Mercury may have shrunk by up to 23 km in diameter, about 30% more than thought
A planet shrinks as it cools. How do cliffs on Mercury's surface reveal by how much, and why does it matter?
Published 11 September 2026. Written by Pratidin from the reports linked at the end; every fact checked by a separate review before publishing. How we work
A study published in the journal Geophysical Research Letters, reported in The Hindu on 11 September 2026, suggests that Mercury has shrunk more than scientists believed. Using data from NASA's MESSENGER spacecraft, which orbited the planet in the 2010s, researchers including scientists at the German Aerospace Center estimate that Mercury's diameter may have contracted by up to 23 km (about 14 miles), roughly 30% more than earlier estimates. Mercury is the smallest planet in the Solar System, with an equatorial radius of about 2,439.7 km, and the innermost, taking about 88 Earth days to orbit the Sun. It has been contracting since it formed about 4.5 billion years ago.
The reason lies in Mercury's unusually large, iron-rich core, which occupies about 57% of the planet's volume, compared with 17% for Earth. As the interior slowly cools, the core and mantle contract. The rigid outer crust must fit over a smaller body, so it breaks and pushes up along thrust faults, forming long cliff-like ridges called lobate scarps. Earlier mapping of these features had suggested that Mercury's radius shrank by about 1 to 7 km. MESSENGER also found small, young-looking fault scarps, evidence that the planet is still contracting today.
The finding matters because contraction is a record of how a rocky planet loses heat, which shapes its geology and its magnetic field. It comes shortly before a new mission reaches Mercury. BepiColombo, a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), was launched on 20 October 2018 and is expected to enter Mercury's orbit in November 2026. Its two orbiters, ESA's Mercury Planetary Orbiter and JAXA's Mio, will map the surface and interior in more detail, allowing scientists to test estimates like this one. Only two spacecraft have visited Mercury before: Mariner 10 in the 1970s and MESSENGER.
Prelims facts
- A study in Geophysical Research Letters using MESSENGER data estimates Mercury's diameter has shrunk by up to 23 km, about 30% more than earlier estimates.
- Mercury's core occupies about 57% of its volume, compared with 17% for Earth; cooling of this large core drives global contraction.
- Lobate scarps are long cliff-like thrust faults formed as Mercury's crust is squeezed by contraction.
- BepiColombo, an ESA-JAXA mission launched on 20 October 2018, is expected to enter Mercury's orbit in November 2026.
- MESSENGER orbited Mercury from 18 March 2011 until it crashed onto the surface on 30 April 2015.
Quick recall
- By how much may Mercury's diameter have shrunk, according to the new study?
- Up to 23 km (about 14 miles), about 30% more than earlier estimates.
- In which journal was the Mercury contraction study published?
- Geophysical Research Letters.
- What share of Mercury's volume does its core occupy?
- About 57% (Earth's core: about 17%).
- What are lobate scarps?
- Long cliff-like ridges formed along thrust faults as a planet's crust is compressed.
- Which was the first spacecraft to visit Mercury?
- NASA's Mariner 10 (1974 to 1975).
- When did MESSENGER orbit Mercury?
- From 18 March 2011 until it crashed on 30 April 2015.
- Which agencies run BepiColombo?
- ESA and JAXA.
- When is BepiColombo expected to enter Mercury's orbit?
- November 2026.
Prelims practice question
BepiColombo, which is expected to enter orbit around Mercury in November 2026, is a joint mission of:
- NASA and ESA
- ESA and JAXA
- ISRO and JAXA
- NASA and JAXA
Show answer
Answer: (b) ESA and JAXA. BepiColombo is a joint mission of the European Space Agency and the Japan Aerospace Exploration Agency. ESA built the Mercury Planetary Orbiter and JAXA built Mio (the Mercury Magnetospheric Orbiter). NASA's earlier Mercury missions were Mariner 10 and MESSENGER.
Asked before in UPSC
Recurring theme: Space science and planetary exploration
Planetary missions like MESSENGER and BepiColombo show how space science builds basic knowledge; use them as global context when discussing India's own space science achievements.
Studies like the Mercury contraction estimate come from sustained university and agency research; the story supports arguments on why basic science careers and funding matter.
Mains practice question
What does the contraction of Mercury reveal about the thermal evolution of rocky planets? Discuss the significance of missions such as BepiColombo for planetary science. (150 words)
Model answer
A new study using MESSENGER data suggests Mercury's diameter has shrunk by up to 23 km, about 30% more than earlier estimates.
What contraction reveals
- Heat loss: Mercury's core fills about 57% of its volume; as it cools, the planet contracts.
- Surface record: lobate scarps, thrust faults in the crust, store the history of that cooling.
- Ongoing activity: young scarps seen by MESSENGER show the planet is still shrinking.
- Comparative insight: a one-plate planet shows how rocky bodies behave without plate tectonics.
Why missions matter
- BepiColombo (ESA-JAXA), due in orbit in November 2026, will map surface, interior and magnetic field to test such estimates.
- Planetary data sharpen models of how Earth-like planets evolve.
- For India, such missions show the value of international collaboration, as in ISRO's lunar and Mars work.
Mercury's shrinking is a clock of planetary cooling, and BepiColombo will help read it more precisely.
The basics
Why this matters
Planets are not fixed objects. A rocky planet is born hot and spends billions of years cooling, and as it cools it changes shape. Mercury shows this more clearly than any other planet. A new study suggests it has shrunk by up to 23 km in diameter, about 30% more than earlier estimates, just before the BepiColombo mission arrives to take a closer look.
Why a cooling planet shrinks
Most materials take up less space when they cool. Mercury has an unusually large metal core, described in Mercury's iron core, so the effect is strong. As the interior shrinks, the crust above has to cover a smaller sphere, and it cracks and piles up along faults. These faults show on the surface as Lobate scarps.
- 1Hot startMercury forms about 4.5 billion years ago with a hot interior
- 2CoolingHeat escapes and the large iron core and mantle slowly cool
- 3ContractionThe interior takes up less space and the planet's radius shrinks
- 4SqueezingThe rigid crust is compressed and breaks along thrust faults
- 5ScarpsLong cliff-like lobate scarps rise across the surface
- Equatorial radius about 2,439.7 km
- Core about 57% of volume
- Orbits the Sun in about 88 days
- Crust shows global contraction faults
- Mean radius about 6,371 km
- Core about 17% of volume
- Orbits the Sun in about 365 days
- Crust recycled through plate tectonics
How we know
Only two spacecraft had visited Mercury before BepiColombo. Mariner 10 flew past in 1974 and 1975. The MESSENGER mission orbited Mercury from 2011 to 2015 and mapped the planet's scarps. Measuring the scarps and adding up how much the crust has been pushed together gives the estimate of shrinkage.
- 1974 to 1975NASA's Mariner 10 makes the first flybys
- 3 August 2004NASA launches MESSENGER
- 18 March 2011MESSENGER enters orbit around Mercury
- 30 April 2015MESSENGER crashes onto Mercury, ending its mission
- 20 October 2018ESA and JAXA launch BepiColombo
- November 2026BepiColombo expected to enter Mercury's orbit
What comes next
BepiColombo will study Mercury's surface, interior and magnetic field from two orbiters, testing whether the new estimate holds.
Go deeper
In one line: Mercury has been shrinking as its large iron core cools, and a new study using MESSENGER data says the shrinkage may be up to 23 km in diameter, about 30% more than earlier thought.
Why it matters for UPSC
Prelims regularly tests planets and space missions. The story also gives a timely hook for BepiColombo, which is due to enter Mercury's orbit in November 2026.
The core idea
The metal heart of the planet, explained in Mercury's iron core, is huge for its size. As it cools, the planet contracts, and the crust is squeezed into Lobate scarps. The MESSENGER mission mapped these features, and the new study used its data to recalculate the shrinkage. BepiColombo will now test the result.
Numbers and dates to remember
- Up to 23 km (about 14 miles) of shrinkage in diameter, about 30% more than earlier estimates.
- Earlier mapping suggested radius shrinkage of about 1 to 7 km.
- Mercury's equatorial radius: about 2,439.7 km; orbital period: about 88 days.
- Core: about 57% of Mercury's volume, against 17% for Earth.
- MESSENGER: launched 3 August 2004, in orbit from 18 March 2011, crashed 30 April 2015.
- BepiColombo: launched 20 October 2018, orbit insertion expected in November 2026.
Where to go next
- Mercury's iron core: why the planet shrinks at all.
- Lobate scarps: the cliffs that record the shrinking.
- MESSENGER mission: the spacecraft whose data drove the new estimate.
- BepiColombo: the ESA-JAXA mission arriving in 2026.
Go deeper: why a shrinking planet matters
A thermometer in stone. Planetary scientists use contraction as a record of heat loss. The more a planet has shrunk, the more heat it has lost, which constrains models of how its interior and core evolved. Mercury is ideal because it has a single rigid outer shell: unlike Earth, it has no plate tectonics to recycle the crust and erase the evidence. The record is written in Lobate scarps.
Why estimates differ. Estimates depend on how many faults are counted and how much each one has shortened the surface. Earlier mapping gave about 1 to 7 km of radius loss. The new study, using MESSENGER mission data, puts diameter loss at up to 23 km. Some of the gap reflects the difficulty of spotting small or eroded faults, which is why independent data matter.
The magnetic field link. Mercury has a global magnetic field, generated in its Mercury's iron core. How fast the core cools affects whether and how long such a field can last. Better contraction estimates therefore feed into the question of how a small planet keeps a magnetic field.
The test ahead. BepiColombo carries two orbiters, ESA's Mercury Planetary Orbiter and JAXA's Mio, with better cameras and instruments. Its data will show whether the higher estimate holds. The mission is also an example of shared international science, the model ISRO has followed with partners in its own lunar and planetary work.
Mercury's iron core
Why the planet shrinks at all
In one line: Mercury has an unusually large metallic core that takes up about 57% of the planet's volume, compared with 17% for Earth.
Why the core drives shrinking
A planet loses heat from its interior over billions of years. As the iron-rich core and the rocky mantle cool, they contract. Because Mercury's core is such a large share of the planet, its cooling has a big effect on the planet's overall size.
Why it is in the news
The new study's larger estimate of contraction means Mercury's interior may have lost more heat than earlier models assumed. The core also produces Mercury's magnetic field, which MESSENGER found to be offset far to the north of the planet's centre.
Where to go next
Lobate scarps
The cliffs that record the shrinking
In one line: Lobate scarps are long, curving, cliff-like ridges formed where the crust has been pushed together and one block has ridden up over another along a thrust fault.
How they form
When a planet's interior shrinks, its crust is compressed. A rigid crust cannot simply crumple, so it breaks, and one side is thrust up over the other. The result is a scarp that can run for hundreds of kilometres and rise hundreds of metres or more.
Why they matter
By measuring how many scarps there are and how much each has shortened the surface, scientists estimate how much the planet has shrunk. MESSENGER also found small scarps that look geologically young, showing Mercury is still contracting.
Where to go next
- MESSENGER mission
- Mercury's iron core
MESSENGER mission
The spacecraft whose data drove the new estimate
In one line: MESSENGER was the NASA spacecraft that became the first to orbit Mercury, from 2011 to 2015.
Key dates
It was launched on 3 August 2004, entered orbit around Mercury on 18 March 2011, and ended its mission by crashing onto the surface on 30 April 2015. Before it, only Mariner 10 had visited Mercury, with flybys in 1974 and 1975.
Key discoveries
MESSENGER mapped the whole planet, found water ice in permanently shadowed craters near the north pole, showed that Mercury's magnetic field is offset to the north, and found young fault scarps proving the planet is still shrinking. Its data underlie the new contraction estimate.
Where to go next
BepiColombo
The ESA-JAXA mission arriving in 2026
In one line: BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to orbit and study Mercury.
The mission
It was launched on 20 October 2018 and carries two orbiters: the Mercury Planetary Orbiter, built and operated by ESA, and Mio (the Mercury Magnetospheric Orbiter), built and operated by JAXA. It made six Mercury flybys between October 2021 and January 2025. A thruster problem reported in May 2024 lengthened its cruise, and it is expected to enter Mercury's orbit in November 2026.
Why it matters
Its instruments will map Mercury's surface, interior and magnetic field in more detail than MESSENGER, allowing scientists to check estimates of how much the planet has shrunk.
Where to go next
- MESSENGER mission
- Mercury's iron core
Take the 11 September 2026 quiz: 54 Prelims-style questions with answers