Astronomy 161 – Introduction to Solar System Astronomy – Autumn 2007

Are we alone in the Universe? This lecture explores the question of how we might go about finding life on planets around other stars. Rather than talking about speculative ideas, like the Drake Equation or SETI, I am instead taking the approach of posing it as a problem of what to look for among the exoplanets we have been discovering in huge numbers in the last decade. I describe the basic requirements for life, and how life on Earth is surprisingly tough (extremophiles). I then give a definition of the Habitable Zone around a star, and present the Goldilocks Problem of how a planet must be neither too hot, too cold (for liquid water) or too big or too small to be hospitable to life. From there I then review the problem of how to go about finding Earth-like planets (Pale Blue Dots) around other stars, and if we do find them, what spectroscopic signatures of life, called biomarkers, we can look for to see if they have some form of life like we understand it on them. Recorded on 2007 Nov 30 in 1000 McPherson Lab on the Columbus campus of The Ohio State University. This is the final lecture for Autumn Quarter 2007.

Are there planets around other stars? Are there Earth-like planets around other stars? Do any of those harbor life? Intelligent life? We'd like to know the answers to all of these questions, and in recent years we've made great progress towards at least answering the first. To date, more than 260 planets have been found around more than 200 other stars, most in the interstellar neighborhood of the Sun, but a few at great distance. This lecture reviews the search for ExoPlanets, discussing the successful Radial Velocity, Transit, and Microlensing techniques. What we have found so far are very suprising systems, especially Jupiter-size or bigger planets orbiting very close (few hundredths of an AU) from their parent stars. Recorded 2007 Nov 29 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Comets are chance visitors from the icy reaches of the outer Solar System. In this lecture I describe the properties of comets, their historical importance, and introduce the "dirty snowball" model of a comet nucleus. At the end of class I created a model of a comet nucleus from common household and office materials, unfortunately I could not arrange for a videographer in time. Recorded 2007 Nov 28 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Beyond the orbit of Neptune lies the realm of the icy worlds, ranging in size from Neptune's giant moon Triton and the dwarf planets Pluto and Eris, all the way down to the nuclei of comets a few kilometers across. This lecture discussed the icy bodies of the Trans-Neptunian regions of the Solar System, discussing the basic properties of Triton (the best studied such object), Pluto, Eris, and the Kuiper Belt, introducing the dynamical families of Trans-Neptunian Objects that record in their orbits the slow migration of Neptune outwards during the early history of the Solar System. The Kuiper Belt is the icy analog of the main Asteroid Belt of the inner Solar System: both are shaped by their gravitational interaction with giant gas planets (Jupiter for the asteroids, Neptune for the KBOs), and are composed of leftover raw materials from the formation of their respective regions of the Solar System. Recorded 2007 Nov 27 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Asteroids are the leftover rocky materials from the formation of the Solar System that reside primarily in a broad belt between the orbits of Mars and Jupiter. This lecture reviews the physical and orbital properties of Asteroids, and discusses the role of Jupiter and orbital resonances in dynamically sculpting the Main Belt of Asteroids. Once again, we see how the history of the dynamical evolution of our Solar System is written in the orbits of its members. Recorded 2007 Nov 26 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

All Jovian planets have rings. We are most familiar with the bright, spectacular rings of Saturn, but the other Jovian planets have rings systems around them. This lecture describes the different ring systems and their properties, and discusses their origin, formation, and the gravitational interactions - resonances, perturbations, and shepherd moons - that govern their evolution. Recorded 2007 Nov 21 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Saturn is attended by a system of 60 known moons and bright, beautiful rings. Today we will explore the moons of Saturn. Among the highlights are Saturn's lone giant moon, Titan, the 2nd largest moon in the Solar System and the only one with a heavy atmosphere. The atmosphere of Titan is mostly nitrogen with a little methane, but the temperature and pressure are such that methane plays the same role on Titan that water plays on the Earth: it can be either a solid, gas, or liquid. The Cassini and Huygens probes have recently shown that there is evidence of liquid methane flows and mudflats, and even liquid methane lakes as big as the Great Lakes or Caspian seas on Earth. The other moon of interest is Enceladus. The shiniest object in the Solar System, Enceladus has spectacular fountains - cryovolcanos - that spew water vapor from reservoirs created in its tidally-heated interior. This ice repaves much of the surface of Enceladus, giving it a young, shiny surface, and builds the E ring of Saturn. Recorded 2007 Nov 20 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Jupiter has its own personal solar system in miniature of 63 known moons. Most are tiny, irregular bodies that are a combination of captured asteroids and comets, but it is the 4 largest, the giant Galilean Moons: Io, Europa, Ganymede, and Callisto, that is of greatest interest to us in this lecture. Each is a fascinating world of its own, with a unique history and properties: volcanically active Io, icy Europa which may hide an ocean of liquid water beneath the surface, the grooved terrain of Ganymede, and frozen dirty Callisto with the most ancient surface of the four. Recorded 2007 Nov 19 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

The Ice Giants Uranus and Neptune are the outermost major planets of our Solar System. Internally they small rocky cores surrounded by deep, slushy ice mantles and shallow hydrogen atmospheres, quite unlike the massive cores and deep metallic hydrogen mantles of Jupiter and Saturn. This lecture describes their basic properties: the origin of their vivid blue/green colors, their composition, structure, and weather. At the end we'll contrast and compare their properties to those of the Gas Giants. Recorded 2007 Nov 15 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

The Gas Giants Jupiter and Saturn are the largest planets in the Solar System. Internally they are deep, heavy Hydrogen/Helium atmospheres on top of dense rock/ice cores without solid surfaces. What we see in our telescopes are just the tops of the clouds. This lecture describes the basic properties of the planets: their composition, atmospheres, weather, and internal structures. Recorded 2007 Nov 14 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Having completed our tour of the Terrestrial Planets, we want to step back and compare their properties. In particular, we will wi review the processes that drive the evolution of their surfaces, their interiors, and their atmospheres. Recorded 2007 Nov 13 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Mars is a cold desert planet with a thin, dry carbon-dioxide atmosphere. The geology of Mars, however, shows signs of an active past, with hot-spot volcanism, and tantalizing signs of ancient water flows. While a cold, dead desert planet today, Mars' past may have been warmer and wetter, with liquid water during the first third of its history. This lecture reviews the properties of Mars, and describes the evidence for its active past. Recorded 2007 Nov 9 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Venus, the second planet from the Sun, is perpetually veiled behind opaque clouds of sulfuric acid droplets atop a hot, heavy, carbon dioxide atmosphere. In size and apparent composition, however, it is a near twin-sister of the Earth. Why is it do different? In this lecture I review the basic properties of Venus, and examine the similarties and differences with the Earth. Recorded 2007 Nov 8 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Mercury, innermost of the planets, is a hot, dead world that has been heavily battered by impacts. In this lecture I review the properties of Mercury, its orbit, rotation, surface, and interior structure. Recorded 2007 Nov 7 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

How did the Solar System form? In this lecture I review the clues for the formation of the solar system in the present-day dynamics (orbital and rotation motions) and compositions of the planets and small bodies. I then describe the standard accretion model for solar system formation, whereby grains condense out of the primordial solar nebula, grains aggregate by collisions into planetesimals, then gravity begins to work and planetesimals grow into protoplanets. What kind of planet grows depends on where the protoplanets form within the primordial solar nebula: close to the Sun only rocky planets form, beyond the Frost Line ices and volatiles can condense out allowing the growth of the gas and ice giants. The whole process took about 100 million years, and we as we explore the solar system in subsequent lectures, we will look for traces of this process on the various worlds we visit. Recorded 2007 Nov 6 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Welcome to the Solar System! We begin our exploration of the Solar System with an overview of the planets, moons, and small bodies that make up our home system. In this lecture I'll introduce many of the themes that will encounter many times as we go through our detailed look at the Solar System in the coming weeks. Recorded 2007 Nov 5 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What physical processes have shaped the Moon? In this lecture, I describe the surface features of the Moon (the Maria and Highlands), how crater density tells us the relative ages of terrains, and what we have learned about Moon rocks returned by astronauts and robotic probes. I will also discuss what is known about the interior of the Moon, and review what we know about lunar history and formation. Like the Earth, the Moon gives us a useful point of comparison with bodies elsewhere in the Solar System. Recorded 2007 Nov 1 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What is the composition and structure of the Earth's atmosphere? Why is it as warm as it is, and how did it form? Today I will describe the composition and structure of the atmosphere, the Greenhouse Effect, the Primordial Atmosphere, and Atmospheric Evolution. The Earth's atmosphere is a complex, dynamic, and evolving system, and we will use it as a point of comparison when we begin to examine other planetary atmospheres in future lectures. Recorded 2007 Oct 31 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What is the interior structure of the Earth? We will start our exploration of the Solar System with our home planet Earth. This lecture discusses the interior structure of the Earth, introducing the idea of differentiation, how geologists map the interior of the Earth using seismic waves, and the origin of the Earth's magnetic field. I describe the basic properties of the crust of the Earth, its division into rigid tectonic plates, and describe how plate motions driven by convection in the upper mantle have shaped the visible surface of our planet over its dynamic history. Recorded 2007 Oct 30 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

How old is the Earth? In this lecture I review the ideas of cyclic and linear time, and how this determines whether or not the question of the age of the Earth is meaningful. I then review various ways people have tried to estimate the age of the Earth, starting with historical ages that equate human history with the physical history of Earth. We then look at physical estimates of the Earth's age that do not make an appeal to human history, but instead seek physical processes that play out over time to make the estimates. This brings us to a discussion of radiometric age dating techniques that use the radioactive decay of isotopes trapped in minerals to identify the oldest Earth rocks and meteorites, and hence establish a radiometric date for the formation of the Earth some 4.55+/-0.05 Billion Years ago. Recorded 2007 Oct 29 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Telescopes outfitted with modern electronic cameras and spectrographs are astronomers' primary tools for exploring the Universe. In this lecture I review the primary types of telescopes and the best observatory sites to locate them, with a brief mention of radio and space telescopes. At the end, I give a brief review of the Ohio State's observing facilities. Recorded 2007 Oct 26 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Why does each element have its own unique spectral signature? how doe emission lines and absorption lines arise? This lecture is the second part of a two-part exploration of matter and light, looking at how the unique spectral-line signatures of atoms are a reflection of their internal electron energy-level structures. Topics include energy level diagrams for atoms, excitation, de-excitation, and ionization. There will be a short demonstration with gas-discharge tubes and slide-mounted diffraction gratings. For podcast listeners, the last portion of the class is the demo, for which we do not unfortunately have the resources to videotape. Recorded 2007 Oct 25 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

How do matter and light interact? This lecture is the first of two that will explore the interaction between light and ordinary matter, and how we measure that with spectroscopy. This lecture introduces the idea of internal energy as quantified by the temperature on the Absolute Kelvin scale, and Kirchoff's empirical Laws of Spectroscopy. We will deal primarily with blackbody spectra emitted by hot solids or hot dense gasses or liquids, the Stefan-Boltzmann and Wien Laws, and introduce emission and absorption line spectra. The next lecture will explain how line spectra arise from atoms and molecules. Recorded 2007 Oct 24 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What is ordinary matter made of? This lecture reviews the basic properties of matter from subatomic to atomic scales, introducing atomic structures, atomic number and chemical elements, isotopes, radioactivity, and half-life, ending with a brief overview of the four fundamental forces of nature: gravitation, electromagnetism, and the weak and strong nuclear forces. Recorded 2007 Oct 23 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What is light? Most astronomical objects are too far away to measure directly. Light is the messenger of the Universe, carrying with it information about objects as near as the Moon and as far away as the most distant objects in the visible Universe. In this lecture we will review the basic properties of light, the electromagnetic spectrum, the inverse square law of brightness, and the Dopper Effect. Recorded 2007 Oct 22 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

How do objects orbit if more than 2 massive bodies are involved? Newton's versions of Keplers 3 Laws of Planetary Motion are only strictly valid for 2 massive bodies. The Solar System, however, clearly has more than 2 massive objects within it. How do we handle this many-body problem? This lecture discusses some of the multi-body gravitational effects seen in our Solar System (and by extension elsewhere). We will describe Lagrange Points for the restricted 3-body problem and consequences like the Trojan Asteroids of Jupiter, long-range gravitational perturbations and their aid in discovering the planet Neptune, close encounters that can dramatically alter the orbits of comets and give us ways to slingshot spacecraft into the outer and inner Solar System without huge expenditures of fuel, and orbital resonances that can amplify small long-range perturbations and either stabilize or destabilize orbits. All of these effects play a role in the Dynamical Evolution of our Solar System that we will see throughout later parts of the course. Recorded 2007 Oct 18 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Why are there two high tides a day? This lecture examines tides caused by the differences in the gravity force of the Moon from one side to the other of the Earth (stronger on the side nearest the Moon, weaker on the side farthest from the Moon). The Sun raises tides on the Earth as well, about half as strong as Moon tides, giving rise to the effect of Spring and Neap tides that correlate with Lunar Phase. We will also discuss body tides raised on the Moon by the Earth, and how that has led to Tidal Locking of the Moon's rotation, which is why the Moon always keeps the same face towards the Earth. We end with a discussion of the combined effects of tidal braking of the Earth, which slows the Earth's rotation by about 23 milliseconds per day century, and causes the steady Recession of the Moon by 3.8cm away from Earth every year. Tidal effects are extremely important to understanding the dynamical evolution of the Solar System, as we'll see time and again in the second half of the class. Recorded 2007 Oct 17 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

Why do Kepler's Laws work? In this lecture I will describe Newton's generalization of Kepler's Laws of Planetary Motion so that they will apply to any two massive bodies orbiting around their common center of mass. I will introduce families of open and closed orbits, the circular and escape speeds, center-of-mass, conservation of angular momentum, and Newton's generalized version of Kepler's 3rd Law. The latter is a powerful tool for using orbital motions as our only way to measure the masses of astronomical objects. Recorded 2007 Oct 16 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.

What is Gravity? Starting with the properties of falling bodies first formulated by Galileo, Newton applied his three laws of motion to the problem of Universal Gravitation. Newtonian Gravity is a mutually attractive force that acts at a distance between any two massive bodies. Its strength is proportional to the product of the two masses, and inversely proportional to the square of the distance between their centers. We then compare the fall of an apple on the Earth to the orbit of the Moon, and show that the Moon is held in its orbit by the same gravity that works on the surface of the Earth. In effect, the Moon is perpetually "falling" around the Earth. Recorded 2007 Oct 15 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.