Inside Science Listener Q&A: Magnetic Reversals, Galactic Collisions, Photosynthesis and More

In this edition of Inside Science, Victoria Gill hosts a panel of experts—Catherine Heymans, Mark Maslin and Penny Sashay—who tackle questions from listeners on topics as varied as the Earth’s magnetic field, why most plants are green, the Milky Way–Andromeda galactic encounter, and the physics behind gravity assists in spacecraft.

The show blends astronomy, earth science and biology to demystify complex ideas, offering clear explanations and inviting further curiosity. From how magnetic reversals affect our planet to whether Mars rocks found on Earth can be traced to their origin, the discussion weaves science into everyday understanding and highlights the big-scale questions that shape our view of the universe.

Overview

In the podcast episode, Victoria Gill hands the microphone to a science panel—Catherine Heymans from Edinburgh University, Mark Maslin from University College London, and Penny Sashay from New Scientist—to respond to questions from curious listeners. The dialogue spans astronomy, earth science, biology and physics, showcasing how a scientific conversation can move fluidly across disciplines while remaining accessible. The discussion is driven by real questions from listeners and a desire to demystify scientific ideas without reducing them to simplifications.

Earth’s magnetic field and pole reversals

The first listener question asks about a reversal of the Earth’s magnetic poles and the potential risks to life and ozone from cosmic radiation. Mark Maslin explains the distinction between rotational axes and magnetic poles, and how the Earth’s magnetic shield arises from the movement of liquid iron in the outer core. Catherine Heymans contributes by noting that Venus and Mars lack a strong magnetic field, helping to contrast Earth’s magnetic shield with other planets. The panel emphasizes that pole reversals occur irregularly—ranging from thousands to tens of thousands of years—and that while such reversals can weaken the field and allow more cosmic radiation to reach the Earth, they do not trigger major mass extinctions. The impact on modern society would be profound primarily due to electronics and communications disruption during periods of weaker shielding.

"Every so often the Earth's magnetic field flips" - Mark Maslin, Professor of Earth System Science, UCL

Chlorophyll, leaves and plant pigments

A classroom question from Laurie asks why plants with non-green leaves are less common and whether other green parts of plants have chlorophyll. Penny Sashay explains that chlorophyll is the main pigment for photosynthesis and that exceptions are mostly parasitic plants that do not rely on photosynthesis. In non-green plants, pigments like anthocyanins can mask chlorophyll, giving plants purple or red appearances while chlorophyll is still present. Leaves evolved to maximize photosynthesis by gathering light efficiently, but chlorophyll can be masked by other pigments. Penny also notes that chloroplasts exist in green tissues beyond leaves, and that breeders have sculpted color for aesthetic or horticultural reasons.

"Green is the default, but sometimes, more rarely, you'll see another color over the top of that" - Penny Sashay, New Scientist

Astronomy and cosmology: Expanding universe and galaxy interactions

Marie’s question about the apparent paradox of universal expansion alongside the eventual Milky Way–Andromeda collision receives a clarifying answer from Catherine. On cosmic scales, the universe is expanding due to dark energy, causing distant galaxies to move away from us. Locally, gravitational attraction dominates, bringing the Milky Way and Andromeda toward a future merger—though the precise timing is debated. The panel explains that two forces govern scales: dark energy accelerates expansion on the largest scales, while dark matter and gravity drive structure formation on smaller scales. The result is a universe where galaxies apart from each other recede, while neighboring galaxies can still collide or merge due to gravitational attraction, highlighting the scale-dependent nature of cosmic motion.

"On small scales, galaxies tend to move towards each other, but on the very, very largest scales, everything is moving away from each other" - Catherine Heymans, Astronomer Royal for Scotland

Space, time and the gravitational slingshot

The discussion moves to space travel and gravity assists, using Voyager missions as a reference. Catherine explains how a moving planet can transfer energy to a spacecraft by approaching from a direction that adds momentum, performing a gravity assist around the planet and exiting with a net speed increase. The panel also confirms that energy is drawn from the planet’s orbital motion, resulting in a negligible slowing of the planet, given its enormous mass compared to the spacecraft. The conversation shows how gravitational slingshots can be used to accelerate, decelerate or redirect spacecraft, illustrating the interplay between orbital mechanics and mission design.

Embryology, nipples in men and basic biology

Penny addresses a long-standing curiosity about why men have nipples. She explains that early fetal development is sexually undifferentiated, with both ovarian and testicular pathways present before later genes drive sex differentiation. Nipples persist in males because they do not significantly impede reproduction, and there is no strong selective pressure to remove them; some studies even suggest potential roles in arousal, though the primary reason is the shared early blueprint.

Plastic recycling: soft plastics and energy recovery

In a segment on plastic pollution, Mark Maslin clarifies what counts as soft plastic—wrappers on bread, crisp packets, bin bags, etc.—and why recycling them is challenging. Soft plastics require breaking down into basic chemical building blocks before reassembly into new plastics. Some can be recycled, but many are diverted to landfill or energy recovery because local authorities are overwhelmed, resulting in a mix of waste management outcomes.

Listener quick-fire round and rapid Q&A

The show finishes with a fast-paced round. Penny answers a hypothetical about whether the principle of photosynthesis could scale to energy production, noting that while photosynthesis underpins fossil fuels and biofuels, solar panels generally outperform natural photosynthesis, though researchers continue to study plant-inspired improvements in photovoltaic efficiency. Catherine then tackles a Martian meteorite origin question using age dating and oxygen isotope signatures, illustrating how scientists prove planetary provenance for meteorites. Finally, Mark and the panel discuss the perception of wind-driven sound propagation, explaining that wind affects the ground-bounce of sound waves, enabling sounds to carry further in certain conditions. The rapid-fire segment demonstrates how quick, structured explanations can illuminate complex topics.

Closing and call for questions

The host thanks the panellists and listeners for their curiosity and invites more questions, underscoring the collaborative and exploratory spirit of Inside Science.

"Yes, we already use plant photosynthesis on an industrial scale to get energy" - Penny Sashay, New Scientist