# Why are there no terrestrial planets with a subsurface ocean?

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Subsurface oceans in satellites are pretty common: Europa, Enceladus, Ganymede, Callisto, maybe Pluto… This is due to tidal heating of their host planet, Jupiter and Saturn, which heats up the inner ice of those satellites. However, planets don't exhibit this inner ice layer, so they don't usually have subsurface oceans (except Pluto or Ceres, if you can call them "planets"). Why is that? Only small bodies like satellites present this inner layer of ice? Is there any Earth-type exoplanets that exhibit this inner layer of ice that could potentially melt down to liquid water? And if there is, why some cold planets have inner ice layers and others don't?

The terrestrial planets are Mercury, Venus, Earth and Mars. Mercury and Venus are too hot for liquid water to exist at any level, Mars has lost nearly all its water and Earth has a surface ocean, not a subsurface one. The inner planets lost most of their volatiles (including water) as they formed, the water on Earth was provided by later icy asteroid impacts.

So none of the terrestrial planets have a sub-surface ocean. The other planets are gas and ice giants. Uranus and Neptune likely have liquid layers surrounding their cores, composed of water, ammonia and other "ices"

To get a subsurface ocean you need a planet that is beyond the frost line (the distance from the sun at which ice is stable in space) and in our solar system the planets beyond the frost line are either dwarfs or giants.

In a sense, the Earth does have a subsurface ocean, only it isn't a water ocean, it is an ocean of molten iron. The outer core of the Earth is highly fluid; it's no more viscous than water.

Among exoplanets, there are several candidate ice planets. Wikipedia lists OGLE-2005-BLG-390Lb, OGLE-2013-BLG-0341L b and MOA-2007-BLG-192Lb. (The principle way of discovering small planets that orbit far from their host star is by microlensing events, hence many of the candidate planets were found by the Optical Gravitational Lens Experiment, or OGLE)

Some hypothesize that the Earth did have a subsurface ocean during the Cryogenian period, which lasted from 720 to 635 million years ago. The Cryogenian saw the two greatest known ice ages in the Earth's history, the Sturtian and Marinoan glaciations. There is some evidence that the Earth was completely covered with ice and snow during those glaciations. (There is also some evidence that it was not.)

Whether even older periods in the Earth's history also succumbed to snowball Earth episodes is even more debatable. The evidence has been wiped out by a billion plus years of tectonic activity. That even older periods of the Earth's history than the Cryogenian did suffer snowball Earth episodes does however make sense.

The Sun is considerably more luminous now than it was shortly after the Earth first formed. Once the Earth cooled from its formation (and that appears to have happened fairly quickly, in a geological sense of "quickly"), that faint young Sun should have resulted in a cool Earth. That the young Earth had periods where it obviously wasn't covered with ice and snow from pole to pole is the faint young Sun paradox. The apparent paradox is almost uniformly explained away via greenhouse gases. But which ones?

That said, once plants started converting carbon dioxide into oxygen, and once the oxygen stopped combining with iron to form most of the world's iron ore deposits, the greenhouse effect that kept the young Earth from freezing over should have dropped significantly. There are some signs that this happened, some that it didn't. Puzzling out what happened well before the Cambrian has always been problematic because rocks that old are hard to find.

As far as I know satellite data from mars observers show significant amounts of ice blow the south pole and pointers to a similar though smaller amount at the northern pole of mars. This is not surprising - Mars' plate tectonics stopped approx 1.2 to 1.5 billion years ago after the planets core cooled down enough. Mars' smaller mass and volume didn't provide enough "insulation" to keep the core hot. Thus the magnetic field of mars pretty much vanished - and was the only protection for the atmosphere against the solar wind. So Mars' atmosphere is now only as thin as earths atmosphere in 48 km height - and easily allows evaporation of surface water, so the only remaining water is sub surface OR in some shadowed craters

There are no terrestrial planets with subsurface oceans because of differentiation. Denser materials move toward the center of the body. Iron is denser than rock which is denser than water which is denser than ice. The icy surface of these moons and dwarf planets is essentially floating on water which is floating on rock. You can actually see this on Earth. We have a partial subsurface ocean at the poles where water ice floats on top of the ocean.

It depends what you mean by ocean. Earth arguably has a subsurface ocean of liquid iron, usually called the "outer core".

## Donut Shaped Planets

I've heard in several different places that a donut shaped planet is (technically) possible, but I'm having a hard time wrapping my brain around it. My intuition tells me that such a system would be incredibly unstable.

We have rings around planets which are kind of like flattened doughnuts but those are not stable over long time scales.

I’m trying to imagine a planetary formation scenario that would result in a toroidal planet forming and I’m having a hard time.

If the forming planet were spinning fast enough to have the requisite angular momentum to have material far from the centre, that would result in large moons or a binary planet, not a toroidal planet.

Even if by some exacting circumstances a toroidal planet were to form, I don’t expect it would be stable enough (like you said) to survive for long.

It has been theorized as possible, and can even be modelled, but the problem is coming up with an explanation that would allow this to form naturally. You would need an extremely delicate balance of gravitational equilibrium and rapidly-rotating matter to meet all of the conditions required to keep a toroidal planet stable. As of yet, there is no reasonable explanation that I know of.

One thing to note, however, is that our galaxy likely houses tens or even hundreds of billions of planets, and there are quite possibly trillions of galaxies out there. When you have some 10 25 planetary systems, it's hard to imagine that there wouldn't be representation of virtually every physically-possible configuration, however improbable said configuration may be. So, if the formation of this system is even remotely possible, it likely exists somewhere.

This has been studied and according to reports a toroidal planet would be perfectly stable. The big problem is that no one has come up with a believable way of forming one. A truly advanced civilization could perhaps build such a planet, but I don't think naturally occurring ones will turn out to be very plentiful. The last time, a couple of years ago, that I heard someone address this the author said it was as much about building up his computer programming skills as any real interest in the actual problem. To me an even more interesting and unlikely (and also probably not technically possible) scenario would have multiple planets orbiting their sun at the same distance all inside a toroidal atmosphere that was itself orbiting the sun. This idea is from an old Edgar Rice Burroughs novel (Beyond the Farthest Star) that is probably about 100 years old now. They could fly airplanes between the planets, just be sure to pack a lunch! LOL

## Where to Look?

### Terrestrial Planets:

The Moon and Mercury have no atmospheres. Nor have they ever had atmospheres (they are not massive enough to retain gases given their location in the relatively warm inner Solar System). So they are out.

• 96% CO2
• 3.5% N2
• 0.5% H2O, H2SO4, HCl, HF (nasty stuff)

The condition of the Venus atmosphere is a result of a Runaway Greenhouse. It is thus a cautionary example to us of how bad things can get if the atmosphere can trap too much heat.

• The early Venus atmosphere was much cooler than it is now. Cool enough for liquid H2O.
• But Venus is closer to the Sun than Earth is, so it was warmer than Earth.
• Because of this, less of the surface water was in liquid form, and the liquid H2O could dissolve less CO2 in it than the water on Earth (not just because there was less of liquid water, but also because CO2 dissolves better in cooler water than in warmer water).
• So there was more CO2 in the atmosphere, and this allowed the atmosphere to trap heat more efficiently.
• This lead to more evaporation of liquid H2O, and the introduction of even more CO2 and H2O to the atmosphere.
• From here the process diverged (hence the term "Runaway Greenhouse"), until all the surface water had evaporated, and all the CO2 was in the atmosphere.

Conditions on Venus and Earth were fairly similar 4 Gyr ago, so life might have arisen on Venus. But it is difficult to imagine how it could have persisted there.

This leaves us with Mars. Mars is a very interesting place to look for signs of both past and present life, as we shall discuss next week.

### Jovian Planets

The Jovian planets have no surfaces. They have very thick atmospheres, that reach all the way to their cores. The internal pressures reach upwards of a few Megabars. There are regions in these atmospheres that have temperatures and pressures that are quite reasonable for life, and this has led to speculation that life might exist there. But there is a big problem. The atmospheres of the Jovian planets have very strong vertical mixing, due to heat convection. Thus a given parcel of gas experiences enormous swings in temperature and pressure as is oscillates from deep in the atmosphere to the upper cloud decks. It is difficult to understand how life could have gotten started under such conditions.

### Outer Satellites

There are a lot of biggish worlds of rock and ice in the outer Solar System. Most of them are too cold and airless to be of interest to us in this context. But two are worth serious study:

### TITAN

A good on-line resource for information and images of Titan is NASA's Space Science Data Center Titan page.

Titan is the largest moon of Saturn. It is larger than Mercury (although, at a density of only 1.9 g/cc, a great deal less massive). Titan is unique among the Solar System's moons in that it has a thick, dense atmosphere. The atmosphere has a surface pressure of about 1.5 times that of Earth's atmosphere, and composed mainly of N2, with 10-15% CH4. We knew that Titan had CH4 in its atmosphere long before the Voyager flybys because we could see it in Titan's spectrum. We couldn't see the N2 because of the N2 in our own atmosphere, so we didn't understand what Voyager would find beforehand. It turns out that at the pressure and low temperatures of the Titan atmosphere (around 100K), methane can condense out and form an aerosol haze. This haze is photoreduced by Solar UV radiation to create smog. As a result, we cannot see the surface of Titan in visible light.

Although we cannot see the surface in visible light, and thus have no Voyager data on the surface features, we CAN see the surface in the Infrared using the Hubble Space Telescope. The color in the linked image is false, but the brightness scale is real. The interpretation of these observations was unclear when they were made in the 1990s.

But we understand the physics and chemistry of the hydrocarbon mixture in the Titan atmosphere to have made some predictions about the surface. The heavy organics will settle out of the atmosphere, to create a layer of organic goo 10m to

1km thick over most of the surface. I am using the word "organic" in its technical sense (composed of carbon-chain compounds). However, the expected existence of this non-biologically produced organic goo has lead people to speculate that Titan could have evolved life. Because Titan was such an interesting place, a key part of the Cassini mission was the Huygens lander probe. This was a small lander that touched down on Titan in 2005, and took data for about 90 minutes before running down.

The physics of methane and ethane (C2H4) at these temperature and pressures led us to believe that Titan should have extensive surface oceans of methane and ethane, and should have methane rainfall. These predictions have been tested by results from the Cassini mission. The first Cassini fly-by of Titan and the Huygens probe landing took place in 2005. We know that the "methane ocean" idea is incorrect. But there is clearly some very interesting chemistry and fluid flow occuring on Titan.

The expected existence of this non-biologically produced organic goo has lead people to speculate that Titan could have evolved life. As discussed earlier, the chemistry of the solvents available (NH3, CH4, C2H6) is very different from that of water. And the chemical reaction rate is much lower at 100K than it is at 300K. Thus the biochemistry of any Titan life must be very different than that of terrestrial life.

### EUROPA

A good on-line resource for information and images of Jupiter's satellites is NASA's Space Science Data Center Jupiter page.

Jupiter has more than 50 known satellites. These fall into two basic groups. There are dozens of small, irregular objects that are probably captured asteroids. And there are four large moons, known as the Galilean Satellites, due to their discovery by Galileo.

Europa is substantially smaller than either Ganymede or Callisto, but it is much denser:

This means Europa must be mostly rock, but also have a significant contribution from volatiles (mostly H2O). The striking thing about the appearence of Europa is that, unlike Ganymede and Callisto, the surface is very bright, there are almost no craters, and there are many features that look like cracks . These features can be very long and complex. Close-ups show terrain that looks the the surface of a frozen pond.

The implication of these observations is that Europa has a deep, subsurface ocean of liquid water. When an object strikes the surface hard enough to leave an impact crater, the impact also breaks the crust and allows water to form fresh ice on the surface, thus erasing the impact crater. The possibility of a liquid ocean is bolstered by the Galileo observation of a magnetic field associated with Europa. To generate a magnetic field, a planet needs some sort of liquid electrical conductor in its interior. On Earth that conductor is liquid iron. Europa has a small iron core, but not nearly enough to support the observed magnetic field. The best prospect is salty water. In other words, a subsurface ocean.

But WHY does Europa have such an ocean?

Consider the case of the Moon and the Earth: The Moon orbits the Earth due to gravity. This causes tides. The essential point about gravity is that it obeys an inverse-square law:

This means that the oceans on the side of the Earth closest to the Moon feel a larger force of gravity due to the Moon than does the center of the Earth. And the center of the Earth feels a larger force than does the ocean on the far side of the Earth from the Moon. The result of this is that two tidal bulges are raised on the Earth. One on the side facing the Moon, and one on the side facing away from the Moon. Tides also occur in solid rock, but they are much less dramatic than those occuring in oceans. The solid body tides on the Earth only produce tidal bulges with amplitudes of about 1cm. Still, this has a dramatic long term effect. The Earth spins much more rapidly than the orbital period of the moon (one day versus 27.3 days), and thus the actual tidal bulge on the Earth is always ahead of the Earth--Moon line. This causes a tidal torque: The Moon is dragging the Earth, slowing its rotation rate down. This is born out by the fossil record: The day was only 22 hours long 400 Myr ago.

Eventually, the tidal torque from the Moon will cause the Earth's rotation rate to slow down to the orbital period of the moon. The Earth is much more massive than the Moon, and gravity is a symmetric force, so the Earth is much more effective at slowing down the Moon than the Moon is at slowing down the Earth. In fact, the Earth has already slowed down the rotation period of the Moon to match its orbital period. This is why we always see the same face of the moon. Tidal Phase Locking is a common phenomenon in the Solar System. Indeed, all the large moons of the outer planets are tidally phase locked.

In the case of the Galilean satellites, the situation is further complicated by a resonance amongst three of the four moons (Ganymede, Europa, and Io). This resonance causes the orbits of the three satellites to be significantly elliptical. Because of this, the distance between a given satellite and Jupiter varies over the course of an orbit, thus Jupiter's gravity is basically stretching the satellites like taffy. If you take a rubber band, and stretch it back and forth for 5 minutes, it gets warm. The same thing happens to the interiors of Ganymede, Europa, and Io. In the case of Io, the energy input is so great that it melts the rock. Europa is further out, and thus only its ice is melted.

The existence of this deep, relatively warm ocean has led to speculation that Europa may have life on it. In fact, it is probably the best place in the Solar System (other than Earth) to look for life. The key question that we do not have an answer to is this: "Is there a mechanism that concentrates the energy enough to drive biochemistry?" To understand what I mean by this, consider the Earth's ocean. Where is the life concentrated? Two places: Near the surface, where there is plenty of sunlight to drive photosynthesis, and near ocean-floor hydrothermal sea vents, where there is geothermal energy. Europa is very far away from the Sun, so even if the ice crust is thin enough to allow sunlight to reach the top of the ocean, that sunlight will be able to drive very little photosynthesis. Are there Europan analogs of the deep ocean vents we find on Earth? We don't know. But we should find out in our lifetimes.

A good on-line resource for information and images of Mars is NASA's Space Science Data Center Mars page.

Mars is about half the diameter of the Earth. As a result, Mars retained its internal heat much longer than the Moon did, and remained geologically active for much of its history. But as it is so much smaller than Earth, it has cooled much faster.

Mars has a number of similarities with Earth. Its rotation period is 24h 40m, nearly the same as Earth's. Its orbital period is 1.88 yr. And its rotation axis is tilted by 25 degrees to its orbital plane (as compared to 23.5 degrees for Earth). Thus Mars has seasons, much like Earth does.

Mars has been a focus of interest in human societies since pre-history. It is very bright every few years, and is then high in the night-time sky. It's reddish color has led many cultures to associate Mars with war.

The more modern interest in Mars dates to the late 1800s, when the Italian astronomer Schiaparelli described features he saw on the surface of Mars as "canali". This translates literally to "channels", but was immediately grabbed onto by the press, and mis-translated as "canals". Modern science fiction was essentially born at that moment.

By the mid-20th century, the scientific community had basically abandoned the "canals" bit. But the public romance with Martians kept the idea going until the Mariner 4 images showed terrain much more like the surface of the Moon than a terrestrial desert.

As we cruise past our sole example of a life-bearing world, we might take a page from an earlier era of planetary exploration, courtesy of Carl Sagan. The astronomer and prize-winning author also was a key member of science teams for a variety of NASA&rsquos solar system exploration missions, including Galileo.

In 1990, as the space probe zipped past Earth for a gravitational kick that would hurtle it toward the outer solar system, it turned its instruments on the home planet. Sagan&rsquos question: Could Galileo detect signs of life on Earth?

And it did. Oxygen. Methane. A spike in the infrared part of the light spectrum, called a &ldquored edge,&rdquo the telltale sign of reflective vegetation on the surface. Galileo even detected what today might be called a &ldquotechnosignature&rdquo &ndash a sign of intelligent life. In this case, powerful radio waves that were unlikely to come from natural sources.

&ldquoIt&rsquos vital to think about what our own planet would look like to an alien,&rdquo said Giada Arney, an astronomer and astrobiologist at NASA&rsquos Goddard Space Flight Center in Greenbelt, Maryland. &ldquoIt&rsquos important to think about what signs of life they could actually see from space.&rdquo

Arney, who says much of her work involves &ldquothinking about Earth as an exoplanet,&rdquo focuses on haze-shrouded worlds. As we search for signs of life around other stars, she reminds us that our own planet would have looked very different at various epochs in the deep past.

The Earth of billions of years ago, in the Archean era, might not even have been Sagan&rsquos &ldquopale blue dot.&rdquo Before the atmosphere became oxygen rich, Earth might occasionally have been a &ldquopale orange dot,&rdquo Arney says, its orange haze created by complex atmospheric chemistry involving methane generated by microbes. A similar haze is found today in the atmosphere of Saturn&rsquos moon, Titan, though in this case, not generated by life.

To find an analog of our own planet out among the stars, we must consider &ldquonot just modern Earth, but Earth through time,&rdquo she said. &ldquoThe kinds of planets that could be (considered) Earth-like may be very different from modern Earth.&rdquo

## Is There Life in Europa’s Subsurface Ocean?

Image Credit: Credit: NASA/JPL

The image above shows the interior structure of Europa, a major moon of the planet Jupiter, but the smallest of its four Galilean moons. The band of blue in the picture depicts a 100 km-thick layer of salty water below its ice crust that many researchers now think could contain life forms that extract energy from the nuclear decay of various elements.

It has long been known that several solar system moons harbor liquid oceans beneath their icy crusts, but despite the total lack of empirical data on conditions at the water/mantle interface on Jupiter’s’ moon Europa, some researchers believe that Europa has a higher potential to host alien life forms than other, similar moons.

According to a report published in the journal Scientific Reports, a team of investigators from the University of São Paulo in Brazil have identified conditions in a South African gold mine that appears to be analogous, if not similar to conditions that might exist at the ocean/mantle interface on Europa. Moreover, the researchers have identified a strain of bacteria known as Desulforudis Audaxviator that lives by means of a process known as water radiolysis, which is the dissociation of water molecules by ionizing radiation. Interestingly, its name derives from a quotation that appears in Jules Verne’s novel Journey to the Center of the Earth (1864) in which the story’s hero, Professor Lidenbrock, finds a secret message in Latin referring to the summit of the Icelandic volcano Snæfellsjökull that reads: “Descende, audax viator, et terrestre centrum attinges” (Descend, bold traveller, and you will attain the center of the Earth).

According to Douglas Galante, a researcher from Brazil’s National Synchrotron Light Laboratory (LNLS) and the Astrobiology Research Center (NAP-Astrobio) of the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences, the organization that oversees the research, the conditions in the South African mine are caused by water seeping through cracks in rocks that contain radioactive uranium.

Essentially, the water molecules are broken apart by the radioactive uranium, which creates free radicals such as H+, OH- and others, that in their turn, break down rocks like iron disulfide (pyrite), in a process that produces sulphate. The sulphate is then used by the bacteria to synthesize adenosine triphosphate (ATP), which is the nucleotide that regulates the storage of energy inside living cells, which means that these bacteria are the first terrestrial organisms that are known to live directly off the by-products of nuclear energy.

However, while it is known that Europa’s interior is relatively hot due to powerful tidal interactions with Jupiter, hot water alone is not enough to sustain even bacterial life. According to the researchers, biological processes depend on there being differences in the amounts of molecules, electrons, and ions in different regions to allow for processes, such as cellular respiration, photosynthesis, and ATP production, to occur.

While nothing is currently known about the actual conditions deep inside Europa, researchers are hopeful that hydrothermal emanations (underwater volcanoes) are present on Europa to provide sources of molecular hydrogen [H2], hydrogen sulphide [H2S], sulphuric acid [H2SO4], methane [CH4], and others to create chemical imbalances that can be transformed into meaningful amounts of biological energy.

Nonetheless, despite the promising results of this study many questions remain unanswered, and will likely stay so until at least 2025 when a dedicated mission, code named “Europa Clipper”, will be launched by NASA to study the actual conditions on Europa. No spacecraft has landed on Europa to date, although the Europa Clipper is expected to perform 45 close flybys of the icy moon, and use a number of instruments to determine whether its conditions are suitable for life, including cameras, spectrometers, magnetometer, a thermal instrument, and an ice penetrating radar.

## Astronomy 115-1 - Mid-Term Sample Answers

1. One of the most serious problems for the study of astrobiology is in defining what we mean by life'' in a way that is useful, but not overly restrictive. One consequence of this is that we often use the nature of life on Earth as a model. One fundamental aspect of terrestrial life is that our biochemistry operates in aqueous solution. Discuss the reasons why we often take this to be universal, and present possible alternatives.

All terrestrial life is based on chemical reactions in aqueous solution. Water is a unique solvent for biochemistry for the following reasons:

1) It exists in liquid phase in a broad and moderate range of temperature. Thus fluctuations in environmental temperature need to be quite large to cause liquid water to freeze or vaporize. Also, water is a liquid in a temperature range that is high enough to foster relatively fast chemical reaction rates, and low enough to allow for the existence of complex chemical compounds.

2) Water is a polar liquid, and thus allows for the construction of things like cell membranes composed of lipid-based compounds that form natural clusters in a water solution.

3) When water freezed, the resulting solid is less dense that liquid water at temperatures just above freezing. This means that ice floats, and thus provides thermal insulation to allow the persistence of liquid water underneath ice layers.

A number of other volatile compounds could, potentially, also be used as liquid solvents for biochemistry. The most common of these are ammonia, methane and ethane. All of these are liquids over much smaller ranges of temperature, and at much lower temperatures than water. Thus they are not as good from a thermal stability standpoint, and any biochemistry occuring in such solutions would have very slow reaction rates. Further, none of these are polar liquids. Thus the nature of the structural chemistry of any life in, say, ammonia solution, would have to be very different that the structural chemistry used for terrestrial life. Finally, ices of these compounds are all more dense than the corresponding liquids. Thus if conditions are sufficiently cool to allow for freezing, the end result is a fully frozen environment, as no surface insulation layer can form.

2. What are lithophilic life forms? What place, other than the Earth, is a potentially interesting place to look for such life, and why is that place a good candidate for such life to exist?

Lithophilic life forms are single-celled organisms that live under the surface of the Earth. They inhabit seams in rocks, and have been identified up to several kilometers under the surface. They obtain water from water that seeps through the Earth's crust, and carbon from dissolved CO2 in the water, and from carbon-bearing minerals. They obtain the energy to run their metabolisms from the breakdown of simple inorganic compounds in the rocks they live in.

It is possible that lithophilic life might persist under the surface of Mars. Although conditions on the Mars surface are now very hostile, the Martian subsurface is likely to be very similar to the terrestrial subsurface. In particular, we know that there is a substantial amount of water in the crust of Mars. This water is mainly in the form of permafrost, although we have recently found at least one large reservoir of liquid water under the Mars surface. As the Mars atmosphere is mainly CO2, the water in the interior is likely to have CO2 dissolved in it. Thus the resources required to drive a lithophilic ecosystem appear to be available on Mars.

While the current Mars surface is very inhospitable, conditions on Mars 3.5 Gyr ago were likely very similar to conditions on the Earth at that time. As we have clear fossil evidence of life on the Earth at that time, it is not unreasonable to suppose that life could have also been present on the Mars surface then. If so, such life could have migrated into the Martian subsurface just as it did on the Earth, and could persist to the present day.

3. Life on the surface of the Earth derives its energy from the Sun. There are several satellites in the outer Solar System that may have conditions suitable for life, but for which sunlight is not a substantial source of energy. The most promising of these is the Jovian moon Europa. Outline the mechanism that is the primary energy source for potential life on Europa.

The primary energy source on Europa is gravitational in origin. Europa is in a phase-locked orbit around Jupiter, with its orbit period equal to its rotational period. But the orbit is elliptical, so the distance between Europa and Jupiter varies over the orbit. Also, although the orbit and rotational periods are equal, the orbital velocity is not constant. Europa orbits faster than average when close to Jupiter and slower than average when farther away.

The variation in Europa's distance from Jupiter means that it is subject to a variation in tidal stretching, with the greatest stretching when it is closest to Jupiter. The variation in orbital velocity adds an additional stretching back and forth as Europa orbits Jupiter. The combination of these two effects lead to the cracking of Europa's ice crust, and also to the frictional heating of its interior. This frictional heating is the energy source that allows for the existence of the subsurface liquid water ocean on Europa.

The energy from this gravitational friction should also heat Europa's deep interior. That could create structures on the Europa ocean bottom that are similar to the hydrothermal sea vents that we find on the Earth's ocean bottom. These are places where mineral-rich superheated water emerges from the crust to form the energy basis for rich colonies of life. If such features do exist at the bottom of the Europan ocean, they may provide the basis for similiar ecosystems there.

4. We now know of at least 3000 planets orbiting other stars. What method was used to discover the majority of these planets, and how does it let us determine the properties of those planets?

Most known extrasolar planets have been discovered using the transit method. If a planet passes directly in front of a star the planet will block a small fraction of the light from the star. Thus, if we monitor the brightness of a star and look for periodic events in which the star becomes slightly dimmer, we can detect the presence of the orbiting planet. This method will only work if the planetary orbit plane is edge-on to our line of sight, so we expect only a few percent of stars with planets to show evidence of transits.

As the transits are periodic, they yield the period of the orbit directly. The amount of the star's light that is blocked tells us the ratio of the size of the planet to the size of the star (the bigger the planet the more light it blocks). If we also know the size of the star, we can use this to determine the size of the planet. An edge-on orbit also allows us to measure the radial velocity variation of the host star accurately. We also know the orbit period, as noted above. If we also know the mass of the star, then we can determine the mass of the planet from the orbit characteristics. Knowing both the size and the mass of the planet allows us to determine its density as well. As rocky planets and gas planets have very different densities, we can use the density to tell us what the nature of the planet is.

## Life in the Solar System

You can summarize this lesson with the common phrase, "Are we alone?" You may associate this question with TV shows, movies, or sci-fi novels, but it is a valid question that researchers have considered alongside topics you might consider more traditional, like, "Why does the sun shine?" There is a growing field of study that is investigating all of the scientific questions associated with the search for life in the Universe, and it is referred to as astrobiology. It is part of our everyday experience that life is prevalent on Earth. But what we do not know for certain is how prevalent life may be in the Solar System, the Milky Way, and the Universe in general.

What is astrobiology? We will be discussing many areas of astrobiology during this lesson, but if you want to start with some pre-reading before you begin, I would recommend the NASA Astrobiology website.

The first task to address in the study of life in the Universe is to define what we are looking for. That is, how do you know that something is living when you find it? It is surprisingly difficult to do this, and so there is no single, universally accepted definition of life. If we compare and contrast living and nonliving things on Earth, we can come up with a set of properties that appear to be common among all living things. These are:

• Living things grow and reproduce
• They evolve and adapt to their environment
• They require liquid water
• They require energy

We can make this list more detailed, but the difficulty with this type of exercise is that you can find examples of nonliving things (for example, fire is often cited) that exhibit some of the properties of life, and you can cite examples (e.g., viruses) that do not fit all of the properties you expect living things to exhibit. While this question continues to be researched, one option that can be pursued in the meantime is for scientists to look for evidence of life elsewhere that shares properties of lifeforms known on Earth. Clearly, this is an assumption (and one that may be wrong), but for the most part, scientists are looking for evidence that simple, microbial life may be present now, or may have been present in the past on other worlds in our Solar System.

The locations considered most likely to harbor life are:

1. Mars
2. Europa and Ganymede, two of the Galilean moons of Jupiter
3. Titan, a moon of Saturn

The reason these locations are considered more likely than, say, Mercury or our Moon, is because there is evidence that each of these worlds either had some liquid on its surface in the past or has subsurface liquid (water in the case of Mars, Europa, and Ganymede, and liquid hydrocarbons in the case of Titan) or on the surface now. This property is considered by many scientists as the single most important requirement for life to exist.

On Mars, we see evidence that liquid water was likely present in the past. Below is an example where scientists believe that the light-colored deposits indicate a brief flow of liquid water that occurred sometime very recently.

The Mars Phoenix Lander saw water ice in a trench it dug as it was studying the Martian soil.

You can find a large number of images and studies of the Martian surface by landers (for example, the Spirit and Opportunity rovers, the Phoenix lander) or orbiters (Mars Global Surveyor, Mars Reconnaissance Orbiter) that suggest that Mars was once a wet world. Given this evidence, NASA has been investing a great deal of time and effort in the study of Mars in a search for life. At the Mars Exploration website, you can find a list of all of the past, present, and future missions to Mars.

The education and outreach group for the Phoenix Mars Mission has put together an excellent resource that compares images of Mars and images of Earth that builds the case that Mars must have been wet in the past.

It was the NASA Galileo mission that gave us the evidence for what may be a subsurface ocean on Europa. And the NASA Cassini mission gave us the evidence for riverbeds and lakes on Titan. A major focus of NASA missions that are currently under consideration is to study these worlds in more detail to see if there is some way that we may verify the presence of life.

## Why is Mars So Dry?

Image credit: NASA/JPL
The MER rovers Spirit and Opportunity, now traveling on the surface of Mars, are exploring a geography drier than the driest desert on Earth. Despite the polar ice caps and suspected pockets of liquid water beneath the martian surface, the amount of water on Mars is but a teaspoon compared to the vast watery reserves of Earth. Why is Mars so dry?

The inner planets of our solar system – Mars, Earth, Venus and Mercury – formed by the accumulation of small rocks and dust that swirled around the sun in its earliest years. If the Earth and Mars are made of the same stardust, they should have been born with about the same ratio of water.

Many scientists think Mars once was very watery, but lost its oceans due to the low mass of the planet. This, combined with a thin atmosphere, allowed most of the water on Mars to evaporate out into space.

But according to a study by Jonathan Lunine of the Lunar and Planetary Laboratory at the University of Arizona, the Red Planet was dry from the very beginning.

Lunine, writing in the journal Icarus in 2003 with colleagues John Chambers, Alessandro Morbidelli, and Laurie Leshin, says that Mars was originally a planetary embryo. In essence, a planetary embryo is a very large asteroid that can be as massive as Mercury or Mars. This pre-Mars embryo existed in the asteroid belt, which at the time was more widely dispersed in the solar system, spread out between 0.5 to 4 AU from the sun. Today the main asteroid belt is roughly at 2 to 4 AU, located between Mars (1.5 AU) and Jupiter (5.2 AU).

Lunine says that Mars grew to its present size from accumulations of smaller asteroids and comets. He says that the more massive Earth, in comparison, mostly formed from large planetary embryos colliding into each other.

“By chance Mars was not struck by giant asteroids while Earth was – the lucky versus unlucky pedestrian,” says Lunine. “But Mars was struck by much smaller bodies because these are so numerous.”

The Earth currently orbits the sun at 1 AU. Lunine says that planetary embryos in this orbit would not have had much water. Early in the sun’s evolution, during planetary formation, the dusty disk that surrounded the young star was very hot. Water-bearing compounds would not have been able to form in this disk at 1 AU.

Since Mars is further away from the sun than Earth, and closer to the cooler, “moist” regions of the asteroid belt, it would seem logical that Mars would have been born with more water. Yet Lunine says that Mars probably acquired only 6 to 27 percent of an Earth’s ocean (1 Earth ocean =1.5 ?1021 kg).

That’s because some of the planetary embryos that eventually constituted the Earth were saturated with water. While 90 percent of the embryos that formed the Earth were from the 1 AU region, and therefore dry, 10 percent were from 2.5 AU and beyond. Embryos coming from this distance would’ve had large supplies of water. Smaller asteroids coming from this distance would’ve contributed to the Earth’s water supply as well. At most, Lunine says that only 15 percent of Earth’s water came from comets.

Mars, meanwhile, had the bad luck to be born as a single dry rock. Mars eventually received some water late in the formation game, after its core had already formed and it had nearly reached its present mass. According to Lunine’s scenario, Jupiter also gained its present day mass around this time. Jupiter’s gravity then either sucked in nearby asteroids or caused them to scatter outwards. The proto-Mars somehow escaped being shifted by Jupiter’s gravity, but was bombarded by the outward-bound asteroids.

“The impacts of small asteroids and comets constituted a “late veneer” which added water to Mars, in contrast to the picture for Earth where water was added through collisions with Mercury-sized embryos throughout a growth period of some tens of millions of years,” the scientists write.

Although Mars doesn’t form in their computer model, the scientists think that may reflect the chaotic nature of planetary formation, where the directions of planetary embryos and asteroids are unpredictable and many outcomes are possible.

“There is a fair amount of randomness involved in building the terrestrial planets, so ending up with a Mars that did not happen to accrete many water-rich planetesimals is a possible occurrence,” says Alan Boss of the Carnegie Institution of Washington. “This may well help explain the paucity of water on modern-day Mars.”

Such differences in planetary formation also could occur among the inner planets of other solar systems. So far, astronomers know of 104 stars that have planets orbiting them. All of the extrasolar planets found so far are gas giants, but it seems likely that terrestrial planets like Mars and the Earth also could orbit distant stars, even though we do not yet have the technology to detect them.

If some inner terrestrial planets are formed by collisions of several planetary embryos, while others are embryos that only gather up moist comets and asteroids, then planets around these other stars could have very different amounts of water. Lunine suggests that the timing and formation of the gas giant planets in each solar system will play an important role in this process, just as Jupiter has influenced the character of our own solar system.

Lunine currently has a paper in Icarus, with Tom Quinn and Sean Raymond of the University of Washington, on the possible variation in water abundance for terrestrial planets around other stars. In addition, he is carefully watching the data collected by the MER rovers Spirit and Opportunity, as well as the satellites currently orbiting Mars.

“Odyssey, MER, and Mars Express will determine how much water exists at present, hopefully, and provide better constraints on past water abundance,” says Lunine. “I am particularly interested in the MARSIS radar results, and those of its successor – SHARAD.”

MARSIS is a radar device on the Mars Express satellite that can look through the top five kilometers of martian crust to search for layers of water and ice. The Italian space agency is planning to fly a shallow subsurface radar, called SHARAD, on NASA’s Mars Reconnaissance Orbiter to see if water ice is present at depths greater than one meter. While MARSIS has a higher penetration capability, it has much lower resolution than SHARAD will have.

## Are all the aliens hanging out in some eldritch realm deep underwater?

As we wonder where all the aliens could be crawling around, maybe it's worth considering that they might not creep and crawl at all. They might be swimming in a subsurface ocean, where they have no idea the universe above even exists.

It sounds almost Lovecraftian, but planetary scientist Alan Stern believes that our best chance of finding extraterrestrial life is beneath layers of rock and ice in the subsurface oceans of bodies like Europa or Enceladus. Because there are a number of these types of worlds in our solar system, they might be common in other star systems. Worlds with surface oceans, like Earth, need to be closer to their stars to be habitable — but life could potentially survive in the depths of frozen orbs that are much further from their stars.

#### More Alien Life

“Because these oceans can be very far away from their parent star, it vastly expands our concept of ‘habitable’,” said Stern, who recently presented a paper that might forever change your mind about aliens at the 52nd annual Lunar and Planetary Science Conference.

Think about it. Earth got lucky. In 4.5 billion years, our planet has survived almost every existential threat except a supernova. It supposedly took a beating from another protoplanet when it was a protoplanet itself (and still has the remnants of that collision deep in its mantle). Through all the cosmic trauma it experienced, from its violent birth to asteroids throwing lethal punches that wiped out more than just the dinosaurs, Earth has held up. Not to mention that the Sun is a middle-aged star, which hasn’t gone red giant and swallowed us up. At least we have another 7 to 8 million years before it opens its fiery jaws and comes for us.

Stern argues that planets like Earth must be rare. Bodies with surface oceans face constant threats from space, and if Mars could speak, it would vouch for that. It is much more likely that life on a cryo-world with liquid oceans beneath miles of ice and rock would survive coronal mass ejections and passing through molecular clouds that bring on a deep freeze. Rogue planets could potentially host life. Anything living so far beneath the skin of a moon or planet would be unfazed by phenomena on the outside, meaning that the surface temperature could plummet to absolute zero and it would still be lurking at the bottom of the ocean like nothing happened.

“Planetary scientists expect to be surprised because we find ourselves constantly taken aback by the planetary types not expressed in our own solar system,” Stern said. “We can expect to find many other kinds of biology out there. Our kind is used to our particular situation, which is why many believe it might exist somewhere else, but we should just keep our eyes open and expect the unexpected.”

There is also the question of what these strange hypothetical worlds would be like beneath the literal and proverbial surface. Stern prefers the analogy of knowing nothing but a desert island and being blown away from all the flora and fauna you would discover if you just got around the rest of Earth for a while, never mind the universe. Now imagine alien beings that might not even grasp the concept of air. They may or may not have subsurface civilizations like Lovecraft’s dreadful and blasphemous R’lyeh, but say these life-forms are more than microbes and actually have a brain.

Spaceflight would be unfathomable for creatures that need to carry so much water with them to stay alive. It would also be highly unlikely that intelligent alien things from the deep would try to reach out to other planets by sending messages, because they would be unaware that any existed. It would not be possible for any technological signals to escape the ice. Because organisms like this would evolve to breathe underwater, they would stay there with no knowledge of anything beyond, so breaking the surface would be a shock. What would they even think if they saw stars?

“If the universe at all follows the pattern in our solar system, with most worlds having oceans on the inside, those oceans are probably more stable environments to life,” said Stern. “Most of the life that would evolve [would] be in those oceans and completely sequestered from knowing there’s a universe.”

Water vapor plumes on Europa could mean there are hydrothermal vents deep beneath the icy crust. Credit: NASA

To find out what lies beneath, what we need is evidence. Plumes of water vapor from Europa and Enceladus could be signs of hydrothermal vents in their darkest depths. Stern, who is involved in the ultraviolet spectrometer team for NASA’s upcoming Europa Clipper mission, believes that we need autonomous remote sensing spacecraft built to withstand perilous waters. Europa Clipper takes off in 2024. He is anxious to see if anything is waiting to be found without actually knowing someone from a faraway planet is trying to find it.

“I really wish I could be around for another 300 years,” he said. “I have a feeling there must be something.”

## Astronomy Without A Telescope – So Why Not Exo-Oceans?

Well, not only may up to 25% of Sun-like stars have Earth-like planets – but if they are in the right temperature zone, apparently they are almost certain to have oceans. Current thinking is that Earth’s oceans formed from the accreted material that built the planet, rather than being delivered by comets at a later time. From this understanding, we can start to model the likelihood of a similar outcome occurring on rocky exoplanets around other stars.

Assuming terrestrial-like planets are indeed common – with a silicate mantle surrounding a metallic core – then we can expect that water may be exuded onto their surface during the final stages of magma cooling – or otherwise out-gassed as steam which then cools to fall back to the surface as rain. From there, if the planet is big enough to gravitationally retain a thick atmosphere and is in the temperature zone where water can remain fluid, then you’ve got yourself an exo-ocean.

We can assume that the dust cloud that became the Solar System had lots of water in it, given how much persists in the left-over ingredients of comets, asteroids and the like. When the Sun ignited some of this water may have been photodissociated – or otherwise blown out of the inner solar system. However, cool rocky materials seem to have a strong propensity to hold water – and in this manner, could have kept water available for planet formation.

Meteorites from differentiated objects (i.e. planets or smaller bodies that have differentiated such that, while in a molten state, their heavy elements have sunk to a core displacing lighter elements upwards) have around 3% water content – while some undifferentiated objects (like carbonaceous asteroids) may have more than 20% water content.

Mush these materials together in a planet formation scenario and materials compressed at the centre become hot, causing outgassing of volatiles like carbon dioxide and water. In the early stages of planet formation much of this outgassing may have been lost to space – but as the object approaches planet size, its gravity can hold the outgassed material in place as an atmosphere. And despite the outgassing, hot magma can still retain water content – only exuding it in the final stages of cooling and solidification to form a planet’s crust.

Mathematical modelling suggests that if planets accrete from materials with 1 to 3% water content, liquid water probably exudes onto their surface in the final stages of planet formation – having progressively moved upwards as the planet’s crust solidified from the bottom up.

Otherwise, and even starting with a water content as low as 0.01%, Earth-like planets would still generate an outgassed steam atmosphere that would later rain down as fluid water upon cooling.

As the Earth formed, water contained in rocky materials either 'outgassed' or just exuded onto the surface - as magma solidified, from the bottom up, to form the Earth's crust. And OK, this is just a nice image of a deep sea volcanic vent - but you get the idea. Credit: Woods Hole Oceanographic Institution.

If this ocean formation model is correct, it can be expected that rocky exoplanets from 0.5 to 5 Earth masses, which form from a roughly equivalent set of ingredients, would be likely to form oceans within 100 millions years of primary accretion.

This model fits well with the finding of zircon crystals in Western Australia – which are dated at 4.4 billion years and are suggestive that liquid water was present that long ago – although this preceded the Late Heavy Bombardment (4.1 to 3.8 billion years ago) which may have sent all that water back into a steam atmosphere again.