Solved: the mystery of why it's impossible to pull apart interleaved phone books

No glue, only friction. Danny Nicholson/Flickr, CC BY-NC-ND

People, trucks and even military tanks have tried and failed the task of pulling apart two phone books lying face up with their pages interleaved, like a shuffled deck of cards. While physicists have long known that this must be due to enormous frictional forces, exactly how these forces are generated has been an enigma – until now.

A team of physicists from France and Canada has discovered that it is the layout of the books coupled with the act of pulling that is producing the force.

The power of approximation

Finding an approximate solution to a complex problem is an essential skill in science (and in life). Often we are faced with questions that we can’t answer exactly, but sometimes good enough is, well, good enough. Enrico Fermi, one of the greatest physicists in the 20th century, has given his name to such “Fermi Questions” – as he was famous for encouraging this skill in his students.

Here’s one example: “How many piano tuners are there in Chicago?”. I have no idea, and I’m not sure Fermi knew either. But by estimating the population of Chicago, the fraction that might play the piano, and how often a piano needs tuning, you can come up with a pretty good guess, without diving into the phone book (it’s probably closer to 100 than to 1,000).

Doing these “back-of-an-envelope” calculations is usually the first step in approaching a scientific question. Sometimes that is as far as you need to go. Sometimes it tells us that the question is worth investigating more to find the exact answer.

Not even Brian Blessed can do it.

This is exactly what the team investigating the friction of phone books did. The back-of-the-envelope answer is friction between the pages. However, assuming the friction is proportional to the number of pages drastically underestimates the total force that is generated (which seems to rise exponentially with the number of pages). But previous attempts to improve this simple model – by including the effects of gravity and air pressure pushing the pages of the books together – have all failed to explain the result.

Surprisingly simple

So, when the back-of-the-envelope calculation fails, things get serious. In this case, the traction instrument was brought out (think the opposite of a vice), it was used to pull books apart while measuring the force required to do so. But not just any books. Rigorously prepared test books with specific numbers of pages, built from paper sheets of exact dimensions, interleaved to high precision.

Data in hand, a mathematical model was put together, and it turned out to be driven by a surprisingly simple fact. The pages of each book are separated by the interleaving and end up “spreading out”, lying at a slight angle from the spine. When the books are pulled away from each other, the pages want to move back closer together and end up squeezing the interleaved pages from the other book. And gripping something tightly greatly increases the friction.

Just impossible.

As an example, imagine a person with long hair in a swimming pool. While floating underwater, their hair can spread out – much like the pages of the books are spread out by the interleaving. Then, if our volunteer swims off, their hair will naturally move close together, following their head which is pulling it along. The pages of our books also want to move close together behind the thing pulling them (the spine of the book), but instead just squeeze more tightly on the pages of the other book, which are in the way. Pulling harder on the books only increases the friction.

This is an example of the geometrical amplification of friction, or how the layout of the books produces forces far beyond what is expected. Knots are another example, looping a rope around itself greatly increases the friction, resulting in a secure grip. The authors point out the recent resurgence of interest in this kind of problem and the general field of tribology, the study of surfaces in relative motion.

This is being driven by the need to understand the structure and behaviour of new micro and nano-engineered materials, which have impact on many aspects of life from medical applications to solar cells. Interleaved carbon nano-tubes as the material of the future anyone?

Why banning the mammoth ivory trade would be a huge mistake

Would a ban on mammoth ivory endanger or save the elephant? Pixabay

There is widely held belief that the only way we can protect globally endangered species that are being poached for the international wildlife trade is to completely ban the trade. This is a dangerous misconception and will speed up extinction rather than prevent it.

Adrian Lister, a mammoth expert from University College London, recently suggested that mammoths should be listed under the convention on international trade in endangered species to keep their ivory from being laundered into an illegal trade in tusks. He argued that the mammoth trade is encouraging the poaching of elephants by keeping up the demand for ivory.

This is madness. Mammoths and mammoth ivory is not rare – it is estimated that there are 10 million mammoths that remain incarcerated within the permafrost of the Arctic tundra. And in any case a ban on mammoth ivory would not stop the trade, it would simply drive it underground and attract the attention of organised crime groups. For example, in my own research I found that prices for illegally caught whale meat rose very quickly when enforcement efforts intensified and this in turn led to the trade being controlled by dedicated “professional” criminals.

In the same way, a ban on mammoth ivory would drive up prices and lead to many mammoth sites being excavated in clandestine fashion, without any associated scientific endeavours to garner knowledge and understanding of these great beasts. In fact the current situation supports collaboration between collectors and academics about new finds, to the benefit of scientific research.

A ban would not save the elephant either. In fact it would do the opposite and probably hasten its extinction in the wild. Although record levels of funding are now being invested in enforcement and anti-poaching measures to tackle the crime, many species such as the rhino remain on the path to extinction in the wild quite simply because bans aren’t working.

Woolly mammoth model at the Royal BC Museum in Victoria (Canada). FunkMonk/wikimedia, CC BY-SA

Around the world, incentives to poach elephants and rhinos are increasing due to rising prices and growing relative poverty between areas of supply and centres of demand, and while trade bans can curtail supply it does not seem to have reduced demand in any measurable way. Indeed, high levels of protection can actually stimulate demand for a species due to something called the anthropogenic allee effect.

Economic theory and research can explain why this happens and why we need to urgently reconsider our reliance on global trade bans. Where there is demand that is not very sensitive to price changes and strong enforcement of a ban, prices for illegal wildlife products will rise steeply, but have little overall effect on supply and consumption. This is especially true where organised criminal networks can circumnavigate the police and customs – a relatively easy trick for countries mired in corruption.

The need for bold moves

In this situation we need to look beyond regulation and consider bold strategies that actually make economic sense. In particular we need policies that drive prices down and reduce the pressure on wild populations. To do this we should be considering introducing sustainable off-take mechanisms such as regulated trade, ranching and wildlife farming. If these new sources of supply are close substitutes, such as mammoth and elephant ivory, these mechanisms will certainly cause prices to fall and pressure on wild populations to reduce.

Cross sectioned mammoth tusk. Cropbot/wikipedia, CC BY-SA

We have seen this happen successfully with crocodilian species, where farmed animals have largely taken over the market and recent economic research in Canada shows that the sale of mammoth ivory into the ivory business in Asia has actually led to lower prices for elephant ivory saving thousands of elephants.

Basic economics tells us that when one introduces a substitute, especially a very close substitute, the price of the alternative product will fall. A recent analysis linked with empirical data predicts that the 84 tonnes of Russian mammoth ivory that was exported to Asia on average per annum over the period 2010-2012 would have actually reduced poaching of wild elephants from 85,000 per year to around 34,000 elephants per year, primarily by reducing elephant ivory prices by about $100 per kilogram.

The policy implication is simple – the mammoth ivory trade should be legal and sustainably managed rather than banned – this will help save both the living elephant and the extinct mammoth.

Airshows are risky – that's why we like them – but they also have a strong safety record

As seen on 24-hour television news. Daniel Leal-Olivas/PA

Whether aircraft are used for travel or for acrobatic displays, it will never be possible to aviate entirely without risk. Airshows are manifestations of our liking for what Anthony Giddens calls deliberately cultivated risk – this excites and sustains those who participate and those who watch.

Deliberately cultivated risk is an outlet for untapped energies and repressed emotions. That is the upside. The downside is that such activities occasionally lead to death and injury.

This was the case at the Shoreham Air Display where a wayward 50-year-old Hawker Hunter two-seat trainer aeroplane ploughed into a road killing at least 11 motorists. Nevertheless, air displays enjoy a remarkably good safety record: the Shoreham crash was the first major loss of life among spectators or the public at an airshow in the UK since Farnborough, 63 years ago, when a prototype De Havilland 110 fighter aircraft disintegrated, its engines scything through the watching crowds killing 29 and injuring a further 63 spectators.

The Hawker Hunter that crashed at Shoreham. Paul Jarrett/EPA

The media view

This generally excellent safety record has been glossed over in much of the reporting of the tragedy. De-contextualised reporting creates an impression of chronic mishap and horror. Photographs of the moment of impact have been repeatedly used, the same jet-explodes-in-orange-fireball footage endlessly replayed across the voracious 24-hour news channels.

Several times an hour eye-witnesses are wheeled out to give their accounts, regardless of the fact that research frequently reveals eye-witness accounts to be highly inaccurate. And commentators have been breaking the golden rule of air crash investigation by indulging in speculation as to the causes.

Within 48 hours of the crash, headlines included: “I drove through fireball and went to work” (The Sun); “Jet crash fireball forces air show safety rethink” (The Times); “Air shows should be over the sea, it should not have been over that road” (The Daily Telegraph); “Why did they have to die?” (Daily Mirror); “Footie pals killed in jet crash carnage” (Daily Star); “How many more fireball dead?” (Daily Mail).

I appreciate why the media is so interested in the story, but there comes a point where legitimate curiosity mutates into self-interested voyeurism. Of course it could be argued that the public gets the press it deserves. That is, that the press merely reflects public morality and behaviour. Consider the following item from the the August 24 edition of Metro:

Steve Barry, assistant chief constable of Sussex Police, warned passers-by [on the A27, site of the Shoreham crash] against taking selfies. In a statement, Sussex Police said: ‘It is not our place to dictate what may or may not be published anywhere, but from a personal perspective I would ask people to consider the feelings of those who have lost loved ones in this incident and, indeed, who may still not have heard from them and are seeking information’.

I have no doubt that Barry’s appeal for decency and restraint will be ignored by some.

If we look at aviation through the prism of an expectation of total safety, we are going to be disappointed. As academics such as Charles Perrow remind us, aviation is an inherently risky activity. Aircraft, especially military aircraft, are “dense” – they contain myriad components packed tightly together.

This density makes unanticipated and uncontrollable interactions between components more likely. Add in an element of deliberately cultivated risk, such as flying a single-engined, five-decade old airframe around the sky in proximity to heavily used transport routes, and there is a window for mishap, error, and potential disaster.

So what is to be done? Two things. First, there must be what Professor Brian Toft calls active learning: aviation is comparatively safe precisely because it learns from its mistakes, applying the lessons of incidents, accidents and near-misses. Whatever can be learned from Shoreham must, and will be, acted upon. And second, the industry must engage the public in a conversation about safety. Aviation, in all its forms, can never be 100% safe, and it’s in the interests of the industry and the public to better manage expectations.

Obesity drug may be on the horizon after study pinpoints genetic mechanism

Help may be on the way. Alan Cleaver/Flicke, CC BY-SA

Nearly half of all Europeans are genetically predisposed to obesity. The condition is a worldwide epidemic affecting more than half a billion people and rising every year in most countries.

Despite this, we know little about the genetic origin of the condition and have no good medical treatment for it other than bariatric surgery. But now a genetic study seems to have cracked the mystery – raising hopes for more efficient treatment.

The global obesity crisis is often blamed on an increasingly sedentary life style and poor eating habits. However, studies have shown that 70-80% of the differences between people in body fat are due to their genes (this is called the heritability).

The first large-scale genetic studies for obesity were launched in 2007, after the initial mapping of the human genome. And one gene, dubbed FTO, made the headlines by popping its head above the other 20,000 genes in the pack. For the past eight years, despite finding nearly 100 other genes linked to obesity, FTO and the area around it have remained the top signals. But scientists around the world have struggled to understand how the gene works and whether it really is behind obesity.

A switch for fat burning?

The new study, published in the New England Journal of Medicine by an international team of researchers, took seven years and cost more than a million dollars. It is based on a wide range of studies of hundreds of patients, cell types and laboratory mice. The researchers also mined a rich public resource of databases for gene expression as well as the heritable changes in gene expression (epigenetics), demonstrating just how complex this field has become.

It turns out that the FTO gene doesn’t do much directly – it influences other nearby genes which cause the changes via regions in the DNA called enhancers and repressors. These can change the precursors of adult fat cells while they are still developing. All fat cells originally come from our bone marrow along with cartilage and bone cells and they pass through different stages as they become fat cells.

Could a simple pill replace invasive and expensive obesity treatments using adjustable gastric bands. JohnnyMrNinja/wikimedia, CC BY-SA

We know from recent research there are different types of fat cells in humans, with the most common being white, then some beige and a few brown – each storing and burning fat differently. Obese people have a greater proportion of white cells, which stores the fat rather than burning it off (getting larger as a result). The susceptibility gene variant that the study uncovered makes people produce less brown and beige fat – although the natural effects are quite small.

The team went on to block this pathway using a gene editing tool called CRISPR and found the effects on cells in culture and in lab mice were actually substantial: with five- to seven-fold effects on the animals' ability to burn fat. In fact, blocking the pathway made the animals 50% thinner.

The implications of this work are that after ten years of knowing about thousands of disease-related genes, we finally have the tools to crack the underlying mechanisms. By understanding how the body changes unhealthy white fat cells to healthy beige cells, we can start to develop new treatments for obesity.

This work also emphasises that the billions of dollars spent on the Human Genome Project and its spinoffs such as ENCODE and Epigenetics Roadmap have not been wasted. But we have redefined the parameters of success.

We know now that identifying genes for the most common diseases is actually pretty useless for prediction or diagnosis. Knowing all the 100 identified obesity genes only explains less than 5% of the genetic effect in an individual. Emerging fields such as epigenetics, metabolomics or microbiomics or the old fashioned way of looking at the health of your parents are much better for personalised medicine.

But if you want to understand how to design a badly needed drug for obesity, gene based studies like this are the key. Full-scale research could start on drugs that increase the relative production of beige and brown fat. Hopefully, trials could be underway in humans within a decade.

Shoreham crash will bring safety changes, but airshows are here to stay

Hawker Hunter WV372, the aircraft that crashed at Shoreham. Guy Gratton, Author provided

The tragic Shoreham Airshow crash has turned the spotlight on the safety of airshows, after a 57-year-old Hawker Hunter T7 failed to pull out of an aerobatic manoeuvre. With 11 and perhaps up to 20 people killed this should cause us to re-examine the industry in detail.

But let’s first knock down a few issues not worth debating. First, several million people each year attend airshows in Britain alone, and many more worldwide. These are very popular spectator events, and that won’t change: the age of the airshow is not over.

Second, that this aeroplane – an obsolete British jet fighter designed by Sydney Camm, responsible for the famous World War II-era Hawker Hurricane and the Hawker-Siddeley Harrier, the world’s first vertical take-off and landing aeroplane – was 57-years-old is irrelevant to its safety. The regulations governing the continued airworthiness of aeroplanes are incredibly strict, and tens of thousands of pounds will have been spent annually on this single aeroplane to ensure its fitness for flight.

A wingtip from the wreckage is removed from the road. REUTERS/Luke MacGregor

Third, whatever we may wish, we cannot eliminate the risk of accidents at airshows. While accident rates should be as low as possible, they cannot reasonably be zero, and I am quite certain that all airshow pilots and most airshow attendees recognise this. Accidents such as that which killed Gulf War veteran Trevor Roche in 2012, or former deputy chief of defence staff Sir Kenneth Hayr in 2001 were tragic – but those killed were participants who fully understood the risks.

What happened at Shoreham, however, where the jet failed to pull out of a dive and crashed onto a nearby busy road filled with traffic, is unacceptable.

Counting the cost

All aviation regulations take a four-tiered approach to safety: all third parties must be protected from death or injury as far as possible; consenting participants, such as airline passengers or airshow audiences, must be protected as far as possible, but not at the expense of protecting the general public; pilots and other aircrew should be protected from injury; and aircraft and structures should be protected from damage.

The crash at Shoreham caused the deaths of people who were not participants in the show, and so had not made any sort of informed decision as to the risks involved. Everybody in the aviation industry recognises that should never happen, and because of that there will be at least two separate investigations, already underway.

The first will be by the Air Accidents Investigation Branch (AAIB), a public body set up to investigate accidents “with a view to the preservation of life and the avoidance of accidents in the future”. There will be a team at Shoreham collecting evidence, aiming to produce a report that identifies the main causes behind the accident and, more importantly, recommends how to prevent a similar accident occurring in the future.

Once the wreckage has been collected, this team will relocate to their base at Farnborough. There will be a great deal of evidence to sift through, although the age and size of the Hunter means there’s no flight data recorder (“black box”), and instead the investigation will mostly involve photographic evidence, eyewitness accounts, and wreckage analysis.

The second investigation will be by the Civil Aviation Authority (CAA), the UK statutory body responsible for regulating all aviation in UK airspace. The CAA has many years experience of regulating airshows, much of which started from the tragedy at Farnborough Airshow in September 1952, when the prototype de Havilland 110 (which would become the Sea Vixen) broke up, killing the pilot, John Derry, his flight test observer, Anthony Richards, and 29 spectators.

Following Farnborough, rules were put in place that defined a strict “display line” away from spectators so that they would not be harmed by a crashing aeroplane. The faster an aircraft is, the further the display line must be from the official spectators, although non-paying spectators lining roads around airfields are a concern. A highly experienced flying display director is also required, usually supported by an expert flying control committee to oversee and approve the display. Those pilots flying in the display must hold individual display authorisations to do so.

This has demonstrated itself to be a robust system, and until Shoreham there has been no loss of life among spectators or the general public at a British airshow since John Derry’s crash in 1952. But clearly on this occasion, something failed and allowed a high energy aircraft to point toward a busy public road, leading to deaths. The CAA has already decided to impose immediate restrictions, grounding all Hawker Hunters, and prohibiting high energy aerobatics at airshows. This is a knee-jerk reaction, although perhaps politically necessary, and should be revised following the investigation.

It is normal that rules found to have failed will be re-evaluated and improved, but politicians and the public have added to the clamour. Neither the AAIB nor CAA will cut corners with their investigations – it’s important to do them right, and those seeking quick answers will probably be disappointed.

The investigation of this awful tragedy – which should never have happened – will result in revisions to the rules that oversee how airshows are conducted. It’s far too early to say with any certainty what those changes will be, but this won’t be brushed under the carpet. What’s equally sure is that the saying often repeated by aviators – that all safety regulations were originally written in blood – remains sadly true.

Privacy watchdog takes first step against those undermining right to be forgotten

It's not erasing the past, just making memories fuzzier. chalboard by sergign/shutterstock.com

The UK’s data privacy watchdog has waded into the debate over the enforcement of the right to be forgotten in Europe.

The Information Commissioner’s Office issued a notice to Google to remove from its search results newspaper articles that discussed details from older articles that had themselves been subject to a successful right to be forgotten request.

The new reports included, wholly unnecessarily, the name of the person who had requested that Google remove reports of a ten-year-old shoplifting conviction from search results. Google agreed with this right to be forgotten request and de-linked the contemporary reports of the conviction, but then refused to do the same to new articles that carried the same details. Essentially, Google had granted the subject’s request for privacy, and then allowed it to be reversed via the back door.

The ICO’s action highlights the attitude of the press, which tries to draw as much attention to stories related to the right to be forgotten and their subjects as possible, generating new coverage that throws up details of the very events those making right to be forgotten requests are seeking to have buried.

There is no expectation of anonymity for people convicted of even minor crimes in the UK, something the press takes advantage of: such as the regional newspaper which tweeted a picture of the woman convicted of shoplifting a sex toy. However, after a criminal conviction is spent, the facts of the crime are deemed “irrelevant information” in the technical sense of the UK Data Protection Act.

The arrival of the right to be forgotten, or more accurately the right to have online search results de-linked, as made explicit by the EU Court of Justice in 2014, does not entail retroactive censorship of newspaper reports from the time of the original event. But the limited cases published by Google so far suggest that such requests have normally been granted, except where there was a strong public interest.

Stirring up a censorship storm

It’s clear Google does not like the right to be forgotten, and it has from early on sent notifications to publishers of de-listed links in the hope they will cry “censorship”. Certainly BBC journalist Robert Peston felt “cast into oblivion” because his blog no longer appeared in search results for one particular commenter’s name.

It’s not clear that such notifications are required at all: the European Court of Justice judgment didn’t call for them, and the publishers are neither subject (as they’re not the person involved) nor controller (Google in this case) of the de-listed link. Experts and even the ICO have hinted that Google’s efforts to publicise the very details it is supposed to be minimising might be viewed as a privacy breach or unfair processing with regard to those making right to be forgotten requests.

The Barry Gibb effect

De-listing notifications achieve something similar to the Streisand effect, where publicity around a request for privacy leads to exactly the opposite result. I’ve previously called the attempt to stir up publisher unrest the Barry Gibb effect, because it goes so well with Streisand. So well, maybe it oughta be illegal.

Some publishers are happy to dance to Google’s tune, accumulating and publishing these notifications in their own lists of de-listed links. Presumably this is intended to be seen as a bold move against censorship – the more accurate “List of things we once published that are now considered to contain irrelevant information about somebody” doesn’t sound as appealing.

In June 2015, even the BBC joined in, and comments still show that readers find salacious value in such a list.

Upholding the spirit and letter of the law

While some reporters laugh at the idea of deleting links to articles about links, this misses the point. The ICO has not previously challenged the reporting of stories relating to the right to be forgotten, or lists of delisted links – even when these appear to subvert the spirit of data protection. But by naming the individual involved in these new reports, the de-listed story is brought straight back to the top of search results for the person in question. This is a much more direct subversion of the spirit of the law.

Google refused the subject’s request that it de-list nine search results repeating the old story, name and all, claiming they were relevant to journalistic reporting of the right to be forgotten. The ICO judgement weighed the arguments carefully over ten pages before finding for the complainant in its resulting enforcement notice.

The ICO dealt with 120 such complaints in the past year, but this appears to be the only one where a Google refusal led to an enforcement notice.

The decision against Google is a significant step. However, its scope is narrow as it concerns stories that unwisely repeat personally identifying information, and again it only leads to de-listing results from searches of a particular name. It remains to be seen whether other more subtle forms of subversion aimed at the right to be forgotten will continue to be tolerated.

Water, water, everywhere – where to drink in the solar system

A frozen lake of water-ice on the floor of a 35 km wide impact crater on Mars. Copyright ESA/DLR/FU Berlin (G. Neukum)

Science fiction movies about aliens threatening the Earth routinely ascribe them the motive of coming here to steal our resources, most often our water. This is ill thought-out, as water is actually extremely common. Any civilisation coming to our solar system in need of water (either to drink or to make rocket fuel) would be foolish to plunge all the way inwards to the Earth, from where they’d have to haul their booty back against the pull of the sun’s gravity.

Until recently, we believed that the Earth was the only body in the solar system that had water in liquid form. While it is true that the Earth is the only place where liquid water is stable at the surface, there’s ice almost everywhere. Many scientists also infer that liquid water may exist beneath the surfaces on several bodies.

But where in the solar system are we likely to find it and in what form? Could we ever get to it and, if so, would we be able to drink it?

Comets and the Kuiper belt

If you are interested in finding places were extraterrestrial microbial life might occur, then you should look for liquid water, or at least “warm” ice within a few degrees of melting. Those places are widespread, if you are prepared to look below the surface of cold bodies or around the edges of patches of permanent shade on hot bodies.

Frozen water can be found everywhere in the Solar System, from the Oort Cloud to Mercury (except on Venus). NASA / JPL-Caltech

Furthest from the sun is the Oort Cloud, a region where most comets spend most of their time some 10,000 times further from the Sun than the Earth is. They are mostly water-ice, with traces of various carbon and nitrogen compounds. Because of those you wouldn’t want to drink comet water neat, but there is probably about five Earth-masses of water out there. We can’t be sure, because only the comets that stray close to the sun can be directly studied.

That’s mostly water jetting off the nucleus of comet 67P/Churyumov-Gerasimenko on 30 July 2015 as the comet drew closer to the Sun. ESA/Rosetta/NAVCAM

Most comets are less than about 10 km across, and out in the Oort Cloud they are separated by vast distances, so if you wanted to harvest a lot of water it might be worth travelling inwards as far as the Kuiper Belt about forty times further from the sun than the Earth is.

Could Pluto hide liquid water far beneath its surface? NASA

Here there are bodies up to just over 2000 km in diameter, like Pluto. These are mostly water-ice surrounding rocky cores, but ices made of more volatile substances may coat their surfaces. A few may even have oceans of liquid water tens or hundreds of kilometres below their surfaces.

The giant planets

Neptune, Uranus, Saturn and Jupiter are the giants of the solar system. Deep inside, and confined by very high pressure, each of these is believed to contain several Earth-masses of water, sandwiched between its rocky core and its outer layers of hydrogen and helium gas.

Europa, a 3130 km diameter moon of Jupiter. There is almost certainly a global ocean of salty water between the surface ice and the rocky interior. NASA/JPL-Caltech/SETI Institute

There is no feasible way to get at that water, but the giant planets each have numerous moons that are made mostly of ice. Far from the sun, the ice contains methane, ammonia and carbon monoxide as well as water. However, at the distance of Jupiter from the sun, only five times further out than the Earth, it was too hot for the more volatile ices to condense, resulting in relatively pure water ice.

There is compelling evidence that several icy moons have internal oceans. The best places to look for life are where the ocean overlies warm rock. This may be the case inside Europa (Jupiter) and Enceladus (Saturn), but chemical reactions with the rock would make the liquid water salty, so not good to drink.

The rocky planets

Closer to the sun, Mars, Earth, Venus and Mercury are in a region that was too hot for ice to condense when the solar system was forming. Consequently the planets are mostly rock, which can condense at higher temperatures than ice. The only water on the rocky planets was either trapped inside minerals and then sweated out from the interior, or was added at the surface by impacting comets.

Mars probably once had at least as much water proportional to its rock as the Earth has, but it is a smaller body with weaker gravity and no magnetic field, allowing most of its water to have been lost to space. However, water certainly flowed on Mars’s surface in the past and there are intriguing signs of water seeping downslope to form gulleys even today. However, in order to survive as a liquid this would probably have to be very salty indeed.

We know for sure that there is water-ice in the polar caps too, but neither setting seems hospitable to life. However, if you took the right kind of terrestrial microbes to the right places on Mars, they might be able to scratch a living. What we are less sure of is whether microbes have already made the trip between planets, hitch hiking on meteorites.

Whereas Mars is too cold, Venus has been too hot for liquid water for most of its history. However, there are water droplets high in its atmosphere. This is not worth collecting as a resource, and a very long shot as a means of supporting microscopic airborne life.

Mercury’s north polar region. The yellow areas are in permanent shadow. ASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory

The last place you might expect to find water is Mercury, because it is mostly far too hot. However, there are craters near the poles onto whose floors the sun never shines. The presence of water-ice in these regions, delivered by impacting comets, has been demonstrated be several techniques and cannot be doubted.

Similarly “cold-trapped” water-ice has also been found inside polar craters on the Moon. This may be one of the first solar system resources that we, rather than visiting aliens, exploit as we leave our home world and make our way into space.