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Welcome back to AI OBSERVER, where we break down the most fascinating scientific mysteries shaping our understanding of the universe.
Today’s edition dives into one of the great unsolved puzzles of planetary science — a world so strange that many astronomers argue it should not exist at all.

Thank you, as always, for reading, sharing, and supporting this newsletter. Your curiosity fuels every edition.

At first glance, Mercury appears unremarkable.
It has no thick atmosphere, no oceans, no signs of ancient life, and no lush geological features like volcano chains or tectonic plates. Its surface is scarred, dry, and heavily cratered — more similar to the Moon than to Earth.

Yet beneath this seemingly dull exterior lies a planetary contradiction so severe that it challenges our most trusted theories of how planets form.

Mercury is:

Individually, each of these traits is strange. Combined, they form a mystery that scientists have struggled with for decades.

Mercury is the smallest planet in our Solar System.
Its mass is roughly 20 times lower than Earth’s, and its diameter is only slightly larger than Earth’s Moon. If placed on Earth, it would span roughly the width of Australia.

According to conventional planet-formation models, worlds that form close to a star should either:

Mercury does neither.

Instead, it remains small, compact, and stable — an outcome that simulations repeatedly fail to reproduce.

Mercury’s most baffling trait is its internal structure.

Despite its small size, it is the second densest planet in the Solar System, surpassed only by Earth. The reason is extraordinary:
👉 Mercury’s metallic core makes up nearly 85% of its radius.

For comparison:

This configuration is unlike any other rocky planet we know.

Planetary scientists openly admit this is deeply problematic. Standard formation models simply do not produce planets with such an extreme core-to-mantle ratio.

Mercury orbits extremely close to the Sun, completing a year in just 88 Earth days.
This region of the Solar System is:

Yet Mercury exists there — alone, isolated, and oddly placed far from Venus.

When researchers simulate early Solar System dynamics, planets tend to form in clusters, not gaps. Mercury’s location appears statistically unlikely, even borderline impossible.

As one planetary dynamicist bluntly put it: “We run the models again and again — and Mercury never shows up.”

🛰️ Early Clues from Mariner 10

The first hints of Mercury’s internal oddities came in the 1970s when Mariner 10 conducted three flybys.
Gravity measurements suggested an unusually massive core — a shock at the time.

🛰️ Messenger Made Things Worse

Decades later, MESSENGER orbited Mercury between 2011 and 2015 and delivered results that deepened the mystery.

Despite extreme temperatures ranging from 430°C during the day to –180°C at night, Messenger detected:

These substances should have been destroyed long ago by solar radiation.

Instead, they survived.

Volatile elements are crucial clues because they act like thermal fingerprints of planet formation.

Their presence suggests one of two things:

Either explanation contradicts classical models.

This is why many scientists argue Mercury may be the closest analogue we have to certain exoplanets — compact, metal-rich worlds orbiting tightly around their stars.

The most widely discussed explanation is the giant impact hypothesis.

In this scenario:

Similar processes are believed to have formed Earth’s Moon.

However, there are major problems:

An alternative idea suggests Mercury was the impactor, not the impacted.

In a “hit-and-run” collision with a larger planet (possibly Venus):

This model solves some mechanical problems but still struggles to explain Mercury’s chemical composition.

Another proposal involves collisional grinding:

This requires extremely high impact rates and efficient dust removal — conditions that may be unrealistic.

A more radical explanation argues Mercury formed exactly where it is today.

In the early Solar System:

While elegant, this theory raises a critical question:
👉 Why did Mercury stop growing?

If iron-rich material was abundant, Mercury should have become much larger.

Answers may finally arrive with BepiColombo, a joint mission by European Space Agency and Japan Aerospace Exploration Agency.

Launched in 2018, the spacecraft is scheduled to enter Mercury’s orbit in late 2026.

Its goals include:

For the first time, scientists may finally distinguish between competing theories.

Solving Mercury’s origin is not just about one planet.

It directly impacts:

In many ways, Mercury is a cosmic laboratory — an extreme case that exposes flaws in our assumptions.

Mercury challenges astronomers precisely because it exists at the intersection of contradictions:

Whether shaped by violence, migration, solar fury, or some mechanism we have not yet imagined, Mercury reminds us of a humbling truth:

👉 The universe does not owe us simple explanations.

🙏 Thank You for Reading

Thank you for spending your time with AI OBSERVER.
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More deep-dive explorations are coming soon.

— AI OBSERVER Team

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