Astrophysics/chemistry/biology at Cambridge

As summer was approaching, I and my fellow Cambridge students were making the most of the sunshine by hunkering down inside, hunched over desks with noses in books, pen in hand and the tables covered in scrawled sheets. Yes indeed, it was exam season!

This summer saw 3 exams for me, which is more than some (couldn't believe it when some students said they were wrapping up their last piece of coursework before putting their feet up), but not as many as others (shoutout to my lawyer friend on 6 exams, and anyone doing natural sciences or medicine!), and they were spread across two highly mathematical modules on planets and stars, and one very broad interdisciplinary module with essays on astrochemistry, astrobiology, origins of life, geology and prebiotic chemistry. Here I'll delve into the core interdisciplinary modules, and some of the cool stuff I learnt.

Origins and Detections of Planets and Habitable Environments

This course was examined last term, and completes the set of modules I studied at Cambridge. I had not studied planets in detail before, so I was keen to learn more about this area of astrophysics, and this module delivered a huge amount of fascinating content. The first half traced the origin of planets, their formation and growth in planetary discs, planetary detection and the architecture of planetary systems. The second half focused on the origin of habitable environments, the climate on the young early Earth, volcanism, Martian geology and the atmosphere of Venus.

Stars form in giant molecular clouds, dense regions of the interstellar medium which grow in mass to become gravitationally bound. Planets form from the dusty debris material around forming stars, a spherical cloudy mess which collapses inwards and rotates with increasing angular momentum. The sphere of dusty debris flattens out over time while it rotates, leaving most mass in the centre (where the star forms), and a small fraction of it orbiting further out in the disc. The remaining disc material feeds the star for about 10,000 years, and discs are undetectable beyond 10 million years, having been depleted. It is somewhere between these two timestamps that planets form.

The inner edge of the disc around the star is hot, and gets colder as the distance from the star increases. Planetary discs (PPDs) have theoretical separations determined by temperature called snowlines, the limit where a molecule will transition from being a solid to liquid or gas. Different molecules have different melting points, so for example in a typical mature PPD any water molecules closer than 3AU to the star will be liquid or gas, and beyond this distance they will be in ice. Meanwhile carbon monoxide has a much lower freezing point, so remains in liquid/gas form until 50AU from the star, until it is so cold even CO freezes. (1AU is the distance from Earth to the Sun).

All this stuff is interesting because depending on the molecular species (especially the ratio of Carbon/Oxygen) on a planet and what state they are in, it is possible to infer where in the protoplanetary disc the planet formed. The gas and debris in PPDs may form pebbles, which for various reasons are the most favourable size to be fast-tracked inwards towards the star, and these are coated in sticky icy molecules, which on reaching their respective snowlines undergo sublimation (transition directly from ice to gas), resulting in regions of the PPD which are concentrated in specific molecular species at specific distances. Dust accumulates in localised regions around the star, forming dust traps, which are the birthplace of rocky planets.

Credit: ALMA (ESO/NAOJ/NRAO). This is the sharpest image ever taken by ALMA . It shows the protoplanetary disk surrounding the young star HL Tauri. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system.

Planets can be detected by several different methods, each with its own bias towards certain planet types, and telescopes exist to cover each method. The most common is the transit method, which works simply by observing a star, recording its brightness, and waiting for the brightness to dip when it is being temporarily partially blocked by a planet passing in front of it. If the star's size is known, from the fraction of light blocked it is easy to work out the planet's size. This has detected the majority of the ~5000 planets discovered to date, however it is biased towards finding massive planets orbiting close to their host stars, as these will block a larger fraction of light than smaller planets orbiting far away from their stars, meaning that our known population of exoplanets is biased towards 'hot-Jupiters', planets of similar gaseous composition and size to Jupiter, orbiting at around 0.1AU or less. One of the challenges for astrophysicists is to improve detection techniques to broaden the types of planets we can detect to better include other types and provide a comprehensive, unbiased overview of planetary populations. This is difficult, but other methods exist to detect exoplanets, such as the radial velocity method which watches for changes in a star's velocity as it ‘wobbles' in the sky, an indication it may be orbited by a planet. Similarly, astrometry looks for periodic shifts in a star's position, and both these methods again favour massive planets. Direct imaging is a method which involves observing a planet directly with a telescope. This sounds simple enough, but stars are several million times brighter than their faint planets, so in order to capture the planet it is necessary to block out the star's light using a device called a coronagraph. Younger, massive planets are the brightest and easiest to spot. Below is an example of a recently imaged exoplanet by the James Webb telescope.

Planet candidate (orange) of similar mass to Saturn orbits the star TWA 7, detected by the James Webb telescope using the direct imaging method, where the host star has been blocked with a coronagraph. Credit: NASA, ESA, CSA, Anne-Marie Lagrange (CNRS, UGA), Mahdi Zamani (ESA/Webb)

Exoplanet architecture was another cool part of the module, which explored the different possible interiors for planets, including some which do not exist in our own solar system. Planets can be rocky, eg. Earth, Mars and gaseous eg. Jupiter, Saturn, but liquid planets are thought to exist too. Potential planets which we don't have in our own solar system include 'hot-Jupiters' as explained earlier, 'Hycean worlds' containing a liquid ocean interior with hydrogen atmosphere, and super-Earths, which are similar to Earth but much bigger. Current studies suggest that there are 2 types of planets which are most common to occur- super-Earths about 1.3x bigger than Earth, and mini-Neptunes around 2-3x bigger than Earth. Possibly, this is because they became 2 distinct populations- those with rocky cores and gas envelopes, and those with rocky cores only, having lost their envelopes to being stripped off by the star's intense ultraviolet radiation, or by the planet radiating its internal energy outwards causing the envelope to erode. Either way, it is an area of ongoing research within planetary science.

The module then transitioned to discussions on habitability- a seemingly vague concept which can pleasingly be narrowed down by some simple maths. Firstly- the habitable zone (HZ) is the distance from a star at which the planet's surface temperature is suitable for liquid water to exist. This of course depends on a variety of factors, such as the star's temperature and size. Atmospheres, chemical composition and stellar conditions also play a role in whether a planet may host a habitable environment. Systems with large numbers of planets orbiting too close to each other are dynamically unstable, suffering close encounters or collisions, which may not be conducive to habitability. Zooming in to planetary conditions, other factors affecting habitability include whether a planet has plate tectonics, volcanism, magnetic fields, atmospheric dynamics and potential delivery of comets. Volcanism and tectonic activity can cause devastation to the environment and any life, however in the long term it has been influential for life on Earth. Subduction zones, where an ocean plate slides under a continental plate, have provided buoyant continents and land surfaces for life. Subduction has also helped provide habitable conditions by locking excess carbon released by volcanism back into the planetary interior, hence preventing runaway greenhouse gas effects, where Earth's temperatures could otherwise have been raised beyond being suitable for life. With more carbon locked in Earth's interior, oxygen was able to rise further into the atmosphere, again providing the habitable environments allowing life to form as we know it today. Volcanoes, generally (but not only) occurring at the boundaries of tectonic plates, can provide nutrient rich environments such as submarine hydrothermal vents, where bacteria can use hydrogen sulfide as 'food' in warm conditions. On land, volcanism can result in fertile soils too.

Earth isn't unique in having volcanos, it is thought that Mars had these too, and Io, one of Jupiter's moons, is the most volcanically active body in the solar system. Mars is broadly split in two halves by its topography (surface), where the northern lowlands are smooth, and the southern highlands appear more battered by impact craters and older. A valley of networks and outflow channels suggests that liquid water may have existed on Mars, eroding its surface. This theory is further informed by large rocky boulders in the surface which may have been delivered by highly energetic floods, and iron sulfate minerals which indicate the drying of a substantial amount of water in Martian lakes. Some mysteries remain around the ancient Martian environment, firstly to do with its climate. Back 10 billion years ago, the Sun was about 3/4 of its current brightness, and rough estimates suggest the Martian surface temperature would have been around -60 degrees Celcius, however geological records indicate water was present in liquid form on the surface around that time, which would not have been possible at such cold temperatures. Secondly, the greenhouse effect of Mars (assuming a water/CO2 atmosphere) would not have been strong enough to raise the temperature by a sufficient amount above water's freezing point. Potentially, both of these mysteries could be resolved if other gases had been present in the atmosphere, such as methane or ammonia, yet these lack obvious formation mechanisms. Multiple missions have been sent to Mars, such as the lander Insight 3 mission which studied Martian seismology from the distribution of marsquakes, as well as various satellites which have observed the gravitational, magnetic and surface properties of the planet from orbit. I had not learnt about planets in the solar system since school, as university level astrophysics tends to focus more on distant exoplanets and their detection methods, but it was lovely to return to a planet closer to home for the first time in over a decade.

Surface of Mars, captured by the NASA Curiosity Rover. Credit: NASA/JPL-Caltech/MSSS

Origins and Detections of Life and Biospheres

This module is one of the reasons I picked this masters course: an interdisciplinary adventure spanning the history of the Earth and the first chemical reactions that resulted in life emerging and evolving to its present form. We started my studying atmospheric dynamics, deducing which chemical reactions are allowed (drawing curly arrows on molecular structures to demonstrate bonds breaking/forming was fun if not complicated), and my favourite part- learning about how prebiotic chemistry could've occurred for life to emerge on Earth. Prebiotic chemistry is the series of reactions that caused the evolution of non-living molecules to living systems. This occurred on Earth between its formation 4.5 billion years ago and life originating ~3.5 billion years ago. My favourite parts of the module were studying potential origin of life scenarios, biosignature detections (to see if there is life on other plants) and generally enjoying dipping my toe in chemistry and biology again for the first time since before university physics, only this time it was tailored to studying planets and life, both of which I find fascinating.

There are a few theories about the environments in which life could've emerged, which are outlined below:

1 - Hydrothermal vents

In the depths of Earth's oceans lie another potential starting habitat for life on our planet, in the form of hydrothermal vents. These bubbling chimneys support diverse marine ecosystems unlike any other life on Earth. Molten magma on the rock/liquid boundary at the bottom of the sea escapes in underwater vents, heating up the surroundings and providing rich nutrients, like an underwater volcano. Most life on Earth relies on sunlight for oxygenic photosynthesis eg. plants, which go on to produce sugars and carbohydrates that form the basis of food chains. At the bottom of the sea however, which can be several kilometers deep, there is no sunlight, which can travel at most to around ~1km, meaning this form of life would not be able to exist at these depths. However, life around hydrothermal vents is unique in that it does not require sunlight at all, relying instead on the rich chemical spewed out by the vents to feed chemosynthetic organisms, a type of microorganism that is able to convert the surrounding chemicals into energy and power entire underwater ecosystems. Methanogens are one such example of this organism, and produce methane as a byproduct of their energy metabolism. The discovery of this type of life in the 1970s has revolutionised our understanding of habitability, and opened up a whole new realm of potential habitats for life, most recently including icy moons. Excitingly, methane has been detected on Saturn's icy moon Enceladus, so there is a chance that beneath its icy crust and ocean layer, hydrothermal vents at the bottom of the sea on Enceladus may also host microbial systems

Hydrothermal vents on Earth at the bottom of the sea, a potential habitats. Credit: NASA

2 - Comets delivering life building blocks

Interstellar chemistry is known to participate in reactions that can result in building blocks for life, such as nitriles which feed molecules for prebiotic chemistry. Some molecules on comets may survive the long journey to Earth from the outer edges of the solar system (impressive given this can include violent impacts) provided the entry is shallow enough and the molecules are sufficiently deep inside the comet to remain protected. Most impacts however are thought to annihilate cosmic chemistry, although it may be possible for some molecules to survive. Another option is that small scale objects, such as micrometer scale interplanetary dust, is more likely to survive the journey to Earth's surface intact. The sketch below illustrates how this occurs. A comet covered in ice travels towards Earth from the the edge of the solar system carrying potential life building block molecules, and collides with asteroids in the Asteroid Belt. The asteroids and comets are ground down into dusty fragments, and this cosmic dust flux drifts towards Earth. While the dust particulates are very small, the buildup in their concentration may be relevant to prebiotic chemistry. Heavier elements in the dust can make the grains dark, so having landed on Earth (billions of years ago Earth was colder and would have been coated in snowy ice), the dark grains would have caused the ice to melt a little, forming small pits with the grains at the bottom. The dust concentration could have built up in multiple small pits over time, resulting in high concentrations of molecules needed for prebiotic chemistry all in the same place. Cosmic dust is certainly an interesting solution to the problem of comets and their molecules being destroyed on impact, and is something we did a podcast about during the course.

3 - Alkaline lakes

Early Earth may have contained alkaline lakes, also known as soda springs, which for those whose GCSE chemistry recollection is hazy, is the chemical opposite of acidic. Alkaline lakes may have played an important role in the origins of life owing to the following favourable properties:

• elevated pH provides ideal conditions for many prebiotic processes

• lakes afford access to UV radiation from sunlight, purifying and selecting products for prebiotic synthesis

• formation of ferrocyanide salts and phosphates provides key molecules for prebiotic synthesis

• minerals are concentrated on evaporation, which again is useful for prebiotic synthesis

Phosphates in particular have posed a problem in origins of life studies on Earth for some time, given that phosphorus is not naturally occurring in water bodies, but necessary for life. Its presence is vital for DNA, RNA, and cell membranes and generally all life as we know it. Other molecules thought to be necessary for life are the CHNOPS elements: carbon, hydrogen, nitrogen, oxygen, phosphorous and sulphur. Early Earth was a more violent place than today, humming with volcanic activity, and volcanic lava is one such source of phosphorus. Alkaline lakes can form in closed basins around volcanos which are filled with runoff from explosions. Since they have no outflow, the lakes only lose water by means of evaporation, allowing the concentration of chemicals such as phosphate to build up over time. Even nowadays, some alkaline lakes on Earth host thriving habitats. Potentially then, alkaline lakes could have accumulated enough phosphates in their shallow ponds to make life possible, which interestingly may also have acted in conjunction with hydrothermal pools. When it comes to the origins of life billions of years ago, it is inevitable there will be uncertainty given we cannot go back in time to see for ourselves. Nevertheless, great progress is being made on potential origins of life scenarios on early Earth combining expertise from across the sciences.

Mono Lake, California, an example of a present day alkaline lake. Credit: GALYNA ANDRUSHKO/ALAMY

The second half of the module was more biology focused, learning about the transitions of cells, the impacts of plants and animals on the biosphere, and how Enceladus could be a candidate for life elsewhere in the solar system. We also covered biosignatures, a key indicator of whether a planet may have signs of life, which was particularly exciting given that not long after lectures finished, one of the astronomers at the institute in Cambridge was all over the news about this very topic. For a detailed post on recent developments on the detection of a potential biosignature on an exoplanet for the first time, read this article published in April here.

All in all, I've learnt a huge amount about astrophysics this year, and although my course is called Planetary Science and Life in the Universe, even that long title doesn't quite do it justice. Nevertheless, hopefully this post offers a snapshot of the fascinating possibilities in astronomy, and the different avenues for research which are all buzzing with possibilities. The year was a whirlwind, and having recovered over the summer, looking back I realise just how much science we covered! The busyness did not stop outside the degree however, as it turns out Cambridge is absolutely bursting with life-changing extracurricular opportunities! I managed to squeeze in multiple swimming competitions (with some medals!), a swim from England to France, a sailing camp in Finland, a trip to South Africa tagging along with the polo team camp while doing my own safari, the varsity ski trip, and a couple of magical balls and formal dinners, and of course many lovely new friends. Many thanks indeed to Murray Edwards College and the Parasol Foundation scholarship for funding my studies at Cambridge, it has truly been an incredible year.

Trinity May Ball at Cambridge 2024, pictured by Ikoroes/instagram.