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Depois de alguma resistência cá está o meu novo blogue

A razão é um elástico

“A razão é um elástico. Vê se consegues não a esticar muito para não rebentar”; Vergílio Ferreira in Pensar. Eu diria mesmo que é preciso cuidado quando achamos que certa afirmação nos dá razão ou nos tira razão. É que nós temos tendência a vermos a razão pelo lado que nos dá mais jeito. E de uma única afirmação podemos tirar mil razões ou a razão de que mais gostamos.

Núcleo de Júpiter

Chego à conclusão que apesar das incertezas sobre a existência de um núcleo em Júpiter grande parte dos investigadores não acredita muito na teoria de Boss. Ora isto significa que a velha teoria da acreção pode estar certa (coisa que ainda não se sabe) e sendo assim Júpiter tem que ter um núcleo. Mas aguardo mais respostas nos próximos dias.

James G. Hill (resposta)

James G. Hill é director do observatório e planetário Rainwater no Mississipi. Uma resposta curta ao encontro do meu pensamento sobre o assunto:

“Like you, I learned that planets formed slowly by accretion and then

differentiation. Current theories have to coincide with more and better

data and at the moment are undergoing adjustments. So far as I know, this

week, Jupiter has a rocky core, but this understanding could change at any

moment. Isn’t science great! Never dull”.

Richard Rand (resposta)

Richard Rand é professor de astronomia na Univesidade do Novo México e outro céptico, embora mais cauteloso:

“One of the main pieces of evidence that Jupiter has a rocky/metallic

core comes from Jupiter’s oblateness: if it didn’t have such a core, it

would be more oblate than it is. Unfortunately, I’m not an expert in

this area so I can’t enlighten much. However, the new evidence seems to

be that the core may have less mass than thought. The range of possible

core masses seems to be about zero to ten Earth masses. So apparently

the data are consistent with no core, but also consistent with a

moderate core. My impression is that it is too early to say that

Jupiter has no core, and more precise data on its rotational figure are

needed”.

J. Scott Miller (resposta)

J. Scott Miller do planetário Rauch da Universidade de Louisville é muito céptico em relação à teoria de Boss. Senão vejamos:

“When reading science from popular press sources, it is always wise to a) read with scepticism, perhaps more so than other sources of science information,

and b) read/understand what the article is actually saying. This particular

article and the standing on this particular topic, is summarized by the

following paragraphs at the bottom of the article:

“Core accretion is still the popular theory for explaining the formation of

Jupiter and other planets in our solar system,” he said.

“People have been thinking in terms of core accretion for the last two

decades, and it takes a while for people to get used to any new idea,

including scientists. But I think that with time, and with continued work on

both core-accretion and disk-instability mechanisms, most scientists will be

able to agree about the likely means for making gas-giant planets.”

In science, to change the “current thinking” or “popular theory”, it is not

just enough to show a new idea works, but that it provides a more correct

interpretation of the data than the current theory does. More importantly, it

must make predictions that the other cannot make that can then be used for

testing the theories against each other to see which survives the testing.

In this article, an idea has been proposed. It is different, so the media,

always hoping to be cutting edge in order to sell advertisement to potential

clients at exhorbitant prices, run stories on these ideas. If they pan out,

then they can point to them when charging their clients, justifying the high

fees by implying that that particular media has its fingers on the pulse of

new science (or new whatever, depending on the interest of the client they are pursuing/wanting to keep). If the idea does not pan, no big deal. The

process of verification can take years and over that time, memories are short

as other new things appear over the horizon. In the meantime, the media have

gotten their advertising fees and are laughing all the way to the bank.

So, for the moment, I wouldn’t worry too much about the core status of

Jupiter. Maybe it does, maybe it does not. The data can be interpreted

either way. And the testing phase is only at its beginnings.

Ainda sobre a teoria de Boss diz o seguinte:

“As indicated in the article, it is a new idea. I have not seen indications of

it even being published in a peer-reviewed journal, though I do not get to

read those with a frequency to say if that is even true. But, for now, it is

an idea that has been put on the table. If it has validity, that will be

discovered. But, it will not happen over night, nor, as the article

indicated, will be selling in to those who have been working in that

particular field.”

Dennis J. Cowles (resposta)

Dennis J. Cowles do planetário de New Orleans dá-me uma resposta curiosa.

“For the last two decades, we’ve assumed that both Jupiter and Saturn have had a large core (10 or 20 Earth masses). The Galileo data imply that the core is smaller than we thought that it was (maybe 5 Earth masses). One group of scientists, Alan Boss among them, argues that the data may indicate that there is no core at all. This is one possible interpretation of the Galileo data, but I’m not sure that most scientists would agree. The moment of inertia implies (as you point out) that there is at least a small core.

What we need to do is send a probe to Jupiter to map the planet’s gravity. We can figure out from a good gravity map what the internal structure is, including whether or not there is a core there.”

William B. Hubbard (resposta)

William B. Hubbard é professor de ciências planetárias no Lunar and Planetary Laboratory da Universidade do Arizona. Escreveu um capítulo inteiro para o New Solar System sobre os interiores dos planetas gigantes, onde diz que Júpiter tem núcleo. Na resposta parece que tem mais dúvidas do que no livro.

“Standard models for the formation of Jupiter require the presence of

a rock-ice core of about 10 Earth masses (ME) to trigger collapse of

nebular hydrogen-helium gas. For solar composition a planet the mass

of Jupiter would include about 5 ME of rock and ice in any case.

The basic problem is that these numbers are only at the level of 1 to

3 percent of the mass of Jupiter, and our uncertainties in the equation

of state of hydrogen are at least as large as this. So it is very

difficult to demonstrate directly from available data on Jupiter that

it has a core. The moment of inertia of Jupiter would not change a

great deal with or without a 10-ME core.

The main evidence that Jupiter must have a 10-ME core is from the

model for its origin. A great deal of effort is being directed at

present to designing geophysical experiments which will give a direct

detection of the core, because the current situation is so uncertain”.

Júpiter

Durante toda a vida aprendi que Júpiter tinha núcleo. Basicamente por duas razões. Uma é o momento de inércia do planeta que nos diz se há ou não material denso concentrado no centro do planeta. E no caso de Júpiter diz que sim. A outra é o modelo de formação do planeta que nos diz que se formou por acreção a partir de um núcleo de gelo e rocha.

Mas há dias que ando numa polémica com o meu amigo Carlos Oliveira sobre este assunto. Um artigo na Space.Com semeou a dúvida. Neste artigo, Alan Boss lança dúvidas sobre a possibilidade de Júpiter ter núcleo, pois este homem é defensor de uma teoria diferente da acreção para a formação dos planetas, onde o núcleo de Júpiter não encaixa muito bem.

Para tirar as minhas dúvidas fiz uma coisa que qualquer divulgador atento deve fazer que é escrever 20 mails a quem sabe do assunto e ver o que eles dizem. Começo pelo John A. Wood do Smithsonian Astrophysical Observatory. Este homem tem um livro interessante sobre o sistema solar (The Solar System da Prentice Hall). E sobre o momento de inércia deu-me uma boa lição. É que apesar do momento de inércia ser um bom indicador não nos diz realmente se existe ou não um núcleo diferenciado.

Hi José,

“A “core” is a central planetary region that is distinctly denser than the

material lying above it. When you descend into the core you cross a sharp

boundary and take a step up in density.

The moment-of-inertia factor I cited means there is denser material at the

core of Jupiter than at smaller depths, but it cannot distinguish between a

core with a well-defined boundary, where there is a sharp change in density,

and a planet where the density simply increases gradually toward the center.

Both models can give you the same moment-of-inertia factor.

The sharp-boundary model has always been more or less assumed, and is cited

in my book, but Alan Boss is saying maybe it’s the other way– density

increases gradually and there is no distinct core boundary. By hindsight,

maybe I should have mentioned that possibility”.

John Wood

Perguntei-lhe depois até que ponto a teoria do Alan Boss tinha adeptos entre a comunidade. Disse que acha a teoria simpática embora seja uma teoria com poucos adeptos.

“You have to go back to why there are these two models. It’s because there

are two models for the formation of Jupiter. They are what is driving the

debate about a Jupiter structure model.

See p. 153 of my (2000) book. Here a model is described in which the core of

Jupiter accreted first, of solid materials; then when the core was massive

enough, it “grabbed” gas from the solar nebula. This model predicts in a

pretty natural way that Jupiter still has a core that is distinctly more

dense than the overlying material. I think most people accept this model of

Jupiter origin, and therefore the distinct-core structure for the planet.

On p. 154, an alternative model is given for the origin of Jupiter: it

formed all at once, from more or less homogeneous nebular material, by

gravitational instability. If that happened, settling-out might have

produced a core with a distinct boundary, but it probably would not have,

leaving Jupiter with an interior that was dense only because pressure has

packed it down, and without a sharp core boundary.

Alan Boss has always (well, for 10 years or more) been the most vocal

advocate of this second formation model. In arguing that Jupiter might have

no core boundary, he is simply stating a consequence of his formation model,

and being consistent with what he has always said. I think his is a minority

opinion, but I personally like it.”

Uma simpatia este homem.

Será que Júpiter tem núcleo?

Será que Júpiter tem núcleo? É uma discussão que vou começar logo à noite.