Of all of the planets, Mars has always been the one place in the solar system where scientists speculate we might find life. Mars is much smaller than the Earth, having an equatorial radius of 3,397 km (about 1/2 that of Earth):
Note the the dramatic difference in season length. This is due to the fact that Mars' orbit is much more eccentric than Earth. Remember that the quantity "eccentricity" measures how out-of-round an orbit is. For the Earth, e = 0.017, almost circular. For Mars e = 0.093, five times larger. This results in interesting effects. The winters in the north are warm and short, as Mars is near perihelion. Obviously, this means that the winters in the South are long and cold. The opposite is true for the summers. During the Southern winters, the atmospheric pressure can drop by 30% as the carbon dioxide freezes-out onto the polar cap. Even though it is much smaller than the Earth, because of our oceans, the total land surface area of Mars is just about the same. Mars has many familiar features that make it almost "feel like home".
Mars has a dramatic topography, with low parts (blue) in the north, higher regions (reddish) in the south, and a large volcanic plateau (Tharsis):
Mars has numerous impact craters that allow us to date the various types of terrain (though some show evidence for subsurface ice/mud--splat!):
Mars has a very tenuous atmosphere, composed mostly of carbon dioxide:
Martian air contains only about 1/1,000 as much water as our air, but even this small amount can condense out, forming clouds that ride high in the atmosphere or swirl around the slopes of towering volcanoes. Local patches of early morning fog can form in valleys. At the Viking Lander 2 site, a thin layer of water frost covered the ground each winter.1
Snow and Ice
The atmosphere is thick enough to allow snow and frost, but not liquid water (it might just be possible that at high noon, in mid-summer, water could exist in the deepest parts of the Hellas basin). There is certainly abundant ice on Mars.
The average temperature on Mars is -63 C (-81o F) with a maximum temperature of 20 C (68o F) and a minimum of -140 C (-220o F).
Barometric pressure varies at each landing site on a semiannual basis. Carbon dioxide, the major constituent of the atmosphere, freezes out to form an immense polar cap, alternately at each pole. The carbon dioxide forms a great cover of snow and then evaporates again with the coming of spring in each hemisphere. When the southern cap was largest, the mean daily pressure observed by Viking Lander 1 was as low as 6.8 millibars; at other times of the year it was as high as 9.0 millibars. The pressures at the Viking Lander 2 site were 7.3 and 10.8 millibars. In comparison, the average pressure of the Earth is 1000 millibars.2
There is certainly water on Mars, some of it locked into the polar caps (though much of what we see in the Martian polar caps is actually frozen carbon dioxide--"dry ice"):
During the darkness of winter, carbon dioxide freezes out on these poles, and they quickly grow in size and depth. The total amount of ice in the polar caps of Mars is similar in mass to the Greenland ice sheet. As soon as "spring" arrives, this carbon dioxide quickly "sublimates", and strong winds (up to 400 km/hr) blow off of the poles. Glacial activity:
There is considerable evidence that just below the Martian surface, there are considerable amounts of frozen water (go here for a recent press release). Here is a picture of a trench dug in the polar region (Mars Phoenix lander):
This, combined with the abundant evidence for water at earlier times in the history of Mars, suggests that it might have been possible for life to have evolved on Mars. We discuss this shortly.
Water on Mars:
There are a large number of features on Mars which indicate that there was flowing water in its past:
Dust and Sand
Here is an image of a dust storm rolling across a small region of Mars:
In general, the darker surface markings are regions that have little dust, or where the dust has been blown away. The reddish regions are those covered in dust.
Volcanoes and Lava Flows
As we have just seen, Mars has a surface that spans features common to the Moon and Earth. For example, like the Moon, there are many impact craters on Mars. But Mars has some of the forces of erosion like those seen on Earth (wind, freezing-thawing cycles), so these features are slowly eroded away. In addition, in the distant past Mars had volcanoes. In fact, the largest volcano in the solar system is on Mars, Olympus Mons:
Olympus Mons (it even has its own website!) has a diameter of more than 500 km, and towers to 25 km above the surrounding terrain (note that the Martian volcano Alba Mons covers a larger area--2,000 km in diameter, but is not as high: 6.8 km). It is a lot like the Hawaiian volcanoes, but Olympus Mons is much larger. For example, the "Big Island" (Hawaii) is only about 120 km across, and only rises 9 km (from the ocean floor). The lower gravity on Mars (due to its small mass) allows the volcanoes to grow beyond what is possible on the Earth:
Olympus Mons is part a complex of volcanoes located in the "Tharis bulge". It is an enormous volcanic plateau. You can explore this region using Google Mars. The Tharis region is believed to have formed much like the Hawaiian volcanoes--but instead of forming a chain due to the movement of the Pacific plate, the lava kept pouring out of the same place. It is so massive, it actually means that Mars is "lopsided" in the sense of mass or gravitational pull. Mars has two distinctive types of topography---southern highlands, with numerous impact craters, and a smooth, northern region covered by lava flows. It has been suggested that the northern hemisphere depression is actually a huge impact crater (or several such features). It has also been suggested that the Martian crust is much thinner here, and that an epoch of plate tectonics resurfaced the region, creating the large, smooth plain. Recent studies indicate that there are large impact features below the lava plains, suggesting that the great depression is at least as old as the southern highlands. These northern plains are lower than the highlands by a few kilometers. To the east of Tharsis is the huge canyon known as "Valles Marineris". It is 4,000 km in length, 200 km wide, and 7 km deep. It is the largest canyon system in the solar system. It is believed to be a rift valley (not unlike the Rio Grande rift), a region of broken crust due to the mass of the Tharsis bulge cracking the crustal surface (go here for more). Here is the scale of this feature:
As we found out when discussing the lunar surface, the presence of many craters can be used to estimate ages, but the erosion, and large scale lava eruption events that have occurred on Mars can make life difficult. Here is the most recent calibration of number of craters vs. age for Mars:
Note that in the plot above, there is evidence for fairly recent volcanic activity ("I", about 100,000 years ago). This is the age of the youngest features seen on Olympus Mons. Most of the Tharis bulge volcanoes seem to have ages that date back to between 1 and 3.5 billion years ago. So these volcanoes stayed active for quite some time.
Because its atmosphere is so thin, an efficient greenhouse effect does not operate on Mars. There may have been periods in the past when Mars was warmer where much of the carbon dioxide locked into the poles (and elsewhere) was released, making it possible for this to happen. This may happen again in the future, but the exact mechanisms driving these long term climatic cycles have not yet been identified. Many people, however, are now proposing the idea of turning Mars into a warmer, wetter place. This is the concept of "terraforming". Through various means, much of the carbon dioxide locked away in the crust/poles is released, creating a thicker atmosphere with a more efficient greenhouse effect. The planet would thus warm up, and the atmosphere would become even denser. Eventually, it should support liquid water. Here is an academic thesis on terraforming Mars (showing how seriously some people take this), and here is a down-to-Earth description of how it might work.
Here is the temperature profile of Mars' atmosphere (remember, 1 Bar is the Earth's mean sea level air pressure):
Mars has a crust, mantle and core similar to the other terrestrial planets:
Like Venus, the magnetic field of Mars appears to be extremely weak, and thus the atmosphere and surface are not buffered from the effects of the solar wind. Many believe that the thin atmosphere on Mars is the result of erosion by the solar wind.
There is evidence, however, that Mars once had a magnetic field, and that it also exibited "flips": There are regions of alternating magnetic polarity frozen-in to the crust ("paleomagnetism"). When we talked about ice ages on Earth, we noted that changes in the tilt of the spin axis of Earth, and how its orbit might change over time, could be the culprits for enhancing ice ages. The Earth is much more stable than Mars, and we find that Mars' spin axis has had huge changes in the past which probably caused enormous glaciation events:
It is believed that the Earth's moon helps keep our spin axis stable. Mars doesn't have a big moon to stabilize it. Given the large eccentricity of the Martian orbit, and the fact that the orbit precesses, and this eccentricity can also change, means that Martian climatic excursions are much larger than seen on Earth.
For Mars, Earth is an "inferior planet, and is only visible in the east before sunrise, or in the west following sunset, here is Earth as a morning star from Mars:
To the naked eye, it would be easy to see the Moon going around the Earth from Mars, but you'd need a telescope to see the phases:
The Moons of Mars
Mars has two tiny moons, Phobos and Deimos (11 and 6 km in radius, respectively). These moons are so small that gravity cannot force them to be spherical, and they look like potatoes, here is Deimos:
An extreme close-up of the surface of Deimos shows that it is covered with a thick layer of dust, burying many of the craters (the smallest features in this photo are about the size of a car):
Even though the moons of Mars are very small, they are also very close to Mars. Phobos orbits at a distance of 9,377 km from the center of Mars. It is almost skimming over the surface of the planet. The orbital period of Phobos, 7 hr 39 min, is shorter than the Martian day, so Phobos would rise in the west, take about 4 hrs to cross the sky, and then set in the East! It would rise in the west again, about 11 hrs later. Deimos, on the other hand, has an orbital period (30 hr) that is very similar to the rotation period of Mars. This results in Deimos keeping up with the rotation, and thus it is visible (above the horizon) for almost three (2.7) straight days! Then it disappears for an equal amount of time. Here is a time lapse (an image every ~ 3 minutes) showing their motion across the Martian sky:
While Phobos is only about 11 km in radius, it is so close to Mars, it almost is as bright as the Earth's moon (when full). Remember, the Earth's moon has a radius of 1738 km, 150X that of Phobos, but it orbits at 384,000 km from the Earth! There are even "eclipses": Phobos Transit
While the two moons of Mars are hard to see, Mars itself is one of the brightest planets in the sky (when it is close to Earth). Because Mars has a similar orbit to Earth, it is a long time between closest approaches. Right now, Mars sets in the west right after sunset, and will soon not be visible from Earth. It will next become visible in Fall 2013, but will not reach "opposition" until April, 2014 when it will be 0.62 AU from Earth.
Some Mars rover images:
Mars also has something we are familiar with in NM: dustdevils. Here is a Dustdevil movie (#1), Dustdevil movie (#2), link.
Comparison of the recent rovers to the first one sent to Mars:
For more on the Mars rovers, go here. Here are views of the rovers and other probes from orbit!
Life on Mars?
As we mentioned above, Mars was always thought to be a possible host of life. Part of this is historical, but we have also begun to realize how adaptable--and tough---life actually is, and even in a hostile place like Mars (dry and very cold) there may be a small chance of finding life. First the historical:
Percival Lowell really got the ball rolling on Mars as having life. Lowell was born into a wealthy New England family. As a kid, he was given a small telescope that he set up in his home, and began to observe the heavens. After his college days and some traveling, Lowell returned to the US in 1877, the year of Schiaparelli's discovery of the "canali" (Italian for "channels") on Mars. After 15 years dabbling in far-eastern culture, Lowell returned to his interest in Mars. After becoming energized by the gift of a book about the planet Mars, he began corresponding and talking to astronomers. This lead to an expedition to observe Mars where he founded a new observatory near Flagstaff, Arizona in 1894 ("Lowell Observatory", and it is still in operation). He quickly began his observations of Mars. He was now able to see for himself the canali described by Schiaparelli--but he now permuted that into "canals"(!), and found many more:
When astronomers demonstrated that the darker regions on Mars were not due to oceans, speculation grew as to their nature. Lowell found that these canals, and the "oases" where two canals intersected, seemed to get darker with the coming of the Martian summer. This led him to propose that that the canals were due to intelligent beings that constructed them to channel water from the Martian polar region (where there was clearly ice), to the arid and warmer equatorial regions:
"Speculation has been singularly fruitful as to what these markings on our next to nearest neighbor in space may mean. Each astronomer holds a different pet theory on the subject, and pooh-poohs those of all the others. Nevertheless, the most self-evident explanation from the markings themselves is probably the true one; namely, that in them we are looking upon the result of the work of some sort of intelligent beings. . . . [T]he amazing blue network on Mars hints that one planet besides our own is actually inhabited now."5
Lowell subsequently published a book entitled "Mars" where all of these ideas were laid out, with illustrations to back up his discoveries. We now know that what Lowell was seeing was probably an optical illusion due to a "connecting of the dots" between craters just below his visibility level. Regardless of its real value, Lowell's work certainly made him famous, and surely influenced H. G. Wells who wrote the infamous "War of the Worlds".
If you go back up to the first whole-disk image of Mars, you can see that there are no linear dark features on the surface of Mars, so we can clearly put these ideas to bed. As we have noted above, Mars has very little water, and is very cold---but Mars does have large amounts of water frozen beneath its surface. It is quite possible that early in its history, Mars had significant bodies of water, or even oceans. In fact, the northern hemisphere of Mars is much lower than the southern, as shown in this topgraphic map:
It is possible that this was once the site of a large ocean, and that this water remains frozen beneath the surface. It is clear (as the images above have shown) that Mars certainly had flowing water at somtime in its history. The rovers have also found evidence for standing basins of water. The question is to whether life had sufficient time to evolve on Mars before it became so cold and dry---perhaps there is life just below the surface. Many of the ideas about present life on Mars are summarized in this document. Strangely, sources of methane have been recently detected on Mars, and some researchers think the only possible source for this methane is life.
Methane map of Mars
To examine the possibilty of life on current Mars, we must return back to Earth. In the regions of the Earth which you and I are familiar, life abounds. But life can stand much tougher environments. These extremely hardy types of life are often referred to as "extremophiles". The native flora and fauna of New Mexico are incredibly hardy---they live in a harsh environment compared to most of the rest of North America: Very little precipitation with hot summers, and relatively cold winters. These plants and animals have evolved to the point where they have little trouble living here. This is the power of evolution. But there are life forms that are even more amazing in their adaptability. These lifeforms exist where there is little free water, extremely high or low temperatures, in highly alkaline or acidic water, with no sunlight and sometimes in environments where there is very little oxygen--the true extremophiles.
The most amazing of these extremophiles are comprised of different types of microorganisms. For example, one family of "archaeal" microorganisms is able to survive and prosper in boiling water! At the opposite extreme are bacteria that have been found growing in the ice in frozen lakes in Antarctica. There are a number of extremophiles that might have played a role in the earliest history of life on Earth. One of these types of bacteria, the "iron eaters", might have been one of the first forms of life on the planet--they truly appear to be able to use iron to metabolize their food in the absence of oxygen. It now appears that these type of bacteria survive throughout the crust of the Earth, and have been found in rock that is two miles below the Earth's surface! Some researchers suggest that these microscopic creatures comprise nearly all of the biotic mass of the Earth.
Other researchers suggests that life on Earth may have begun near "black smokers ", thermal vents deep in the ocean:
Near these vents, whole ecosystems have developed based on primitive bacteria that eat the abundant sulfur that is spewed out of these vents. Other plants and animals live on the bacteria, and these in-turn are eaten by larger creatures such as the giant tube worms:
Other species such as shrimps, lobsters, crabs, mussels, and clams also live here. The reason these are such important findings is that the early Earth contained little food, or free oxygen. So when life arose, it had to find other sources of food such as iron and sulfur.
Perhaps the most striking evidence for how robust microscopic life can be was demonstrated in 1969. Before the manned Apollo missions to the Moon, NASA launched several unmanned missions that successfully landed on the Moon. Surveyor 3 landed on the Moon in April of 1967. In November of 1969, the astronauts Pete Conrad and Alan Bean landed the lunar module of Apollo 12 within 500 ft of the Surveyor 3 landing site:
As part of this mission, they recovered the camera of Surveyor 3 and brought it back to Earth (taking care to be sterile). When the camera lens was analyzed in the lab, they found that 50 to 100 microorganisms had survived! This was after nearly three years in a complete vacuum, high exposure to radiation, and incredibly cold temperatures with no water and no food. Clearly, life is not as fragile as one might first suspect.
In 1996, a stunning announcement was made by a group of NASA researchers: they had found fossil evidence for life inside a Martian meteorite--that is a rock from the surface of Mars that had found its way to Earth:
First, how do we know its from Mars? Analysis of the gases within the meteor show they are identical with the composition of the Martian atmosphere as revealed by the two US Viking probes that landed there. The chemical composition of the rock itself is also consistent with what we know about the Martian surface (for more on these aspects of martian meteorites go here, or here. This piece of Martian crust appears to have been traveling for about 16 million years in space on its eventual voyage to the Earth, landing in Antartica about 13,000 years ago.
When the researchers looked deep inside the rock, they found peculiar chemical structures that not only looked like fossilized forms of earthly bacteria, but also had chemical compositions consistent with these types of bacteria:
Obviously, it remains difficult to completely prove that these structures are due to fossilized microscopic life, and the debate about their true nature lives on. It will take future missions to Mars to retrieve rock samples that have led a little less eventful life--more sedentary sedimentary rocks--not one that had to be blasted off of Mars to get here in the first place. Clearly, however, we have found that there are forms of life on Earth that live inside rock, and that finding these types of structures in rocks from elsewhere in the solar system would no longer be the least bit surprising. We will again return to these ideas next week when we talk about life in the moons that orbit Jupiter and Saturn.
5From The Planet Mars: A History of Observation and Discovery by William Sheehan, Chapter 7: Lowell