Transcript: The truth about Mars began to emerge in the 1960s and 1970s. In 1965 the Mariner IV space probe became the first probe to reach the red planet. It took a few close-up photographs as it zipped by the surface revealing an impact cratered surface looking much like the Moon. In 1976 the Viking I spacecraft landed on the surface of Mars. It descended through the apricot colored sky and landed on a rocky plain. Its cameras viewed a surface strewn with boulders and red sand dunes. ...

Transcript: Radar mapping of the surface of Venus has given us some clues as to the volcanic history of our sister planet. Venus has huge volcanoes ranging in size up to 5 to 6 miles high. There is evidence that the entire surface of Venus was re-surfaced by volcanoes and magma 500 to 800 million years ago. This is quite unlike the situation on the Earth, where although there is volcanic activity, it’s possible to find relatively young and relatively old surfaces. Almost the entire...

Transcript: Venus, second planet out from the Sun, is the sister planet to the Earth, closest among all the planets to the Earth in size and mass. Its atmosphere is composed of 96 percent carbon dioxide and 4 percent nitrogen with very small amounts of water vapor, argon, sulfur dioxide, and other trace gases. Due to the high concentration of carbon dioxide in the atmosphere, Venus has been subject at ancient times in its history to a runaway greenhouse effect where the Sun’s radiation is...

Transcript: Mercury takes 88 days to orbit the Sun and 59 days to spin once on it’s axis with respect to the stars. Thus a Mercury day is exactly two thirds of a Mercury year, and this simple fraction is not a coincidence. It’s related to tidal forces and resonances acting to align the spin rate of Mercury with its orbit. Mercury has a non-circular orbit, and the point closest to the Sun is called the perihelion. A hundred and fifty years ago it was noted that the position of the...

Transcript: Mercury, the closest planet to the sun, is a relatively small planet, only one third the size of the Earth and only one third larger than Earth’s moon. Mercury is so small that its gravity cannot retain an atmosphere. With no atmosphere there is no erosion caused by gases, and there is no liquid water on the surface because of the extremes of temperature. As Mercury’s surface preserves a perfect record of cratering over the history of the solar system, in this way Mercury is...

Transcript: At first glance the four terrestrial planets, Mercury, Venus, Earth, and Mars, have few similarities. Mercury is a hot, airless, moon-like planet. Venus has a dense carbon dioxide atmosphere and no satellite. Earth has a unique atmosphere of nitrogen and oxygen with extensive liquid water oceans and a large moon. Mars has two small moons, a thin carbon dioxide atmosphere, and polar caps made of solid carbon dioxide. Nonetheless, there are similarities among these planets and...

Transcript: Although we will never know for sure, because no one was around to witness it, there's good indirect evidence that the Moon formed in a spectacular way by a giant impact on the Earth early in its history. Several pieces of evidence point to this conclusion. First, as a whole the Moon is deficient in iron compared to the Earth, and has a very small iron core. Second, the moon is made out of material that's mostly similar to that of the Earth's mantle. Third, the lunar surface...

Transcript: The fact that gravity is an inverse square law leads to the idea of tides and tidal forces. Essentially the Moon exerts its gravity on the Earth as the Earth does on the Moon. The force of gravity from the Moon on the near side of the Earth to the Moon is larger than on the far side. This causes a stretching force. For a liquid like water that can flow freely it leads to the piling up of water on both the near side of the Earth towards the Moon and on the far side. If you...

Transcript: The only celestial object beyond the Earth that humans have set foot on is the Moon. The Apollo program was started in response to the Russian space program. Manned exploration of space was a direct outcome of the cold war. President Kennedy challenged the United States and its scientists and engineers to develop a vehicle and a payload and a life support system to send humans to the Moon and bring them back safely. Thus we have the Apollo program. During the years 1969 to...

Transcript: It is thought that the Moon formed in a very unusual way by being splashed off the still molten Earth by a huge impact early in the Earth’s history. We know that the Moon is the same age of the Earth, and in fact age dating of the Moon rocks by radioactive techniques gave us our first reliable estimate of the Earth’s age. The formation process of the Moon is manifested in the very different geology of the Moon from the Earth. The Moon has a very small metallic core and an...

Transcript: Earth's moon is an unusual satellite, one of the largest in the solar system. The Moon is only 30 percent smaller than Mercury, about half the size of Mars, and about a quarter of the size of the Earth itself. Such a ratio of 4 to 1 in planet to moon size is unique in the solar system. At a quarter the size of the Earth, the surface gravity of the Moon is one-sixth that of the Earth's and has about one-eightieth of the mass. The gravity of the Moon is insufficient to hold an...

Transcript: One of the most important scientific issues of the day is global warming. Global warming is subject to a debate, and there is no full consensus yet, but there is growing scientific evidence of greater and greater reliability that human activity is fundamentally altering the atmospheric composition of the entire planet with a possible increase in temperature as a result. The evidence for global warming is growing, but it's not yet totally unambiguous. While it is true that...

Transcript: The Greenhouse Effect occurs when the Sun’s radiation hits the Earth's surface and is reradiated at longer infrared wavelengths. Those infrared waves are preferentially absorbed by certain gases such as carbon dioxide, water vapor and methane. These greenhouse gases therefore cause a heating effect. In the case of Venus a runaway greenhouse effect raised the temperature of the planet to eight or nine hundred degrees. In the case of the Earth carbon dioxide is a trace gas, but...

Transcript: The first evidence that human activity could globally affect the Earth’s atmosphere occurred for the situation of the ozone hole. Ozone is trace gas, an isotope of oxygen that occurs 20 to 30 kilometers above the Earth’s surface. It’s a trace gas but an important gas because ozone protects us from ultraviolet radiation. In the 1970s and ‘80s, the ozone hole, a seasonally varying depletion in the ozone layer seen most clearly in Arctic and Antarctic regions, started growing. By...

Transcript: The history of life on Earth is not simply a smooth progression governed by natural selection. There have been times when life has been eradicated by cosmic intruders, catastrophes from space in the form of large impactors. These do not occur very often. During the first half billion years of the Earth’s history, the era of heavy bombardment, objects a kilometer or larger hit more than once every million years. Since then the impact rate has been much lower, and an impactor...

Transcript: The Earth's atmosphere has changed its chemical composition systematically over the history of the Earth, but most of these changes occur on very long time scales, hundreds of millions or even billions of years. There's growing evidence for climate change caused by changes in atmospheric composition on times scales of a hundred years or less caused, we think, by human activity on this Earth. This is a profound implication for how we run our affairs because climate change could...

Transcript: Fossil record of life on Earth is incomplete, and it only dates back about 500 million years. But there’s evidence at several points in time for mass extinctions in the species record. One of these occurs about 250 million years ago at the end of the Permian period. The other occurs 65 million years ago between the Cretaceous period and Tertiary period. The evidence in the latter case is very good that a cosmic intruder, a giant meteoric impact, caused the extinction of about...