We’re a species that likes to leave evidence of itself in new places. In Greenland, for example, the Kingittorsuaq runestone, dating from the 14th Century, offers inscriptions that help chart Norse exploration of the region. The oldest inscription at New Mexico’s El Morro dates from 1605, though many explorers left their names and stories on the cliffs there. Apollo 11’s plaque, with its “We came in peace for all mankind” is justly famous, as are the Golden Records of the two Voyagers and the Pioneer plaques, even if the latter were dogged with controversy at the time of their unveiling.

Image: The Kingittorsuaq runestone. Credit: Ukendt /Nationalmuseet, Danmark, CC BY-SA 2.5 DK , via Wikimedia Commons.

Clearly the Solar System is wide open for future plaques and markers, so that NASA’s inclusion of a plaque aboard Europa Clipper comes as no surprise. The poem it carries focuses, of course, on that intriguing moon, and I rather like poet Ada Limón’s “In Praise of Mystery: A Poem for Europa” except for its first stanza. I snag on the word ‘expansiveness,’ and the notion of a sky inky with it. The word ‘expanse’ is itself so liminal, especially as applied to an inky night sky, that it carries its own freight of awe.

To this jaded ear, ‘expansiveness’ is bloated. I can’t imagine saying ‘the sky is certainly inky with expansiveness tonight.’ So I’ll pass on stanza 1, but go for the rest of the poem, which is a deft evocation of water’s place in our evolution and our explorations:

Arching under the night sky inky
with black expansiveness, we point
to the planets we know, we

pin quick wishes on stars. From earth,
we read the sky as if it is an unerring book
of the universe, expert and evident.

Still, there are mysteries below our sky:
the whale song, the songbird singing
its call in the bough of a wind-shaken tree.

We are creatures of constant awe,
curious at beauty, at leaf and blossom,
at grief and pleasure, sun and shadow.

And it is not darkness that unites us,
not the cold distance of space, but
the offering of water, each drop of rain,

each rivulet, each pulse, each vein.
O second moon, we, too, are made
of water, of vast and beckoning seas.

We, too, are made of wonders, of great
and ordinary loves, of small invisible worlds,
of a need to call out through the dark.

Those last three stanzas form a fine conclusion; the poem ends with a satisfying click describing its mission, which is to fly aboard Europa Clipper and wind up hurtling past the target world again and again, battered by radiation as it hunts for information about an alien sea. What a good thing it is to put human artifacts on spacecraft. There is scant likelihood, of course, that the Europa poem, flying along with a microchip containing 2.6 million names submitted by the public, will one day be read by anyone, but the impulse is to commemorate and inspire ourselves. It’s something we humans do.

Image: The lower half of Europa Clipper’s vault plate, showing the poem by U.S. poet laureate Ada Limón (lower right), a drawing representing the Jovian system that will host the names of 2.6 million people flying with the mission on a microchip (top right), a tribute to planetary scientist Ron Greely (bottom left), and the radio emission lines known as the ‘Water Hole’ (center). Credit: NASA/JPL-Caltech.

As a collector and user of vintage fountain pens, I am particularly pleased to see that the poem is inscribed in the author’s handwriting, a nice touch in an era increasingly learning that writing by hand, though rarely taught these days, is actually a powerful way to explore and retain ideas. The vault plate, which you can explore here, likewise contains Frank Drake’s handwriting. Drake (1930-2022), among much else in a magnificent career, contributed the first SETI search, at Green Bank in West Virginia, and the seminal Drake Equation, which estimates the probability of finding life elsewhere in the cosmos and describes the factors critical to the discussion.

Image: The upper half of Europa Clipper’s vault plate, showing the Drake Equation in Frank Drake’s own handwriting. Credit: NASA/JPL-Caltech

The Europa Clipper vault plate is small, measuring 1 millimeter in thickness and 18 X 28 centimeters. What I’ve described so far is the inner-facing plate. The outer side contains a visual representation of the word for ‘water’ spoken in 103 languages, with the central symbol the sign for water in American Sign Language. Water, after all, is why we are probing Europa. Audio renditions of these words are contained as visual waveforms representing each sound. They look a bit runic to me.

Image: The art on this side of the plate, which will seal an opening of the vault on NASA’s Europa Clipper, features waveforms that are visual representations of the sound waves formed by the word “water” in 103 languages. At center is a symbol representing the American Sign Language sign for “water.” Credit: NASA/JPL-Caltech.

We’re going to get 49 close passes of Europa if all goes well when the spacecraft arrives in 2030. The commemorative plate seals an opening in the metal vault which will protect the craft’s sensitive electronics from the sleet of particles produced by interactions between the planet and its magnetic fields. We can hope to learn a good deal more about the thickness of the moon’s icy crust, its interactions with the ocean below, the composition of that ocean, and the geology of the surface. We’re getting close to Europa Clipper’s launch, slated for October at Kennedy Space Center.

Jupiter’s radiation belts, we’ve recently learned, may play a role in what goes on in the ocean below. As a paper in Nature Astronomy explains, the bombardment of ionized particles can split any water molecules encountered on the surface, producing oxygen that could find its way into the ocean. Thus lead author Jamey Szalay (Princeton University):

“Europa is like an ice ball slowly losing its water in a flowing stream. Except, in this case, the stream is a fluid of ionized particles swept around Jupiter by its extraordinary magnetic field. When these ionized particles impact Europa, they break up the water-ice molecule by molecule on the surface to produce hydrogen and oxygen. In a way, the entire ice shell is being continuously eroded by waves of charged particles washing up upon it.”

The Juno spacecraft’s Jovian Auroral Distributions Experiment (JADE) instrument flew within 354 kilometers of the surface in September of 2022, measuring the hydrogen and oxygen ions created by the particle bombardment. The work allowed a calculation of the rate of oxygen being produced at Europa, which turns out to be about 12 kilograms per second (previous estimates have reached as high as 1000 kilograms per second). “[W]hat we didn’t realize,” adds Szalay, “is that Juno’s observations would give us such a tight constraint on the amount of oxygen produced in Europa’s icy surface.” How much of this oxygen, if any, works its way into the ocean to provide potential metabolic energy is something Europa Clipper should help us understand.

Image: This illustration shows charged particles from Jupiter impacting Europa’s surface, splitting frozen water molecules into oxygen and hydrogen molecules. Scientists believe some of these newly created oxygen gases could migrate toward the moon’s subsurface ocean, as depicted in the inset image. Credit: NASA/JPL-Caltech/SWRI/PU.

The paper is Szalay et al., “Oxygen production from dissociation of Europa’s water-ice surface,” Nature Astronomy 04 March 2024 (full text).