Up close and personal with Ganymede
Jupiter’s largest moon Ganymede was imaged in great detail by the JunoCam imager aboard NASA’s Juno spacecraft when it made a flyby of the icy moon on 7 June 2021. This historic flyby marked the closest approach to Jupiter’s largest moon – closer than any other spacecraft has achieved in over 20 years. The probe flew within just 1,038 kilometres (645 miles) of the moon’s frigid surface. A cratered landscape with distinctly bright and dark terrain is visible in this image, along with long striations that could possibly be linked to tectonic faults. Juno is expected to reveal details about the Jovian moon’s composition, magnetosphere and ionosphere after its close encounter with the icy celestial body.
There’s a mystery brewing at the centre of the Earth. Scientists can only see it when they study seismic waves passing through the planet’s solid-iron inner core. For some reason, waves move through the core significantly faster when they’re travelling between the North and South Poles than when they’re travelling across the equator.
Researchers have known about this discrepancy, known as seismic anisotropy, for decades, but have been unable to come up with an explanation that’s consistent with the available data. Now, using computer simulations of the core’s growth over the last billion years, a recent study offers a solution that finally seems to fit: every year, little by little, Earth’s inner core is growing in a ‘lopsided’ pattern, with new iron crystals forming faster on the east side of the core than on the west side.
“The movement of liquid iron in the outer core carries heat away from the inner core, causing it to freeze,” says Daniel Frost, a seismologist at the University of California, Berkeley. “This means the outer core has been taking more heat from the east side [under Indonesia] than the west [under Brazil].”
To visualise this lopsided growth in the core, imagine a tree trunk with growth rings radiating out from a central point, Frost said, but “the centre of the rings is offset from the centre of the tree” so that rings are spaced farther apart on the east side of the tree and closer together on the west side. A cross section of Earth’s inner core might look similar to that. However, this asymmetric growth doesn’t mean that the inner core itself is misshapen or at risk of becoming imbalanced.
On average, the inner core’s radius grows evenly by about one millimetre every year. Gravity corrects for the lopsided growth in the east by pushing new crystals towards the west. There the crystals clump into lattice structures that stretch along the core’s north-south axis. These crystal structures, aligned parallel with Earth’s poles, are seismic superhighways that enable earthquake waves to travel more quickly in that direction.
But what’s causing this imbalance in the inner core? That’s hard to say without looking at all the
“The inner core is freezing out of the liquid outer core, like a snowball adding more layers”
other layers of our planet, Frost said. “Every layer in the Earth is controlled by what’s above it, and influences what’s below it,” he said. “The inner core is slowly freezing out of the liquid outer core, like a snowball adding more layers. The outer core is then cooled by the mantle above it, so to ask the question of why the inner core is growing faster on one side than the other might be asking the question of why one side of the mantle is cooler than the other.”
Tectonic plates could be partially to blame. As cold tectonic plates dive deep below the Earth’s surface at subduction zones – places where one plate sinks below another – they cool the mantle below. However, whether mantle cooling could impact the inner core is still a subject of debate.
An explosion on the Sun is helping uncover new information about what causes powerful solar eruptions. In March 2016, scientists used NASA’s Solar Dynamics Observatory and the Solar and Heliospheric Observatory, a joint mission of
NASA and the European Space Agency (ESA), to observe an explosion on the Sun. The event showed characteristics of three different types of solar eruptions that usually happen separately, but occurred together this time. “This event is a missing link where we can see all of these aspects of different types of eruptions in one neat little package,” said Emily Mason, a solar scientist at NASA’s Goddard Space Flight Center in Maryland.
There are typically three different varieties of eruptions that can take place on the Sun: coronal mass ejections (CMEs), jets or partial eruptions. CMEs and jets are explosive and blast particles and energy out into the vacuum of space, whereas partial eruptions originate from the Sun’s surface but don’t make it all the way out into space
– the material that erupts just falls back onto the Sun. The March 2016 event seems to have characteristics of all three different types of solar eruptions, so scientists think that they could all be caused by the same phenomenon. By finding the mechanism behind this event, they could explain the origins of all solar eruptions.