What is an Ice Age?

Scientists have known for some time that the Earth goes through cycles of climatic change. Owing to changes in Earth’s orbit, geological factors, and/or changes in Solar output, Earth occasionally experiences significant reductions in its surface and atmospheric temperatures. This results in long-term periods of glaciation, or what is more colloquially known as an “ice age”.

These periods are characterized by the growth and expansion of ice sheets across the Earth’s surface, which occurs every few million years. By definition we are still in the last great ice age – which began during the late Pliocene epoch (ca. 2.58 million years ago) – and are currently in an interglacial period, characterized by the retreat of glaciers.

Definition:

While the term “ice age” is sometime used liberally to refer to cold periods in Earth’s history, this tends to belie the complexity of glacial periods. The most accurate definition would be that ice ages are periods when ice sheets and glaciers expand across the planet, which correspond to significant drops in global temperatures and can last for millions of years.

The Antarctic ice sheet, which expanded during the last ice age. Credit: Wikipedia Commons/Stephen Hudson

During an ice age, there are significant temperature differences between the equator and the poles, and temperatures at deep-sea levels have also been shown to drop. This allows for large glaciers (comparable to continents) to expand, covering much of the surface area of the planet. Since the Pre-Cambrian Era (ca. 600 million years ago), ice ages have occurred at widely space intervals about about 200 million years.

History of Study:

The first scientist to theorize about past glacial periods was the 18th century Swiss engineer and geographer Pierre Martel. In 1742, while visiting an Alpine valley, he wrote about the dispersal of large rocks in erratic formations, which the locals attributed to the glaciers having once extended much further. Similar explanations began to emerge in the ensuing decades for similar patterns of boulder distribution in other parts of he world.

From the middle of the 18th century onward, European scholars increasingly began to contemplate ice as a means of transporting rocky material. This included the presence of boulders in coastal areas in the Baltic states and the Scandinavian peninsula. However, it was Danish-Norwegian geologist Jens Esmark (1762–1839) who first argued the existence of a sequence of world wide ice ages.

This theory was detailed in a paper he published in 1824, in which he proposed that changes in Earth’s climate (which were due to changes in its orbit) were responsible. This was followed in 1832 by German geologist and forestry professor Albrecht Reinhard Bernhardi speculating about how the polar ice caps may have once reached as far as the temperate zones of the world.

Overlook of the Grinnell Glacier in Glacier National Park, Montana. Credit: USGS

At this same time, German botanist Karl Friedrich Schimper and Swiss-American biologist Louis Agassiz began independently developing their own theory about global glaciation, which led toSchimper coining the term “ice age” in 1837. By the late 19th century, ice age theory gradually began to gain widespread acceptance over the notion that the Earth cooled gradually from its original, molten state.

By the 20th century, Serbian polymath Milutin Milankovic developed his concept of Milankovic cycles, which linked long-term climate changes to periodic changes in the Earth’s orbit around the Sun. This offered a demonstrable explanation for ice ages, and allowed scientists to make predictions about when significant changes in Earth’s climate might occur again.

Evidence for Ice Ages:

There are three forms of evidence for ice age theory, which range from the geological and the chemical to the paleontological (i.e. the fossil record). Each has its particular benefits and drawbacks, and has helped scientists to develop a general understanding of the effect ice ages have had on geological record for the past few billion years.

Geological: Geological evidence includes rock scouring and scratching, carved valleys, the formation of peculiar types of ridges, and the deposition of unconsolidated material (moraines) and large rocks in erratic formations.  While this sort of evidence is what led to ice age theory in the first place, it remains temperamental.

For one, successive glaciation periods have different effects on a region, which tends to distort or erase geological evidence over time. In addition, geological evidence is difficult to date exactly, causing problems when it comes to getting an accurate assessment of how long glacial and interglacial periods have lasted.

Horseshoe-shaped lateral moraines at the margin of the Penny Ice Cap on Baffin Island, Nunavut, Canada. Lateral moraines are accumulations of debris along the sides of a glacier formed by material falling from the valley wall. Credit: NASA/Michael Studinger

Chemical: This consists largely of variations in the ratios of isotopes in fossils discovered in sediment and rock samples. For more recent glacial periods, ice cores are used to construct a global temperature record, largely from the presence of heavier isotopes (which lead to higher evaporation temperatures). They often contain bubbles of air as well, which are examined to assess the composition of the atmosphere at the time.

Limitations arise from various factors, however. Foremost among these are isotope ratios, which can have a confounding effect on accurate dating. But as far as the most recent glacial and interglacial periods are concerned (i.e. during the past few million years), ice core and ocean sediment core samples remain the most trusted form of evidence.

Paleontological: This evidence consists of changes in the geographical distribution of fossils. Basically, organisms that thrive in warmer conditions become extinct during glacial periods (or become highly restricted in lower latitudes), while cold-adapted organisms thrive in these same latitudes. Ergo, reduced amounts of fossils in higher latitudes is an indication of the spread of glacial ice sheets.

This evidence can also be difficult to interpret because it requires that the fossils be relevant to the geological period under study. It also requires that sediments over wide ranges of latitudes and long periods of time show a distinct correlation (due to changes in the Earth’s crust over time). In addition, there are many ancient organisms that have shown the ability to survive changes in conditions for millions of years.

As a result, scientists rely on a combined approach and multiple lines of evidence wherever possible.

Ice ages are characterized by a drop in average global temperatures, resulting in the expansion of ice sheets globally. Credit: NASA

Causes of Ice Ages:

The scientific consensus is that several factors contribute to the onset of ice ages. These include changes in Earth’s orbit around the Sun, the motion of tectonic plates, variations in Solar output, changes in atmospheric composition, volcanic activity, and even the impact of large meteorites. Many of these are interrelated, and the exact role that each play is subject to debate.

Earth’s Orbit: Essentially, Earth’s orbit around the Sun is subject to cyclic variations over time, a phenomenon also known as Milankovic (or Milankovitch) cycles. These are characterized by changing distances from the Sun, the precession of the Earth’s axis, and the changing tilt of the Earth’s axis – all of which result in a redistribution of the sunlight received by the Earth.

The most compelling evidence for Milankovic orbital forcing corresponds closely to the most recent (and studied) period in Earth’s history (circa. during the last 400,000 years). During this period, the timing of glacial and interglacial periods are so close to changes in Milankovic orbital forcing periods that it is the most widely accepted explanation for the last ice age.

Tectonic Plates: The geological record shows an apparent correlation between the onset of ice ages and the positions of the Earth’s continents. During these periods, they were in positions which disrupted or blocked the flow of warm water to the poles, thus allowing ice sheets to form.

The Earth’s Tectonic Plates. Credit: msnucleus.org

This in turn increased the Earth’s albedo, which reduces the amount of solar energy absorbed by the Earth’s atmosphere and crust. This resulted in a positive feedback loop, where the advance of ice sheets further increased the Earth’s albedo and allowed for more cooling and more glaciation. This would continue until the onset of a greenhouse effect ended the period of glaciation.

Based on past ice-ages, three configurations have been identified that could lead to an ice age – a continent sitting atop the Earth’s pole (as Antarctica does today); a polar sea being land-locked (as the Arctic Ocean is today); and a super continent covering most of the equator (as Rodinia did during the Cryogenian period).

In addition, some scientists believe that the Himalayan mountain chain – which formed 70 million years ago – has played a major role in the most recent ice age. By increasing the Earth’s total rainfall, it has also increased the rate at which CO² has been removed from the atmosphere (thereby decreasing the greenhouse effect). Its existence has also paralleled the long-term decrease in Earth’s average temperature over the past 40 million years.

Atmospheric Composition: There is evidence that levels of greenhouse gases fall with the advance of ice sheets and rise with their retreat. According to the “Snowball Earth” hypothesis – in which ice completely or very nearly covered the planet at least once in the past – the ice age of the late Proterozoic was ended by an increase in CO² levels in the atmosphere, which was attributed to volcanic eruptions.

Image of the Harding Ice Field on Alaska’s Kenai Peninsula. Credit: US Fish and Wildlife Service

However, there are those who suggest that increased levels of carbon dioxide may have served as a feedback mechanism, rather than the cause. For example, in 2009, an international team of scientists produced a study – titled “The Last Glacial Maximum” – that indicated that an increase in solar irradiance (i.e. energy absorbed from the Sun) provided the initial change, whereas greenhouse gases accounted for the magnitude of change.

Major Ice Ages:

Scientists have determined that at least five major ice ages took place in Earth’s history. These include the Huronian, Cryogenian, Andean-Saharan, Karoo, and the Qauternary ice ages. The Huronian Ice Age is dated to the early Protzerozoic Eon, roughly 2.4 to 2.1 billion years ago, based on geological evidence observed to the north and north-east of Lake Huron (and correlated to deposits found in Michigan and Western Australia).

The Cryogenian Ice Age lasted from roughly 850 to 630 million years ago, and was perhaps the most severe in Earth’s history. It is believed that during this period, the glacial ice sheets reached the equator, thus leading to a “Snowball Earth” scenario. It is also believed that ended due to a sudden increase in volcanic activity that triggered a greenhouse effect, though (as noted) this is subject to debate.

The Andean-Saharan Ice Age occurred during the Late Ordovician and the Silurian period (roughly 460 to 420 million years ago). As the name suggests, the evidence here is based on geological samples take from the Tassili n’Ajjer mountain range in the western Sahara, and correlated by evidence obtained from the Andean mountain chain in South America (as well as the Arabian peninsula and the south Amazon basin).

Floating ice at the calving front of Greenland’s Kangerdlugssuaq glacier, photographed in 2011 during Operation IceBridge. Credit: NASA/Michael Studinger

The Karoo Ice Age is attributed to the evolution of land plants during the onset of the Devonian period (ca. 360 to 260 million years ago) which caused a long-term increase in planetary oxygen levels and a reduction in CO² levels – leading to global cooling. It is named after sedimentary deposits that were discovered in the Karoo region of South Africa, with correlating evidence found in Argentina.

The current ice age, known as the Pliocene-Quaternary glaciation, started about 2.58 million years ago during the late Pliocene, when the spread of ice sheets in the Northern Hemisphere began. Since then, the world has experienced several glacial and interglacial periods, where ice sheets advance and retreat on time scales of 40,000 to 100,000 years.

The Earth is currently in an interglacial period, and the last glacial period ended about 10,000 years ago. What remains of the continental ice sheets that once stretched across the globe are now restricted to Greenland and Antarctic, as well as smaller glaciers – like the one that covers Baffin Island.

Anthropogenic Climate Change:

The exact role played by all the mechanisms that ice ages are attributed to – i.e. orbital forcing, solar forcing, geological and volcanic activity – are not yet entirely understood. However, given the role of carbon dioxide and other greenhouse gas emissions, there has been a great deal of concern in recent decades what long-term effects human activity will have on the planet.

For instance, in at least two major ice ages, the Cryogenian and Karoo Ice Ages, increases and decreases in atmospheric greenhouse gases are believed to have played a major role. In all other cases, where orbital forcing is believed to be the primary cause of an ice age ending, increased greenhouse gas emissions were still responsible for the negative feedback that led to even greater increases in temperature.

The addition of CO2 by human activity has also played a direct role in climatic changes taking place around the world. Currently, the burning of fossil fuels by humans constitutes the largest source of emissions of carbon dioxide (about 90%) worldwide, which is one of the main greenhouse gases that allows radiative forcing (aka. the Greenhouse Effect) to take place.

In 2013, the National Oceanic and Atmospheric Administration announced that CO² levels in the upper atmosphere reached 400 parts per million (ppm) for the first time since measurements began in the 19th century. Based on the current rate at which emissions are growing, NASA estimates that carbon levels could reach between 550 to 800 ppm in the coming century.

If the former scenario is the case, NASA anticipates a rise of 2.5 °C (4.5 °F) in average global temperatures, which would be sustainable. However, should the latter scenario prove to be the case, global temperatures will rise by an average of 4.5 °C (8 °F), which would make life untenable for many parts of the planet. For this reason, alternatives are being sought out for development and widespread commercial adoption.

What’s more, according to a 2012 research study published in Nature Geoscience – titled “Determining the natural length of the current interglacial” – human emissions of CO² are also expected to defer the next ice age. Using data on Earth’s orbit to calculate the length of interglacial periods, the research team concluded that the next ice (expected in 1500 years) would require atmospheric CO² levels to remain beneath around 240?ppm.

Learning more about the longer ice ages as well the shorter glacial periods that have taken place in Earth’s past is important step towards understanding how Earth’s climate changes over time. This is especially important as scientists seek to determine how much of modern climate change is man-made, and what possible counter-measures can be developed.

We have written many articles about the Ice Age for Universe Today. Here’s New Study Reveals Little Ice Age Driven by Volcanism, Did a Killer Asteroid Drive the Planet into an Ice Age?, Was There a Slushball Earth?, and Is Mars Coming out of an Ice Age?

If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth and Episode 308: Climate Change.

Source:

Astronomy Cast Ep. 415: Temperature of the Universe

The temperature of the Universe can vary a dramatic amount from the hot cores of stars to the vast cold emptiness of deep space. What’s the temperature of the Universe now, and what will it be in the future?

Visit the Astronomy Cast Page to subscribe to the audio podcast!

We record Astronomy Cast as a live Google+ Hangout on Air every Monday at 12:00 pm Pacific / 3:00 pm Eastern. You can watch here on Universe Today or from the Astronomy Cast Google+ page.

Could We Terraform a Black Hole?

Is there any possible way to take a black hole and terraform it to be a place we could actually live?

In the challenge of terraforming the Sun, we all learned that outside of buying a Dyson Spaceshell 2000 made out of a solar system’s worth of planetbutter, it’s a terrible idea.

Making a star into a habitable world, means first destroying the stellar furnace. Which isn’t good for anyone, “Hey, free energy! vs. Let’s wreck this thing and build houses!”

Doubling down on this idea, a group of brilliant Guidensians wanted to crank the absurdity knob all the way up. You wanted to know if it would be possible to terraform a black hole.

In order to terraform something, we convert it from being Britney Spears’ level of toxic into something that humans can comfortably live on. We want reasonable temperatures, breathable atmosphere, low levels of radiation, and Earthish gravity.

With temperatures inversely proportional to their mass, a solar mass black hole is about 60 billionths of a Kelvin. This is just a smidge over absolute zero. Otherwise known as “pretty damn” cold. Actively feeding black holes can be surrounded by an accretion disk of material that’s more than 10 million degrees Kelvin, which would also kill you. Make a note, fix the temperature.

There’s no atmosphere, and it’s either the empty vacuum of space, or the superheated plasma surrounding an actively feeding black hole. Can you breathe plasma? If the answer is yes, this could work for you. If not, we’ll need to fix that.

You’d be hard pressed to find a more lethal radiation source in the entire Universe.

Black holes can spin at close to the speed of light, generating massive magnetic fields. These magnetic fields whip high energy particles around them, creating lethal doses of radiation. There are high energy particle jets pouring out of some supermassive black holes, moving at nearly the speed of light. You don’t want any part of that. We’ll add that to the list.

Black holes are known for being an excellent source of vitamin gravity. Out in orbit, it’s not so bad. Replace our Sun with a black hole of the same mass, and you wouldn’t be able to tell the difference.

So, problem solved? Not quite. If you tried to walk on the surface, you’d get shredded into a one-atom juicy stream of extruded tubemanity before you got anywhere near the time traveling alien library at the caramel center.

Reduce the gravity. Got it.

Artist rendering of a supermassive black hole. Credit: NASA / JPL-Caltech.
Artist rendering of a supermassive black hole. Credit: NASA / JPL-Caltech.

As we learned in a previous episode on how to kill black holes, there’s nothing you can do to affect them. You couldn’t smash comets into it to give it an atmosphere, it would just turn them into more black hole. You couldn’t fire a laser to extract material and reduce the mass, it would just turn your puny laser into more black hole.

Antimatter, explosives, stars, rocks, paper, scissors…black hole beats them all.

Repeat after me. “Om, nom, nom”.

All we can do is wait for it to evaporate over incomprehensible lengths of time. There are a few snags with this strategy, such as it will remain as a black hole until the last two particles evaporate away. There’s no point where it would magically become a regular planetoid.

That’s a full list of renovations for the cast and crew of “Pimp my Black Hole”.

Let’s look at our options. You can move it, just like we can move the Earth. Throw stuff really close to a black hole, and you get it moving with gravity. You could make it spin faster by dropping stuff into it, right up until it’s rotating at the edge of the speed of light, and you can make it more massive.

With that as our set of tools, there’s no way we’re ever going to live on a black hole.

It could be possible to surround a black hole with a Dyson Sphere, like a star.

Freemon Dyson theorized that eventually, a civilization would be able to build a megastructure around its star to capture all its energy. Credit: SentientDevelopments.com
Freemon Dyson theorized that eventually, a civilization would be able to build a megastructure around its star to capture all its energy. Credit: SentientDevelopments.com

It turns out there’s a way to have a pet black hole pay dividends aside from eating all your table scraps, shameful magazines and radioactive waste. By dropping matter into a black hole that’s spinning at close to the speed of light, you can actually extract energy from it.

Imagine you had an asteroid that was formed by two large rocks. As they get closer and closer to the black hole, tidal forces tear them apart. One chunk falls into the black hole, the smaller remaining rock has less collective mass, which allows it to escape. This remaining rock steals rotational energy from the black hole, which then slows down the rotation just a little bit.

This is the Penrose Process, named after the physicist who developed the idea. Astronomers calculated you can extract 20% of pure energy from matter that you drop in.

There’s isn’t much out there that would give you better return on your investment.

Also, it’s got to have a similar satisfying feeling as dropping pebbles off a bridge and watching them disappear from existence.

Terraforming a black hole is a terrible idea that will totally get us all killed. Don’t do it.

If you have to get close to that freakish hellscape I do recommend surrounding your pet with a Dyson Sphere and then feeding it matter and enjoying the energy you get in return.

A futuristic energy hungry civilization bent on evil couldn’t hope for a better place to live.

Have you got any more questions about black holes? Give us your suggestions in the comments below.

In Fact It’s Cold As Hell: Mars Isn’t As Earthlike As It Might Look

The slopes of Gale Crater as seen by Curiosity are reminiscent of the American southwest (NASA/JPL-Caltech)

“Mars ain’t no kind of place to raise your kids; in fact it’s cold as hell” sang Elton John in “Rocket Man”, and although the song was released in 1972 — four years before the first successful landing on Mars — his weather forecast was spot-on. Even though the fantastic images that are being returned from NASA’s Curiosity rover show a rocky, ruddy landscape that could easily be mistaken for an arid region of the American Southwest one must remember three things: this is Mars, we’re looking around the inside of an impact crater billions of years old, and it’s cold out there.

Mars Exploration Program blogger Jeffrey Marlow writes in his latest “Martian Diaries” post:

Over the first 30 sols, air temperature has ranged from approximately -103 degrees Fahrenheit (-75 Celsius) at night to roughly 32 degrees Fahrenheit (0 Celsius) in the afternoon. Two factors conspire to cause such a wide daily range (most day-night fluctuations on Earth are about 10 to 30 degrees Fahrenheit). The martian atmosphere is very thin; with fewer molecules in the air to heat up and cool down, there’s more solar power to go around during the day, and less atmospheric warmth at night, so the magnitude of temperature shifts is amplified. There is also very little water vapor; water is particularly good at retaining its heat, and the dryness makes the temperature swings even more pronounced. 

In that way Mars is like an Earthly desert; even after a blisteringly hot day the temperatures can plummet at night, leaving an ill-prepared camper shivering beneath the cold glow of starlight. Except on Mars, where the Sun is only 50% as bright as on Earth and the atmosphere only 1% as dense, the nighttime lows dip to Arctic depths.

“Deserts on Earth have very extreme temperature ranges,” says Mars Science Laboratory Deputy Project Scientist, Ashwin Vasavada. “So if you take a desert on Earth and put it in a very thin atmosphere 50% farther from the Sun, you’d have something like what we’re seeing at Gale Crater.”

And although the afternoon temperatures in Gale may climb slightly above freezing that doesn’t mean liquid water will be found pooling about in any large amounts. Curiosity’s in no danger from flash floods on Mars… not these days, anyway.

With atmospheric pressure just above water’s thermodynamic triple point, and temperatures occasionally hovering around the freezing point, it is likely that local niches are seeing above-zero temperatures, and Vasavada acknowledges, “liquid water could exist here over a tiny range of conditions.” But don’t expect a Culligan water plant in Gale Crater any time soon. “We wouldn’t expect for Curiosity to see liquid water, because it would evaporate or re-freeze too quickly,” explains Vasavada. “With so little water vapor in the atmosphere, any liquid water molecules on the surface would quickly turn to gas.”

So when on Mars, drink your coffee quickly. (And pack a blanket.)

“Gale Crater may look like the dusty, basaltic basins of the American southwest, but one look at the thermometer will send you running for the winter coat.”

– Jeffrey Marlow, Martian Diaries

Read Marlow’s full article here.

Image: Sunset on Mars seen by the MER Spirit from Gusev Crater in 2005 (NASA/JPL-Caltech)

What is the Boltzmann Constant?

There are actually two Boltzmann constants, the Boltzmann constant and the Stefan-Boltzmann constant; both play key roles in astrophysics … the first bridges the macroscopic and microscopic worlds, and provides the basis for the zero-th law of thermodynamics; the second is in the equation for blackbody radiation.

The zero-th law of thermodynamics is, in essence, what allows us to define temperature; if you could ‘look inside’ an isolated system (in equilibrium), the proportion of constituents making up the system with energy E is a function of E, and the Boltzmann constant (k or kB). Specifically, the probability is proportional to:

e-E/kT

where T is the temperature. In SI units, k is 1.38 x 10-23 J/K (that’s joules per Kelvin). How Boltzmann’s constant links the macroscopic and microscopic worlds may perhaps be easiest seen like this: k is the gas constant R (remember the ideal gas law, pV = nRT) divided by Avogadro’s number.

Among the many places k appears in physics is in the Maxwell-Boltzmann distribution, which describes the distribution of speeds of molecules in a gas … and thus why the Earth’s (and Venus’) atmosphere has lost all its hydrogen (and only keeps its helium because what is lost gets replaced by helium from radioactive decay, in rocks), and why the gas giants (and stars) can keep theirs.

The Stefan-Boltzmann constant (?), ties the amount of energy radiated by a black body (per unit of area of its surface) to the blackbody temperature (this is the Stefan-Boltzmann law). ? is made up of other constants: pi, a couple of integers, the speed of light, Planck’s constant, … and the Boltzmann constant! As astronomers rely almost entirely on detection of photons (electromagnetic radiation) to observe the universe, it will surely come as no surprise to learn that astrophysics students become very familiar with the Stefan-Boltzmann law, very early in their studies! After all, absolute luminosity (energy radiated per unit of time) is one of the key things astronomers try to estimate.

Why does the Boltzmann constant pop up so often? Because the large-scale behavior of systems follows from what’s happening to the individual components of those systems, and the study of how to get from the small to the big (in classical physics) is statistical mechanics … which Boltzmann did most of the original heavy lifting in (along with Maxwell, Planck, and others); indeed, it was Planck who gave k its name, after Boltzmann’s death (and Planck who had Boltzmann’s entropy equation – with k – engraved on his tombstone).

Want to learn more? Here are some resources, at different levels: Ideal Gas Law (from Hyperphysics), Radiation Laws (from an introductory astronomy course), and University of Texas (Austin)’s Richard Fitzpatrick’s course (intended for upper level undergrad students) Thermodynamics & Statistical Mechanics.

Sources:
Hyperphysics
Wikipedia

Earth Surface Temperature

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The average Earth surface temperature is 14° C. That’s 287 kelvin, or 57.2° F.

As you probably realize, that number is just an average. The Earth’s temperature can be much higher or lower than this temperature. In the hottest places of the planet, in the deserts near the equator, the temperature on Earth can get as high as 57.7° C. And then in the coldest place, at the south pole in Antarctica, the temperature can dip down to -89° C.

The reason the average temperature on Earth is so high is because of the atmosphere. This acts like a blanket, trapping infrared radiation close to the planet and warming it up. Without the atmosphere, the temperature on Earth would be more like the Moon, which rises to 116° C in the day, and then dips down to -173° C at night.

We’ve written several articles about the temperature of the planets. Here’s an article about the temperature of all the planets, and here’s an article about the temperature of the Moon.

If you’d like more information on the Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an entire episode of Astronomy Cast all about Earth. Listen here, Episode 51: Earth.

What is Absolute Temperature?

If you measure temperature relative to absolute zero, the temperature is an absolute temperature; absolute zero is 0.

The most widely used absolute temperature scale is the Kelvin, symbolized with a capital K, which uses Celsius-scaled degrees (there’s another one, the Rankine, which is related to the Fahrenheit scale). We write temperatures in kelvins without the degree symbol; absolute zero is 0 K.

Another name for absolute temperature is thermodynamic temperature. Why? Because absolute temperate is directly related to thermodynamics; in fact it is the Zeroth Law of Thermodynamics that leads to a (formal) definition of (thermodynamic) temperature.

Roughly speaking, the temperature of an object (or similar, like the gas in a balloon) measures the kinetic energy of the particles (atoms, molecules, etc) of the matter it’s made up of … in an average sense, and macroscopically. Note that blobs of matter have far more energy than just the kinetic energy of the atoms in the blob – there’s the energy that holds the atoms together in molecules (if there are any), the binding energy of the nuclei (unless the blog is pure hydrogen, with no deuterium), and so on; none of these energies are counted in the blob’s temperature.

You might think that at absolute zero a substance would be in its lowest possible energy state, especially if it is a pure compound (or isotopically pure element). Well, it isn’t quite that simple … leaving aside zero point energy (something quite counter-intuitive, from quantum mechanics), there’s the fact that many solids have several different, stable crystal structures (even at 0 K), but only one with minimal energy. Then there’s helium, which is a liquid at 0 K (the solid phase of a substance has a lower energy than the corresponding liquid phase), unless under pressure.

The Kelvin is one of the International System of Units (SI) base units (there are seven of these), and is defined with reference to the triple point of water (“The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water” is the 1967/8 definition; the current one – adopted in 2005 – expands on this to take account of isotropic variations).

Why is it called the Kelvin? Because William Thompson – Lord Kelvin – was the first to describe an absolute temperature scale, in a paper he wrote in 1848; he also estimated absolute zero was -273o C.

Project Skymath has a nice introduction to absolute temperature.

Some Universe Today material you may find interesting: Absolute Zero, Coldest Temperature Ever Created, and Planck First Light.

Sources: Wikipedia, Hyperphysics