Are we alone? If not, where are "they"?

In our solar system?

In other stellar systems?

Here we will begin investigating the possibility of life existing in our own solar system. It is not only easier to look for sings of life (present and past) locally, but what we learn about it will help direct our searches elsewhere.

Because Venus and Mars are the nearest planetary neighbors to the Earth, and both share some characteristics with it, it is natural to look at these two planets first.

Venus is a planet whose size and mass are similar to that of the Earth. Being covered with clouds, early speculations were that it was a warm, water-covered world. On the Earth, such a warmer supposedly wet period coincided with the age of the carboniferous forests and amphibious animal life, eventually leading to the rise of the dinosaurs. While some envisioned a world of prehistoric creatures, all the evidence really said was: "we can't see anything!"

With the discovery that the atmosphere was composed mostly of CO₂, it was thought that perhaps the oceans were made of carbonated water (the seltzer-sea model).

In the 1950's, the serious investigation of Venus really began. Observations of the energy emitted by the planet at radio wavelengths (which easily penetrate the clouds) indicated that the surface temperature of Venus was close to 750 K (almost 900 F). This has been confirmed with space probes that have visited there. The surface pressure is 90 times that of the Earth as well. The atmospheric composition is 96.5% CO₂, and almost no H₂O at all. The clouds themselves are composed mostly of droplets of concentrated sulfuric acid (H₂SO₄).

The structure of the cloud layers in the atmosphere of Venus

This is not a very hospitable place at all!

Why is it so hot?

Venus suffers from an extreme greenhouse effect. It causes it to be hotter than Mercury, even though it is farther from the Sun and more reflective. The details of how the process operates on Venus was first worked out by Carl Sagan, and refined by his student Jim Pollack, whose specialty was the travel and interaction of light in atmospheres (what astronomers usually call radiative transfer).

We can understand the process if we look at it in steps.

First, radiation from the Sun reaches the top of the planet's atmosphere, where some of the radiation may be reflected back outward into space by clouds and scattering by individual molecules and other particles. The fraction of the light that is reflected is called the albedo.

Most of the light that is not reflected hits the surface of the planet, because the atmospheric gases are generally transparent to the sunlight, which is predominantly visible radiation. At the surface it is either reflected or absorbed.

The absorbed radiation warms the surface, which in turn radiates energy like a blackbody, and is governed by Wien's Law and the Stefan-Boltzmann Law. But the surface of the planet is much cooler than the surface of the Sun, and radiated mostly at infrared wavelengths (Wien's Law).

However, the atmospheric gases, while generally transparent to visible light, are very opaque to the infrared light, so that the radiation cannot leave the planet.

Since there is more energy being absorbed by the planet than it is emitting, the temperature rises. The planet is not in thermal equilibrium.

Eventually, the emitted spectrum shifts toward shorter wavelengths where there are a few transparent "windows" for the energy to leak out, and equilibrium is established. Bit it is only done at the "expense" of a higher value of T.

The Earth also has a greenhouse effect, but it is mild, only raising the temperature by a few tens of degrees, not hundreds as on Venus.

[Please do not confuse the Greenhouse Effect with the ozone layer. TOTALLY DIFFERENT THINGS! Ozone is a minor greenhouse gas. Most of the greenhouse effect on the Earth comes from water vapor, and the majority of the rest from carbon dioxide.]

In the case of Venus, we refer to a runaway greenhouse effect to describe the probable history of the process. Venus began with a very thick CO₂ atmosphere, providing initial GE heating. If Venus possessed any water in the past, the heat would have begun to evaporate it. Water vapor is a greenhouse gas, so adding it to the atmosphere would increase the GE, causing more evaporation, raising the GE even more, etc. A runaway process ensues.

Most of the water vapor is subsequently broken apart (dissociated) by solar ultraviolet light into H and D (deuterium - heavy hydrogen - ²H) and OH (and OD). H and D are light enough to escape from the planet. Since D is heavier of the two, its rate of escape is less, and analysis of the ratio D/H allows planetary scientists to make an educated guess as to the amount of water Venus once had.

Why is Venus so different from the Earth? Why isn't the Earth suffering a runaway GE?

The answer seems to be that the Earth was cool enough to have liquid water that did not immediately evaporate. On the Earth, some of the CO₂ dissolved in the oceans, while the rest of it was removed by a weathering process called the Urey Reaction, after Harold Urey, a Nobel Prize-winning chemist with an interest in planetary science. The reaction can be summed up as follows:

Today this process is also accomplished by living organisms who make carbonate shells.

The CO₂ on the Earth is currently locked up in its rocks. If the outer layers of the Earth were to become molten, enough CO₂ would be released to increase the surface pressure to about 30 Atm and we would suffer a runaway GE just as Venus did!

The Earth does recycle some CO₂, as crustal plate subduction leading to melting and release of CO₂ in volcanic eruptions. Luckily, the amount is small enough that the Urey Reaction can take it care of it.

The D/H ratio of the atmosphere suggests that there may have been a lot of water on Venus in the past, at least in the form of vapor - a Wet Greenhouse. The models do not tell us for sure if any of it was in the liquid form (possible at high temperatures if the pressure was high enough).

The surface has been mapped with imaging radar techniques. We see impact craters, volcanoes, lava domes, "coronae", and what may be lava channels. There is no unambiguous evidence for liquid water in the past from these data, nor from surface lander probes.

It doesn't look good for life ever having existed on Venus. We should look elsewhere.

With a period of rotation of 24 1/2 hours, an axial tilt of 23 1/2 degrees, and visible polar caps, Mars shares many characteristics with Earth. The polar caps and surface markings exhibit seasonal changes.

Throughout the centuries since the development of the astronomical telescope, observations of Mars became more and more detailed in nature. By the late 1800's Schiaparelli recorded numerous strait-line markings he called "canali", Italian for "channels". However, many individuals took this term to mean "canals" which are of artificial origin. Among these was Percival Lowell, who built his own observatory (he was a member of the wealthy Lowell family of Massachusetts) just outside Flagstaff Arizona. He found canals everywhere, and developed a hypothesis of the "dying Mars". Here, the atmosphere was leaking away and the planet was drying out. The intelligent Martians had obviously build a world-girdling network of canals to carry water from the polar caps to irrigate the desiccated regions so that the crops would not fail.

Belief in life on mars was so prevalent at this time that a prize was offered by the Paris Academy of Sciences to anyone who first succeeded in establishing communication with a planet other than Mars. Apparently, Mars was considered too easy!

However, not everyone saw canals. Furthermore, it was pointed out that liquid water could not exist under the conditions believed to exist on Mars.

So what do we know?

Let's look at the planet, working from the outside to the surface.

Moons of Mars

Mars has 2 moons, both of which have been photographed by spacecraft. They are Phobos ("Fear") 27x21x19 km in size, and Deimos ("Panic") 15x12x11 km in size. Their albedos are about 0.02 - almost totally black! Both appear just like the pictures we have of asteroids, and are believed to be asteroids captured by the gravity of Mars.

The atmosphere of Mars is 95% CO₂, 3%N₂, 0.01 mm of precipitable H₂O vapor. The surface pressure is 1/150 Atm.

Clouds - CO₂ ice dominates at higher altitudes and latitudes, while H₂O ice is present at lower altitude and latitudes. Occasionally dust storms are visible. A few grow to be so large that they cover the entire planet!

The thermal structure of the atmosphere of Mars

Liquid water is only marginally possible, briefly, at low altitudes, where the atmospheric pressure is high.

Imagine a closed container, half-filled with water. All molecules above T=0 K are in motion. The natural jiggling motion of the molecules allows some to break free of the water - the process of evaporation (which also carries the heat with it). Eventually, there are so many molecules that as many as leave the surface hit it again and stick, going back into the liquid. Equilibrium is established.

All the water is in the liquid form	Water is evaporating faster than it recondenses	Equilibrium - the rate of evaporation and consensation are equal

Now, remove the lid, and the molecules are free to escape, never to find their way back. Eventually all the water will evaporate. This is what happens when the pressure is low.

[Pressure cookers use this principle to maintain water in the liquid form at a temperature much higher than the boiling point of water - that's why they can cook so fast!]

The same would happen on Mars.

Under some conditions, even ice will sublime (evaporate directly from the solid to the gas phase). This is why an ice tray left in the freezer will be empty if left alone for months.

The phase (solid, liquid, or gas) that water or any other substance will be in can be illustrated with a phase diagram. Here, the regions occupied by various phases are shown as functions of temperature and pressure. Given the surface temperatures and pressures on Mars, we see that liquid water can just barely exist where T and P are greatest.

We see a number of different topological features on the surface of the planet:

1. Craters

Many are flat-bottomed

They are often eroded

They are found mostly in the Southern Hemisphere



Craters on mars

2. Volcanoes

Mostly in the Northern Hemisphere

Mostly in the Tharsis region

They are Shield volcanoes, similar to those that make up the Hawaiian Islands



Olympus Mons

The largest is Olympus Mons, 700 km across and 25 km high.

3. Valleys

Small sinuous runoff channels

Longer wider outflow channels

Canyons or Rift Valleys - Vallis Marineris is 4000 km long, about 200 km wide, and 7 km deep in places. These seem to be due largely to crustal faulting.

Sinuous runoff channels

A portion of Vallis Marineris

The first two types have downhill flow patterns. Then first one has meandering tributary structures. Sandbar and other flow structures are present. The Mars Pathfinder images from the surface shows further evidence that liquid water once flowed on Mars. We do not see it today. Where is it?

We know that the North Polar Cap, which never disappears during the summer, is made largely of water ice. (The South Polar Cap may have some, but not as much as the NPC). Furthermore, water ice can be frozen into the ground as subsurface permafrost at latitudes away from the equator.

To learn and see more, visit the at JPL Mars Missions Site and the SEDS (Students for the Exploration and Development of Space) Mars site.

Mars appears to have suffered from a reverse runaway GE. Cooling the planet removes by freezing almost all the water vapor from the atmosphere, which reduces the GE, which causes yet more water to be removed, cooling it further, etc. Should the planet warm up to the point where liquid water could exist, it causes the Urey Reaction to remove CO₂, cooling the planet back down. The planet has its thermostat stuck on "frozen". Because plate subduction does not seem to be operating on Mars, the lost CO₂ is not recycled fast enough to warm the planet.

The appearance of most of the water flow structures is consistent with being caused by short-duration catastrophic flooding (fed by impacts and internal thermal events) plus slow sapping of groundwater. However, a couple of recently-discovered features look more like river-cut channels of long duration, suggesting that water might have been stable for long periods of time. This requires that Mars was wetter and warmer for a significant period of its history, which is significant for the possibility of life on Mars in the past. Stay tuned as more results come in from the current and future Mars missions!

Most of the surface is covered with sandy and rocky (gassy basaltic) material. This surface regolith (Greek for "blanket of stone") contains iron-rich clays and iron oxides (rust). In the winter, CO₂ frost is often present.

The planet has an overall low density and weak magnetic field, indicating it lacks an iron core as significant as the Earth. What iron it has seems to be mixed throughout (that's why the surface is iron-rich). Since only one working seismometer has landed on the planet (and 2 are needed to pinpoint marsquakes), this technique has not told us anything significant about the interior structure.

Formation by accretion of planetesimals

Partial differentiation of iron and rock, placing some iron in the core, but leaving a lot near the surface

Lithosphere solidifies. Aesthenosphere grows but is never substantial. May be gone today. No plate tectonics.

Crust fractures as internal heat expands the interior 5-10 km

H₂O frozen or lost, followed by runaway reverse GE.

During this time, the proximity of Mars to the Asteroid Belt would have resulted in (perhaps) significant erosion of its atmosphere through impacts.

It is possible that Mars went through a period when it was much warmer and wetter than today. Was there enough water for Mars to have had lakes and oceans? Possibly, but the evidence is rather minimal at the present time. However, some features that look like dried lakebeds are seen.

Furthermore, recent analysis of a meteorite from Mars contains a collection of elements suggeting that Mars had oceans whose chemistry was similar to that of the Earth's. The Nahkla meteorite, recovered in Egypt in 1911, has been found to contain sodium, chlorine, magnesium, and calcium. These may have been left in the rock when salt water leaked into it, and then evaporated.

In June 2000, scientists working with the Mars Global Surveyor anounced that they had found evidence of mass wasting on the sides of some craters that suggested the action of liquid water more recently than a million years, and possibly only a few years. Liquid water should exist more easily at low elevations near the equatorial regions, but the outflows occur only 30 degrees from the equator, and at high elevations.

Gullies at 70 Degrees South Latitute in Polar Pit Walls

These temperate and polar latitudes are precisely the ones that show evidence for subsurface water in the past, being where the craters with lobate ejecta are found, and where the abrupt souces of outflow channels are located. Modles of the heat flow in Mars suggested that water would be expected to be frozen down to depths of mant kilometers in these regions.

If these new MGS findings are correct, something in these regions is allowing the water to be liquified. This water percolates upward to the surface, where an ice barrier forms, but eventually the pressure breaks the ice, resulting in the outflow features observed.

Canals on Mars?

What about the canals? Well, there aren't any. As Carl Sagan said, "Where we have strong emotions, we are liable to fool ourselves." The ONLY topographic feature on Mars that matches the position of one of the canals is Vallis Marineris. It's so big that even Lowell couldn't miss it!