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Moon formation

A giant impact is the leading explanation for the Moon’s origin. Its exact sequence must account for the Moon’s composition, orbit and relationship to Earth.

Early Mars climate

Ancient Martian rocks and landforms record liquid water under a fainter Sun. What climate sustained it, and how persistent were those wet conditions?

Uranus’s internal heat

Uranus does release heat from its interior, despite older claims that it does not. How does its interior lose heat and produce the seasonal energy budget we observe?

Glass transition

A cooling liquid can become enormously slower without developing a crystal’s obvious order. What microscopic change explains its transition into a glass?

Primordial lithium problem

Old, metal-poor stars contain less lithium than standard early-universe calculations predict. Did the stars destroy it, or is something missing from the calculation or inference?

Core-collapse supernova mechanism

The collapse of a massive star can launch an explosion. Which stars explode, and can models predict the explosion and remnant from the star’s pre-collapse structure?

Diffuse interstellar bands

Starlight passing through interstellar gas picks up many absorption bands. Some have known molecular carriers, but most still lack a secure identification.

Planetesimal formation

Planetary systems start with small solid grains. How do these become gravitationally bound bodies before collisions or inward drift remove them?

Onset of plate tectonics

Earth’s moving plates are well established today. How and when did its early surface develop the connected system of recycling and motion we recognize as plate tectonics?

Large low shear velocity provinces

Two enormous regions near the bottom of Earth’s mantle slow seismic waves. What mixtures of heat and material create them, and how have they changed through time?

Aerosol–cloud interactions

Airborne particles change how clouds form and reflect sunlight. How much cooling do those changes produce, across different clouds and weather conditions?

Leptonic CP violation

Do neutrinos and antineutrinos change flavour differently in a way that violates charge–parity symmetry?

Neutron lifetime puzzle

Proton-counting beam measurements and stored-neutron measurements give incompatible neutron lifetimes.

QCD critical point

Does hot, baryon-rich nuclear matter have a critical endpoint where the character of its phase transition changes?

r-process sites

Which events supplied the rapid neutron-capture elements found throughout our Galaxy?

Solar dynamo

Where and how does the Sun regenerate the magnetic field underlying its activity cycle?