SOLAR SYSTEM

SOLAR SYSTEM NEWS

Meteor Impact Caught by Door Cam: On July 24 last year, a doorbell camera of a home in Charlottetown, Prince Edward Island, caught the noise and ejecta from what proved to be the impact of a small meteor. Fragments of the meteor were collected and its origin confirmed, but it didn’t hit the news until recently. It was the first time an impact event was recorded. See the video and more information about it here: https://www.sciencealert.com/potential-world-first-wild-doorbell-video-records-sound-of-meteorite-crash . Buildings have been hit by meteors on a number of occasions, and there is one confirmed incident where a person was hit, but this is the first time such an event, and the noise it made, was recorded!

Huygens Probe lands on Titan: I missed a rather significant round-number anniversary last week: January 14 was the 20^th anniversary of the landing of the Huygens probe on Saturn’s large moon, Titan. My bad. Huygens had been carried to Saturn on the Cassini spacecraft, and both were amazingly successful as described in a previous Item of the Week (here). A paragraph from that Item states that “Titan was found to have features akin to those on Earth due the hydrologic cycle. The Titan equivalent involves methane, not water, however. Huygens imaged lakes and rivers of liquid methane, made by methane rain. Detailed measurements show that Titan’s is the most chemically-complex atmosphere in the Solar System. Sunlight and methane make complex molecules that make the atmosphere opaque to visible light.”

This ambitious mission to Saturn was a joint effort by NASA, who provided the Cassini orbiter, and ESA, who provided the Huygens lander. The entry sequence video, available on the ESA mission site (here) and elsewhere is spectacular. Lakes and integrated networks of drainage channels were seen during entry. The images Huygens acquired on the surface show rounded “rocks” of methane ice, possibly transported and shaped by transport in a river of methane, which can exist as a liquid at Titan’s -170 °C surface temperature. See more images and videos of Titan in the JPL Photojournal, here. NASA celebrated the 10^th anniversary of Huygens’ landing with a summary, here.

SOLAR SYSTEM FYI

BepiColumbo at Mercury: Only two spacecraft have explored Mercury significantly to date: NASA’s Mariner 10, which flew by three times in the early-mid 1970s, and MESSENGER, a successful orbital mission that ended in 2015. ESA’s BepiColumbo mission was designed as a follow-up, launching in October, 2018. Moving inward in the Solar System takes a lot of energy, just like moving outward does, and NASA trajectory planners use gravity assists to move inward economically. Six gravity assists were necessary to get BepiColumbo ready for final orbital insertion via rocket in November, 2026. The first assist was from the Earth, the next two were from Venus, and three were required at Mercury, the last of which was last week. It showed the south polar region of Mercury in better detail than ever before. For more info about the mission, see: https://sci.esa.int/web/bepicolombo; for more about this third fly-by, see: https://skyandtelescope.org/astronomy-blogs/astronomy-space-david-dickinson/bepicolombo-mission-returns-to-mercury-for-best-flyby-views-yet.

A Mars Rover Retrospective: When NASA fans look back on Mars exploration in 2024, three items stick out. Most important was the detection of possible biosignatures on a rock named “Cheyava Falls,” in July by the Perseverance rover. Also important was Percy’s SHERLOC instrument cover failing then working again, and Percy’s climb out of Jezero Crater to Witch Hazel Hill, where the rocks pre-date the crater’s formation. For more on these developments, see: https://science.nasa.gov/blog/a-rover-retrospective-turning-trials-to-triumphs-in-2024.

Pluto and Charon: Kiss and Capture? Why does Pluto have such a large moon, relative to its own size? A new mechanism of the Pluto/Chiron system’s formation is that two icy bodies collided in the outer Solar System, but with rather low relative speeds, with the consequence that they glommed onto one another rather than destroyed each other (as was the case when the proto-Earth was hit by another body), spinning like a demented snowman. Eventually the two pulled apart, leaving a scar that today forms part of Pluto’s “Heart” (Sputnik Planitia), a scenario supported by recent computer modeling. For a summary of this work, see: https://phys.org/news/2025-01-newly-capture-mechanism-formation-pluto.html

Pluto and Charon: Large Impact Mascon? An alternative process for the creation of Sputnik Planitia is that it is a mega-impact feature the filled with a slab of “excess” mass – water ice, which is heavier than nitrogen ice prevalent there. See the proposed explanation in more detail here: https://skyandtelescope.org/astronomy-news/sputnik-planitia-drives-plutos-wandering-heart.

2025 Solar System Exploration Events: Zhenbo Wang describes the plethora of upcoming Solar System exploration missions, including surveying the sky with SPHEREx, studying LEO with Space Rider, exploring the Moon with M2/Resilience, investigating an asteroid with Tianwen-2, and more. For the full story, see: https://www.astronomy.com/space-exploration/space-missions-coming-in-2025.

2060 Chiron is a Unique Hybrid: Chiron is a member of a class of bodies called “Centaurs.” Just as a centaur is a man/horse hybrid in mythology, Centaurs have characteristics of both asteroids and comets in reality, and their orbits are confined to the belt between Jupiter and Neptune. Detailed observations from the JWST show the presence of CO2 and CO ice on Chiron’s surface and CO2 and CH4 in its comet-like coma, a shroud of gas and dust released from Chiron. CO2 and CH4 ices have also been detected on some Trans-Neptunian Objects. Chiron was discovered in 1977, and has been observed in more detail than the other Centaurs. Trans-Neptunian Objects tend not to have a coma, because they are so far from the Sun that their surface ice cannot sublimate. Classical asteroids have no surface ices. Centaurs do have surface ices, but sublimation is limited to those times when they are closer to the Sun. The details of the chemistry of ices and gases has implications for the evolution of the Solar System. For a summary of this work and its implications, see: https://www.sciencedaily.com/releases/2024/12/241218131526.htm; for the paper in Nature Geoscience, see: https://www.nature.com/articles/s41561-024-01612-0.

Chiron isn’t the only Centaur of Note! We’ve all seen some amazing pictures of the deepest of deep-space objects acquired by the JWST. Numerous objects now bear (perhaps temporary) labels as the “most distant known.” But the JWST makes important observations much closer to home, too!

Chiron is the oddest Centaur (see news above), but the largest Centaur asteroid is 10199 Chariklo, discovered in 1997 and named for the wife of the centaur Chiron and (perhaps) a daughter of Apollo.

Recall how the rings of Uranus were discovered – it’s a classic case of learning things by looking at them in front of other things. Astronomer James Elliot, aboard the Kuiper Airborne Observatory, was measuring the brightness of a background star as Uranus passed in front of it, hoping to gain information about the uranian atmosphere (a la Mariner 4 at Mars). Just prior to the occultation, Elliot and his team saw the light from the background star dim slightly five times, and then they saw the same thing just after the star was occulted. Elliot knew those dips were caused by Uranus’ hitherto unknown ring system!

Well, the same thing happened in 2013 with Chariklo. Astronomers calculated that Chariklo would pass in front of a minor star, and they wanted to observe that occultation closely from many locations on Earth in order to be able to refine estimates of Chariklo’s size and shape. Those observations went well but were overshadowed (sorry) by a scene straight out of Jim Elliot’s experience.

Seven seconds before the occultation would begin, astronomers saw two small dips in the star’s light, and another two seven seconds after the occultation. Rings!

Finding rings around one of the Sun’s larger planets was a wonderful accomplishment, but rings around an asteroid?!?

Chariklo was in the news again recently. The JWST was able to see it occult star named Gaia DR3 6873519665992128512, the first time it had observed any occultation event and a portent of JWST’s ability to “do science” much closer to home than the very-distant objects it was built to study.

The leading hypothesis so far as to why an asteroid could have rings are that the rings are remnants of a larger debris field created by an impact with another icy body. For more info an Chariklo and the JWST, see here: https://phys.org/news/2023-01-webb-spies-chariklo-high-precision-technique.html.

ENCELADUS

Thermal Vents: Several lines of evidence indicate that Saturn’s moon, Enceladus, has a deep iced-over ocean with the chemistry potentially amenable to biological activity. The same sort of gravitational/tidal tug-of-war that makes Jupiter’s moon, Io, so volcanically active keeps the ocean liquid, and causes geyser-like spewing from fractures in the ice cover. We know about the tides, and about the chemistry of the spray from the fractures because the Cassini spacecraft was steered thorough! The situation on Jupiter’s moon, Europa, is almost certainly similar; investigating it further is the purpose of the recently-launched Europa Clipper spacecraft. But we know that there is interesting chemistry at Enceladus, so further exploration is needed there, too. One way to do so is to study further such hydrothermal vents and the environment they create in their vicinities. We’ve known about the vents and the life they support on Earth for some time now, but work is ongoing, and we are learning more all the time in preparation for the next step, direct exploration is needed; see: https://eos.org/articles/arctic-hydrothermal-vents-may-resemble-those-on-enceladus. And under development….

NASA’s Swimming Robots: Jupiter’s Europa and Saturn’s Enceladus almost certainly have large-scale oceans, iced over, but still liquid. Ganymede and Callisto, Mimas, and some of the bodies in the outer Solar System likely do, too. We know that Enceladus has active hot spring activity that could keep its ocean liquid, because the Cassini spacecraft was able to sample the plume of one of its large geysers. Everywhere on Earth oceanographers have been able to find deep-water hot springs, they’ve found abundant life. The likelihood that such conditions elsewhere might also support abundant life makes exploring those ice-covered extraterrestrial oceans a high priority. The Europa Clipper spacecraft, presently en route to Jupiter, is designed to examine Europa more closely, but what would really be useful would be to get a remotely-controlled submersible to ply those dark and deep seas, so far away. And that is just what NASA is planning to do!

NASA is experimenting with small submersible prototypes that could be delivered to a subsurface ocean, carried through the ice by a lander with a heat source that allows it to melt its way through the ice crust above. See: https://www.jpl.nasa.gov/news/nasa-ocean-world-explorers-have-to-swim-before-they-can-fly for more information (when the JPL News site is working). And, …

Enceladus May Someday Have EELS! The Planetary Society has produced a radio program about a concept to explore the subsurface ocean of Enceladus. The concept is called the “Exobiology Extant Life Surveyor” (EELS). The acronym is particularly apt, because EELS would be a snake-like, AI-enabled robot capable of navigating the surface fractures of Enceladus and accessing the liquid ocean at depth. Check out the radio program info here: https://www.planetary.org/planetary-radio/2024-eels and the JPL webpage about the EELS concept here: https://www.jpl.nasa.gov/robotics-at-jpl/eels! But wait, there’s more!

MARS

Active Processes on Mars: One of the most exciting observations of Mars made by the Mariner 9 orbiter was the presence of surface features that changed with time. Several ideas for them had been proposed, but it quickly became apparent that the changes were caused by dust and the thin martian winds. Studies of wind-related sand/dust movement on Earth followed, to help better understand such things on both planets. Craters, mountains, and other topography and the wind can make specific erosion patterns. Having an obstacle of known size and shape cause redistribution of surface materials would help. And the now-retired InSight lander is serving science one more time. Images acquired by the HiRISE camera aboard the Mars Reconnaissance Orbiter show a pattern of dust erosion around the lander that allow scientists to estimate the deposition and erosion of dust under observable weather conditions. For more on this study, see: https://www.jpl.nasa.gov/news/nasa-mars-orbiter-spots-retired-insight-lander-to-study-dust-movement.

Springtime on (Boom!) Mars: Mars has seasons like the Earth does, but they all last much longer because of Mars’ longer year. Mars has polar caps like Earth does, too, but they are made of both frozen CO2 and H20. Both ices sublimate (turn from solid to gas without a liquid phase in between), and if gaseous CO2 builds up beneath and H20 ice layer, an explosion can occur, making distinctive surface features. For more on this unusual surface process, see: https://www.jpl.nasa.gov/news/avalanches-icy-explosions-and-dunes-nasa-is-tracking-new-year-on-mars!

Perseverance Reaches the Top of Jezero Crater! Percy reached “Lookout Hill” on the rim of Jezero Crater recently. It had originally landed on the crater’s floor, and one of its primary missions was to make a detailed examination of a delta built within Jezero when the crater was partially-filled with water, in the distant martian past. It has completed the delta operations, exploring the “fan front,” “upper fan,” and “margin unit.” It’s now on its fifth campaign, the “Northern Rim.” Until now, Percy had been exploring terrain that post-dated the emplacement of Jezero Crater. Now that it is starting to drive away from the rim, Percy will begin to encounter formerly-deep-seated rocks ejected by the formation of Jezero, and rocks that were there when Jezero was formed. Additional exploring may get Percy to large ejecta blocks from the formation of the Isidis Basin. For more on these exciting results, and for a couple of cool NASA YouTube videos of Percy’s travels, see: https://www.nasa.gov/missions/mars-2020-perseverance/perseverance-rover/nasas-perseverance-rover-reaches-top-of-jezero-crater-rim.

Mars: CO2 Polar Caps and Icy Rivers Beneath? An interesting hypothesis has been proposed by Peter Buhler of the Planetary Science Institute regarding the evolution of Mars’ climate and the appearance of many landforms indicative of water flow at a time when Mars’ atmospheric pressure was too low to allow it. In a nutshell, the climate became cool enough for the CO2 in Mars’ atmosphere to freeze out, covering the surface with a thick layer of insulative dry ice. Thermal heat from Mars’ interior was enough to melt any water ice present and allow it to flow beneath the dry ice “glacier.” For more on this interesting idea, see: https://skyandtelescope.org/astronomy-news/solar-system/icy-rivers-may-have-flowed-on-ancient-mars.

OTHER

Ryugu Dust Yields Big Science: Japan’s Hayabusa spacecraft visited the asteroid 162173 Ryugu (“Dragon Palace”), arriving in June, 2018. Hayabusa orbited, observed, and then collected a sample of its surface material and returned it to Earth on December 5, 2020. Scientists have been studying the material intensely ever since. One of their discoveries was that the grains returned had condensed from the proto-solar nebula and had a weak magnetic signal from the field that pervaded the space in which they formed. While weak, that magnetic field was strong enough to affect the formation of the Solar System. "We're showing that, everywhere we look now, there was some sort of magnetic field that was responsible for bringing mass to where the sun and planets were forming," says study author Benjamin Weiss, the Robert R. Shrock Professor of Earth and Planetary Sciences at MIT. "That now applies to the outer solar system planets." (source of quote, and details on this advance.: https://www.sciencedaily.com/releases/2024/11/241106132119.htm).

Researchers eagerly await examination of material collected from asteroid 101955 Bennu by NASA’s OSIRIS-REx spacecraft.

More on NASA’s Juno Mission - at Io: Since Juno is nearing its operational limit, NASA is more willing to take risks to get scientific observations too risky for a mission with a long life ahead. One of the interesting objects of study in the Jupiter system is its innermost large Moon, Io, where tidal heating is so severe that Io has many active volcanoes (predicted a week before their discovery in a “called shot” greater than Babe Ruth’s famous 1932 home run).

The radiation environment in Io’s location is intense, and could damage Juno, but a series of progressively-close fly-bys are planned for the coming months, necessary to observe Io’s ever-changing surface closely. The first such occurred on July 30, and Juno survived getting to within 22,000 km of Io’s surface without damage, and is presently returning interesting data to Earth. Fly-bys planned for December 30 and then February 3 next year will get to within 1,500 km of Io. For more information about the Juno mission, see here and here. For more on the Juno mission, in a style unusual for NASA, see here.

Voyager 2 was an amazing mission. Check out the excitement of its encounters with all four gas giants, conveyed as only Al Hibbs could, see here, the very first Item of the Week in the Archive. And don’t miss my retirement missive about those days, too! And Juno’s extended mission has paid off, because it recently found a …

Bennu is Shedding; NASA Style; and How “Science” Operates: The line between “asteroid” and “comet” has been blurred significantly by recent discoveries.Decades ago, solar system astronomers discovered that an asteroid (Phaeton), not a comet, was the source of the Geminid meteor shower. As a consequence, some would come to refer to it as a “rock comet” (Somewhere Bill Haley is smiling!). Then, in early 2019, the OSIRIS-REx spacecraft, then in orbit around asteroid Bennu showed that it was shedding considerable material, albeit at a low rate.

But are asteroids (and comets) the only source of debris in the inner Solar System?

A few months ago, authors of a paper published in JGR: Planets make the case that the very small particles responsible for the Zodiacal Light (the “False Dawn” of Omar Khayyam) actually come from either Mars or its two tiny moons (or perhaps martian moon(s) no long in existence). The data upon which that conclusion was based came primarily from the analysis of micro-meteoroid impacts on the solar panels of the Juno spacecraft when it was en route to Jupiter!

Longtime A+StW fans know that I frequently cite NASA’s know-how, and how they not only do the extremely difficult, they do it with style. The radio science experiment aboard Mariner 4 was one case in point; the item above is another example. Not only is Juno actively returning data and accomplishing its mission objectives, creative scientists have figured out a way to squeeze very interesting information from an unanticipated source!

Here’s another example of how the process of scientific inquiry works, too. The data from Juno’s micrometeoroid hits very strongly suggest the Mars system is the source of the impactors. However, neither the researchers or other planetary scientists have come up with a mechanism that would remove material from Mars, Phobos, or Deimos and get it into the interplanetary medium to cause the ZL. But the observational data will now drive more investigation, and Science will march on!

We had a similar situation when the first identification of the martian origin of some of the meteorites on Earth was announced. The observational data were overwhelming, but nobody thought impact could remove material from Earth’s gravity field, that is, until the data spurred them to investigate further.

Remember, this thing we call “Science” in not just a body of accumulated knowledge, it’s more importantly the process through which that knowledge was acquired!

For more info, see:

Summary of Bennu activity: https://eos.org/editors-vox/up-close-with-an-active-asteroid

JGR Planets paper: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JE006381

Mimas Joins the “Moons with Subsurface Oceans Club!” Wow! When planetary scientists first saw close-up images of Jupiter’s moon, Europa, and Saturn’s moon, Enceladus, they were deeply suspicious that both had a deep, liquid ocean with a thick ice cover. That proved to be correct, with the reason for them having an underground ocean was the same tidal heating mechanism proposed for Jupiter’s moon, Io, which was proven so dramatically when Voyager 1 fly by it (see more of that story here). Subsequent studies have shown that two of Jupiter’s other large moons, Ganymede and Callisto, also have large underground bodies of liquid (mostly) water. Saturn’s large moons, Titan and Enceladus do, too. The Cassini spacecraft even flew through plumes of water spewing from cracks on the surface of the latter (and found chemistry akin to that of “black smoker” hot springs in deep ocean locales on Earth – which by the way teem with life). And there is strong evidence that dwarf planets Ceres and Pluto; Neptune’s large moon, Triton; and several of the moons of Uranus have them, too. All these bodies make quite a club!

A moon doesn’t have to be big to have enough internal heating from tidal forces to make an underground ocean (but it helps). It turns out that a small moon can, too, provided it’s close enough to larger bodies to be subject to sufficient tidal stresses. Planetologists analyzing the motion of Mimas, a small but close-in moon of Saturn, also has underground liquid water. [Mimas was noteworthy when the first up-close pictures of it were acquired, because it has one giant crater that makes it a dead ringer for Star Wars’ Death Star, which was fresh on everyone’s mind when the fly-by took place.]

But wait, there’s more! Detailed tracking of the Cassini spacecraft provides data that indicates that the subsurface ocean on Mimas is very, very young, geologically speaking, only a few million years old.

For more on Mimas and its hidden ocean, see: https://www.sciencedaily.com/releases/2024/02/240207120512.htm and https://www.astronomy.com/science/evidence-grows-for-a-young-ocean-lurking-under-mimas-icy-crust

Uranian Moons Harbor Subsurface Oceans: Mimas, Europa, and Enceladus aren’t the only outer planet moons with a subsurface ocean! The National Academy’s 2023 Planetary Science and Astrobiology Decadal Survey identified the further exploration of Uranus and its larger moons as a priority goal. NASA has only visited Uranus once before, a fly-by by the Voyager 2 spacecraft in January, 1986 (the excitement the real-time release of the images of the fly-by is related here). The need for information to assist planning the recommended mission led scientists to revisit the 37-year-old data. Computer modeling not possible back then shows that the four largest moons (Ariel, Umbriel, Titania, and Oberon) are large enough to have Uranus’ gravity generate internal tidal heating that could lead to their having a subsurface ocean of liquid, likely water. This is the same mechanism driving internal geologic processes on Jupiter’s large satellites and Saturn’s Titan. For more information on this study, see: https://www.jpl.nasa.gov/news/new-study-of-uranus-large-moons-shows-4-may-hold-water.

Expanding Knowledge, Expanding Nomenclature: We all know about the “demotion” of Pluto from planetary status, even if we do not agree about it. Personally, I believe that making such changes in nomenclature is a necessary and proper reflection of our expanding knowledge of the nature of the Solar System, and I like to use as an analogy the actions taken during a spring cleaning of my garage (as long-time A+StW readers will recall).

Ancient astronomers knew the five major planets quite well, and could predict their locations quite accurately, enough so to forecast eclipses and other astronomical events/movements. But occasionally a strange interloper would make a temporary appearance, causing consternation all around. Most of these objects received the name, “comet,” or “hairy stars,” based on their appearance. And Galileo showed that at least some planets have smaller objects, moons, orbiting them. So the nomenclature had to expand from “stars and planets” in the sky to “stars, planets, moons, asteroids, and comets” in the sky.

The objects we then called “asteroids” reflect another aspect of the nomenclature changes that are a natural attendant of learning more about our Solar System. Thousands of them have been discovered and had their orbits calculated in considerable detail. Most of them are confined to the “Main Belt” between the orbits of Mars and Jupiter; the few outliers were initially considered inconsequential oddballs.

Astronomers also knew that something strange was going on in the Solar System’s outer reaches, even if they at first didn’t know enough to “clean out the garage.” Uranus’ obliquity was unlike any other planet’s, Neptune’s large moon, Triton, has a retrograde orbit that is decaying, and Pluto’s orbit is more elliptical than the orbits of the planets and is inclined to the Plane of the Ecliptic much more than any of the planets.

The discovery of Kuiper Belt objects “muddied the crick” considerably. So did the recognition of “active asteroids” that blur the line between “asteroids” and “comets.” So did the recognition that Jupiter’s gravity has an enormous effect on the evolution of the Solar System. But that’s a good thing, too, because it reflects an increase in our understanding of the nature of the Solar System as a whole.

Astronomers are now considering a more comprehensive classification scheme.

Trans-Neptunian planets are more like Pluto, Charon, Arrokoth, and other distant objects (KBOs). They are rich in volatile materials because they have never been heated by the Sun to any extent. Comet C/2014 UN271 Bernardinelli-Berntein may be another example. Those that do approach the Sun have highly-elliptical orbits, at least at first.

Blame Jupiter. Its gravity can either eject first-timers (as Comet B-B likely will be) or make their orbits much less elliptical, exposing their surfaces to periodic solar heating and devolatilization. Some will eventually lose so much of their volatiles that they are no longer comets, but rather more asteroidal in nature.

Jupiter’s gravity tends to force shorter-period comets more and more toward the Main Belt. By the time that happens, they are comets no more but asteroids, some still capable of shedding meteoroids, some not.

Those bodies in the transition phase often show characteristics of both comet and asteroid. They are rare because the overall time taken in transition is short compared to the age of the Solar System. Now called “Centaurs,” their discovery played an important role in astronomers figuring out this evolutionary process.

Most meteor showers we see today are the result of comets shedding rocky debris as they devolatilize, a process that could continue over many orbits. Two bodies very near the end of their activity are the parent bodies for two different showers: the Geminids last month are debris from the asteroid 3200 Phaeton, and the Quadrantids appearing now are debris from the near-Earth asteroid 2004 EH.

And the whole Uranus, Triton, Pluto thing will continue to refine our understanding. Science marches on! The January, 2022, issue of Sky and Telescope (pages 14-19) has an excellent description of this whole issue, written by Kat Volk at the University of Arizona’s Lunar and Planetary Laboratory.

SOLAR SYSTEM LINKS AND OTHER INFORMATION

Mars Odyssey

https://mars.nasa.gov/odyssey

https://mars.nasa.gov/odyssey/files/odyssey/Odyssey0302.pdf

https://themis.asu.edu

Mars Express

https://sci.esa.int/web/mars-express/-/31021-summary

https://sci.esa.int/web/mars-express/-/55263-beagle-2-lander-found-on-mars

Mars Science Laboratory (aka Curiosity)

https://mars.nasa.gov/msl/home

https://www.jpl.nasa.gov/missions/mars-science-laboratory-curiosity-rover-msl

Robert P. Sharp: https://en.wikipedia.org/wiki/Robert_P._Sharp