Sean McMahon

@seanhmcmahon

Astro/geo/palaeo/biology @ UK Centre for Astrobiology:

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Sean McMahon@seanhmcmahon· 5d

“Life… finds a way”. Freshly broken rock reveals green microbes hiding inside, sheltering from the harshness of the Arctic desert. Svalbard.

Held in a gloved hand, a freshly broken fist-sized rock showing bright green strip 1 cm below what was the outer surface. Photographed outdoors; snow patches visible in background.

Sean McMahon@seanhmcmahon· 8d

Climate change is revealing new geology in Svalbard. This layer of fossil microbial colonies (stromatolites), tilted almost vertical, formed nearly 800 million years ago in a tropical sea. Until just a few years ago, nobody had seen this view: ice covered everything here.

Sean McMahon@seanhmcmahon· 11d

Fossil microbial colonies: some spectacular 800 million-year-old stromatolites photographed this week in Spitsbergen.

Orange rock with strange blobby bulbous protuberances on top surface, somewhat like mushrooms or lichens in shape but part of the rock. A scale-bar indicates that he blocks are 5–20 cm across. Rock is a boulder resting on mossy tundra.

Orange boulder on black tundra with blue fjord in the background and snow-capped mountains/glaciers on the horizon. The boulder shows pale vertical stripes crossed by finely etched curved parallel lines. A glove for scale shows that these stripes are a couple of fingers' width. They are fossilized columnar stromatolites.

Sean McMahon@seanhmcmahon· 18d

Up in Svalbard for the next couple of weeks on fieldwork. Did not expect to find fossil plants on the glacier above Longyearbyen!

A photograph from the Longyearbreen glacier, Svalbard. Foreground: a boulder with dark fossil fern leaves resting on the glacier ice. Middle distance: some scientists and snowmobiles. Background: The glacial U-shaped valley leading down to the coast where the town of Longyearbyen hugs the shore. There are mountains beyond the water in the distance.

A photograph from the rocky debris around the Longyearbreen glacier, Svalbard. Foreground: a boulder with a fossil leave, clearly showing veins. Background: higher up, the glacier nestled in a valley.

Scientists traipsing between streams on the glacier, below an overcast sky.

Sean McMahon@seanhmcmahon· 19d

Another ammonite from North Yorkshire, this one found (and proudly displayed) by my young niece!

A smiling young girl holds an ammonite fossil in two parts (from a split nodule). The fossil is a spiral about the size of a ping pong ball.

Sean McMahon@seanhmcmahon· 20d

Just back from North Yorkshire Jurassic coast. Few things satisfy like opening one of these!

Photograph showing part and counterpart of a split nodule bearing a beautiful ammonite in positive and negative relief respectively. Ammonite about as wide as a tennis ball. A geological hammer is shown for scale.

Sean McMahon@seanhmcmahon· Jul 14

Here's a taster from my segment on Monday's episode of The Sky at Night, now available on BBC iPlayer:

Sean McMahon@seanhmcmahon· Jul 9

"Aliens Next Door?" — You can see me chatting to @chrislintott.bsky.social on the BBC's Sky at Night next Monday about some of our experiments @theukca.bsky.social to understand recent observations by Perseverance Rover on Mars. @severinhryn.bsky.social gets a shout-out! #astrobiology

Sean McMahon@seanhmcmahon· Jun 30

Outside? Bad news for your hypothesis! Inside? Good news! Similar thinking applies (we argue) to many possible "biosignatures" in astrobiology. Even if alien biology is very weird, we can ask: is this a Pareto-optimal way for evolution to balance the trade-offs in this context? 8/10

A cartoon diagram plotting Trait 2 (y axis) against Trait 1 (x axis) for an imaginary bean-shaped organism, highlighting an imaginary Pareto region in this 2-dimensional trait-space. Beans outside the region are labelled "less likely to be biogenic". Those inside it are labelled "more likely to be biogenic". At the corners of the region are "archetypes" representing the most extreme Pareto-optimal combinations of traits.

Sean McMahon@seanhmcmahon· Jun 30

It has been shown that curved bacterial rod shapes on Earth form a Pareto set balancing swimming speed, foraging efficiency, and material cost (see Schuech et al: doi.org/10.1073/pnas...). The same laws of physics on Earth and Mars constrain that set. Are the Martian rods inside or outside it? 7/10

Figure from Schuech et al. (2019). A plot with curvature on the y axis and elongation on the X axis. Hundreds of black dots cluster within a large multicoloured region on the plot. Curved rods are drawn around the edges of the region to illustrate the combinations of curvature and elongation defining the edge. Caption from the paper: Pareto-optimal curved-rod morphologies. Colored regions represent the set of shapes that are Pareto-optimal resulting from trade-offs between Swimming Efficiency, Chemotactic SNR, and Construction Ease; white region represents suboptimal shapes; and dots represent observed species medians. Selected simulated morphologies are plotted along the main optimal/suboptimal Pareto front as well as at the centroid of the disconnected Pareto-optimal “island.” RGB color values were assigned by normalizing performances in each task within the optimal region between 0 and 1. Color thus signifies relative, not absolute, trade-offs between tasks as indicated by the color triangle (Inset). Not all colors in the triangle are realized because not all possible trade-offs are realized (e.g., the shapes that excel at Construction Ease also excel at Swimming Efficiency).

Sean McMahon@seanhmcmahon· Jun 30

There is lots of controversy about how to recognise Pareto sets in real biological data. But there are dozens of likely examples in the literature, from the shapes of bat-wings, ammonite shells, sperm cells and reptile jaws to the structures of nervous systems, fungal networks, and proteins. 5/10

Scatter plot from a paper about proteins in E. coli by Loren Koçillari et al. (2018, Scientific Reports). Solubility is plotted on the y axis; hydrophobicity on the x axis; hundreds of data points fall within a triangular region of the plot. The authors argue that proteins in the repertoire of E. coli are constrained to a Pareto-optimal combination of solubility and hydrophobicity. Source: https://www.nature.com/articles/s41598-018-27287-3

Sean McMahon@seanhmcmahon· Jun 30

🤔 Can a concept widely used in economics help us recognise alien life, from fossils on Mars to molecules in the outer solar system? A quick thread to explain a new paper from my research group : Can Pareto Optimality be Evidence of Life?, by Rodgers et al. 🧵 1/10 doi.org/10.1177/1531...

Cartoons of (a) swimming rod-shaped bacteria, which look like long beans with wriggly strings at the ends; (b) a branching network that could be a kind of plant, alga, or fungus; (c) the structure of a protein molecule.

Sean McMahon@seanhmcmahon· May 23

sorry @stevebrusatte.bsky.social but you appear to be on display in terminal 3 at Chicago O’Hare

A table of books on display in an airport bookshop, including the story of birds by Steve Brusatte

Sean McMahon@seanhmcmahon· May 22

I was lucky enough to hold a fragment of Mars the other day. Sometimes the fourth planet feels very close.

A black flat irregular stone in the palm of a man’s hand with a label identifying the stone as part of the meteorite fall of June 1911 at Nakhla, Egypt.

Sean McMahon@seanhmcmahon· May 20

The history of the world between thumb and forefinger: 4.6 billion-year-old minerals on one side; on the other a scab of black glass formed when this #meteorite began to melt while falling through Earth’s atmosphere in 1969.

Dark sliver of rock held in the hand in a museum backroom with label identifying it as Allende meteorite

Sean McMahon@seanhmcmahon· Apr 3

Away we go! — and what care we For treasons, tumults, and for wars? We are as calm in our delight As is the crescent-Moon so bright Among the scattered stars (William Wordsworth, "Peter Bell", 1798)

Sean McMahon@seanhmcmahon· Jan 14

🚨New preprint (not peer reviewed)! Last year, NASA announced "potential biosignatures" on Mars: bleached spots in rust-red rocks. Here, we show that rust-dissolving bacteria really can make spots like these. Next step: see if we can make them without bacteria! #astrobiology doi.org/10.64898/202...

Sean McMahon@seanhmcmahon· Jan 9

Escape to the sandy carbonate lakeshores of ancient Mars with new study from Perseverance team: "The favored depositional model... is a lacustrine shore zone environment [with] wave action along a paleoshoreline"... Features mystery domes! agupubs.onlinelibrary.wiley.com/doi/10.1029/...

Sean McMahon@seanhmcmahon· Dec 7

Looking forward to speaking at unibremen.bsky.social next week: www.uni-bremen.de/en/universit...

Sean McMahon@seanhmcmahon· Nov 21

It was a great pleasure this week to welcome renowned astrobiologist Dr Frances Westall back to her alma mater @edinburgh-uni.bsky.social to receive the honorary DSc degree in recognition of her many contributions to astrobiology (Photographs by www.andrewperry.co.uk).

Sean McMahon@seanhmcmahon· Nov 15

My friend Alex Liu is offering TWO postdoc positions in #paleontology and #sedimentology at Cambridge focused on some spectacular new Ediacaran-age localities in southern Namibia... www.cam.ac.uk/jobs/researc... www.cam.ac.uk/jobs/researc...

Geologists in the field in Namibia: a wide brown desert and a clear blue sky

Sean McMahon@seanhmcmahon· Nov 9

‼️New PhD project in Edinburgh: "Animals, traits, and oxygen in the Ediacaran Nama Group, Namibia". Supervised by Rachel Wood (Amelia Penny and I co-supervising) — big UK+Namibian team — help us understand early animal evolution! Apply by Dec 14th #paleontology e5-dtp.ed.ac.uk/project?item...

Members of Rachel Wood's research group conducting geological fieldwork in Namibia. Geologists in hats gather around a rocky outcrop under a clear blue sky, with a hilly, sparsely vegetated Namibian landscape in the background.

Sean McMahon@seanhmcmahon· Oct 27

Congratulations to Ruby Allen-Brown for winning the UKCA-Harding Essay Prize 2025. You can read Ruby's winning entry at: www.astrobiology.ac.uk/opportunitie...

Sean McMahon@seanhmcmahon· Sep 16

New paper led by my PhD student, Edwin Rodriguez! Building on previous work, we present a systematic description of the ONE BILLION YEAR OLD fossils of the phosphates of the Diabaig Formation, NW Scotland — including some early eukaryotes and new species... #paleontology #fossils #geology

Microscope image of fossils showing new genus and species Minimarmilla multicatenaria, with strap-like bundles comprising filamentous strands of rectangular cell-like units. The entire specimen is about half a millimetre across.

Sean McMahon@seanhmcmahon· Sep 13

Just discovered this absolute gem of a time-saver. Thanks @anboa4.bsky.social!

Sean McMahon@seanhmcmahon· Aug 28

Contact (1997) played tonight at the UK Centre for Astrobiology's monthly film night. Third or fourth time I've seen it... but so few movies capture the wonder, emotion, and excitement of science and exploration this well. There's a bit of Jodie Foster's Ellie Arroway in every scientist I know.

Sean McMahon@seanhmcmahon· Aug 20

Hello! I'm Sean, an astrobiologist and palaeobiologist at the University of Edinburgh. You can read about my group's work on fossilization, false biosignatures, Mars and early life here: www.astrobiology.ac.uk/mcmahon-group. Here's me in Namibia — and a billion-year-old Scottish fossil...

Man in the desert holding a pair of oryx horns on top of his hat. The horns are almost as long as his arms.

Sean McMahon@seanhmcmahon· May 6

A great pleasure to meet collaborators from @naturalis.bsky.social the other week to kick off our @hfspo.bsky.social -funded collaboration on biosignatures and life detection!