The impact that formed Mercury’s spectacular 100-km (62-mile) wide Hokusai crater, named after the famous Japanese artist, who created the Great Wave off Kanagawa, could have delivered the billions of tonnes of water ice stored at its poles. That’s according to a new study, which could also help explain why, despite superficial similarities, our Moon seems so dry in comparison to the first rock from the Sun.
Mercury, the Solar System’s innermost resident is a mostly dull grey, heavily cratered world of rocky rubble and organic dust scorched by day time temperatures of up to 430 degrees Celsius (800 degrees Fahrenheit).
However, in the last 10 years evidence from Earth-based radar and NASA’s MESSENGER spacecraft suggested something more interesting, if perhaps counterintuitive, can be found at its poles — billions of tonnes of water ice. Just last month, the BepiColombo mission will return to map these areas in more detail.
Such contrasting environments on a planet, not much bigger than our Moon might seem unlikely. However, the axis on which Mercury spins runs almost straight up and down. This leaves pockets inside craters at the poles which the Sun’s rays never reach.
With no significant atmosphere to retain and distribute heat, these areas of permanent shadow act as cold traps for water molecules.
Interestingly, our Moon also has permanently shadowed craters at its poles, though investigations to look for water have provided only tantalising hints but nothing more.
“On Mercury the signals are like wow — that is water!” says Carolyn Ernst, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory. “The difference between Mercury and the Moon needs to be explained.”
Forming in such a hot part of the Solar System with very little water around, exogenic models which deliver the water later appear more likely. And there are a few options.
Micro-meteorites are tiny but they are continuously landing on all the planets, and over billions of years could provide sufficient water.
Alternatively, you could have a single, water-bearing impactor — a comet or a very hydrous asteroid. Whilst on impact, a lot of the water will vaporise and escape, some would remain, and a fraction of that could reach the cold traps, either travelling through a temporary, thin water-based atmosphere created by the impact, or by ‘hopping’ there overtime.
Thirdly, you could have a bit of both — a multiple impact scenario with a mixture of large impactors and micrometeorite contributors.
A clue to the most likely scenario came from MESSENGER’s images of those shadowy pockets, which revealed strikingly sharp albedo boundaries between reflective, mostly pure water ice and dull regolith. There was not much evidence of mixing.
“If that [ice] is sitting there for a long time, bombardment by little meteorites should garden the surface, making the boundary more diffuse,” says Ernst.
That points to a single recent deposition millions, rather than billions, of years ago.
Ernst decided to search for a smoking gun.
With current estimates based on neutron spectrometer data of anywhere between 10 billion and 1 trillion tonnes of Mercurian water ice, if one single impact was the source, it had to have been a major impact event.
Whilst there are several large craters on the planet’s surface, one in particular is speculator.
Hokusai is just under 100 km in diameter with a horseshoe-shaped central peak and rays of ejecta that encircle much of the northern hemisphere.
The lack of superposed younger impacts makes it ones of Mercury freshest, with the latest predictions suggesting an age of less than 300 million years.
Named by John Harmon, a radio astronomer who authored or co-authored many of the Earth-based radar results for Mercury, Ernst doesn’t see any indication the name was a subtle, cryptic suggestion of the possible events surrounding its formation.
“I believe he just liked the artist. Cool coincidence that it’s a candidate water-bringer,” she says.
In a paper published in the Journal of Geophysical Research: Planets, Ernst and her colleagues asked if the Hokusai impact really does measured up to the challenge. However, they faced several unknowns.
What was the impactor made out of? How fast was it travelling and at what angle did it arrive?
Searching for clues in those beautiful rays, they mapped their distribution, before honing in on the crater itself to get an overall understanding of the angle of approach required to account for this distribution of ejected material.
Their analysis revealed rays extending 7,000 km (4,350 miles) from the crater centre, with the largest concentration toward the southwest.
Closer in they found more evidence of an asymmetrical distribution, with the least extensive deposits to the east.
With the level of asymmetry directly linked to the angle of approach, an oblique (but not too oblique) angle of around 30-40 degrees was deemed most likely.
Unfortunately, the velocity of the incoming object proved more tricky to nail down.
Whilst space rocks approach Earth at a fairly consistent 20 km/s (12 miles per second), Mercurian impactors arrive at a much wider range of speeds, due in part to the high eccentricity of the planet’s orbit, and the gravitational influence of the Sun, which can speed up incoming objects significantly, given the right orbital alignments.
Without a narrower fix on the incoming speed, a range of objects could account for Hokusai’s crater and ray pattern, including small and relatively dry rocky asteroids, or more encouragingly, much larger, water rich comets. These were shown to be able to account for the observed icy pole composition, even when you added in standard figures for the % lost through impact or during migration.
A 30 km/s (19 miles per second) approach speed limit proved key. Any faster and not only does the size or water ice component of the projectile decrease in order to produce the same size crater, but the mass of water retained post-impact also drops off steeply.
Around a quarter to a third of the Mercury’s impacts arrive at speeds less than this critical value, however that figure is lower for the generally faster moving comets. Ernst though is still positive.
“This could just mean it [the comet] was coming in the same direction as the planet is spinning,” she says.
So-called prograde objects catch up with their eventual targets, with the relative speed of the impact reduced by the fact that planet is spinning in the same direction.
“Just because something is less likely doesn’t mean we should discount it,” she argues.
In fact, the rarity of such an event may actually be useful when we compare Mercury to the Moon.
“The Moon has likely experienced large planet altering impact events in the past, but perhaps nowhere near as recently. The more we understand the water at the poles of Mercury and how it got there, the more we understand how it got anywhere in the Solar System, and this has implications for Earth and us.”
“This [paper] takes us a step closer to addressing two compelling mysteries with broad implications for our understanding of the evolution of the inner Solar System — where Mercury’s water ice came from, and why the poles of Mercury and the Moon appear so different,” says Parvathy Prem, an expert in comet impacts and lunar water, who also works at the Johns Hopkins Applied Physics Lab but wasn’t involved in this paper.
“It verifies that the idea that impact-delivery of Mercury’s polar deposits holds water.”
Further confirmation may come from the BepiColombo mission as it fills gaps left by MESSENGER, whose highly elliptical orbit meant it never got close enough for high resolution mapping of the south pole.
Comparing any differences in water ice build-up and proximity of potential impact sites between the two poles might add more weight to the impact delivery theory.
This is work is important. The odds are we will land a craft at the poles of the Moon before we do on Mercury, and lunar water is an essential component of planned Moon bases and permanent habitation of our orbiting companion. Understanding where water ice resides on Mercury will inform these lunar explorations.
“Are you going to find it in giant chucks, buried under the surface, or in some small parts per million, in each bucket of regolith? The answers might lie at Mercury poles,” says Ernst.
Carolyn M. Ernst et al. Examining the Potential Contribution of the Hokusai Impact to Water Ice on Mercury. Journal of Geophysical Research: Planets, published online September 1, 2018; doi: 10.1029/2018JE005552