The news that NASA recently reconfirmed the existence of carbon atoms in Martian soil led me to wonder: where does carbon come from anyway?
The bulk of Mars, like Earth, is composed of rock: i.e., a dense, central core of iron and nickel enveloped in a thick mantle of silicates (silicon + oxygen), with some potassium and phosphorus thrown in. Mars has proportionally more iron than Earth does, which is why early astronomers tagged it as the ‘Red Planet’ – from the oxidation of iron on its surface. Carbon is present on Mars chiefly as carbon dioxide (CO2) which comprises 96% of its atmosphere (creating dry ice at the poles). In 2003, tiny amounts of methane (CH4) were detected for the first time by NASA, creating a scientific curiosity. Methane breaks down quickly under ultraviolet radiation from the Sun, so its presence suggests a source on the Martian surface someplace. Where? Volcanoes have been ruled out for lack of geologic activity, as have living microorganism, for obvious reasons. The methane source might be non-biological carbon, called carbonates (such as limestone), but their role on Mars is still unclear. So, it’s a methane mystery.
Scientists do know, however, that carbon arrives on the Red Planet via bombardment by meteorites, which often contain graphite, diamonds, and other carbon compounds. Mars, like Earth and the Moon, have been pummeled by meteorites over the eons, largely from the asteroid belt located between the Red Planet and Jupiter. This belt is likely the remnants of an ancient pre-planet that never got its act together before being pulled apart by Jupiter’s immense gravitational fluxes. 75% of all asteroids that have been studied so far by scientists are carbonaceous (i.e., they are rich in carbon) and chemically they match primordial materials formed during the early solar system, which makes these asteroids approximately four billion years old!
A word about words: asteroids are space-bound objects, some of which are big enough to be called ‘planetoids,’ while meteorites are any rocky object that strikes the surface of a planet. When an asteroid of any size enters a planet’s atmosphere it becomes a meteor and when it hits the surface it becomes a meteorite. Capiche?
As another aside, bits of Mars have been discovered on Earth. In 2011, desert nomads watched a cluster of small meteorites fall to Earth near a town called Tissint in southern Morocco. Upon inspection, scientists discovered that the fragments were from Mars! Apparently they were blasted off the Red Planet 700,000 years ago by a gigantic meteorite in what must have been a spectacular explosion. There are at least sixty other “Martian meteorites” known to researchers and most appear to be from the same cataclysmic collision. The Tissint fragments excited scientists for two reasons: they were “fresh” from the heavens, and therefore uncontaminated by Earth-bound chemicals; and second, they showed clear evidence of weathering by water – further proof that Mars was a wet place once-upon-a-time.
This story illustrates the role meteorites may have played in the creation of life on Earth. In 2011, NASA announced there was a reasonable chance that meteorites brought source materials for DNA to our home. Scientists used advanced mass spectrometry instruments to scan eleven carbonaceous asteroids for nucleobases, which are part of the building blocks for DNA and RNA. They discovered three that, while widely distributed in the asteroids, were rare or absent on Earth. “Finding nucleobase compounds not typically found in Earth’s biochemistry strongly supports an extraterrestrial origin,” said Dr. Jim Cleaves, one of the scientists. The plot thickens!
Here’s a photo of the Tissint meteorite:
Carbon is also transported through the solar system by comets, though impacts with planets are much less frequent than asteroids. For many years, scientists harbored doubts about the carbon content of comets – a view that changed dramatically in 2005 when NASA’s Deep Impact probe scored a direct hit on comet Tempel 1, producing a plume of gas and dust far richer in carbon compounds than scientists expected. This discovery meant that comets could also have contributed the chemical raw materials that produced life on Earth so long ago.
Comets, sometimes called ‘dirty snowballs,’ are composed of rock, dust, water ice, and frozen gases, including methane, carbon dioxide, carbon monoxide and ammonia. As the Tempel 1 experiment revealed, they also contain a variety of organic compounds, including long-chain hydrocarbons and amino acids, the latter confirmed by NASA’s Stardust mission in 2009. Comets, of course, wander periodically into the inner solar system from deep space, and some scientists speculate they may have brought life-starting water and organic compounds to Earth and Mars. There are over 4000 known comets, though this number is likely only a fraction of the potential comet population. Some researchers speculate that the ‘reservoir’ of comet-like bodies in the outer Solar System may number one trillion. To us on Earth, the number of comets visible to the naked eye averages only one per year, and many of these are faint.
There are spectacular exceptions, however, including what scientists call the Great Comets, the most famous of which is Halley’s Comet. Named for the British astronomer who correctly computed its periodicity – the comet makes an appearance every 75-76 years – the comet has been observed and recorded for over two thousand years. An appearance in 1066, just prior to the Battle of Hastings, was taken as a bad omen for the King of England (who was subsequently shot in the eye by a Norman arrow). In America, the writer Mark Twain was born in 1835, as Halley’s Comet cruised across the night sky. In his autobiography, written in 1909, Twain said, “I came in with Halley’s comet in 1835. It is coming again next year, and I expect to go out with it.” Twain died on 21 April 1910.
Halley’s Comet last appeared in 1986 and will next appear in mid-2061.
It’s not just meteors and comets, carbon can be found in abundance throughout the solar system. Our sun has a great deal of carbon and exhibits elements of a carbon fusion cycle – in which carbon takes over from hydrogen as the main source of fusion energy. The atmosphere of Venus is 96% carbon dioxide, though unlike Mars its surface temperature is boiling, not freezing. There appears to be a hydrocarbon ‘sea’ on Titan, the largest moon orbiting Saturn. Every planet in the solar system contain lots of carbon and if you toss in the asteroids and comets, not to mention the carbon cornucopia called Earth, then you have a strong sign that carbon is one of the ‘founding’ elements of our solar system. We are carbon – carbon is us.
This takes us back a mere 4.5 billion years, however. Where did the solar system’s carbon originate? I’ll cover that next.
In the meantime, here’s a photo of Halley’s Comet in 1910: