Venus Compared to Earth

Venus is often referred to as “Earth’s Twin” (or “sister planet”), and for good reason. Despite some rather glaring differences, not the least of which is their vastly different atmospheres, there are enough similarities between Earth and Venus that many scientists consider the two to be closely related. In short, they are believed to have been very similar early in their existence, but then evolved in different directions.

Earth and Venus are both terrestrial planets that are located within the Sun’s Habitable Zone (aka. “Goldilocks Zone”) and have similar sizes and compositions. Beyond that, however, they have little in common. Let’s go over all their characteristics, one by one, so we can in what ways they are  different and what ways they are similar.

Continue reading “Venus Compared to Earth”

How Do We Terraform Mars?

As part of our continuing “Definitive Guide To Terraforming” series, Universe Today is happy to present our guide to terraforming Mars. At present, there are several plans to put astronauts and ever settlers on the Red Planet. But if we really want to live there someday, we’re going to need to do a complete planetary renovation. What will it take?

Despite having a very cold and very dry climate – not to mention little atmosphere to speak of – Earth and Mars have a lot in common. These include similarities in size, inclination, structure, composition, and even the presence of water on their surfaces. Because of this, Mars is considered a prime candidate for human settlement; a prospect that includes transforming the environment to be suitable to human needs (aka. terraforming).

That being said, there are also a lot of key differences that would make living on Mars, a growing preoccupation among many humans (looking at you, Elon Musk and Bas Lansdorp!), a significant challenge. If we were to live on the planet, we would have to depend rather heavily on our technology. And if we were going to alter the planet through ecological engineering, it would take a lot of time, effort, and megatons of resources!

The challenges of living on Mars are quite numerous. For starters, there is the extremely thin and unbreathable atmosphere. Whereas Earth’s atmosphere is composed of 78% nitrogen, 21% oxygen, and trace amounts of other gases, Mars’ atmosphere is made up of 96% carbon dioxide, 1.93% argon and 1.89% nitrogen, along with trace amounts of oxygen and water.

Artist's impression of the terraforming of Mars, from its current state to a livable world. Credit: Daein Ballard
Artist’s impression of the terraforming of Mars, from its current state to a livable world. Credit: Daein Ballard

Mars’ atmospheric pressure also ranges from 0.4 – 0.87 kPa, which is the equivalent of about 1% of Earth’s at sea level. The thin atmosphere and greater distance from the Sun also contributes to Mars’ cold environment, where surface temperatures average 210 K (-63 °C/-81.4 °F). Add to this the fact that Mars’ lacks a magnetosphere, and you can see why the surface is exposed to significantly more radiation than Earth’s.

On the Martian surface, the average dose of radiation is about 0.67 millisieverts (mSv) per day, which is about a fifth of what people are exposed to here on Earth in the course of a year. Hence, if humans wanted to live on Mars without the need for radiation shielding, pressurized domes, bottled oxygen, and protective suits, some serious changes would need to be made. Basically, we would have to warm the planet, thicken the atmosphere, and alter the composition of said atmosphere.

Examples In Fiction:

In 1951, Arthur C. Clarke wrote the first novel in which the terraforming of Mars was presented in fiction. Titled The Sands of Mars, the story involves Martian settlers heating up the planet by converting Mars’ moon Phobos into a second sun, and growing plants that break down the Martians sands in order to release oxygen.

In 1984, James Lovelock and Michael Allaby wrote what is considered by many to be one of the most influential books on terraforming. Titled The Greening of Mars, the novel explores the formation and evolution of planets, the origin of life, and Earth’s biosphere. The terraforming models presented in the book actually foreshadowed future debates regarding the goals of terraforming.

Kim Stanley Robinson's Red Mars Trilogy. Credit: variety.com
Kim Stanley Robinson’s Red Mars Trilogy. Credit: variety.com

In 1992, author Frederik Pohl released Mining The Oort, a science fiction story where Mars is being terraformed using comets diverted from the Oort Cloud. Throughout the 1990s, Kim Stanley Robinson released his famous Mars TrilogyRed Mars, Green Mars, Blue Mars – which centers on the transformation of Mars over the course of many generations into a thriving human civilization.

In 2011, Yu Sasuga and Kenichi Tachibana produced the manga series Terra Formars, a series that takes place in the 21st century where scientists are attempting to slowly warm Mars. And in 2012, Kim Stanley Robinson released 2312, a story that takes place in a Solar System where multiple planets have been terraformed – which includes Mars (which has oceans).

Proposed Methods:

Over the past few decades, several proposals have been made for how Mars could be altered to suit human colonists. In 1964, Dandridge M. Cole released “Islands in Space: The Challenge of the Planetoids, the Pioneering Work“, in which he advocated triggering a greenhouse effect on Mars. This consisted of importing ammonia ices from the outer Solar System and then impacting them on the surface.

Since ammonia (NH³) is a powerful greenhouse gas, its introduction into the Martian atmosphere would have the effect of thickening the atmosphere and raising global temperatures. As ammonia is mostly nitrogen by weight, it could also provide the necessary buffer gas which, when combined with oxygen gas, would create a breathable atmosphere for humans.

Scientists were able to gauge the rate of water loss on Mars by measuring the ratio of water and HDO from today and 4.3 billion years ago. Credit: Kevin Gill
Scientists were able to gauge the rate of water loss on Mars by measuring the ratio of water and HDO from today and 4.3 billion years ago. Credit: Kevin Gill

Another method has to do with albedo reduction, where the surface of Mars would be coated with dark materials in order to increase the amount of sunlight it absorbs. This could be anything from dust from Phobos and Deimos (two of the darkest bodies in the Solar System) to extremophile lichens and plants that are dark in color. One of the greatest proponents for this was famed author and scientist, Carl Sagan.

In 1973, Sagan published an article in the journal Icarus titled “Planetary Engineering on Mars“, where he proposed two scenarios for darkening the surface of Mars. These included transporting low albedo material and/or planting dark plants on the polar ice caps to ensure they absorbed more heat, melted, and converted the planet to more “Earth-like conditions”.

In 1976, NASA officially addressed the issue of planetary engineering in a study titled “On the Habitability of Mars: An Approach to Planetary Ecosynthesis“. The study concluded that photosynthetic organisms, the melting of the polar ice caps, and the introduction of greenhouse gases could all be used to create a warmer, oxygen and ozone-rich atmosphere.

In 1982, Planetologist Christopher McKay wrote “Terraforming Mars”, a paper for the Journal of the British Interplanetary Society. In it, McKay discussed the prospects of a self-regulating Martian biosphere, which included both the required methods for doing so and ethics of it. This was the first time that the word terraforming was used in the title of a published article, and would henceforth become the preferred term.

This was followed in 1984 by James Lovelock and Michael Allaby’s book, The Greening of Mars. In it, Lovelock and Allaby described how Mars could be warmed by importing chlorofluorocarbons (CFCs) to trigger global warming.

Artist's concept of a possible Mars terraforming plant. Credit: National Geographic Channel
Artist’s concept of a possible Mars terraforming plant, warming the planet through the introduction of hydrocarbons. Credit: nationalgeographic.com

In 1993, Mars Society founder Dr. Robert M. Zubrin and Christopher P. McKay of the NASA Ames Research Center co-wrote “Technological Requirements for Terraforming Mars“. In it, they proposed using orbital mirrors to warm the Martian surface directly. Positioned near the poles, these mirrors would be able to sublimate the CO2 ice sheet and contribute to global warming.

In the same paper, they argued the possibility of using asteroids harvested from the Solar System, which would be redirected to impact the surface, kicking up dust and warming the atmosphere. In both scenarios, they advocate for the use of nuclear-electrical or nuclear-thermal rockets to haul all the necessary materials/asteroids into orbit.

The use of fluorine compounds – “super-greenhouse gases” that produce a greenhouse effect thousands of times stronger than CO² – has also been recommended as a long term climate stabilizer. In 2001, a team of scientists from the Division of Geological and Planetary Sciences at Caltech made these recommendations in the “Keeping Mars warm with new super greenhouse gases“.

Where this study indicated that the initial payloads of fluorine would have to come from Earth (and be replenished regularly), it claimed that fluorine-containing minerals could also be mined on Mars. This is based on the assumption that such minerals are just as common on Mars (being a terrestrial planet) which would allow for a self-sustaining process once colonies were established.

This image illustrates possible ways methane might be added to Mars' atmosphere (sources) and removed from the atmosphere (sinks). NASA's Curiosity Mars rover has detected fluctuations in methane concentration in the atmosphere, implying both types of activity occur on modern Mars. A longer caption discusses which are sources and which are sinks. (Image Credit: NASA/JPL-Caltech/SAM-GSFC/Univ. of Michigan)
NASA’s Curiosity Mars rover has detected fluctuations in methane concentration in the atmosphere, implying that it is added and removed all the time. (Image Credit: NASA/JPL-Caltech/SAM-GSFC/Univ. of Michigan)

Importing methane and other hydrocarbons from the outer Solar System – which are plentiful on Saturn’s moon Titan – has also been suggested. There is also the possibility of in-situ resource utilization, thanks to the Curiosity rover’s discovery of a “tenfold spike” of methane that pointed to a subterranean source. If these sources could be mined, methane might not even need to be imported.

More recent proposals include the creation of sealed biodomes that would employ colonies of oxygen-producing cyanobacteria and algae on Martian soil. In 2014, the NASA Institute for Advanced Concepts (NAIC) program and Techshot Inc. began work on this concept, which was named the “Mars Ecopoiesis Test Bed“. In the future, the project intends to send small canisters of extremophile photosynthetic algae and cyanobacteria aboard a rover mission to test the process in a Martian environment.

If this proves successful, NASA and Techshot intend to build several large biodomes to produce and harvest oxygen for future human missions to Mars – which would cut costs and extend missions by reducing the amount of oxygen that has to be transported. While these plans do not constitute ecological or planetary engineering, Eugene Boland (chief scientist of Techshot Inc.) has stated that it is a step in that direction:

“Ecopoiesis is the concept of initiating life in a new place; more precisely, the creation of an ecosystem capable of supporting life. It is the concept of initiating “terraforming” using physical, chemical and biological means including the introduction of ecosystem-building pioneer organisms… This will be the first major leap from laboratory studies into the implementation of experimental (as opposed to analytical) planetary in situ research of greatest interest to planetary biology, ecopoiesis and terraforming.”

The "greening of Mars" would be a multi-tiered process, Credit: nationalgeographic.com
The “greening of Mars” would be a multi-tiered process, involving the importation of gases and terrestrial organisms to convert the planet over the course of many generations. Credit: nationalgeographic.com

Potential Benefits:

Beyond the prospect for adventure and the idea of humanity once again embarking on an era of bold space exploration, there are several reasons why terraforming Mars is being proposed. For starters, there is concern that humanity’s impact on planet Earth is unsustainable, and that we will need to expand and create a “backup location” if we intend to survive in the long run.

This school of though cites things like the Earth’s growing population – which is expected to reach 9.6 billion by mid-century – as well as the fact that by 2050, roughly two-thirds of the world’s population is expected to live in major cities. On top of that, there is the prospect of severe Climate Change, which – according to a series of scenarios computed by NASA – could result in life becoming untenable on certain parts of the planet by 2100.

Other reasons emphasize how Mars lies within our Sun’s “Goldilocks Zone” (aka. “habitable zone), and was once a habitable planet. Over the past few decades, surface missions like NASA’s Mars Science Laboratory (MSL) and its Curiosity rover have uncovered a wealth of evidence that points to flowing water existing on Mars in the deep past (as well as the existence of organic molecules).

Project Nomad, a concept for terraforming Mars using mobile, factory-skyscrapers. 2013 Skyscraper Competition. Credit: evolo.com/Antonio Ares Sainz, Joaquin Rodriguez Nuñez, Konstantino Tousidonis Rial
Project Nomad, a concept for the 2013 Skyscraper Competition that involved mobile factory-skyscrapers terraforming Mars. Credit: evolo.com/A.A. Sainz/J.R. Nuñez/K.T. Rial

In addition, NASA’s Mars Atmosphere and Volatile EvolutioN Mission (MAVEN) (and other orbiters) have provided extensive information on Mars’ past atmosphere. What they have concluded is that roughly 4 billion years ago, Mars had abundant surface water and a thicker atmosphere. However, due to the loss of Mars’ magnetosphere – which may have been caused by a large impact or rapid cooling of the planet’s interior – the atmosphere was slowly stripped away.

Ergo, if Mars was once habitable and “Earth-like”, it is possible that it could be again one day. And if indeed humanity is looking for a new world to settle on, it only makes sense that it be on one that has as much in common with Earth as possible. In addition, it has also been argued that our experience with altering the climate of our own planet could be put to good use on Mars.

For centuries, our reliance on industrial machinery, coal and fossil fuels has had a measurable effect Earth’s environment. And whereas this has been an unintended consequence of modernization and development here on Earth; on Mars, the burning of fossil fuels and the regular release of pollution into the air would have a positive effect.

Credit: nationgeographic.com
Infographic showing a cost-estimate and time frame for the terraforming of Mars. Credit: NASA/National Geographic Channel/Discovery Channel

Other reasons include expanding our resources base and becoming a “post-scarcity” society. A colony on Mars could allow for mining operations on the Red Planet, where both minerals and water ice are abundant and could be harvested. A base on Mars could also act as a gateway to the Asteroid Belt, which would provide us with access to enough minerals to last us indefinitely.

Challenges:

Without a doubt, the prospect of terraforming Mars comes with its share of problems, all of which are particularly daunting. For starters, there is the sheer amount of resources it would take to convert Mars’ environment into something sustainable for humans. Second, there is the concern that any measure undertaken could have unintended consequences. And third, there is the amount of time it would take.

For example, when it comes to concepts that call for the introduction of greenhouse gases to trigger warming, the quantities required are quite staggering. The 2001 Caltech study, which called for the introduction of fluorine compounds, indicated that sublimating the south polar CO² glaciers would require the introduction of approximately 39 million metric tons of CFCs into Mars’ atmosphere – which is three times the amounts produced on Earth between 1972 and 1992.

Artist's conception of a terraformed Mars. Credit: Ittiz/Wikimedia Commons
Artist’s conception of a terraformed Mars. Credit: Ittiz/Wikimedia Commons

Photolysis would also begin to break down the CFCs the moment they were introduced, which would necessitate the additional of 170 kilotons every year to replenish the losses. And last, the introduction of CFCs would also destroy the Martian any ozone that was produced, which would undermine efforts to shield to surface from radiation.

Also, the 1976 NASA feasibility study indicated that while terraforming Mars would be possible using terrestrial organisms, it also recognized that the time-frames called for would be considerable. As it states in the study:

“No fundamental, insuperable limitation of the ability of Mars to support a terrestrial ecology is identified. The lack of an oxygen-containing atmosphere would prevent the unaided habitation of Mars by man. The present strong ultraviolet surface irradiation is an additional major barrier. The creation of an adequate oxygen and ozone-containing atmosphere on Mars may be feasible through the use of photosynthetic organisms. The time needed to generate such an atmosphere, however, might be several millions of years.”

The study goes on to state that this could be drastically reduced by creating extremophile organisms specifically adapted for the harsh Martian environment, creating a greenhouse effect and melting the polar ice caps. However, the amount of time it would take to transform Mars would still likely be on the order of centuries or millennia.

Mars-manned-mission vehicle (NASA Human Exploration of Mars Design Reference Architecture 5.0) feb 2009. Credit: NASA
Artist’s concept for a NASA manned-mission to Mars (Human Exploration of Mars Design Reference Architecture 5.0, Feb 2009). Credit: NASA

And of course, there is the problem of infrastructure. Harvesting resources from other planets or moons in the Solar System would require a large fleet of space haulers, and they would need to be equipped with advanced drive systems to make the trip in a reasonable amount of time. Currently, no such drive systems exist, and conventional methods – ranging from ion engines to chemical propellants – are neither fast or economical enough.

To illustrate, NASA’s New Horizons mission took more than 11 years to get make its historic rendezvous with Pluto in the Kuiper Belt, using conventional rockets and the gravity-assist method. Meanwhile, the Dawn mission, which relied relied on ionic propulsion, took almost four years to reach Vesta in the Asteroid Belt. Neither method is practical for making repeated trips to the Kuiper Belt and hauling back icy comets and asteroids, and humanity has nowhere near the number of ships we would need to do this.

On the other hand, going the in-situ route – which would involve factories or mining operations on the surface to release CO², methane or CFC-containing minerals into the air – would require several heavy-payload rockets to get all the machinery to the Red Planet. The cost of this would dwarf all space programs to date. And once they were assembled on the surface (either by robotic or human workers), these operations would have to be run continuously for centuries.

There is also several questions about the ethics of terraforming. Basically, altering other planets in order to make them more suitable to human needs raises the natural question of what would happen to any lifeforms already living there. If in fact Mars does have indigenous microbial life (or more complex lifeforms), which many scientists suspect, then altering the ecology could impact or even wipe out these lifeforms. In short, future colonists and terrestrial engineers would effectively be committing genocide.

NASA's Journey to Mars. NASA is developing the capabilities needed to send humans to an asteroid by 2025 and Mars in the 2030s. Credit: NASA/JPL
NASA’s Journey to Mars. NASA is developing the capabilities needed to send humans to an asteroid by 2025 and Mars in the 2030s. Credit: NASA/JPL

Given all of these arguments, one has to wonder what the benefits of terraforming Mars would be. While the idea of utilizing the resources of the Solar System makes sense in the long-run, the short-term gains are far less tangible. Basically, harvested resources from other worlds is not economically viable when you can extract them here at home for much less. And given the danger, who would want to go?

But as ventures like MarsOne have shown, there are plenty of human beings who are willing to make a one-way trip to Mars and act as Earth’s “first-wave” of intrepid explorers. In addition, NASA and other space agencies have been very vocal about their desire to explore the Red Planet, which includes manned missions by the 2030s. And as various polls show, public support is behind these endevours, even if it means drastically increased budgets.

So why do it? Why terraform Mars for human use? Because it is there? Sure. But more importantly, because we might need to. And the drive and the desire to colonize it is also there. And despite the difficulty inherent in each, there is no shortage of proposed methods that have been weighed and determined feasible.In the end, all that’s needed is a lot of time, a lot of commitment, a lot of resources, and a lot of care to make sure we are not irrevocably harming life forms that are already there.

But of course, should our worst predictions come to pass, we may find in the end that we have little choice but to make a home somewhere else in the Solar System. As this century progresses, it may very well be Mars or bust!

We have written many interesting articles about terraforming here at Universe Today. Here’s The Definitive Guide To Terraforming, Could We Terraform the Moon?, Should We Terraform Mars?, How Do We Terraform Venus?, and Student Team Wants to Terraform Mars Using Cyanobacteria.

We’ve also got articles that explore the more radical side of terraforming, like Could We Terraform Jupiter?, Could We Terraform The Sun?, and Could We Terraform A Black Hole?

Astronomy Cast also has good episodes on the subject, like Episode 96: Humans to Mar, Part 3 – Terraforming Mars

For more information, check out Terraforming Mars  at NASA Quest! and NASA’s Journey to Mars.

And if you like the video, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

The Aquarius Constellation

Welcome back to Constellation Friday! Today, we will be dealing with one of the best-known constellations, that “watery” asterism and section of the sky known as Aquarius. Cue the soundtrack from Hair!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the-then known constellations. This work (known as the Almagest) would remain the definitive guide to astronomy and astrology for over a thousand years. Among the 48 constellations listed in this book was Aquarius, a constellation of the zodiac that stretches from the celestial equator to the southern hemisphere.

Also known as the “Water Carrier”, Aquarius is bordered by Pegasus, Equuleus and Delphinus at the north, Aquila to the west, Capricornus to the south-west, Piscis Austrinus and Sculptor to the south, Cetus to the east and Pisces to the north-east. Today, it is one of the 88 constellations recognized by the International Astronomical Union (IAU), and is perhaps the most referenced and recognized of all the constellation.

Continue reading “The Aquarius Constellation”

What is a Terrestrial Planet?

In studying our Solar System over the course of many centuries, astronomers learned a great deal about the types of planets that exist in our universe. This knowledge has since expanded thanks to the discovery of extrasolar planets, many of which are similar to what we have observed here at home.

For example, while hundreds of gas giants of varying size have been detected (which are easier to detect because of their size), numerous planets have also been spotted that are similar to Earth – aka. “Earth-like”. These are what is known as terrestrial planets, a designation which says a lot about a planet how it came to be.

Definition:

Also known as a telluric or rocky planet, a terrestrial planet is a celestial body that is composed primarily of silicate rocks or metals and has a solid surface. This distinguishes them from gas giants, which are primarily composed of gases like hydrogen and helium, water, and some heavier elements in various states.

The term terrestrial planet is derived from the Latin “Terra” (i.e. Earth). Terrestrial planets are therefore those that are “Earth-like”, meaning they are similar in structure and composition to planet Earth.

Earth-like planets. Image Credit: JPL
Artist’s concept for the range of Earth-like extrasolar planets that have been discovered in recent years. Credit: NASA/JPL

Composition and Characteristics:

All terrestrial planets have approximately the same type of structure: a central metallic core composed of mostly iron, with a surrounding silicate mantle. Such planets have common surface features, which include canyons, craters, mountains, volcanoes, and other similar structures, depending on the presence of water and tectonic activity.

Terrestrial planets also have secondary atmospheres, which are generated through volcanism or comet impacts. This also differentiates them from gas giants, where the planetary atmospheres are primary and were captured directly from the original solar nebula.

Terrestrial planets are also known for having few or no moons. Venus and Mercury have no moons, while Earth has only the one (the Moon). Mars has two satellites, Phobos and Deimos, but these are more akin to large asteroids than actual moons. Unlike the gas giants, terrestrial planets also have no planetary ring systems.

The Earth's layers. Credit: discovermagazine.com
The Earth’s interior structure, shown here as consisting of multiple “layers”. Credit: discovermagazine.com

Solar Terrestrial Planets:

All those planets found within the Inner Solar System – Mercury, Venus, Earth and Mars – are examples of terrestrial planets. Each are composed primarily of silicate rock and metal, which is differentiated between a dense, metallic core and a silicate mantle. The Moon is similar, but has a much smaller iron core.

Io and Europa are also satellites that have internal structures similar to that of terrestrial planets. In the case of the former, models of the moon’s composition suggest that the mantle is composed primarily of silicate rock and iron, which surrounds a core of iron and iron sulphide. Europa, on the other hand, is believed to have an iron core that is surrounded by an outer layer of water.

Dwarf planets, like Ceres and Pluto, and other large asteroids are similar to terrestrial planets in the fact that they do have a solid surface. However, they differ in that they are, on average, composed of more icy materials than rock.

Extrasolar Terrestrial Planets:

Most of the planets detected outside of the Solar System have been gas giants, owing to the fact that they are easier to spot. However, since 2005, hundreds of potentially terrestrial extrasolar planets have been found – mainly by the Kepler space mission. Most of these have been what is known as “super-Earths” (i.e. planets with masses between Earth’s and Neptune’s).

Examples of extrasolar terrestrial planets include Gliese 876 d, a planet that has a mass 7 to 9 times that of Earth. This planet orbits the red dwarf Gliese 876, which is located approximately 15 light years from Earth. The existence of three (or possibly four) terrestrial exoplanets was also confirmed between 2007 and 2010 in the Gliese 581 system, another red dwarf roughly 20 light years from Earth.

The smallest of these, Gliese 581 e, is only about 1.9 Earth masses, but orbits very close to the star. Two others, Gliese 581 c and Gliese 581 d, as well as a proposed fourth planet (Gliese 581 g) are more-massive super-Earths orbiting in or close to the habitable zone of the star. If true, this could mean that these worlds are potentially habitable Earth-like planets.

The first confirmed terrestrial exoplanet, Kepler-10b – a planet with between 3 and 4 Earth masses and located some 460 light years from Earth – was found in 2011 by the Kepler space mission. In that same year, the Kepler Space Observatory team released a list of 1235 extrasolar planet candidates, including six that were “Earth-size” or “super-Earth-size” (i.e. less than 2 Earth radii) and which were located within their stars’ habitable zones.

Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range. As of January, 2013, 2740 planet candidates have been discovered.

Categories:

Scientists have proposed several categories for classifying terrestrial planets. Silicate planets are the standard type of terrestrial planet seen in the Solar System, which are composed primarily of a silicon-based rocky mantle and a metallic (iron) core.

Iron planets are a theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass. Planets of this type are believed to form in the high-temperature regions close to a star, and where the protoplanetary disk is rich in iron. Mercury is possible example, which formed close to our Sun and has a metallic core equal to 60–70% of its planetary mass.

Coreless planets are another theoretical type of terrestrial planet, one that consists of silicate rock but has no metallic core. In other words, coreless planets are the opposite of an iron planet. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common. Though the Solar System has no coreless planets, chondrite asteroids and meteorites are common.

And then there are Carbon planets (aka. “diamond planets”), a theoretical class of planets that are composed of a metal core surrounded by primarily carbon-based minerals. Again, the Solar System has no planets that fit this description, but has an abundance of carbonaceous asteroids.

Until recently, everything scientists knew about planets – which included how they form and the different types that exist – came from studying our own Solar System. But with the explosion that has taken place in exoplanet discovery in the past decade, what we know about planets has grown significantly.

For one, we have come to understand that the size and scale of planets is greater than previously thought. What’s more, we’ve seen for the first time that many planets similar to Earth (which could also include being habitable) do in fact exist in other Solar Systems.

Who knows what we will find once we have the option of sending probes and manned missions to other terrestrial planets?

Universe Today has articles on smallest terrestrial exoplanet and gas planets. For the latest information on confirmed extrasolar planets, be sure to check out the Kepler’s Planet Candidates.

For a full list of all confirmed and potential planets, consult the Extrasolar Planet Encyclopaedia.

Astronomy Cast has episodes on the terrestrial planets including Mars, and an interview with Darin Ragozzine, one of the Kepler Space Mission scientists.

What is Mars Made Of?

For thousands of years, human beings have stared up at the sky and wondered about the Red Planet. Easily seen from Earth with the naked eye, ancient astronomers have charted its course across the heavens with regularity. By the 19th century, with the development of powerful enough telescopes, scientists began to observe the planet’s surface and speculate about the possibility of life existing there.

However, it was not until the Space Age that research began to truly shine light on the planet’s deeper mysteries. Thanks to numerous space probes, orbiters and robot rovers, scientists have learned much about the planet’s surface, its history, and the many similarities it has to Earth. Nowhere is this more apparent than in the composition of the planet itself.

Structure and Composition:

Like Earth, the interior of Mars has undergone a process known as differentiation. This is where a planet, due to its physical or chemical compositions, forms into layers, with denser materials concentrated at the center and less dense materials closer to the surface. In Mars’ case, this translates to a core that is between 1700 and 1850 km (1050 – 1150 mi) in radius and composed primarily of iron, nickel and sulfur.

This core is surrounded by a silicate mantle that clearly experienced tectonic and volcanic activity in the past, but which now appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, magnesium, aluminum, calcium, and potassium. Oxidation of the iron dust is what gives the surface its reddish hue.

Composite image showing the size difference between Earth and Mars. Credit: NASA/Mars Exploration
Composite image showing the size difference between Earth and Mars. Credit: NASA/Mars Exploration

Magnetism and Geological Activity:

Beyond this, the similarities between Earth and Mars’ internal composition ends. Here on Earth, the core is entirely fluid, made up of molten metal and is in constant motion. The rotation of Earth’s inner core spins in a direction different from the outer core and the interaction of the two is what gives Earth it’s magnetic field. This in turn protects the surface of our planet from harmful solar radiation.

The Martian core, by contrast, is largely solid and does not move. As a result, the planet lacks a magnetic field and is constantly bombarded by radiation. It is speculated that this is one of the reasons why the surface has become lifeless in recent eons, despite the evidence of liquid, flowing water at one time.

Despite there being no magnetic field at present, there is evidence that Mars had a magnetic field at one time. According to data obtained by the Mars Global Surveyor, parts of the planet’s crust have been magnetized in the past. It also found evidence that would suggest that this magnetic field underwent polar reversals.

This observed paleomagnetism of minerals found on the Martian surface has properties that are similar to magnetic fields detected on some of Earth’s ocean floors. These findings led to a re-examination of a theory that was originally proposed in 1999 which postulated that Mars experienced plate tectonic activity four billion years ago. This activity has since ceased to function, causing the planet’s magnetic field to fade away.

Map from the Mars Global Surveyor of the current magnetic fields on Mars. Credit: NASA/JPL
Map from the Mars Global Surveyor of the current magnetic fields on Mars. Credit: NASA/JPL

Much like the core, the mantle is also dormant, with no tectonic plate action to reshape the surface or assist in removing carbon from the atmosphere. The average thickness of the planet’s crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi). By contrast, Earth’s crust averages 40 km (25 mi) and is only one third as thick as Mars’s, relative to the sizes of the two planets.

The crust is mainly basalt from the volcanic activity that occurred billions of years ago. Given the lightness of the dust and the high speed of the Martian winds, features on the surface can be obliterated in a relatively short time frame.

Formation and Evolution:

Much of Mars’ composition is attributed to its position relative to the Sun. Elements with comparatively low boiling points, such as chlorine, phosphorus, and sulphur, are much more common on Mars than Earth. Scientists believe that these elements were probably removed from areas closer to the Sun by the young star’s energetic solar wind.

After its formation, Mars, like all the planets in the Solar System, was subjected to the so-called “Late Heavy Bombardment.” About 60% of the surface of Mars shows a record of impacts from that era, whereas much of the remaining surface is probably underlain by immense impact basins caused by those events.

The North Polar Basin is the large blue low-lying area at the northern end of this topographical map of Mars. Its elliptical shape is partially obscured by volcanic eruptions (red, center left). Credit: NASA/JPL/USGS
The North Polar Basin is the large blue low-lying area at the northern end of this topographical map of Mars. Credit: NASA/JPL/USGS

These craters are so well preserved because of the slow rate of erosion that happens on Mars. Hellas Planitia, also called the Hellas impact basin, is the largest crater on Mars. Its circumference is approximately 2,300 kilometers, and it is nine kilometers deep.

The largest impact event on Mars is believed to have occurred in the northern hemisphere. This area, known as the North Polar Basin, measures some 10,600 km by 8,500 km, or roughly four times larger than the Moon’s South Pole – Aitken basin, the largest impact crater yet discovered.

Though not yet confirmed to be an impact event, the current theory is that this basin was created when a Pluto-sized body collided with Mars about four billion years ago. This is thought to have been responsible for the Martian hemispheric dichotomy and created the smooth Borealis basin that now covers 40% of the planet.

Scientists are currently unclear on whether or not a huge impact may be responsible for the core and tectonic activity having become dormant. The InSight Lander, which is planned for 2018, is expected to shed some light on this and other mysteries – using a seismometer to better constrain the models of the interior.

Hellas Planitia extends across about 50° in longitude and more than 20° in latitude. From data from the Mars Orbiter LaserAltimeter (MOLA). Credit: NASA

Other theories claim that Mars lower mass and chemical composition caused it to cool more rapidly than Earth. This cooling process is therefore believed to be what arrested convection within the planet’s outer core, thus causing its magnetic field to disappear.

Mars also has discernible gullies and channels on its surface, and many scientists believe that liquid water used to flow through them. By comparing them to similar features on Earth, it is believed these were were at least partially formed by water erosion.  Some of these channels are quite large, reaching 2,000 kilometers in length and 100 kilometers in width.

Yes, Mars is much like Earth in many respects. It’s a rocky planet, has a crust, mantle, and core, and is composed of roughly the same elements. As our exploration of the Red Planet continues, we are learning more and more about its history and evolution. Someday, we may find ourselves settling on that rock, and relying on its similarities to create a “backup location” for humanity.

We have many interesting articles on the subject of Mars here at Universe Today. Here’s How Long Does it Take to Get to Mars?, How Far is Mars from Earth?, How Strong is the Gravity on Mars?, What is the Weather like on Mars?, The Orbit of Mars. How Long is a Year on Mars?, How Do We Colonize Mars?, and How Do We Terraform Mars?

Ask a Scientist answered the question about the composition of Mars, and here’s some general information about Mars from Nine Planets.

Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.

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How Many Miles Around the Earth?

Planet Earth, which we humans and all currently-known forms of life call home, is the third planet from the Sun, and the largest of the terrestrial planets. With a mean radius of 6,371 km (3,958.8 miles), it is slightly larger than Venus (which has a radius of approx. 6,050 km), almost twice the size of Mars (~3,390 km), and almost three times the size of Mercury (~2,440 km).

Basically, Earth is a pretty big world. But just how big if one were to measure it from end to end? If one were to just start walking, how many kilometers (and/or miles) would they have to go before they got back to where they started. Well, the short answer is just over 40,075 km (or just over 24,901 miles). But as always, things get a little more complicated when you look closer.

Continue reading “How Many Miles Around the Earth?”