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A new look at old data is giving scientists a fresh reason to view Europa, a moon of Jupiter, as a leading candidate in the search for life beyond Earth, with evidence of water plumes shooting into space.
A bend in Europa's magnetic field observed by NASA's Galileo spacecraft during a 1997 flyby appears to have been caused by a geyser gushing through its frozen crust from a subsurface ocean, researchers who re-examined the Galileo data reported.
Galileo was passing about 200 kilometres above Europa's surface when it apparently flew through the plume.
"We know that Europa has a lot of the ingredients necessary for life, certainly for life as we know it. There's water. There's energy. There's some amount of carbon material. But the habitability of Europa is one of the big questions that we want to understand," said planetary scientist Elizabeth Turtle of Johns Hopkins University Applied Physics Laboratory.
"And one of the really exciting things about detection of a plume is that that means there may be ways that the material from the ocean — which is likely the most habitable part of Europa because it's warmer and it's protected from the radiation environment by the ice shell — to come out above the ice shell.
"And that means we'd be able to sample it."
The research, headed by University of Michigan space physicist Xianzhe Jia, was published in the journal Nature Astronomy.
The findings support other evidence of plumes from Europa, whose ocean may contain twice the volume of all Earth's oceans.
NASA's Hubble Space Telescope collected ultraviolet data suggestive of a plume in 2012.
NASA will get a close-up look from a new spacecraft during its Europa Clipper mission that could launch as soon as June 2022, providing a possible opportunity to sample plumes for signs of life, perhaps microbial, from its ocean.
Europa is considered among the prime candidates for life in our solar system, but is not the only one.
For example, NASA's Cassini spacecraft sampled plumes from Saturn's ocean-bearing moon Enceladus that contained hydrogen from hydrothermal vents, an environment that may have given rise to life on Earth.
A bit smaller than Earth's moon, Europa's ocean resides under an ice layer 15 to 25km thick, with an estimated depth of 60 to 150km.
As seasons come and go on Mars, NASA's Curiosity rover has been diligently sniffing and digging away, looking for signs the planet could have supported life.
The latest data shows huge swings in the level of methane in the atmosphere as the seasons change, and new types of organic molecules capable of preserving life just beneath its surface.
The discoveries, reported today in two papers in the journal Science, while not evidence of life, provide more tantalising clues about what's happening on Mars, for future missions to investigate.
Spikes and plumes of methane in Mars's thin atmosphere have been detected by several missions over more than a decade, but they have been frustrating to study, said Christopher Webster, who led the methane study.
"These spikes and plumes … they never come back," he said.
On Earth, methane is mainly produced by biological processes. But the origin of methane on Mars has long been debated.
While it could be produced by microorganisms under the surface of Mars, it could also be produced by non-biological processes such as chemical reactions in rocks, or the breakdown of organic matter in dust delivered by comets or meteors, by UV radiation.
Dr Webster said the difference was much larger than what you would expect if the methane was produced by the breakdown of organic matter from space.
"We've been able to rule out some of the more simple or accepted ideas of Mars's methane," Dr Webster said.
"The only thing that fits the data is that you have a source of methane below the surface.
"We cannot rule out its creation from biological activity … [But] it could also be rock chemistry."
NASA's rover hasn't just been sniffing the air as it's trundled across the Gale Crater — it has also been drilling just below the surface of the 3.5 billion-year-old lakebed in search of organic molecules.
In 2013, it confirmed organic compounds in rocks in a deep part of the crater called Yellowknife Bay, said Jennifer Eigenbrode, who led the organic molecule study published in Science.
But the deposits were much smaller than they had anticipated.
"A lot of us were left scratching our heads trying to figure out, 'What does this mean?' Then we turned around and realised, 'Let's just go and find more'," said Dr Eigenbrode of NASA's Goddard Space Flight Centre.
They hit pay dirt about 6.5 kilometres away, at two sites near Pahrump Hills at the base of Mt Sharp.
"The rock powder was dark grey. Oh that was such a great feeling when we found those," Dr Eigenbrode said.
Thimble-sized samples of material baked slowly in temperatures of between 500–840 degrees Celsius, revealed the grey mudstone contained different carbon-based molecules to those found at Yellowknife Bay.
These molecules appeared to come from a much larger molecule, and contained high levels of sulfur.
"Those two features are very important for the preservation of organic material in rock on Earth," Dr Eigenbrode said.
Although there is not enough information to know whether the carbon molecules were created by biological or non-biological processes, it is possible that they could be a source of methane, Dr Eigenbrode said.
"They could be changed from something like we've observed at the base of the mountain into methane that eventually makes its way back in to the atmosphere," she said.
For a minor planet, Pluto throws up some pretty big surprises. The icy world has dunes, but unlike Earth's sandy versions, Pluto's seem to be composed of flecks of methane ice.
Published today in the journal Science, it's the latest in a swag of new discoveries uncovered by NASA's New Horizons spacecraft.
When New Horizons flew past Pluto in July 2015, it sent back detailed images of the icy world's surface.
Matt Telfer at the University of Plymouth in the United Kingdom was one of many planetary scientists and geologists who scoured the pictures for interesting features such as dunes.
One location stood out: a north-west patch of Pluto's vast heart-shaped plain of ice dubbed Tombaugh Regio.
New Horizons took photos of dune-like ripples roughly a kilometre wide and tens of kilometres long on the plain, nestled against mountain peaks standing more than 3 kilometres tall.
"There were a few areas that looked interesting," said Dr Telfer, who co-authored the paper.
"But this one spot, the more we looked at it, we thought 'they really, really do look like dunes'."
Upon closer examination, their location, orientation and spacing all pointed to them being dunes.
But there was a problem: planetary scientists thought Pluto's atmosphere was too thin to sustain the winds needed to sculpt the ripples that were visible.
So how did they appear?
Think of dunes, and you probably picture sweeping swathes of hot sandy desert. Winds whisk sand grains into the air, which fall to the ground in ordered ridges.
On Earth, this happens largely because our atmosphere is thick enough to keep the wind blowing.
But Pluto has a comparatively thin atmosphere — only 0.001 per cent of what we have on Earth. Little atmosphere means weak winds.
Pluto is extremely cold — around minus 230 degrees Celsius — and orbits the sun at an average distance of 6 billion kilometres.
But even the meagre solar energy that falls on its icy surface is enough to warm nitrogen ice on its frosty mountains.
The nitrogen ice turns into a gas in a process called sublimation. Still-frozen methane grains from the slopes are shaken loose and lofted into the atmosphere.
"On the order of a stiff Earth breeze, methane grains around a quarter of a millimetre across will blow around," Dr Telfer said.
Size wise, the methane grains are "on the order of typical Australian dune sand from the Simpson or Strzelecki [deserts]", he added.
Temperature differences in the atmosphere trigger "thermal winds", which keep the methane grains moving along and eventually dump them on the plain below.
Those grains, in turn, kick up more grains and so on, to eventually produce long, thin dunes.
It turns out that Pluto's thin atmosphere, plus its low gravity, may help the dune-forming process. With little gravity or wind resistance to drag or slow them down, methane grains can stay airborne longer.
The dunes are no more than half a million years old, because that's how long it takes the slab of ice they sit on to completely turn over.
But Dr Telfer suspects the dunes are much younger — perhaps on the scale of hundreds of Earth years.
And that particular site is, at the moment, the only place known on Pluto that ticks all the dune-building boxes: a supply of granular material, enough wind to push them around and atmosphere — even a little bit.
A mysterious bow-shaped "gravity" cloud that hovers over mountains on Venus could speed up the planet's day by a couple of minutes, according to a study.
The study, published today in Nature Geoscience, may help explain variations in the planet's rotation seen by different spacecraft over the years.
Venus spins at a leisurely pace, taking an average of 243 Earth days to complete one full rotation.
Measurements taken 16 Earth years apart by the Magellan and Venus Express missions differ by up to seven minutes, said Thomas Navarro, a planetary scientist at the University of California, Los Angeles.
"We have to average the displacement over a long period of time to get an estimate.
"We kind of know what the average duration of the day is, what we don't know very well is how the fluctuations of the rotation rate could make the day vary."
Venus is also hard to study because it is shrouded in thick clouds of sulfuric acid.
Unlike the leisurely pace taken by the planet itself, these clouds rip right around Venus in just four to five Earth days.
"We do not understand much about why the atmosphere is super rotating," Dr Navarro said.
But not all the atmosphere appears to super rotate.Recently, the Japane
In a massive shot in the arm for the search for life on Mars, a giant lake of liquid water seems to be buried beneath the ice near the Red Planet's south pole.
Using ground-penetrating radar on an orbiting spacecraft, an Italian team picked up signs of a 20-kilometre-wide body of liquid water, hidden 1.5 kilometres under the ice cap.
They published their observations in Sciencetoday.
Whether that body of water is a relic of past oceans or part of a bigger network of subterranean lakes is still a mystery, said Roberto Orosei, a planetary scientist at the Italian National Institute of Astrophysics and lead author of the paper.
"We need to determine if this discovery is unique or if it's something that exists elsewhere on Mars," he said.
Whether it's a one-off or not, he added, has important implications for finding life lurking on Mars today.
But whether the Martian subsurface lake is mostly pure water or sludgy sediment remains to be seen.
The MARSIS instrument can't tell us how deep the water is or how much stuff is mixed with it.
It could be pure water, maybe a metre deep or so, or water-saturated sediments tens of metres thick, Dr Orosei said. Both scenarios would produce the same radar signal.
So what can we deduce about the lake?
To stay liquid in such frigid conditions, the lake must be salty.
Dissolved salts lower water's freezing point — seawater on Earth, for instance, freezes at minus 2 degrees Celsius.Applying pressu
Curiosity and awe have greeted a complete lunar eclipse, the longest one of this century and visible in much of the world.
The so-called "blood moon," when it turns a deep red, was visible at different times in Australia, Africa, Asia, Europe and South America when the sun, Earth and moon lined up perfectly, casting Earth's shadow on the moon.
The total eclipse lasted one hour, 42 minutes and 57 seconds, though a partial eclipse preceded and followed it, meaning the Moon spent a total of three hours and 54 minutes in the Earth's umbral shadow, according to NASA.
A total lunar eclipse occurs when the Earth's shadow moves across the Moon, blocking out light from the Sun.
"What it actually shows is the solar system in action, and it gives you a bit of a better perspective of your own place in the scheme of things, in the universe," he said.
Astrophysicist at the Australian National University Brad Tucker said a total lunar eclipse is something even experts get excited about.
"The Moon passes into the Earth's shadow and that red colour really is the atmosphere of our Earth covering the light to bed.
"So you're actually seeing … the sunrise and the sunset of the Earth simultaneously," he said.
Pictures to follow