Geological Time

When we talk about something being “recent”, what do we mean?

In day to day terms recent can mean this week, this month, or this year. It implies that an event is contemporary, that it didn’t occur too long ago. Context is all important. When we talk about history we consider “recent events” to be those that have had a direct impact on the present. In human terms that could means the last century or so. 

Geologists have a very different concept of time. When we study the way landscapes have evolved “recent” can mean a few million years. These time scales are very hard for us to wrap our heads around. It is easy to imagine a year, because we all experience them. A century is about the length of the longest human lifespans, and that probably isn’t a coincidence. It is a long period of time, but one which is easy for most of us to visualise and understand. We know how long we have lived, and can easily imagine the lives of our parents and grandparents. It’s when we try to imagine the passage of time before then that we can run into trouble.

Once we get into longer timescales everything becomes a bit harder to picture. How many generations have passed since the dawn of civilisation?  How long ago did the dinosaurs die out? We can work out the answers to these questions, but it takes a lot of practice to put historical events in perspective. One oft cited example from the field of archaeology is that Cleopatra VII lived closer to the present day than to the time when the pyramids were built. This takes a lot of people by surprise; after all they were both part of a period which we, in the popular consciousness at least, define quite loosely as “Ancient Egypt”. The fact that this “period” lasted for thousands of years, including numerous different civilisations and cultures is easy for those of us who aren’t historians to forget. 

We run into similar problems when talking about the far larger spans of time associated with palaeontology and geology. Anatomically modern humans have only actually existed for around 200,000 years. Our various ancestor species existed before that, and it is believed that human like species have been using tools for around 2.5 million years. This seems like an amazing length of time, but in the grand scheme of things it is the blink of an eye when compared to the periods over which other successful groups of species have existed. 

The first dinosaurs are believed to have emerged during the middle Triassic period; a staggering 232 million years ago. This means that the last dinosaurs, which lived around 65 million years ago lived closer to the modern day than they did to the time when their earliest ancestors evolved. Humanity, and its descendant species, will have to do very well if we are to last for even a fraction of this length of time. 

Millions of years are all very well but the age of the earth is far, far longer. The earth formed 4.5 billion years ago. Just to confuse things there are actually two definitions of “billion”. What we’re talking about here is 1x10⁹ a one with nine zeros after it.  to write this out the long way the world is 4,500,000,000 years old. 

It is likely that life formed quite “rapidly”, probably within the first 0.5 billion years. Many of the processes that shape the planet move quite slowly. One good example of this is the movement of the tectonic plates. Many dinosaurs lived on a massive supercontinent called Pangea, at a time when all of the landmass of the earth formed one massive block. From around 200 million years ago Pangea gradually split apart forming the continents as we know them today. 

The movement that results in this dramatic change is so slow that it is hard to observe. In order to figure out the past arrangement of the continents we have to examine numerous lines of evidence to reconstruct a jigsaw puzzle millions of years in the making. The further back in time we got the harder this process becomes, as evidence such as the fossil record becomes more fragmentary. 

But some of you will likely have noticed that the breakup of Pangea is pretty recent in geological terms. There is evidence to suggest that Pangea was the latest of eight supercontinents which have existed over the course of Earth’s history. The earliest, usually referred to as “Ur” likely formed around 3 billion (3x10⁹) years ago. Earth had been around for 1.5 billion years before this first supercontinent formed, but the processes which build up continental crust are very slow. Ur was the first time enough of this landmass came together to form a full continent. It took numerous cycles to produce the amount of landmass which we know today. Continents gradually merged and rifted, with more landmass being produced all the time. 

Looking back through time is never easy, and there is frequently a lot of uncertainty involved. Fortunately the science of geology has developed numerous tools for peering back in time. We can date objects using the ratios of radioactive elements, and extract cores of soil or ice to determine what was happening thousands of years ago. We can use the fossil record, and the stratigraphic layers built up over time to go back even further. These techniques and many others allow us to work out what processes are occurring on scales which no one human could observe.

Guess the Planet 4: Cratered Plane

This heavily cratered plane looks like a classic space image. But on what planet or moon can it be found, and what is the interesting feature of this area? Check back on Friday to find out...

“Spiders” on Mars

In this weeks guess the planet I showed you these spiderlike landforms.These features are actually called spiders or "araneiforms". They are found near the south pole of Mars, an area covered with carbon dioxide ice. On Earth Carbon Dioxide is only really found in its gaseous form, but because temperatures on Mars are so much colder the icecaps consist of large amounts of solid CO₂. This has the potential to produce landforms which are not found on earth at all. 

There is still a fair amount of debate surrounding the formation of these spider-like features, but it seems likely that they are created when geysers erupt from the ice. These eruptions erode the spiderweb like channels that can be seen in the image, and deposit dark material on the surface, making them stand out from the bright terrain on which they form. 

So what causes these geysers? There are a variety of theories for what is warming up the subsurface and causing the eruptions to occur. The leading theory is that these slabs of CO₂ are largely translucent, so sunlight can penetrate through the ice. It is possible that the ice thus acts like a greenhouse, trapping head and allowing the subsurface to warm up. Of course warm ice doesn’t stay in the solid phase very long, and as we know from Earth you have to keep CO₂ very cold if you want it to remain as ice. 

Phase changes such as boiling, melting, and freezing are controlled by two factors; temperature and pressure. Earth is very unusual, in that our mean temperature and pressure conditions lie close to what is called the “triple point” of water. This is the point at which it can easily exist in any of the three common states of matter, as liquid water, as ice, or as water vapour in the atmosphere. Earth’s temperature and pressure conditions are a long way from the triple point of carbon dioxide. It can only be stable as a gas on Earth. 

On Mars temperatures are much colder so CO₂ Ice can form much more readily. However liquid CO₂ still isn’t stable under these conditions. When CO₂ ice undergoes a phase change it goes straight from its solid state to a gaseous one, a process called sublimation. We can actually tell that the fluid which carved the spider channels was a gas rather than a liquid, because some of the channels appear to flow up hill. 

This theory explains several aspects of the spider features, but there are a wide range of other hypotheses as well, including that the heat is generated by biological processes under the ice. Many scientists think that a greenhouse effect is more likely, but until we visit the south pole of Mars and observe these features directly we won’t know for sure. This sort of uncertainty is very common in planetary science, and is why rover missions are so important. 

Further reading

NationalGeographic 

NASA

 Image credit: HiRISE image: ESP_011408_0930, NASA/JPL/University of Arizona

What is a planet?

Many of you spotted that last week’s “guess the planet” was actually a moon. Let’s take a look at the definitions of the different objects of the solar system, and how they relate to one another. In many cases the category into which we put an object tells us a lot about it, In others it can seem almost arbitrary. For example the “demotion” of Pluto from planet to dwarf planet status in 2006, proved unpopular with the public, but was an important change which allows us to better classify Pluto like objects. 

So what are the different objects in the solar system and how are they classified? Clearly we have the Sun at the centre. It is our only star, and everything else revolves around it. We then have the planets and dwarf planets with which most of us probably have at least a passing familiarity. In terms of mass the contents of the solar system is mostly Jupiter. This figure,from Wikipedia shows how it dwarfs everything else, including Saturn which is our next most massive object. 

Jupiter and Saturn are both gas giants. They are composed primarily of hydrogen and helium. These are the lightest elements of the periodic table, so the fact that Jupiter and Saturn are so much more massive than everything else drives home how huge they really are. 

Uranus and Neptune are also gas planets, but they have a somewhat different composition, and so are classified as Ice Giants. They still have a lot of hydrogen and helium in them, but are mostly composed of heavier elements, including oxygen, nitrogen, and carbon. These were incorporated into the planets in the form of ices, although they don’t remain ice like today. These planets were originally classified as gas giants, but were separated into their own class once it became apparent that they were substantially different to Jupiter and Saturn. 

Although they are fascinating, we probably won’t be looking at the gas planets much in this blog. They kind of lack the solid surface needed for geomorphology. That said I might throw them into the guess the planet column every so often...

The giant planets might have most of the mass of the solar system, but it is the smaller objects which introduce most of the variety. There are four terrestrial planets; Mercury, Venus, Earth and Mars. For much of the history of astronomy “planet” had been fairly loosely defined and so Pluto was also part of that list. However as more Pluto like objects were discovered in the outer solar system it became clear that a more rigorous classification scheme was needed. The new definition of a planet is that must orbit the sun, be large enough to be roughly spherical, and have “cleared the neighbourhood around its orbit” of smaller bodies. 

Let’s look at each of these statements in turn.

Orbiting the sun is fairly straight forwards; this is what separates planets from satellites or moons. We usually think of moons as being smaller than the planets, but this doesn’t mean that they are small objects, just smaller than the object they orbit. Some, such as Ganymede and Titan are actually larger than Mercury, and vastly bigger than Pluto, if they orbited the sun they would be considered planets on a par with anything in the inner solar system. Pluto’s moon Charon is almost as large as the planet it orbits, and in some ways they could be considered to be a binary planetary system. The solar system has 181 known moons, orbiting most of the main planets and a few of the dwarf planets. 

The second criteria in the definition of a planet is that it “has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape” this sounds complicated, but just means that when the planet was forming, its gravity was strong enough to pull it into a spherical shape, rather than remaining irregular like many of the smaller moons and asteroids. Planets are never perfectly spherical; they are actually what is termed an “oblate sphereoid”. 

The dwarf planets meet these first two criteria, they are roughly spherical, and they are in orbit around the sun. However they don’t meet the last of the three, which is that they have cleared the neighbourhood of their orbit. This means that they have swept up all of the other material in the same part of space as them, with it either impacting as meteorites, or going into orbit as satellites. 

The eight planets have largely cleared their orbits, although there is so much very small material floating around that it is quite hard to be entirely certain. The dwarf planets have not, and share their regions of space with numerous asteroids and planetesimals. 

There are currently five dwarf planets; Ceres, Pluto, Haumea, Makemake and Eris. Four of these are transneptunian objects, while Ceres is the largest body in the asteroid belt. As many as 200 other transneptunian objects are likely to meet the criteria to be dwarf planets, but assessing these is a long and arduous process, so relatively few have so far been accepted as such. This is the reason for Pluto being reclassified. If we were to continue to consider it a planet then there are a great many objects which also qualify. 

The roster of planets has never been set in stone; it has been continually expanded as new objects were discovered. It is worth noting that Pluto wasn’t always the ninth planet, and is not the first planet to be reclassified as something else. Ceres is the largest body in the asteroid belt. When first discovered, prior to Pluto, it was classified as a planet. However it was later demoted to an asteroid. It is now considered a dwarf planet again. 

Pluto may have been considered a planet for a long time, but it is very different to the other terrestrial planets and never quite fit into that classification. It is much smaller, it is much icier and it is located in the outer solar system. This gives it dramatically different environmental conditions to the rocky planets and moons.  The current classification makes it the largest of an interesting new class of objects, which we are only beginning to explore. A lot of people dislike this, because for cultural reasons we are used to thinking of it as part of a select club. But I would argue that Pluto deserves its own category. 

So what about the objects that don’t meet the second or third criteria? The remainder of objects in the solar system are officially classified as “small solar system bodies”. They are much smaller than even the littlest of the dwarf planets, and there are a vast number of them. There are 552,894 known asteroids and 3,083 known comets. Like many of the smaller moons they don’t have sufficient mass to become spherical. They are frequently found in regions of space such as the asteroid belt, where numerous other small objects occur.

Over the course of this blog I’ll be trying to show you interesting things from as many of these worlds as possible, be they planets, moons or not. We have an amazing amount of instruments out in the solar system studying these bodies. So this shouldn’t be too difficult to achieve. 

References:

Definition of the Planet at the IAU

Solar System Stats

Image credit: 

Planet Size Comparison CC BY 3.0

Guess the Planet 3: Spider-like features

What are these strange, spider-like features sprawled across the surface of another world? On which planet are they found and what do you think might cause them? Tune in on Friday to find out...
Happy Halloween!

The Seas of Titan

As many of you have spotted this week’s guess the planet doesn’t actually come from a planet. This image is from Titan, the largest of Saturn’s moons. We’ll be seeing a few moons and dwarf planets in this series, and maybe the odd asteroid and comet. 

This is quite a famous image, so it’s good that I had a few responses from people who knew it straight away. It shows Ligeia Mare, the second largest of Titans seas. Here is an image of the northern hemisphere of Titan to put these seas into context. The North Pole of Titan is at the centre of the image, which extends out to 50 degrees north. The credit for this image goes to the Cassini Team, NASA/JPL-Caltech/ASI/USGS More information on this area can be found on the Cassini website. The image from Monday's post can be found here.

The result is a detailed map of this strange moon, showing that the northern hemisphere is covered by numerous lakes and seas. This is actually a false colour image, because Titan is shrouded in thick layers of cloud, so it isn’t possible to photograph the surface from orbit as we can with Mars, the Moon, and when the weather permits, Earth. Instead this image was produced using radar. Different materials respond to the radar in different ways, allowing the Cassini team to determine which areas are solid, which are liquid, map the topography of the landscape, and even determine the roughness of the surface. 

Ligeia Mare itself is 420 by 350 km across, so covers a sizable area. It is surrounded by many smaller lakes and may be connected to the other large seas that occupy this part of the Titan’s surface. We are still not certain why this region of Titan has these large bodies of liquid, when other regions do not. 

While it might look like a terrestrial sea, Ligeia Mare wouldn’t be much fun to swim in, since it is composed of liquid hydrocarbons rather than water. It is so cold on Titan that water never thaws and is only found as solid ice. The liquid that makes up these lakes is composed of Methane and Ethane. On Earth we know these chemicals as gasses, but at low temperatures in the outer solar system they exist only in their liquid state. 

Titan is the only body besides Earth to have stable liquid oceans on its surface which makes it a fascinating opportunity to compare how “hydrology” and coastal processes have evolved separately using different materials in two very different temperature regimes.