Landing sites on Europa identified

Galileo got there first (Image: NASA/JPL/University of Arizona/University of Colorado)

A RIGOROUS analysis of the jagged terrain of Jupiter's moon Europa is helping to identify safe landing strips for future missions.

Europa is thought to have an ocean of water beneath its icy shell. The latest study is the first to use images from the Galileo spacecraft, which orbited Jupiter from 1995 to 2003, to generate measurements of Europa's slopes. "This is the first quantitative sampling that gives hard numbers, real numbers that you can believe," says Paul Schenk of the Lunar and Planetary Institute in Houston, Texas.

Schenk used shadows, plus pictures taken from two different angles, combined into 3D images, to calculate the slopes of various regions of Europa. He examined four different kinds of terrain: ridged plains that make up the majority of the surface; impact craters; so-called "chaos" regions where icebergs appear to float in a frozen soup; and long smooth stripes called dilational bands.

Chaos regions and impact craters are particularly exciting for planetary scientists since liquid water from a subsurface ocean may have burst through at these points, making it possible to search for evidence of life without having to drill below the surface.

These sites are bad news for landers, though. Up to half the landscape in these regions was tilted more than 10 degrees, a similar incline is making life difficult for the Mars rover Spirit. The steepest slopes can reach 20 or 30 degrees. Even the ridged plains have rounded tops that could pose problems for landers.

The only smooth features were the dilational bands, which slope at about 5 degrees or less. These broad tracks, tens of kilometres wide and hundreds of kilometres long, form when cracks in the ice shell open in response to the gravitational pull of Jupiter and the other large moons. The cracks then fill with water and open even further, leaving smooth tracks in between. "It's a little bit like a mid-ocean ridge spreading on Earth," Schenk says.

These areas could be smoother because they didn't form as violently as impact craters, or because the upwelling water smoothed over whatever rough patches were there. Thanks to regular flooding, the cracks could also harbour life. "These bands are one of the places that a future project might decide it wanted to land," Schenk says.

Europa was also recently selected as the target for an orbiting mission. The orbiter will finish mapping Europa's surface, where Galileo left off. "The Galileo antenna malfunctioned," says Schenk. "They could only map about 15 per cent of the moon at resolutions that are worth mapping."

"The issue of topography is very important as we put together the objectives for the Europa orbiter mission," says Bob Pappalardo at NASA's Jet Propulsion Laboratory in Pasadena, who is working on the Jupiter Europa Orbiter scheduled for launch in 2020.

Muscular blob shows new direction for tissue engineering

A microscope view of the new, controllable blob of muscle proteins (Image: Harvard University)

A quivering blob of muscle proteins in a Harvard lab could lead to controllable biomaterials to replace damaged body tissue.

Under a microscope, the "active gel" looks like a throbbing tangle of fibres immersed in jelly. Created by David Weitz and his colleagues at Harvard University, it is made from a molecular net of the muscle protein actin held into shape by another protein, filamin. Each actin strand has around 300 molecules of another muscle protein, myosin, attached.

The gel stiffens when exposed to ATP, the chemical that cells use to store and release energy. It becomes 1000 times firmer, a change in elasticity of the same order as Jell-O setting, says Weitz.

The myosin molecules flex like miniature biceps, bunching up the actin strands and causing the network to "tense up".

Natural mover

"What we're trying to do is unravel the design principles that nature uses to make mechanical structures," says Weitz.

Unlike the materials typically used by engineers, which have fixed properties, many natural materials and structures can adapt theirs as circumstance require. Muscle is a good example, says Weitz, and the network he has created is a step toward replicating such properties.

"This bridges ideas that have been out there," says Margaret Gardel, a researcher at the University of Chicago not involved with the work. The blob is similar to the adaptable but tough protein skeleton that as well as holding cells in shape also allows them to shape-shift as required, she says.

Weitz thinks his active gel design could be used to give a new twist to tissue engineering, which usually involves using a static scaffold to guide the growth of replacement tissues from stem cells.

Scaffolds with tunable elasticity could allow more complex structures to be grown, says Weitz. For example, a floppy, untensed blob could be moved into position and then set in place with a pulse of ATP.

Because the physical properties of nearby surfaces are known to affect what kind of tissue stem cells grow into, a scaffold with controllable stiffness could direct a collection of stems cells to grow into different cell types to sculpt more intricate tissues that contain different kinds of cell.

Disrupt emergency exits to boost evacuation rates

Obstructing the exit could save lives (Image: Scott Craig/cancerbot/StockXchng)

Need to evacuate people quickly through a narrow opening? Put something in their way.

Physicists timed a crowd of 50 women as they exited as fast as possible through a door, and then repeated the experiment with a 20-centimetre-wide pillar placed 65 centimetres in front of the exit to the left-hand side.

The obstacle improved the exit rate by an extra seven people per minute – from 2.8 people to 2.92 people per second.

Daichi Yanagisawa at the University of Tokyo, Japan, who led the research team, explains that the pillar creates a relatively uncrowded area where it's needed most – just in front of the exit.

Usually, the exit becomes clogged by people competing for the small space, and the crowd is slowed. The pillar blocks pedestrians arriving at the exit from the left so effectively that the number of people attempting to occupy the space just in front of the exit is reduced, says Yanagisawa. With reduced crowding there are fewer conflicts and the outflow rate increases.

But the positioning of the pillar is crucial, says Yanagisawa. When the researchers moved the pillar so that it stood directly in front of the exit's centre, rather than to the left, the outflow rate dropped to 2.78.

That's because there's a second factor influencing outflow rate, dubbed the turning function. As pedestrians approach the busy doorway they weave and duck to squeeze through the crowd. With every turn they lose momentum and their walking speed decreases, which reduces the rate of outflow through the exit

With the pillar offset to the left, the turning function of pedestrians approaching the exit from the left increases. Although they take longer to reach the exit, the total effect is an increase in outflow rate since those approaching from the centre or the right have a comparatively free and empty route to the exit.

But if the pillar is central, it affects the turning function of most pedestrians approaching the exit. Because more pedestrians are slowed down by the obstacle, the total outflow rate drops.

Mystery of the missing mini-galaxies

Something missing? (Image: NASA/ESA/STScI/AURA/A. Aloisi)

LIKE moths about a flame, thousands of tiny satellite galaxies flutter about our Milky Way. For astronomers this is a dream scenario, fitting perfectly with the established models of how our galaxy's cosmic neighbourhood should be. Unfortunately, it's a dream in more ways than one and the reality could hardly be more different.

As far as we can tell, barely 25 straggly satellites loiter forlornly around the outskirts of the Milky Way. "We see only about 1 per cent of the predicted number of satellite galaxies," says Pavel Kroupa of the University of Bonn in Germany. "It is the cleanest case in which we can see there is something badly wrong with our standard picture of the origin of galaxies."

It isn't just the apparent dearth of galaxies that is causing consternation. At a conference earlier this year in the German town of Bad Honnef, Kroupa and his colleagues presented an analysis of the location and motion of the known satellite galaxies. They reported that most of those galaxies orbit the Milky Way in an unexpected manner and that, taken together, their results are at odds with mainstream cosmology. There is "only one way" to explain the results, says Kroupa: "Gravity has to be stronger than predicted by Newton."

Challenging Newton's description of gravity is controversial. But regardless of where the truth lies, the Milky Way's satellite galaxies have become the latest battleground between the proponents of dark matter and theories of modified gravity.

Our standard picture of the universe comes from many decades of observations. It asserts that visible matter - the kind of stuff that you, me, the planets and stars are made of - is outweighed by a factor of 6 or 7 by invisible, cold dark matter. No one knows what dark matter is made of, but its existence has been postulated to explain how the stars in spiral galaxies can orbit at such breakneck speeds without being flung off into the void. There isn't enough ordinary matter out there to hold on to everything, so the extra gravitational grip provided by large amounts of dark matter stops these speeding stars flying off into space.

Dark matter is also thought to have played a key role in shaping the early universe. In the aftermath of the big bang, it was the dark stuff that first began to clump together under the force of gravity because its lack of interaction with light meant it was not blasted apart by the big-bang fireball. Later on, normal gaseous matter fell into these clumps - dubbed dark matter haloes - where it congealed into stars to make visible galaxies.

A key feature of this dark matter scenario is that dark matter haloes of all sizes form. According to the standard model of cosmology, a halo as large as the one thought to have seeded the Milky Way should be surrounded by thousands of mini haloes, which themselves should have seeded small satellite galaxies.

So why don't we see them? It could simply be because most of the satellite galaxies contain only a few thousand stars and their faintness makes them extremely hard to spot (see New Scientist, 15 August, p 10).

Another problem is that it is not obvious to the human eye that an apparent group of stars in the sky is a bound collection rather than a chance alignment of stars at wildly different distances. Proving their connectedness requires computerised search techniques and detailed analyses of the colours of the stars to give their relative distances and types - a painstaking and expensive business.

Tidal dwarfs

Nevertheless, the rate of discovery of satellite galaxies has been boosted in the past five years by a detailed search by the Sloan Digital Sky Survey. Whereas only nine satellites were discovered in the 30 years before SDSS, another 15 have been found since. The biggest are about 1000 light years across - less than 1 per cent of the diameter of the Milky Way's disc - and the smallest about 150 light years across. Despite this progress, the total number of satellites known falls far short of that predicted by the cold dark matter paradigm.

The missing-satellites problem is not the only puzzle. Kroupa and his Bonn colleague Manuel Metz, together with Gerhard Hensler at the University of Vienna, Austria, and Helmut Jerjen of Mount Stromlo Observatory near Canberra, Australia, have studied the location and motion of the small number of known satellite galaxies. They found that a high proportion of the galaxies appear to be confined to a plane perpendicular to the disc of the Milky Way. What's more, most of the galaxies orbit the Milky Way in the same direction. "This is completely incompatible with the dark matter model of the Milky Way's formation," says Kroupa. He points out that the satellites should be more like a swarm of bees, moving on random orbits and distributed in a spherical shell around our galaxy.

Robot with bones moves like you do

Like looking in the mirror (Image: The Robot Studio)

YOU may have more in common with this robot than any other - it was designed using your anatomy as a blueprint.

Conventional humanoid robots may look human, but the workings under their synthetic skins are radically different from our anatomy. A team with members across five European countries says this makes it difficult to build robots able to move like we do.

Their project, the Eccerobot, has been designed to duplicate the way human bones, muscles and tendons work and are linked together. The plastic bones copy biological shapes and are moved by kite-line that is tough like tendons, while elastic cords mimic the bounce of muscle.

Mimicking human anatomy is no shortcut to success, though, as even simple human actions like raising an arm involve a complex series of movements from many of the robot's bones, muscles and tendons. However, the team is convinced that solving these problems will enable the construction of a machine that interacts with its environment in a more human manner.

Simple human actions like raising an arm involve a complex series of movements for the robot

"We want to develop these ideas into a new kind of 'anthropomimetic robot' which can deal with and respond to the world in ways closer to the ways that humans do," says Owen Holland at the University of Sussex, UK, who is leading the project.

The team also intends to endow the robot with some human-like artificial intelligence.

Second robot deployed to help free stuck Mars rover

In the struggle to free the Mars rover Spirit from a sand trap, NASA engineers are bringing out the reserve troops. A second, lighter duplicate rover slid into a sandbox for testing this week, delaying any attempt to free Spirit by as much as three weeks, to mid-September.

Spirit has been stuck in a sandpit for nearly four months. Since late June, engineers have been trying to determine the best moves to extricate it by driving a test rover around a sandbox at NASA's Jet Propulsion Laboratory (JPL) in California.

Rover engineers had earlier announced that they were almost done with the testing and would be ready to move Spirit around 10 August, but they backtracked in a meeting on 6 August.

"We've come up with additional tests that we want to do, and additional computer modelling that we want to do as well," says rover project manager John Callas at JPL. "Now we're looking at the middle of September."