Planetary materials and Astrobiology
Have you ever heard of rocks falling from the sky? Rocks that come from space? Those rocks are known as meteorites and they belong to a class of materials called planetary materials. In this class of materials we can also find very small pieces of dust, also coming from space, and known as cosmic or interplanetary dust. Space rocks come from four different sources, namely the Moon, asteroids, comets and other planets. At present scientists and museums have in their collections rocks from Mars, the Moon, a variety of asteroids and one comet, comet Wild 2. Scientists care about these space rocks because they contain clues about our origins. Indeed, these rocks are very ancient and have suffered very little changes since they formed, 4.5 billion years ago. They contain information about the formation of the solar system and the origin of life that, on Earth, was erased due to the effects of erosion. These rocks are for scientists the Rosetta stone, the document that contains information to decipher our past. Astrobiology is the study of the origin and evolution of life in the universe. It is a science made of several sciences because it requires the interaction of several disciplines to understand what happened to the Earth to become what it is and how life appeared and evolved. Geologists, astronomers, biologists, chemists, evolutionists, biochemists, microbiologists, atmospheric scientists, and environmental scientists, to name a few, interact to understand a very complex scientific question: How did life start in the first place?
This question can, and should, be answered from several angles. One approach is the study of ancient rocks that come from space. Indeed, rocks from space often contain some of the ingredients that life uses to exist, namely molecules made of carbon, oxygen, nitrogen and hydrogen and that we call organic molecules. The job of an astrobiologist working with this type of rocks it to try to find organic molecules, decipher what kind they are, and make sure the molecules are from space and not contamination from their lab or the place where the rock originally landed. Wow! What a tough task! And yet, it is fascinating!
This is what I do. Or rather, what I did during 14 years. Now I have reinvented myself as a science communicator, but during all those years I was at all times fascinated by all these questions and the possible answers we can find in those rocks. Because those rocks are a real Pandora box; you begin looking inside them and you start finding things that are, literally, “out of this world”.
Among the most interesting planetary materials we have for astrobiology, we can count with a class of meteorites, originating from asteroids, that contain carbon. This class of meteorites is called carbonaceous chondrites and because they are rich in the element carbon they look dark. They also contain small spherical droplets of glass that are called “chondrules”. Chondrules are considered the most pristine rocky material within a meteorite because they formed short after the Sun formed, and have since gone through little changes. Carbonaceous chondrites are the most important group of meteorites for the study of the origin of life, because they are the only group, so far, that contains organic molecules. In fact, the word “chondrit
The meteorite in this picture is called Murchison, named after the small village Murchison, Australia where it fell. It is the most famous and studied rock within carbonaceous chondrites because it contains many types and large amounts of organic molecules, amongst which we count about 70 different types of amino acids.
Comets area another type of planetary object that presents interest for astrobiology. Comets are small bodies made of rock and ice. They are often referred to as “dirty snowballs”. Comets originate from two reservoirs: the Oort cloud and the Kuiper Belt. The Kuiper Belts is relatively close from the Sun, just beyond Neptune, while the Oort Cloud is very distant. Comets that originate from the Kuiper Belt have usually short periods (period is the time it takes a comet to go once around the Sun), in the order of several dozens of years. By contrast, long-period comets originate from the Oort Cloud and their periods are in the order of several hundreds to thousands of years. In our group we investigate samples from a short-period comet named Wild 2.
The samples were brought back to Earth by a NASA space mission called Stardust.
The third group of planetary materials that present interest for astrobiology is the cosmic, or interplanetary dust. These are dust particles that measure less than one millimeter and can originate from several events:
- collisions among bodies in the planetary environment
- particles found between planets, remnants from the dynamic events that formed the solar system
- particles that detach from larger meteorites upon entry in the Earth’s atmosphere
- particles from the tails of comets
We collect these dust particles from two different areas: the stratosphere (the layer of the atmosphere that is found at 20-30 km altitude) and the Earth’s surface. Cosmic dust particles collected in the stratosphere are often called interplanetary dust particles (acronymed IDPs). These particles are usually very small; their sizes range between 1-50 µm (for a reference a human hair is 70-100 µm wide).The dust is collected with special plastic collectors covered with silicon oil that helps the particles to stick. The collectors are flown on the outside of NASA airplane’s wings for several hours at an altitude of 20-30 km. The dust collected on the ground is often collected in Antarctica and is known as micrometeorites.
Just like meteorites, both comets and cosmic dust have a variety of organic molecules. Planetary materials rich in organic molecules are thought to have seeded the prebiotic Earth with the first molecules to form life. Scientists think that planetary materials played an essential role for the origin of life because they fall to Earth and deposit every year approximately 80,000 tonnes of extraterrestrial material, much of which is made of organic molecules essential to form life.
In my next blog I will write about the Stardust mission and some results obtained through the study of cometary particles and related to astrobiology.