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Today we have some exciting news from out there in the solar system [checks notes] er, excuse me, I guess I mean “in there” in the solar system, since today’s news concerns from the planet closest to the sun. Yet, even though it is first, Mercury is shockingly unknown compared to the other planets of the solar system. This past Friday (October 1st 2021), the joint ESA/JAXA space mission BepiColombo (which launched back in 2018) finally made it to the innermost world. Since 2018, the transfer module has been slaloming around Earth and Venus in order to make it to Mercury. Indeed, the mission is named after the brilliant Italian astrophysicist who first proposed interplanetary gravity assist maneuvers as a mechanism for altering the velocity and trajectory of interplanetary spacecraft. BepiColombo took the picture at the top of this post, a view of part of Mercury’s northern hemisphere taken at a distance of about 2,420 kilometers (1,500 miles) from the world.

In 2025, the spacecraft will deploy two orbiters to truly comprehensively map Mercury and attempt to unlock the mysteries of the planet’s ancient face. Mercury’s surface seems to be made of a very dark lava which has been bombarded by meteorites for the last several billion years. Although Mercury is dinky in volume (smaller than some of the solar system’s moons) it is quite dense and presumably contains a metallic core suitable for a much larger planet. Interestingly (albeit unsurprisingly, for anyone who lives here) the densest planet in the solar system is Earth!

Not since the end of the Messenger mission has Mercury been the thrilling center of attention in astronomy. I can hardly wait for subsequent discoveries about the fast, hot, tiny planet. Some of the secrets of the making of the solar system have been locked away on the little world, waiting billions of years for the right orbiter to come along.

Hi everyone! Sorry that the posts were thin on the ground last week. The head druid told me that I needed to honor the solstice by taking some time to reflect on the meaning of things [citation needed]. Anyway…since I didn’t blog last week, I failed to post these astonishing pictures of Jupiter’s giant moon Ganymede, which were photographed by NASA’s Juno spacecraft as it slaloms though the Jovian system.

Ganymede as imaged by NASA probe Juno

Although its lack of atmosphere and pockmarked plains of dust make it superficially resemble Earth’s moon, Gannymede is a very strange and unique heavenly object Of the 200 known moons in the solar system, it is the largest. Indeed it is 26% larger than the planet Mercury by volume (although it is only 45% as massive as the metalliferous first planet). Ganymede has a diameter of 5,268 km (3,273 mi), so each pixel in the full size image of the Jovian moon is equal to a kilometer (although you may want to check out the NASA image to really savor that scale–since WordPress has a noteworthy penchant for scrunching up my images in incomprehensible ways).

A photo of the dark side of Ganymede taken by Juno’s incredibly light sensitive navigational camera

Alone among moons in the solar system, Ganymede has a magnetic field, albeit a rather meager one compared to Earth or Jupiter. Scientists surmise that the magnetic field is created by convection within the liquid iron core of the moon–although answers are not forthcoming as to why it has a liquid iron core to begin with (these planetary cores seem to be the real determinant of what planets are like, but I feel like we know precious little about them). Thanks to its size (and maybe thanks also to its magnetosphere), Ganymede has a very thin oxygen atmosphere…but that just creates more question, since elemental Oxygen has a tendency to instantly bond to all sorts of other elements. The 20 percent or so of oxygen in Earth’s atmosphere did not become a mainstay until about 1.5 billion years ago when photosynthesizing bacteria finally became so prevalent that they overcame the constant loss of atmospheric oxygen thanks to oxidation. Hopefully Juno’s survey will help us solve atmospheric mysteries on Ganymede. Ganymede is also believed to have a vast subsurface ocean of icy water tucked away somewehere beneath its surface. Astronomers have reasonably speculated that this Ganymede underworld ocean may contain more water than all of Earth’s oceans combined!

This is the largest version of this interesting cross section which I could find

Ganymede is a Galilean moon–which means it was discovered by the great scientist, and is one of the first objects ever discovered to orbiting another planet (I still sometimes imagine the thrill Galileo must have felt when he realized what he was seeing). I wonder what surprises Juno will send back for us!

The North Pole of Enceladus during the October 30th, 2015 Cassini Flyby (NASA/ESA/ASI)

The North Pole of Enceladus during the October 30th, 2015 Cassini Flyby (NASA/ESA/ASI)

Since 2004, the Cassini spacecraft has been in orbit around Saturn. The robot probe (a joint effort of NASA, ESA, and the Italian space agency) received the most press when it launched a flying saucer lander onto Saturn’s planet-like moon Titan, but it is still out there doing amazing work. Last week, while I was busy writing about Halloween themes, the probe made its closest pass yet to Saturn’s ice moon, Enceladus. Enceladus is only 500 kilometers in diameter and it is coated in ice, but it is of great interest to scientists because ice plumes venting from the moon’s south pole seem to indicate a large polar subsurface ocean of liquid water. Warmed above freezing by tidal flux, this ocean beneath the ice probably has a thickness of around 10 km.

View of Enceladus’ south pole geyser, backlit by Saturn

View of Enceladus’ south pole geyser, backlit by Saturn

On October 30th, Cassini flew by the icy moon at the dangerously close distance of 30 kilometers (18.6 miles). The probe was directly above the south pole of Enceladus and it collected a little flake of ice to analyze (which strikes me as incredibly amazing and beautiful). It will take some time for the ship’s devices to assay the drop of water from an alien ocean, but Cassini also snapped some photos which we already have. These are taken from point blank range above the south pole. The ocean is down there beneath the scratches and scars. What is the nature of this icy ocean? How long has it been there? Could it possibly harbor life?

Artist's redition of New Horizons approaching Pluto and Charon

Artist’s redition of New Horizons approaching Pluto and Charon

After years and years and years of waiting, NASA’s New Horizons mission is officially in its “flyby” stage. As I write, the robot probe is desperately snapping pictures and taking readings of Pluto and its moon Charon. The closest pass-by will arrive next Tuesday when New Horizons will be a mere 12,500km from the dwarf planet.

Hmm, I can sort of see a heart, a whale, and a donut (Photo courtesy of NASA, New Horizons)

Hmm, I can sort of see a heart, a whale, and a donut (Photo courtesy of NASA, New Horizons)

Today’s post serves to alert you to keep your eyes peeled next week! I will be eagerly awaiting news of the developments and I will relay them to you as quickly as possible–although Pluto is 320 light minutes away from us (give or take a few hundred million kilometers) so nobody is going to be caught up in real time. In the meantime, New Horizons is already learning more about the dwarf planet than we have ever known before: this is a mission to a world almost wholly unknown to us despite the fact that we are neighbors in the same star system! Pluto has a distinctive reddish pinkish hue and features an array of high-contrast features (presumably composed of layers of exotic ices) which, to human eyes, superficially resemble familiar shapes. Most notable is a large cardiod-shaped feature in the southern hemisphere unsurprisingly dubbed “the heart”. There is also a planet sized stain resembling a whale and a smaller stain which looks like a donut. No doubt we will get a better idea about these bright/dark areas during the close-up approach next week. Right now I hope people are appreciating my artistic prescience!

Mister SETI (Wayne Ferrebee, 2012, oil on panel)

Mister SETI (Wayne Ferrebee, 2012, oil on panel)

The main thing which is currently striking to scientists (who have better things to worry about then whether methane ice looks like a whale) is how dissimilar Pluto is from its moon Charon. The two objects are closer size-wise than any other planet/moon system in the solar system, yet Charon is completely unlike Pluto in appearance and make-up. The moon, which is named after the ferryman of the underworld, is gray and nearly featureless and has no atmosphere (I should have mentioned that Pluto does have an atmosphere—at least at this phase of its strange orbit).


Hooray for New Horizons! Considering where it is and what it is currently doing, I almost find it hard to think of it as real, but it most assuredly is. Also hooray for us! We have some bad moments, but we can launch a highly functional robot out of Earth’s gravity well to the edge of the solar system! It isn’t a space colony on Venus—but it’s a start. Our arms are growing longer and our apprehension keener. I almost can’t wait for next week, yet somehow I think I’ll still manage to enjoy the weekend.

Ye Olde Ferrebeekeeper Archives

December 2021