Introduction

This chapter focuses on the peculiarities of astrometry applied to Solar System objects. This is a whole subject by itself and within the scope of this book we will restrict ourselves to an overview, emphasizing the features that make Solar System observations definitely distinct from stellar astrometry.

A beginner would naively think that Solar System astrometry deals primarily, if not only, with the major planets. Actually, the field is much broader and the planets are only a very small subset of the whole subject.

The list of potential sources is conveniently broken down into large categories as follows:

1. • The major planets from Mercury to Neptune. They are much too big, let alone too bright, to be observed by conventional astrometric techniques (meaning to determine their positions) in a direct way, although this remark does not fully apply to transit instruments. Their true positions are eventually obtained from the positions of their faint satellites against the background stars combined with the theory of their motion. In this chapter we omit physical observations of the planets necessary to determine their rotation and establish a local reference frame on their surfaces.

2. • The planetary satellites, Table 26.1, comprising three subcategories:

3. (i) The small inner satellites, orbiting very close to the planetary surface and, for the giant planets, connected to their ring system.

4. (ii) The classical satellites, usually the largest, orbiting at a few planetary radii, with low inclination to the planet's equatorial plane and nearly circular orbits.

5. (iii) The faint outer satellites, orbiting on eccentric irregular orbits, prograde or retrograde.

6. […]

Introduction

The fundamental task of measuring stellar positions, and the derived properties of trigonometric distances and space motions, has preoccupied astronomers for centuries. As one of the oldest branches of astronomy, astrometry is concerned with measurement of the positions and motions of planets and other bodies within the Solar System, of stars within our Galaxy and, at least in principle, of galaxies and clusters of galaxies within the Universe as a whole.

While substantial advances have been made in many areas of astrometry over the past decades, the advent of astrometric measurements from space, pioneered by the European Space Agency's Hipparcos mission (operated between 1989 and 1993), has particularly revolutionized and reinvigorated the field. Considerable further progress in space and ground measurements over the next decade or so collectively promise enormous scientific advances from this fundamental technique.

At the very basic level, accurate star positions provide a celestial reference frame for representing moving objects, and for relating phenomena at different wavelengths. Determining the changing displacement of star positions with time then gives access to their motions through space. Additionally, determining their apparent annual motion as the Earth moves in its orbit around the Sun gives access to their distances through measurement of parallax. All of these quantities, and others, are accessed from high-accuracy measurements of the relative angular separation of stars. Repeated measurements over a long period of time essentially provide a stereoscopic map of the stars and their kinematic motions.

Introduction

The observations of gamma-ray bursts (GRBs) with the Compton Gamma Ray Observatory/Burst and Transient Source Experiment (CGRO/BATSE; see Chapter 3) pointed toward an extragalactic origin of GRBs; however, a direct measurement of the distance was still missing. Faster and much more precise positions were needed to find the counterparts at other wavelengths, the “Holy Grail” of GRB scientists. However, whether any “remnant” (afterglow) emission actually followed the GRB was unknown. A number of breakthrough discoveries were enabled by the launch of BeppoSAX in 1996. The fundamental questions about the nature of GRBs addressed with BeppoSAX and their implications, can be summarized as follows:

• Where are GRBs located? – distance-scale measurement; properties of the host galaxy; properties of the GRB formation site.

• What is the source of the GRB energy? – nature of their progenitors; how the progenitors produce the initial GRB energy (collapsars or mergers?).

• How is this energy transformed into radiation?

BeppoSAX

BeppoSAX was an Italian X-ray satellite with Dutch participation, nicknamed Beppo after the physicist Giuseppe “Beppo” Occhialini (Piro et al. 1995a, Boella et al. 1997a). One of the key science goals of BeppoSAX was the study of transient sources in the sky, including GRBs. Scientific instruments and ground operations were specifically designed to search for transient events over wide areas of the sky, and to follow up their detection with fast observations using sensitive X-ray telescopes.

Introduction

The dynamics of celestial bodies is an important topic for astrometry. First, as we have seen in the previous chapter, the position and velocity of the observer with respect to the BCRS as well as the positions and velocities of Solar System objects are necessary to reduce observations for aberration, parallax, and gravitational light deflection. Second, astrometric observations represent an important source of information allowing us to model the dynamical behavior of various celestial systems: Earth satellites, interplanetary stations, major and minor planets of the Solar System, binary and multiple stars, exoplanetary systems, etc.

A modern introduction in the various techniques of celestial mechanics can be found in the books of Murray and Dermott (1999), Beutler (2005) and Roy (2005). In this chapter we give a short overview of the most important results and concentrate on the practical aspects of the N-body problem relevant to obtaining and processing high-accuracy astrometric data.

Introduction

The Doppler detection of the Jupiter-mass planet around the nearby, solar-type star 51 Pegasi (Mayor and Queloz 1995) heralded the new era of discoveries of extrasolar planets orbiting normal stars. Four different techniques have been successfully used for the purpose of exoplanet detection. Decade-long, high-precision (1–5 m/s) radial-velocity surveys of ˜3000 F-G-K-M dwarfs and subgiants (e.g. Butler et al. 2006, Udry and Santos 2007, Eggenberger and Udry 2010) in the solar neighborhood (d ≤ 50 pc) have yielded so far the vast majority of the objects in the present sample (a total of over 760 planets in ˜600 systems, as of June 2012). Ground-based photometric transit surveys (e.g. Char-bonneau et al. 2007, Collier Cameron 2011) are uncovering new transiting systems at a rate of ˜30 per year, while space-borne observatories such as CoRoT and particularly Kepler hold promise of increasing by an order of magnitude the present yield (over 230 transiting systems known to date). Finally, over three dozen sub-stellar companions have also been detected so far by means of gravitational microlensing (e.g. Bond et al. 2004, Beaulieu et al. 2006, Gaudi et al. 2008, Muraki et al. 2011), direct-imaging surveys (e.g. Chauvin et al. 2005, Kalas et al. 2008, Marois et al. 2008, 2010), and timing techniques (Silvotti et al. 2007, Lee et al. 2009, Beuermann et al. 2010).

Introduction

This chapter offers practical advice to anyone developing, or supporting, astrometric computer applications related to celestial coordinate systems and positions. It identifies sources of ready-made software, useful in its own right or as benchmarks, and references to the underlying algorithms. This will be found useful in realizing many of the transformations detailed in Chapter 7, in which the theoretical bases and definitions of the transformations are provided. We shall limit ourselves to the sequence of transformations for ground-based astrometry of stars that links catalog coordinates with the direction of the incoming light as seen by the terrestrial observer. The catalog information with which the sequence begins typically comprises ICRS (International Celestial Reference System) right ascension and declination [α, δ], and space motion in the form of proper motions in α and δ plus parallax and radial velocity. The final observed coordinates [A, E] stand for azimuth and altitude, the latter informally called elevation. The sequence of transformations in this chapter are meant to aid the user who needs to take information in an astrometric catalog and predict the observed coordinates, in contrast to the situation in Chapter 19, where the observer wishes to transform in the opposite sense, i.e. from the observed coordinates to the final catalog positions.

Introduction

The study of gamma-ray bursts (GRBs) remains highly dependent on the capabilities of the observatories that carry out the measurements. The large detector size of BATSE produced an impressively large sample of GRBs for duration and sky distribution studies. The burst localization and repointing capabilities of BeppoSAX led to breakthroughs in host and progenitor understanding. The next phase in our understanding of GRBs is being provided by the Swift mission. In this chapter we discuss the capabilities and findings of the Swift mission and their relevance to our understanding of GRBs. We also examine what is being learned about star formation, supernovae, and the early Universe from the new results. In each section of the chapter, we close with a discussion of the new questions and issues raised by the Swift findings.

The Swift observatory

Swift (Gehrels et al. 2004) carries three instruments: a wide-field Burst Alert Telescope (BAT; Barthelmy et al. 2005a) that detects GRBs and positions them to arcminute accuracy, a narrow-field X-Ray Telescope (XRT; Burrows et al. 2005a) and a UV–Optical Telescope (UVOT; Roming et al. 2005) that observe their afterglows and determine positions to arcsecond accuracy, all within about 2 minutes. BAT is a coded aperture hard X-ray (15–350 keV) imager with 0.5 m2 of CdZnTe detectors (32 000 individual sensors; ~2400 cm2 effective area at 20 keV including mask occultation) and a 1.4 sr half-coded field of view.

Introduction

For thousands of years people have wondered about the existence of habitable worlds. We frame the discussion in terms of a hierarchical series of ancient questions: “Do other Earths exist?” and “Are they common?” and “Do any have signs of life?” With hundreds of known exoplanets of increasingly smaller mass and size, we are on the verge of answering these questions. Thousands of years from now, people will look back and see as one of the most significant, positive accomplishments of our early twenty-first century society the first discoveries of exoplanets, and the human foray into finding and characterizing habitable worlds.

How to discover Earths

Do other Earths exist? Our Galaxy, the Milky Way, has about 100 billion stars. The universe has upwards of 100 billion galaxies. The chance therefore that another Earth exists is extremely high, even if Earths are rare.

Finding Earths to confirm their logical existence, however, is another matter. We will not believe that other Earths exist until we have robust evidence. The biggest challenge in detecting another Earth is that Earth-like planets are miniscule compared to their adjacent parent star. Our own Earth is so much smaller than the Sun (100 times), so much less massive (1000 times), and so much less bright (107–1010, depending on wavelength). Any planet-detection technique (see Figure 11.1) is challenged to find another Earth.

Introduction

We saw in the previous chapter that Earth developed a climate that supported liquid water at its surface very early in its history, probably within the first few hundred million years. That was good for life, of course, because all life that we know about on Earth requires liquid water at least episodically. There are good chemical reasons for thinking that this requirement might be universal, some of which were discussed earlier in this volume.

Here we are concerned with a somewhat later stage in Earth's history, starting from when the rock record begins, around 3.8–4 billion years ago, or 3.8–4 Gyr ago, and continuing on until the rise of atmospheric oxygen, around 2.3 Gyr ago. This time interval overlaps almost precisely with the geologic time period called the Archean Eon. Although not formally defined as such, the beginning of the Archean corresponds with the beginning of the rock record, as marked by the oldest dated fragments of continental crust at Earth's surface. We know that Earth had older rocks, based on crystals of the mineral, zircon, which survive today as sand grains in mid-Archean quartzites from Western Australia, and can be dated as far back as 4.4 Gyr ago (Valley et al. 2002). Defining the end of Archean time to be precisely 2.5 Gyr ago was a somewhat arbitrary decision of the geological community: at about this time, the dominant type of sedimentary basin switches from the granite-greenstone belt configuration (dominated by ultramafic, magnesium-rich volcanics and chemically immature sedimentary rocks) to basins controlled by thermal subsidence along passive margins. This switch in rock types may or may not have influenced the rise of O2, as discussed later in this chapter. We will henceforth use the term “Archean” to refer to the time period preceding the rise of O2, recognizing that astrobiological and geological terminologies have slightly different meanings.

Introduction

Earth records its own history in the physical, chemical, and biological features of sedimentary rocks. In particular, the history of life is recorded by the remains of organisms buried and preserved in accumulating sediments, by physical traces of organisms’ activity in sediments (e.g. burrowing), and by chemical changes wrought by organisms (e.g. oxygen produced by land plants, algae, and cyanobacteria). The process of sediment accumulation, so essential to preservation, has biased the fossil record: organisms that lived in environments where burial was likely are relatively well represented in the geologic record, whereas organisms that lived in habitats characterized by net erosion seldom become fossils.

There is a second bias to the fossil record. The organisms most likely to be preserved as fossils are those that produce “hard parts,” mineralized skeletons or decay-resistant organic compounds such as the lignin in wood. In contrast, organisms with no readily preservable components fossilize only under exceptional circumstances, although some leave a record in the form of “trace” fossils such as tracks and burrows. Some microorganisms produce walls, spores, and extracellular envelopes that also preserve well in accumulating sediments; thus, we have a fossil record of bacteria and unicellular eukaryotes that predates the conventional record of animals and land plants. As in the case of animals and their skeletons, some microorganisms routinely produce preservable structures, whereas others never do. There are also microbial trace fossils, recorded by the influence of microbial mat communities on bedding and stromatolites, distinctive three-dimensional structures formed where large colonies of microbes influenced or controlled the formation, texture, and/or mechanical properties of sediments. In general, then, the fossil morphologies that document early life largely record microorganisms that (1) lived where burial facilitates preservation and (2) made decay-resistant organic walls or sheaths. Cyanobacteria are well represented in Proterozoic sedimentary rocks; Archaea are unknown as microfossils (e.g. Knoll 2003).

Introduction

The astrobiological relevance of small bodies has been acknowledged for several decades with regard to their role in delivering volatiles to Earth and the inner Solar system (see Lunine 2006 for a review). However, until recently these objects were considered too small to sustain a deep liquid layer and hydrothermal activity over the long term. The last decade has been marked by a dramatic evolution of our understanding of small bodies, from observational constraints and theoretical arguments. The discoveries of geological activity on Saturn's satellite Enceladus and Pluto's satellite Charon have prompted theoreticians to develop new approaches for modeling the interiors of these objects, some of which are larger and/or warmer than Jupiter's satellite Europa, considered an archetype of a potentially habitable icy world. The purpose of this chapter is to evaluate the habitability potential of certain small bodies, i.e. their potential for sheltering life, whether life could develop in these environments, or was brought in from a different source.

This chapter focuses on large wet asteroids, small icy satellites, and trans-Neptunian objects (e.g. see the representatives of each class in Figure 10.1). We evaluate the occurrence in each class of objects of certain parameters that determine their capacity to sustain habitable conditions: the energy necessary to support chemical activity and chemical conditions amenable to the thriving of living organisms. The latter aspect is difficult to fully fathom as life has been found in the most surprising environments and based on unexpected nutrient systems. The question of the origin of life in favorable environments is considered in Chapter 2.

Introduction

What is this thing called “life” that seems to interest everyone? And why are we looking for it so hard? Why, for example, are we seeking “Earth-like planets” in “habitable zones,” or the spectroscopic signatures of amino acids and carbohydrates in the interstellar dust? Why not just accept the perspective of Dr. Manhattan from The Watchmen: most of the universe appears uninhabitable, yet seems to get along quite well.

Certainly, it is easy to understand the motivation of SETI astronomers who seek intelligent life. They seek someone to talk to, someone who can teach us something, if it does not consume us, or destabilize our society with technological wizardry. If we physically encounter life of the type that SETI seeks, we will almost certainly recognize it as life. SETI-style life will have the attributes that we most value in life, whether it explains the secrets of dark matter or simply applies 40 of us to a recipe for Rigelian stew. But the higher probability seems now that we will not find anything so complex. The life that we seem most likely to encounter will be “primitive,” single-celled organisms visible only under the microscope. And the life that we now seem most likely to encounter will be detectable only by its chemical signatures, no longer able to grow after its first encounter with us.

In this chapter we will look at the environment of the early Earth as a habitat for life and at the primitive life forms that inhabited it. The early Earth was a very different planet from today's Earth. Hotter, much more volcanically active, with an oxygen-poor atmosphere and ocean waters that were probably slightly more acidic and more salty than today's ocean, at first glance the early Earth seems to have been an inhospitable planet. But this was the Earth upon which life first appeared. In fact, life could not have appeared on today's Earth because of the ubiquitous presence of oxygen – an active molecule that effectively destroys the organic ingredients of life by oxidation. Despite its apparent inhospitality, the early Earth was habitable because it had conditions that were conducive to the appearance of simple life forms: it had liquid water, carbon molecules, energy sources, and the elements necessary for both the building bricks of cells and for its metabolic processes (HNOPS, plus transition metals). And this early, different planet apparently teemed with primitive forms of life.

The environment of the early Earth

After consolidation of the planetesimals forming the proto-planet, early radiogenic heat from short-lived radiogenic species, such as 26Al, fused the accreted planetesimals into a molten mass, producing a magma ocean, which allowed differentiation of the heavier elements, iron and nickel, into the core and the lighter elements, forming silicate minerals, into the mantle. Degassing of the early mantle expelled the lighter elements (volatile elements) that were originally contained in the planetesimals to create a weakly reducing atmosphere of N2, CO2, and water, with traces of other gases (Kasting and Brown 1998). About 40 My after the consolidation of the proto-Earth, it was impacted by another smaller planet having a composition not too different from that of the Earth. It is possible to estimate the timing of this impact from the age of differentiation of the cores of the Earth and the Moon. Using the ratio of the quantity of the radiogenic isotope 182H and its daughter 182W remaining in the mantle of the Earth and the Moon, the impact has been dated to approximately between 40 and 100 My after accretion (Yin et al. 2002, Kleine et al. 2009). The planetary material issuing from this glancing impact produced the Earth's satellite, the Moon. The existence of the Moon had a number of consequences.

Introduction

The “classical” criteria for habitability can be summarized as the presence of liquid water, energy sources to sustain metabolism, and “nutrients” over a period of time long enough to allow the development of life. The concept of a “habitable zone” (HZ) around each star defines where water can be stable at the surface as a result of the equilibrium temperature of the planet in the star's radiation field. Liquid water may exist on the surface of planets orbiting a star at a distance that does not induce tidal lock. But habitability conditions can be found not only on the surfaces of Earth-like planets: a subsurface ocean within a planet or the satellite of a gas giant may be habitable for some life form that may be very different from Earth-like life. Indeed, icy surfaces may cover liquid oceans, move and fracture like plate tectonics, and exsolute the internal material and energy through an interconnected system. With the discovery of planets beyond the Solar System and the search for life in exotic habitats such as Mars, Europa, Titan, and Enceladus, habitability in general needs a better and broader definition.

Liquid water has been recognized as the best solvent for life to emerge and evolve, although other possibilities have been suggested (e.g. Bains 2004). Water, an abundant compound in our galaxy (e.g. Cernicharo and Crovisier 2005), is liquid within a large range of temperatures and pressures and is also a strong polar–nonpolar solvent. This dichotomy is essential for maintaining stable biomolecular and cellular structures (Des Marais et al. 2002). A large number of organisms is capable of living in water. However, in a body of pure water, life will probably never spontaneously originate and evolve. This is because, while there are many organisms living in water, none we know of is capable of living on water alone because life requires other essential elements such as nitrogen and phosphorus in addition to hydrogen and oxygen. Besides, no organism we know of is made entirely of water. So obviously “just water” is not an auspicious place for starting life and evolution in.

It is with great pleasure that I welcome all the distinguished scientists convened here, at the invitation of the Pontifical Academy of Sciences, to discuss a theme that is as new as it is difficult and fascinating: Astrobiology. It is a field which requires a range of all but the most profound of scientific knowledge, as well as highly refined research techniques, and it means often proceeding on the basis of scarce evidence and formulating hypotheses requiring strict verification, which in turn can be diversely configured. It means resorting to results of research based on extreme aspects of the possibility of life on Earth, and to study how to verify its presence on other planets or exoplanets. It means – at its limit – studying if and how one could verify the existence of extraterrestrial forms of intelligence and how to enter in contact with them. This is a task that demands scientific integrity, not to be confused with science fiction.

In your study, which represents, I would say, an intense and indispensable case of vast multidisciplinary research, I don't doubt that you will find yourself accompanied and stimulated by that human atmosphere of collegiality and friendship offered by the Pontifical Academy of Sciences.