Tag: Europa Clipper

• Out there: using a moon to spot dark matter

  The search for dark matter, the invisible ‘glue’ that holds galaxies together, has long focused solely on subatomic particles and this could be a mistake if a new study Physical Review D is to be believed. Its sole author, William DeRocco, a postdoctoral scholar at the Maryland Centre for Fundamental Physics at the University of Maryland, has proposed that dark matter could exist as “macroscopic” objects and the best place to find evidence of them might be — get this — the surface of Jupiter’s largest moon, Ganymede.

  Most scientific experiments look for less-than-microscopic dark matter particles. Yet there’s a large gap in scientists’ knowledge regarding dark matter ‘composites’ that weigh between 10¹¹ and 10¹⁷ grams, roughly the mass of a large mountain. Because these objects would be very rare, scientists will need a detector that’s both enormous and has been operating for billions of years.

  According to DeRocco, Ganymede fits this description perfectly. It’s larger than the planet Mercury and has a surface that has remained largely unchanged for more than 2 billion years. Unlike Earth, which has weather and tectonic plates that erase impact craters, Ganymede still bears signs of almost every meteor that has slammed into it, so its surface is like a geological history book.

  In the study DeRocco used a computer program called iSALE to simulate what happens when a piece of macroscopic dark matter hits an icy moon.

  Traditional asteroids are relatively fragile. When they collide with a planetary body like Ganymede, they explode and leave behind a bowl-shaped crater. Macroscopic dark matter on the other hand would be extremely dense, potentially as dense as an atomic nucleus (which is around 100 trillion times more dense than liquid water). And DeRocco found that instead of exploding, these objects would act like a cosmic needle, punching a deep “borehole” through Ganymede’s 12-km-thick outer ice shell.

  The iSALE simulations thus revealed a unique signature that could help scientists distinguish a dark matter strike from a normal asteroid. High-speed dark matter, travelling at about 270 km/s, would easily pierce the conductive ice crust. As the hole later collapses under the moon’s gravity, it will throw up a jet of liquid water and slush from the deep subsurface. This jet could ultimately bring up material from Ganymede’s subsurface ocean, such as mineral salts, and deposit them on the surface.

  The result would be a relatively small crater, no wider than 10 km, surrounded by a large quantity of material with a chemical composition unlike the rest of the surface ice.

  That the study has been published now isn’t incidental: the spacecraft of two major missions, NASA Europa Clipper and ESA JUICE, are currently en route to Jupiter. Both missions carry high-resolution infrared cameras and radar capable of penetrating ice. While their primary goal is to search for signs of life, DeRocco’s study suggests they could also help look for dark matter composites.

  University of Cantabria, Spain, astrophysicist Bradley Kavanaugh told New Scientist that DeRocco’s idea is promising if also there’s no evidence yet that dark matter composites of the sort that Ganymede could ‘detect’ exist: “It’s more about trying to look at all the possibilities. I would say these are quite exotic objects. They’re incredibly dense and would be held together by very strong forces in some dark sector.”

  Indeed, considering scientists are pretty sure dark matter exists even as empirical evidence of the substance remains out of reach, it may not be a bad idea to follow up on bold ideas — more so if the instruments it needs are already on the way.

  Featured image: A view of an equatorial region of Ganymede (between 57 N and 57 S). Credit: USGS.

  2026.01.11

• Europa’s ice shell could be quaking

  Even before astronomers noticed last year that Europa was spouting jets of water vapor from its icy surface, they thought there was something shifty about Jupiter’s moon. While the 66 other Jovian moons are pitted with craters, Europa sports some unusual blemishes: an abundant crisscrossing of ridges tens of kilometres long. Many are abruptly interrupted by smooth ice patches.

  Two geologists think they can explain why. Backed by photos taken by the Galileo space probe, they suggest Europa’s thick shell isn’t continuous but is made up of distinct plates of ice. These plates move away from each other in some places, exposing gaps which are then filled by deeper ice rising upward. In other places they slide over each other and push surface ice downward and form ridges.

  “We knew that stuff has been moving over the surface, and up from beneath and breaking through, but we weren’t able to figure where all the older stuff was going,” said study coauthor Dr. Louise Prockter, a planetary scientist at Johns Hopkins. “We’ve found for the first time evidence that material is going back into the interior.” The study was published last month in Nature Geoscience.

  On Earth, this kind of tectonic activity replenishes compounds necessary for life, such as carbon dioxide, by letting them move up from the interior through fissures to the surface. Now, scientists say a similar mechanism could apply to Europa. Astronomers think the moon harbors a subsurface ocean of liquid water that feeds the vapor plumes, and could be habitable.

  “It’s certainly significant to find another solid body in the solar system that undergoes some kind of surface recycling,” said Peter Driscoll, a planetary scientist at the University of Washington who was not involved in the study.

  Prockter, together with Simon Kattenhorn, a geologist at the University of Idaho, Moscow, worked with photographs of a part of Europa’s surface covering 20,000 km². The pictures were shot by Galileo when it orbited Jupiter from 1995 to 2003.

  “We go in using something like Photoshop and start cutting the image up,” Dr. Prockter explained. They then pieced them back together so that the crisscrossing ridges lined up end-to-end, and compared what they had to the surface as it is today.

  “Once we started doing the reconstruction, we ended up with a big gap right in the middle,” she said.

  The researchers concluded the missing bit had dived beneath another plate.

  Although only some of Galileo’s photographs were at a resolution high enough to be useful for the study, Dr. Prockter said it was unlikely that their finding was a one-off because signs of displacement were visible all over Europa’s surface.

  Nevertheless, Dr. Driscoll cautioned against using Earth’s tectonic activity as a model for Europa’s. “There are a number of missing features” that define tectonics on Earth, he said, such as arc volcanos and continents. “And many of the properties of Earth’s features may not be expected for an icy shell like Europa, where the materials are extremely different.”

  A better gauge of these disparities might be a probe to the Jovian moon that NASA has planned for the mid-2020s.

  “I think the timing right now is very important,” said Candice Hansen, a member of NASA’s Planetary Science Subcommittee. She says the Europa study will help scientists working on the probe secure the requisite funding and commitment from Congress.

  “I am very enthusiastic about a mission to Europa, and this exciting result is one more reason to go,” she said.

  

  2014.10.16