I don’t know if it’s because I’m feeling particularly old this week (20-26 September 2018) but somehow the digest is looking at things on a large scale. By that I mean both in terms of time and in terms of distance, the latter taking us off the earth altogether and out into the Solar System.
Both of these are relevant because, as I keep saying, although our Earth is at one level a closed system, at another it isn’t. It’s part of a solar system and a wider universe, where external influences have an impact over how the system works and where there’s so much more to learn about the processes that may have operated in the past or may do so in the future, much closer to home.
On a Dwarf Planet a Long, Long Way Away…
I’m not a space scientist and I don’t present to be, but there are relationships between what we see happening on our planet and what happens on planets elsewhere.
On Earth, volcanism occurs as a result of the melting of rock — in other words it’s based on silicate chemistry (though there are such things as mud volcanoes, which are more to do with liquefaction under pressure). But logic surely tells us that where anything can become warm enough, and liquid enough, to rise to the surface above a rigid crust, the process termed volcanism can occur.
I’m not too proud to use a reference book for a clear definition, and though my handy Earth science dictionary doesn’t define volcanism (which it probably deems too obvious), it does define cryovolcanism as “volcanism occurring at low temperatures where liquid erupts through an overlying crust of ice”.
Cryovolcanism is very much an extraterrestrial thing, and there’s evidence that it exists on planetary moons such as Enceladus and Ganymede and on other, largely icy bodies. One of these, considered in a study published this week in Nature, is Ceres, a dwarf planet (or large asteroid, depending on your point of view).
NASA has a spacecraft studying Ceres, which means that it’s giving us a look into a subject which is poorly understood but which, the study (based on the data returned from the spacecraft) notes, “may be an important planetary phenomenon in shaping the surfaces of many worlds in the outer Solar System and revealing their thermal histories”.
The data emerging from this space mission have revealed not only that cryovolcanism is ongoing on Ceres, but that there’s evidence that it’s been going on for billions of years, and that the remains of extinct cryovolcanoes can still be seen.
On Our Planet a Long, Long Time Ago…
When did life begin on Earth? It’s a question for which we don’t yet have an answer, other than that life must be at least as old as the oldest fossil — currently dated back to around 3.5 billion years ago — but a new study suggests that in fact it’s very much older than that, possibly even going back before the period of the Earth’s history known as the Late Heavy Bombardment, in which meteorite strikes were frequent and which, as the study notes, “was believed to have destroyed all potential for life”.
Dating fossil evidence is the key part of the process of identifying the beginning of life, but it has its flaws — not least that, had fossils existed prior to the LHB, much of it would surely have been destroyed and much more will have been lost by the constant recycling of the crust. This leaves the possibility that life may be older than the oldest rocks on earth — but how do we know?
The answer, as is so often the case, lies in geochemistry, the analysis of the constituent bodies of the Earth. Rock might be weathered and consumed but its constituent parts are not, and some of the hardier minerals can be dated and their chemistry — and some of their history identified.
The researchers were able to use particularly resistant mineral grains, zircons, to identify the type of rock from which they derived — and the chemical composition of this mineral indicates that the conditions for life may have existed up to 4.1 billion years ago.
As it happens, both of the stories above relate to minerals involved in the mineral cup, the second round of which is almost complete. One is a loser and one, still in the completion, is among the favourites.
A lot of interesting and attractive minerals went out this week — minerals such as biotite and amphibole (found in igneous rocks), the metamorphic mineral serpentine and the mercury ore, cinnabar.
Apatite went out, too, despite getting my vote. Apatite isn’t one of the more visibly attractive minerals — no glowing like opal or sparkling like diamond here — but it is important to all of us. You may never have heard of it but it’s in your teeth and your bones, and it’s the principal component of the fossils which help us to understand something about early life.
Zircon is still in the game, and it’s this mineral’s extraordinary durability which makes it a geochemical time-keeper. That’s why it was so important in the second of the studies outlined above. I expect we’ll see a lot more of it.