seeding” the population at large, and influencing group formation in the next generation. Although there are ancestor-descendant relations between individual organisms, you can’t identify any one group in generation T+1 as the descendant of any particular group in generation T. Are the groups in this case meaningful evolutionary units? It depends on which biologist you talk to.

The curve of binding energy

9.86 – 9.33 billion years ago

More on stardust and us.

Looking at the abundance of different elements in the universe, we get the following:

element abundances

Note that the vertical scale is logarithmic, so there is vastly more hydrogen and helium in the universe than any other element. As noted in the last post, all the elements except hydrogen and helium were formed after the Big Bang, spewed out by supernovas and the collisions of neutron stars. In general, heavy elements are less abundant because it takes more steps to produce heavy elements than light ones. But the curve is not smooth. The lightest elements after hydrogen and helium (lithium, beryllium, boron) are relatively rare, because they get used up in the nucleosynthesis of heavier elements. And there is a saw tooth pattern in the chart, because nucleosynthesis favors atoms with even numbers of protons. So we get lots of oxygen, magnesium, silicon, and iron, the main constituents of our planet. Lots of carbon too. Finally, iron (Fe) is more than 1000 times more abundant than might be expected based on a smooth curve. Iron nuclei are especially stable because binding energy, the energy that would be required to take the nucleus apart into its constituent protons and neutrons, reaches a maximum with iron. Here’s the famous curve of curve of binding energy (nucleons are protons and neutrons):

curve of binding energy

An implication of this curve is that if you can split a really heavy nucleus, of Uranium-235 say, into smaller nuclei (but still heavier than iron), you will release energy equal to the vertical difference between U-235 and its lighter fission products (not shown) on the vertical scale. This is lots of energy, way more than you get from breaking or forming molecular bonds in ordinary chemical reactions. And if you can fuse two light nuclei, of hydrogen say, into a larger nucleus, you can get even more energy. When we split uranium, we are recovering some of the energy that a star put into synthesizing elements heavier than iron, before or as it went supernova. When we fuse hydrogen, we are extracting energy left over from the Big Bang that no star got around to releasing. (This doesn’t violate the Law of Conservation of Energy, because the negative gravitational potential energy of the universe cancels the positive energy represented by the matter. So the total energy of the universe is zero.)

Starting to figure this all out was part of a scientific revolution that made physics in 1950 look very different from physics in 1900. The new physics resolved a paradox in the study of prehistory. Geologists were pretty confident, based on rates of sedimentation, that the Earth had supported complex life for hundreds of millions of years. But physicists couldn’t see how the sun could have kept shining for so long. The geologists were right about deep time; it took new physics to understand that the sun got its energy from fusing hydrogen to helium (via some intermediate steps).

As the scientific revolution in atomic physics was picking up steam, it was natural to assume that it would be followed by a revolution in technology. After all, earlier scientific revolutions in the understanding of masses and gases, atoms and molecules, and electrons and electromagnetism, had been followed by momentous innovations in technology: the steam engine, artificial fertilizers, electrification, radio, to name just a few. But in some ways, the Atomic Age hasn’t lived up to early expectations. The atom bomb brought an earlier end to the Second World War, but didn’t change winners and losers. The bomb was never used again in war, and it’s a matter for debate how much the atom bomb and the hydrogen bomb changed the course of the Cold War. Nuclear energy now generates a modest 11% of the world’s electricity (although this number had better go up in the future if we’re serious about curbing carbon dioxide emissions). And a lot of ambitious early proposals for harnessing the atom never got anywhere. Project Plowshare envisioned using nuclear explosions for enormous civil engineering projects, digging new caves, canals, and harbors. Even more audacious was Project Orion, which developed plans for a rocket propelled by nuclear explosions. Some versions of Orion could have carried scores of people and enormous payloads throughout the solar system. Freeman Dyson, a physicist who worked on the project, said “Our motto was ‘Mars by 1965, Saturn by 1970.’”

On the purely technical side these plans were feasible. There were concerns about fallout, but the problems were not insurmountable. Nevertheless both Plowshare and Orion were cancelled. Regarding Orion, Dyson said “… this is the first time in modern history that a major expansion of human technology has been suppressed for political reasons.” The history of the Atomic Age  and its missed opportunities is one more refutation of pure technological determinism. How or even whether a new technology is exploited depends on social institutions, politics, and cultural values.

We are stardust

10.4-9.9 billion years ago

The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.

Carl Sagan (h/t to commenter remanandhra)

There’s a long gap between the origin of the universe, the first stars, and early galaxies, and the origin of our Solar System and our planet Earth. If we were using a linear scale for our calendar, the Solar System would get started in September. Even on our logarithmic scale, Sun and Earth wait until late January. A spiral galaxy like the Milky Way is an efficient machine for turning dust into stars over many billions of years. But the earliest stars it produces are poor in “metals” (to an astronomer, anything heavier than helium is a metal). It takes generations of exploding stars producing heavier elements and ejecting them into space before a star like the Sun — 2% metal – can form.

And just three years back, a spectacular discovery provided support for another mechanism of heavy element formation. Astronomers for the first time detected gravitational waves from the collision of two neutron stars, 300 million light-years away. Such collisions may be responsible for the formation of some of the heaviest atoms around, gold and silver in particular. So your gold ring may be not just garden-variety supernova stardust, but the relic of colliding neutron stars. Here’s a chart showing where the elements in our solar system come from:


Alchemists thought they could change one element into another – lead into gold, say. But it takes more extreme conditions than in any chemistry lab to transmute elements. The heart of a star makes heavy elements out of hydrogen and helium; it takes a supernova to make elements heavier than iron. So it’s literally true, not just hippy poetry, that “we are stardust” (at least the part of us that isn’t hydrogen).