Earth Science, Planetary Science, Space Science

Wednesday

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Earth and the moon in one frame as seen from the Galileo spacecraft 6.2 million kilometers away. (from Picture of Earth from Space by Fraser Cain)

It is ironic, though not particularly paradoxical, that the earth sciences as we known them today only came into being as the result of the emergence of space science, and space science was a consequence of the advent of the Space Age. We had to leave the Earth and travel into space in order to see the Earth for what it is. Why was this the case, and what do I mean by this?

It has often been commented that we had to go into space in order to discover the earth, which is to say, to understand that the earth is a blue oasis in the blackness of space. The early images of the space program had a profound effect on human self-understanding. Photographs (as much or more than any theory) provided the theoretical context that allowed us to have a unified perspective on the Earth as part of a system of worlds in space. Once we saw the Earth for what it was, What Carl Sagan called a “pale blue dot” in the blackness of space, drove home a new perspective on the human condition that could not be forgotten once it had been glimpsed.

To learn that our sun was a star among stars, and that the stars were suns in their own right, that the Earth is a planet among planets, and perhaps other planets are other Earths, has been a long epistemic struggle for humanity. That the Milky Way is a galaxy among galaxies, a point that has been particularly driven home by recent observational cosmology as with the Hubble Ultra-Deep Field (UDF) image (and now the Hubble eXtreme-Deep Field (XDF) image), is an idea that we still today struggle to comprehend. The planethood of the Earth, the stellarhood of the sun, the galaxyhood of the Milky Way are all exercises in contextualizing our place in the universe, and therefore an exercise in Copernicanism.

But I am getting ahead of myself. I wanted to discuss the earth sciences, and to try to understand what they are and how they have become what they are. What are the Earth sciences? The Biology Online website has this brief and concise definition of the earth sciences:

The Earth Sciences, investigating the way our planet works and the mechanisms of nature that drive it.

The geology.com website has a more detailed definition of the earth sciences that already hints at their relation to the space sciences:

Earth Science is the study of the Earth and its neighbors in space… Many different sciences are used to learn about the earth, however, the four basic areas of Earth science study are: geology, meteorology, oceanography and astronomy.

For a more detailed overview of the earth sciences, the Earth Science Literacy Initiative (ESLI), funded by the National Science Foundation, has formulated nine “big ideas” of earth science that it has published in its pamphlet Earth Science Literacy Principles. Here are the nine big ideas taken from their pamphlet:

1. Earth scientists use repeatable observations and testable ideas to understand and explain our planet.

2. Earth is 4.6 billion years old.

3. Earth is a complex system of interacting rock, water, air, and life.

4. Earth is continuously changing.

5. Earth is the water planet.

6. Life evolves on a dynamic Earth and continuously modifies Earth.

7. Humans depend on Earth for resources.

8. Natural hazards pose risks to humans.

9. Humans significantly alter the Earth.

Each of these “big ideas” is further elaborated in subheadings that frequently bring out the planethood of the Earth. For example, section 2.2 reads:

Our Solar System formed from a vast cloud of gas and dust 4.6 billion years ago. Some of this gas and dust was the remains of the supernova explosion of a previous star; our bodies are therefore made of “stardust.” This age of 4.6 billion years is well established from the decay rates of radioactive elements found in meteorites and rocks from the Moon.

Intuitively, we would say that the earth sciences are those sciences that study the Earth and its natural processes, but the rapid expansion of scientific knowledge has made us realize that the Earth is not a closed system that can be studied in isolation. The Earth is part of a system — the solar system, and beyond that a galactic system, etc. — and must be studied as part of this system. But we didn’t always know this, and this comprehensive conception of earth science is still in the process of formulation.

The realization that the processes of the Earth and the sciences that study these processes must ultimately be placed in a cosmological context means that contemporary earth science is now, like astrobiology, which seeks to place biology in a cosmological context, a fully Copernican science, though not perhaps quite as explicitly as in the case of astrobiology. The very idea of Earth science as it is understood today, like planetary science and space science, is essentially Copernican; Copernicanism is now the telos of all the sciences. Copernican civilization needs Copernican sciences. As I said in my presentation to this year’s 100YSS symposium, the scope of an industrial-technological civilization corresponds to the scope of the science that enables this civilization.

What this means is that the sciences that generations of Earth-bound scientists have labored to create in order to describe the planet upon which they have lived, which was the only planet that they could know prior to the advent of space science, are all planetary sciences in embryo — all potentially Copernican sciences that can be extended beyond the Earth that was their inspiration and origin. Before space science, all science was geocentric and therefore essentially Ptolemaic. Space science changed that, and now all the sciences are gradually becoming Copernican.

In the case of earth science, this is a powerful scientific model because the earth sciences have been, by definition, geocentric. That even geocentric sciences can become Copernican is a powerful lesson and provides a model for other sciences to follow. I have often quoted Foucault as saying that “A real science recognizes and accepts its own history without feeling attacked.” I think it can be honestly said that the geosciences recognize and accept their history as geocentric sciences and this in no way inhibits their ability to transcend their geocentric origins and become Copernican sciences no longer exclusively tied to the Earth. I find this rather hopeful for the future of science.

Another way to conceptualize earth science is to think of the earth sciences as those sciences that have come to recognize the planethood of the Earth. This places the Earth in its planetary context among other planets of our solar system, and it also places these planets (as well as the growing roster of exoplanets) in the context of planetary history that we have learned first-hand from the Earth.

To a certain extent, earth science and planetary science (or planetology) are convertible: each is increasingly formulated and refined in reference to the other. What is planetary science? Here is the Wikipedia definition of planetary science:

Planetary science (rarely planetology) is the scientific study of planets (including Earth), moons, and planetary systems, in particular those of the Solar System and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history. It is a strongly interdisciplinary field, originally growing from astronomy and earth science, but which now incorporates many disciplines, including planetary astronomy, planetary geology (together with geochemistry and geophysics), atmospheric science, oceanography, hydrology, theoretical planetary science, glaciology, and the study of extrasolar planets.[1] Allied disciplines include space physics, when concerned with the effects of the Sun on the bodies of the Solar System, and astrobiology.

The Division for Planetary Sciences of the American Astronomical Society doesn’t give us the convenience of a definition for planetary science, but in its offerings on A Planet Orbiting Two Suns, A Thousand New Planets, Buried Mars Carbonates, The Lunar Core, Propeller Moons of Saturn, A Six-Planet System, Carbon Dioxide Gullies on Mars, and many others, give us concrete examples of planetary science which examples may, in certain ways, be more helpful than an explicit definition.

Jupiter’s moon Europa may have liquid water beneath its icy surface, kept warm inside by the enormous gravitational forces of Jupiter. Planet science is endlessly fascinating, and we learn new things about planetology almost every day.

The “aims and scope” of the journal Earth and Planetary Science Letters also give something of a sense of what planetary science is:

Earth and Planetary Science Letters (EPSL) is the journal for researchers, policymakers and practitioners from the broad Earth and planetary sciences community. It publishes concise, highly cited articles (“Letters”) focusing on physical, chemical and mechanical processes as well as general properties of the Earth and planets — from their deep interiors to their atmospheres. Extensive data sets are included as electronic supplements and contribute to the short publication times. EPSL also includes a Frontiers section, featuring high-profile synthesis articles by leading experts to bring cutting-edge topics to the broader community.

A recent (2006) controversy over the status of Pluto as a planet led to an attempt by The International Astronomical Union (IAU) to formulate a more precise definition of what a planet is. The definition upon which they settled demoted Pluto from being a planet to being a dwarf planet. While this decision does not have complete unanimity, it is gaining ground in the literature. Here is the IAU of planets, dwarf planets, and small solar system bodies:

(1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies.”

With this greater precision of definition than had previously been the case in regard to planets, we could easily define planetary science as the study of celestial bodies that (a) are in orbit around the Sun, (b) have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (nearly round) shape, and (c) have cleared the neighbourhood around their orbits. Of course, this ultimately won’t do, because a comprehensive planetary science will want to study all three classes of celestial bodies detailed above, and will especially want to study the mechanisms of planet formation, dwarf planet formation, and small object formation for the light that each shines on the other. Like the Earth, that is part of a larger system, all the planets are also part of a larger system, and how they relate to that system will have much to teach us about solar system formation.

This more comprehensive perspective brings us to the space sciences. What is space science? The Wikipedia entry on space sciences characterizes them in this way:

The term space science may mean:

●The study of issues specifically related to space travel and space exploration, including space medicine.

●Science performed in outer space (see space research).

●The study of everything in outer space; this is sometimes called astronomy, but more recently astronomy can also be regarded as a division of broader space science, which has grown to include other related fields.

It is interesting that this definition of space science does not mention cosmology, which is more and more coming to assume the role of the master category of the sciences, since it is ultimately cosmology that is the context for everything else, but we could easily modify that last of the above three stipulations to read “cosmology” in place of “astronomy.” As the definition notes, the space sciences have grown to include other related fields, and in the future it may well be that the space sciences become the most comprehensive scientific category, providing the conceptual infrastructure in which all other scientific enterprises must be contextualized.

Since the Earth is a planet, and planets are to be found in space, one might readily assume that the Earth sciences, planetary sciences, and space sciences might be arranged in a nested hierarchy as follows:

Conceptually this is correct, but genetically, i.e., in terms of historical descent, it is obvious that the sciences that we have created to study our home planet are the sciences that, when generalized and applied beyond the surface of the Earth, are the sciences that become planetary science and space science.

Before space science and planetary science, there were of course the familiar sciences of geology (later geomorphology), atmospheric science or meteorology (later climatology), oceanography, paleontology, and so forth, but it was only when the emergence of space science and planetary science placed these terrestrial sciences into a cosmological context that we came to see that our sciences that study the planet that we call our home together constitute the Earth sciences in contrast to, and really in the context of, space science and planetary science. Great strides have been made in this direction, but further work remains to be done.

Geologic timescales for Earth and Mars with rocks plotted at the age of their emplacement. The age of soil samples analyzed by landed missions to Mars are too uncertain to plot on Fig. 4, and since no rocks were analyzed at the Viking 1 landing site in Chryse Planitia, that site is not shown. Martian geologic timescale of Hartmann and Neukum (2001), with subdivisions indicating the early, middle, and late Noachian, early and late Hesperian, and early, middle, and late Amazonian.

We know that the Earth and its solar system is about 4.6 billion years old, and most recent estimates for the age of the known universe put it at about 13.7 billion years. This means that the Earth has been around for almost exactly a third of age of the entire universe, which is not an inconsiderable length of time. Our sun and its solar system stands in relation to other stars of a similar age, and these stars and solar systems with significant traces of heavier elements stand in certain relationships to earlier populations of stars. The whole history of the universe is present in the rocks of the Earth, and we have to keep this in mind in the expanding knowledge base of the earth sciences.

While geological time scales are essentially geocentric, it would be possible to formulate an astrogeography and an astrogeographical time scale, extrapolating earth science to planetary science and thence to space science, that not only placed Earth’s geological history into cosmological context but also placed all planetary bodies and planetary systems and their geology in a cosmological context. For such an undertaking the generations of stars and planetary formation would be of central concern, and we could expect to see patterns across stars and solar systems of the same generations, and across planets within a given solar system.

This work has already begun, as can be seen in the above table laying out the geological histories of the Earth, the Moon, and Mars in parallel. Since one of the major theories for the formation of the Moon is that most of its substance was ripped out of the Earth by an enormous collision, the geological histories of the Earth and the Moon may ultimately be shown to coincide.

Stars and planets formed from the same dust and debris clouds filled with the remnants of the nucleosynthesis of earlier poulations of stars. This is now familiar to everyone. Galaxies, in turn, formed from stars, and thus also reflect a generational index reflecting a galaxy’s position in the natural history of the universe.

Since we now also believe that all or almost all spiral galaxies (and perhaps also other non-spiral or irregular galaxies) have a supermassive black hole at their centers, I have lately come of think of entire galaxies as the vast “solar systems” of supermassive black holes. In other words, a supermassive black hole is to a galaxy as a star is to a solar system. As planetary systems formed around newly born stars, galaxies formed around newly born black holes (if their gravity was sufficiently strong to form such a system). This way of thinking about galaxies introduces another parallelism between the microcosm of the solar system and the macrocosm of the universe at large, the structure of which is defined by galaxies, clusters of galaxies, and super clusters.

All of this falls within a single natural history of which we are a part.

Our history and the history of the universe are one and the same.

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