The Martian moon Phobos is cratered, lumpy and about 16.8 miles long, or 3 miles longer than the island of Manhattan. According to a recent study, the moon is also unusually light. Planetary scientists found that Phobos is probably not a solid object, and that as much as 30 percent of the moon’s interior may be empty space.

That doesn’t mean that Phobos is an empty shell where we could, say, set up a rest stop for spaceships on their way to the outer planets. But the new finding probably does mean that Phobos was not an asteroid that got caught in Mars’ gravity as it floated by the planet.

A giant earthquake that triggered a deadly southwest Pacific tsunami was actually two great temblors, finds a pair of new studies in the Aug. 19 Nature. These results uncover an unusual sequence of geological events that is the first of its kind to be observed by scientists, the study authors say.

The earthquakes, which likely struck within two minutes of each other on September 29, 2009, spawned a tsunami that killed nearly 200 people in Samoa, American Samoa and Tonga. Scientists assumed that a single quake under the ocean floor had caused the devastation, but the pattern of far-flung aftershocks, aberrant tsunami waves and the inexplicable movement of a Tongan island cast doubt on that simple explanation.

“We knew right off the bat that something was weird about this earthquake,” says geophysicist Eric Geist of the U.S. Geological Survey in Menlo Park, Calif. Geist wasn’t involved in the current studies but has puzzled over the anomalous signs produced by the quake. “This is a very complicated event, and these studies, for me, really helped explain a lot.”

The earthquake that everybody knew about was a whopper: a magnitude-8.1 quake in which the ground was pulled apart along a fault. The hidden quake was a different type.  It happened about 70 kilometers from its predecessor on a thrust fault where the west-moving Pacific plate dives under the east-bound Tonga block of the Australia plate, an event called subduction. These two plates scrape past each other 24 centimeters each year, “the fastest plate tectonics on the planet,” says study coauthor John Beavan of GNS Science in Lower Hutt, New Zealand.

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A magnitude-8.1 quake in the southwest Pacific triggered a second quake, comprised of two magnitude-7.8 subevents, where the east-moving Tonga block of the Australia plate meets the west-moving Pacific plate. Keith Koper/University of Utah Seismograph Stations

The two research teams separately uncovered the presence of a second earthquake using different data. Beavan and his colleagues found that tsunami gauges on the ocean floor measured a big positive pressure wave, a suspicious sign since a ground-extending earthquake would cause a drop in pressure.

Another big clue came from GPS stations on the northern Tongan island of Niuatoputapu, which has been steadily moving east. On its own, the first earthquake would have sent the island slightly back to the west. But instead, the island jumped about 40 centimeters to the east. “It was completely out of kilter,” Beavan says. The best explanation was that a second thrusting fault earthquake had caused the motion.

The other research group, led by seismologist Thorne Lay of the University of California, Santa Cruz, spotted abnormalities in seismic data that led them to dig deeper into the seismic records.

“We were able to pull together a self-consistent story of the triggered thrust earthquake, and clearly it was as big as the first event,” Lay says. “It was a magnitude-8 hidden earthquake. And you would think, ‘Well, aren’t seismologists a bunch of idiots. They can’t even find a magnitude-8 earthquake,’ but it was obscured by the strong shaking from the first one.”

Lay and his team built a model in which the normal fault earthquake happened first, triggering the hidden thrust-fault quake, Lay says. Beavan’s team’s study didn’t have the resolution to parse the timing of the earthquakes, but he says that he suspects Lay and his colleagues’ timeline is correct.

Both studies peg the second earthquake at a magnitude 8.0. Lay and his colleagues were able to distinguish two distinct but nearly simultaneous energy releases from the second earthquake, which they call subevents. Each of these subevents, they estimate, was magnitude 7.8, which combined to hit the 8.0 mark.

“We pretty well understand what’s going on with these earthquakes,” Beavan says. “The fact that these two studies, which use completely different techniques, both come up with the same answer is really nice.”

Understanding all of the forces involved may be useful in building better models of how earthquakes can trigger one another and tsunamis. This is the first example of a normal fault earthquake setting off a thruster fault quake, Geist says, which “will set off a lot of interesting research on the mechanics of subduction zones and how they behave.”

Pumping up is easier for people who have been buff before, and now scientists think they know why — muscles retain a memory of their former fitness even as they wither from lack of use.

That memory is stored as DNA-containing nuclei, which proliferate when a muscle is exercised. Contrary to previous thinking, those nuclei aren’t lost when muscles atrophy, researchers report online August 16 in the Proceedings of the National Academy of Sciences. The extra nuclei form a type of muscle memory that allows the muscle to bounce back quickly when retrained.

The findings suggest that exercise early in life could help fend off frailness in the elderly, and also raise questions about how long doping athletes should be banned from competition, says study leader Kristian Gundersen, a physiologist at the University of Oslo in Norway.

Muscle cells are huge, Gundersen says. And because the cells are so big, more than one nucleus is needed to supply the DNA templates for making large amounts of the proteins that give muscle its strength. Previous research has demonstrated that with exercise, muscle cells get even bigger by merging with stem cells called satellite cells, which are nestled between muscle fiber cells. Researchers had previously thought that when muscles atrophy, the extra nuclei are killed by a cell death program called apoptosis.