— Katie Joy | 04 Jan 2019
Whilst we sit and wait for the weather to improve in the field it gives a good opportunity for a sciencey blog post about the meteorite science side of the project. If you take a look over at the main ‘Science’ tab and follow the links to the ‘Meteorite Science’ tab it should give you some background information about why meteorites are scientifically important.
As an overview — meteorites provide us with direct samples of other rocky Solar System bodies, and, therefore, we can use them as probes to answer lots of different questions about how our Solar System formed and changed with time. The type of question we ask depends on the type of meteorite that we are studying and the lab equipment that we can use. Hopefully, any samples we find will be worked on by lots of different scientists in the meteorite community who are specialists in their particular meteorite group or laboratory method. Some questions we can ask of meteorite samples include:
What types of stars existed in the local area before our Sun formed?
Tiny (micron-sized) mineral grains trapped within very primitive dusty meteorites provide hints to what was happening prior to the Sun forming. These presolar grains — often made of minerals like diamond and silicon carbide — are chemically distinct from any of the material we have found within our own Solar System. These chemical (isotopic) anomalies can be related to how these grains formed in other stars, before they were included in the starting materials of our own Solar System.
How old is the Solar System?
We can date the earliest minerals that formed in our Solar System using a range of mass spectrometry isotopic techniques. This technology means that we have a very good precision on the timing of the oldest solid materials to form around our Sun. These grains — called calcium aluminium inclusions — are found in carbonaceous chondrite meteorites and have ages of 4567 million years old (4.56 billion years). This is the reference point we have for understanding the timing of other major events that occurred like the formation of the planets and the Earth itself.
What types of planetary bodies existed early on in the Solar System at the time the planets were forming?
Meteorites provide us with an insight to the diversity, size and number of the earliest formed planetesimals (small planetary bodies) that would have grown through run away collisions to form larger bodies. Recent estimates suggest that the 60,000 or so meteorites we have in the collection originate from only ~110 parent bodies (mostly asteroid-like), when you relate different groups together. We know that some of these parent bodies represent the very earliest assemblages of dust that first formed around the Sun (take a look at the Osiris-Rex space mission and Hayabusa-2 mission that are currently en route to these types of asteroids ready to collect material to return to the Earth, others have come from bodies that must have been larger and were heated from the inside out by radioactive decay-driven heating. Some meteorites are completely unique examples of potentially quite large parent bodies that must have been >200 km in size and melted completely to differentiate into an iron-rich core, a silicate mantle and a silicate crust. These bodies may have remained intact to still exist as large bodies in the asteroid belt (we think that a group of meteorites likely originated from a very large asteroid called Vesta), others were smashed apart leaving smaller asteroids formed of just a part of an original larger one (the NASA Pschye mission hopes to visit an asteroid we think is made of iron-metal, like the iron meteorites we have in the sample collection, formed from a core of an early planetessimal body). Every meteorite we find has the potential to come from a previously recognised rocky planetary body, giving us the motivation to keep on collecting and studying the populations of bodies that exist in the asteroid belt and those bodies that have broken apart early in the Solar System’s history.
How did the Earth get its volatiles including water?
One of the big questions we have is how did the Earth form, what were its starting materials, and why is it similar and different to the other large rocky Solar System bodies (Mercury, Venus, the Moon and Mars). Meteorites help to chemically constrain the starting materials for the Earth, although we have no perfect chemical match to known meteorite groups — there is similarity to some of the enstatite chondrites, but we also need contributions of other starting chondritic groups as well. We also know that different asteroid groups likely later delivered some of the Earth’s highly siderophile element chemical component and also likely some of its volatiles (water and other elements) to help form our planet’s atmosphere and hydrosphere. We can compare and contrast the chemical makeup of the different meteorite groups to understand how much of Earth’s chemical budget is original, and how much has been added early in its history.
What is the geological history of the Moon?
We have about 300 stones of lunar meteorites that originated from the Moon. We know they are from the Moon as they are chemically similar to the samples that were collected by the Apollo missions. Each one potentially provides us with a new region of the Moon to study and has allowed us to identify new types of rock samples, and help test our ideas of how the Moon has geologically evolved through time. We have some meteorites that were formed very early on in the Moon’s history, others than were made in impact cratering events and some that were formed in volcanic eruptions.
What is the geological history of Mars?
To date we have not yet collected any Mars rocks by sending a spacecraft there and returning it to Earth (there are sample return missions planned for about 10-15 years’ time). Therefore, martian meteorites are our most direct way to investigate Mars’s past. We know that this group of samples come from Mars as the gas trapped within them matches that measured by orbiting satellites and the Mars Curiosity Rover. Most of the martian meteorites we have were formed in volcanic eruptions that occurred on Mars’s surface about 600 million years ago, and we have some older intrusive magmatic examples as well. One special meteorite — nicknamed Black Beauty — found in Northwest Africa, gives us insights to Mars’s volcanic and impact evolution over a very long period of time from before 4 billion years ago to as recently as 1.1 billion years ago.
In addition to these planetary science questions – meteorites in Antarctica can also provide an indication of the history of the ice flows they are sitting in, and give a constraint on the age of the ice itself. This allows us to potentially use meteorites as probes of very recent terrestrial cryosphere processes, allowing us to understand the history of Antarctic glaciers and ice flow movement.