Earth and Mars orbit the Sun within the `Habitable Zone’ where liquid water can potentially be present on the surface, depending on the atmospheric pressure and composition (and the presence of sufficient water). We therefore focus on the properties of these two planets and review the properties of Venus and Mercury only to contrast the conditions for planets too close to their star to be habitable.

Earth

From a distance, Earth appears as a rotating sphere with oceans of liquid H_2O and brown rocky continents , covered with a thin gaseous atmosphere with swirling clouds of H_2O clouds. Polar caps of frozen H_2O . Continents show mountain ranges, volcanos, rivers and occasional impact craters. Below are two typical views of Earth, one from space and the other from the surface. See some beautiful images of Earth at NASA’s Visible Earth catalog.

But if one looks under the oceans, another remarkable pattern of mid-oceanic ridges  and ocean trenches are seen! Watch a beautiful 2-minute video of the slowly spinning Earth here .

Earth’s interior is still mostly molten from its formation 4.5 billion years ago. The solid crust of light silicates is very thin: only 5-50 km (vs. total radius of 6400 km). Under the thin crust is a thick mantle of melted silicates, a liquid outer core, and an inner core of solid iron/nickel.

Earth’s surface features are explained by plate tectonics . First introduced by Alfred Wegener in the 1930s and largely disparaged at the time, it is now completely confirmed by GPS and radio astronomical measurements of continental motions. The Earth’s surface is broken into ~20 crustal plates , which are moved in different directions by convection currents in the mantle. New crust forms by uplift and spreading of mantle material at mid-oceanic ridges, with the newest rock along the ridge line. Old crust is either subducted into the mantle along a trench, or is crumpled upward into mountain ranges (e.g. the Himalaya Mountains being produced today as the Indian plate crashes into the Asian plate). Plate motions and subduction are violent processes and produce volcanoes , mountain ranges and earthquakes along plate boundaries.

Typical plate motions are 1-5 cm per year, roughly the rate at which fingernails grow, amounting to roughly a thousand kilometers every 10⁸ years. The diagrams below are two representations of convection from the mantle and its effects on the surface tectonic plates.

Earth’s active plate tectonics serve a key role in the “carbon cycle” that helps maintain the carbon dioxide concentration of Earth’s atmosphere.

Earth’s crustal motions today include:

• Atlantic Ocean is widening, separating South America from Africa and North America from Europe. North American & Europe have collided repeatedly in the past. The Appalachian Mountains around us now are from 2 collisions ago – its counterparts on the other side are the Scottish highlands and Atlas Mountains in Morocco.
• Australia and India are pushing into Asia. India’s collision with Asia continues to raise the Himalaya Mountains.
• A sliver of western California is on the Pacific plate, which is sliding northward past the North American Plate, i.e., Los Angeles is moving towards San Francisco. This produces earthquakes (e.g. along the San Andreas fault in California) and volcanos (e.g. Mt. St. Helens in Washington State).

Since the Earth is 4.5 billion years old, the continents have collided and rearranged their configurations dozens of times. For example, 200 million years ago, all continents were stuck together into one super-continent, Pangaea. Note that if there were no plate tectonics, volcanism, and mountain building, rain would erode the continents into oceans and there might be no “dry land” on Earth today.

Earth is unique among the planets in having a relatively large-sized moon - the Moon, which is 1⁄80 the mass of Earth and orbits at a distance of 55 Earth radii. For this reason the Earth-Moon system is occasionally referred to somewhat humorously as a “binary planet”. The most likely way for such a massive and nearby Moon to have been formed is by a cataclysmic collision, towards the end of the formation of the Solar System, between the proto-Earth and another roughly Mars-sized planetesimal. The collision would have ejected a large amount of Earth’s crust into orbit, where it could have collected into the spherical body we see today. Key consequences of Earth’s large Moon for life on Earth are that it raises large tides in our oceans, and stabilizes the tilt of Earth’s axis (which is responsible for the seasons) over very long timescales. Tides in the Earth-Moon system are also slowing down Earth’s rotation, and hence, the length of a day: At the time of the dinosaurs, each day was ~23 hours long, and currently the day is lengthening by 2 milliseconds per century.

Mars

The picture below is a mosaic assembled from over 100 images of Mars taken by the Viking Orbiters. It shows the Valle Marineris, two large volcanos, streambeds, and craters.

Mars’ rotation rate (1⁄24.6 hours) and axial tilt (25.19 deg) give it a day and seasonal temparature variations. However, Mars is significantly smaller and so has weaker gravity (40% of Earth’s) and a much thinner atmosphere (less than 1% Earth’s surface pressure), with a predominantly carbon dioxide (3% nitrogen) composition which offers little evidence of current life.

The surface of Mars consists of lots of red-colored dust covering rocks (often a grey-greenish color), plus small polar ice caps of frozen CO_2 and H_2O . The red color is from oxidized iron-rich minerals (i.e. rust). Some regions are heavily cratered - these are more ancient surfaces that have not changed in billions of years, while other lightly-cratered regions have been renewed more recently by lava (volcanism) or water erosion. The region of the planet imaged by the Viking probe (image in center, and exaggerated model on right) exhibits four huge extinct volcanos and a huge canyon system. Olympus Mons : largest volcano in the solar system (size of PA and 25 km high). Valles Marineris is the deepest and longest canyon system in the Solar System, as big as the U.S. It may have resulted from the separation of two tectonic plates long ago when the interior of the planet was still molten.

Mars is now mostly cooled to a solid, but had a molten interior for much of its history. For some period (up to ~2 billion years ago?), active volcanism brought molten rock up from the interior. Most remarkably, small features around certain canyons and craters suggest that liquid water once flowed on Mars. Possibly due to a greenhouse effect and global warming from gases ejected by volcanos. Today nearly all the water on the surface is frozen. Only very recently have scientists found evidence for liquid water briefly being exposed on the surface of Mars (before sublimating or freezing). Carl Sagan and others suggest that life may have formed on Mars during its wet period. Following are some of the remarkable photos taken by Mars orbiters during the past decades showing the complex surface of the planet.

Several robots roved Mars during the 2000s, and the “Curiosity” mission is still  active after 9 years. They all found remarkable small-scale evidence for past lakes and flowing water on the Martian surface. On the left is a 3cm x 3cm closeup of small “blueberry”-like rocks that must have formed under water. On the right is an image released from NASA’s Curiosity mission showing sedimentary clasts, rock formed under water.

Browse some of the remarkable pictures taken from orbit around Mars using the European Space Agency’s Mars Express interactive image browser or images of NASA’s Mars Global Surveyor . The Mars Science Laboratory, nicknamed Curiosity, was launched in November 2011 and landed on Mars on August 6 2012. There are lots of fascinating photos here . More NASA science findings about Mars appear here .

The interior of Mars is mostly solid, unlike Earth’s interior that is molten. This is because Mars is smaller and less massive (about 1⁄10 the mass of Earth) and hence has cooled faster since its formation 4.5 billion years ago. (Mercury and the Moon are even smaller and have entirely cooled.) Earth’s surface is very young, constantly remade by erosion (rain, rivers), volcanic lava flows, and plate tectonics. Mars’s surface is partly old with many impact craters, and partly newer with lava flows and water erosion. The surfaces of planets thus reflect conditions in the planetary interiors.

The Jovian planets - Jupiter, Saturn, Uranus and Neptune - probably have cores (suspected to be liquid metal for Jupiter and Saturn, but primarily ice for Uranus and Neptune), but are “mostly atmospheres,” comprised of dense gas. Below are photographs of the planets by robotic probes, and an artist’s diagram of their interiors. The Jovian interiors are composed primarily of H_2 and and He, with important amounts of methane ( CH_4 ), water ( H_2O ), ammonia ( NH_3 ) and related molecules. The Jovian planets thus reflect better the original material than the terrestrial planets where most of the gases have escaped.

Given the extremely high pressures at the interface with any surface, it seems unlikely that the Jovian planets harbor living organisms. We will therefore review them briefly and concentrate instead on their moons which seem more likely sites for life.

Jupiter has more mass than all of the other planets put together, and is only about 1⁄10 the mass it would need to initiate deuterium fusion reactions in its core (the “easiest” type of fusion for a star). Thanks to its mass and 10-year orbit, Jupiter would likely be the first planet discovered by an alien civilization searching for planets around our Sun using techniques like those we use today (as we will discuss in the final unit of the course). Jupiter shines more brightly in the night sky than any star, and is outshined only by the Moon and Venus. Its strong magnetic field causes occasional bright radio flares brighter than any other radio phenomenon in the Solar System apart from the Sun. Its atmosphere exhibits bands caused by high-altitude winds and gargantuan hurricane-like stoms, including the famous “Red Spot” storm, which is roughly the size of ten Earths and has persisted in its atmosphere for more than three centuries. Jupiter has four large moons, discovered by Galileo, that rival Mercury in size and some of which may offer suitable habitats for life, as we discuss below and will explore further in the weeks ahead.

Saturn is a less massive, more distant version of Jupiter exhibiting similar visual colors, including banded high-velocity clouds and storms, and having a similar atmospheric composition. Its appearance in telescopes is the most dramatic of all the planets, thanks to its giant ring system, likely the disrupted remains of a moon that wandered too close to the planet in the relatively recent past (less than 1 Ga ago) and was torn apart by Saturn’s tidal forces. Saturn has a single massive moon, Titan, roughly the size of Mercury, the only planetary satellite in the Solar System to have a substantial atmosphere and weather. Titan is another possible habitat for life which we will explore in more detail in the weeks ahead. Saturn, easily observed with the naked eye, is the most distant (slowest moving) of the planets known to ancient astronomers.

Uranus and Neptune have been called the “ice giants” because they exist in the cold outer reaches of the Solar System and exhibit atmospheres with significant ice content and a different mix of gases than Jupiter and Saturn, giving them their blue-green (or if you prefer, cyan) appearance in visual-light images. Both are too faint to be seen with the naked eye (with ocassional exceptions for Uranus), and were not discovered until after telescopes were in widespread astronomical use. Uranus is noted for the unusual 97 degree tilt of its rotational axis, which means it effectively rolls around the Solar System on its side instead of spinning like a top as the other planets do (Venus can be considered to be upside down, and spins slowly, which is also unusual). Neptune has a single massive moon, Triton, which orbits in the “wrong” (retrograde) direction and so is thought to be a captured Kuiper Belt object. Triton’s properties, including its very tenuous nitrogen atmosphere, are very similar to those of Pluto and other massive Kuiper Belt objects, and with current understanding it is too cold and too far from the Sun to possibly support life.

Our current picture of the interiors of these gas giants is shown in the figure below.

Moons of Jupiter

Jupiter has four large inner moons discovered by Galileo: Io, Europa, Ganymede, and Callisto. The pictures here are from the Galileo robot probe launched by NASA that orbited Jupiter and photographed its moons from 1995-2003.

Io is closest to Jupiter and, subjected to enormous tidal forces of the giant planet, should have its spin locked to its orbital period like our Moon’s spin is locked to its orbit, with the same side always facing its planet. But the pull of the next moon out, Europa, disturbs Io’s orbit, with the result that Io is constantly being “massaged” by Jupiter’s tidal forces, which heat and melt its icy and rocky interior. As a result, Io is the most volcanically active body in the Solar System (below). With many constantly erupting volcanos, its surface covered with recent lava flows, and its complete lack of any atmosphere, Io’s surface and interior are probably too harsh and unstable for life.

Europa (below, left) has an icy surface with long dark cracks and no craters. These surfaces show evidence of having been repeatedly melted and refrozen. Models of this phenomenon suggest there is a large ocean beneath a thick icy crust that may contain more liquid water than exists in all the oceans of Earth. As with Io, tidal forces of Jupiter likely provide sufficient heat to keep this water from freezing. The evidence for existence of these oceans, their likely properties, and their potential for harboring life will be a focus for us in the weeks ahead.

Ganymede (below, right) and Callisto exhibit relatively young surfaces that, again, suggest the presence of large quantities of liquid water - or, at the very least, “slush” - below the surface of both moons, again because of Jupiter’s tidal influence, combined with the tidal forces of the other inner moons. We will discuss prospects for life on these moons as well.

Moons of Saturn

Saturn’s moons, discovered by Christian Huygens in the 1600s, are now being investigated by the Cassini probe which arrived there in 2005. There are dozens of moons, most with ancient icy surfaces covered in craters. Some have large cracks suggesting heating and expansion from large collisions. Saturn’s largest moon is Titan, the only moon with a substantial atmosphere and observed weather patterns. The surface features are invisible due to clouds (below, left), and models suggest that Titan may have liquid N_2 (nitrogen), CH_4 (methane), or C_2H_6 (ethane) oceans.

Cassini dropped a research probe named Huygens (built by the European Space Agency) onto Titan in 2005. It took some remarkable photos during its descent (below, center) and after landing on the surface (below, right) - the only visible light photos we have of the surface of this alien world. The last photo shows stream-beds kilometers in length and surface material with a mix of light and dark colors. It is much too cold for liquid water; the liquid seen flowing on Titan’s surface (center) is likely to be methane or ethane.

Saturn’s moon Enceladus is another example of an icy moon like the Galilean satellites of Jupiter. The existence of liquid water under its surface has been confirmed by the Cassini mission’s observations of jets of water vapor erupting from its surface. We will discuss Enceladus along with Europa, Ganymede, and Callisto in the context of habitability.

Moons of Uranus and Neptune

Uranus and Neptune have a number of prominent moons, most with ancient, heavily cratered icy surfaces. Uranus’ moon Miranda ** (below, left) is remarkable: it looks like different terrains have been pieced together. Giant cliff separates two regions. Perhaps Miranda was shattered into pieces by collision with an asteroid/planetisimal, and reformed itself. Triton (below, right) is the largest of Neptune’s moons and has very strange charateristics: it orbits in the wrong direction, and its surface has two different terrains (smooth melted material, and rough colored material). The moon is extremely cold and the surface is mainly nitrogen ice ( N_2 , bluish) on one side and methane ice ( CH_4 , pinkish) on the other. It appears to have (active?) volcanos (erupting nitrogen? water? soot?). Triton is the only large moon with a retrograde orbit, so it may have melted during its capture by Neptune long ago. But the surface has very few craters, so it must have been altered many times since the original capture.