Mercury A to Z: Caloris Basin

Hello, friends!  For this year’s A to Z Challenge, we’re exploring the planet Mercury.  In today’s post, C is for:


It seems like just about every planet has its thing.  Saturn has her rings.  Jupiter has his Great Red Spot.  Mars has both Olympus Mons and Valles Marineris, the largest volcano and the largest canyon, respectively, in the entire Solar System.  And as for Mercury, Mercury has Caloris Basin, an absurdly large crater in Mercury’s northern hemisphere.

So how did Mercury end up with such a big crater?

Based on what science currently knows about the history of the Solar System in general, and the history of Mercury in particular, Caloris Basin most likely formed during an event known as the Late Heavy Bombardment.

About four billion years ago, the Solar System looked a little different than it does today.  The gas giants (Jupiter, Saturn, Uranus, and Neptune) were engaged in these gravitational tug-of-wars with each other, pulling each other into new orbits, swapping places with each other, and generally causing chaos in the outer Solar System—and generally making a mess of the inner Solar System, too.  All those gravitational tug-of-wars in the outer Solar System sent tons and tons and tons of stray asteroids hurtling toward the inner Solar System.  All the inner planets (Mercury, Venus, Earth, and Mars) took a beating.  Earth’s Moon took a beating, too.

A particularly large asteroid must have slammed into Mercury near the end of the Late Heavy Bombardment.  We know this must have been near the end of the Late Heavy Bombardment because Caloris Basin has only a few smaller, younger-looking craters inside it, while the surrounding terrain is thoroughly peppered with older-looking craters.  That impact must have been a truly Earth-shattering Mercury-shattering event, sending ripples and shockwaves all the way around the planet, leaving geological marks that can still be seen to this day.

Caloris Basin was discovered in 1974 by NASA’s Mariner 10 space probe.  At the time of the discovery, Caloris Basin was only half in daylight, so the full size of the crater was unknown.  You may recall from yesterday’s post that Mariner 10 visited Mercury three times, but due to an unfortunate coincidence of orbital mechanics, Caloris Basin was only half in daylight every single time Mariner 10 showed up.  And it was always the same half of Caloris Basin, too.  So the full size of the crater remained uncertain until the 2010’s, when the MESSENGER Mission entered orbit of Mercury and finally imaged the entire crater in full daylight.

Based on Mariner 10’s data, scientists originally guessed that Caloris Basin was 1300 km (810 miles) in diameter, making it larger than Texas.  MESSENGER revealed that its actually 1550 km (960 miles) in diameter, making it even more larger than Texas.


Here’s an article from, going into a little more detail about how Caloris Basin formed and what we currently know about it.

And here’s an article from Wondrium about the Late Heavy Bombardment, how it happened, and how we know about it.

Also, I thought I read somewhere that Caloris Basin was the largest impact basin in the Solar System, and an early draft of this blog post included that detail.  But that’s not true.  Apparently the largest impact basin in the Solar System is Utopia Planitia on Mars.  For anyone interested, here’s a Wikipedia page listing all the largest craters known to exist in the Solar System.

6 thoughts on “Mercury A to Z: Caloris Basin

    1. Mars is kind of greedy about having all the biggest things in the Solar System, isn’t he? For years, I really thought Mercury had Mars beat for biggest crater, but I guess I was wrong.


    1. Getting into the history of science is one of my favorite parts of learning science. It’s one thing to learn about the things we know, but quite another to hear the stories of how scientists figured this stuff out, along with the mistakes that were made along the way.


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