Thus vast headlines in the Independent, the Guardian and the Times this morning, all of them devoting all of the front page above the fold to results from Europe's Mars Express. Late editions of yesterday's Evening Standard gave the whole front page to the story:
Eureka! We've found water on Mars
The other UK broadsheets also have big front page pix, though their headlines don't scream the news quite as large (the tabloids appear to have other fish to fry). It's all a) very exciting, b) beautifully illustrated and c) rather disingenuous.
To begin with, it's important to distinguish between liquid water and water ice. Liquid water is the really important stuff. Liquid water appears to be a necessary condition for life on the earth, and thus probably for earth-like life elsewhere. Since the key to Mars's attraction is its possible suitability as an abode for life, this makes water on Mars a big deal. There's lots of geomorphological evidence for liquid water in the past, both the relatively recent past (the gullies discovered in images from Mars Global Surveyor) and the incredibly ancient past (the larger valley and flood features, some now being beautifully re-imaged by the Mars Express camera). There are hints that there may be traces of liquid water around today - the best, I think, is the study by Oded Aharonson and his colleagues suggesting that liquid water in tiny traces may play a role in the "slope streaks" found in some places. (The paper on this is available as a pdf here) But no one has discovered liquid water on Mars, nor would anyone, I think, claim to have done so.
What the Mars Express people are talking about is water ice. To an earthly imagination - the imagination, for example, of a news editor - the distinction between water and water ice may seem a bit silly: ice is water, really, after all, isn't it. This point of view is not stupid, just understandably parochial. On the earth ice is a temporary condition water finds itself in before melting back into a liquid, and so to find ice is more or less the same as finding liquid water, as long as you're willing to wait a bit. But on a cold planet with a very thin atmosphere water doesn't work this way. Ice on Mars can be stable indefinitely - and when some external change makes it unstable, it may well turn straight into water vapour without ever passing through a liquid phase, a process called sublimation. To find water ice on Mars certainly raises the possibility of liquid water, but it doesn't entail it as a logical necessity.
The fact that it raises the possibility of liquid water, though, still makes the discovery of water ice a historical step forward in our understanding of Mars. But this is where the disingenuousness comes in. Take Charles Arthur's piece in the Independent. "Since [the Viking missions of the late 1970s] a succession of missions have failed even to confirm the presence of water there, a prerequisite for life. Until yesterday." It's interesting to note Charles's choice of the word "confirm". Because though he doesn't say it, the fact that there is water ice on Mars was convincingly demonstrated by the Vikings almost thirty years ago.
The man who did the most to take this historical step was Hugh Kieffer of the US Geological Survey. Kieffer started working on Mars with the Mariner 6 and 7 missions in the late 1960s, and he's still at it today. In the 1970s he was in charge of the Viking Orbiters' infrared instruments, which measured the the surface temperature of the planet below. At that point, the ice caps were thought to be made of dry ice -- solid carbon dioxide. In winter the caps expanded as carbon dioxide from the atmosphere froze out on to the ground; in the spring they retreated as the seasonal component of the caps sublimed back into the thin air. Kieffer's measurements showed that at the beginning of the northern spring, when the seasonal cap was retreating, its surface was at a temperature of about 150K. This is the temperature at which solid carbon dioxide sublimes under martian conditions, which makes sense -- that is what was going on.
However, in the summer, the temperature of parts of what was left of the cap rose to about 200K. That was too hot for carbon dioxide ice; it was just the right temperature, however, for water ice. Kieffer reasoned that the change reflected the fact that a seasonal layer of carbon dioxide had gone away and a more permanent layer of water ice had been revealed beneath it (the point in the spring on which this transition happens for a given locale is now known, rather charmingly, as that place's "crocus date").
The clinching confirmation of Kieffer's argument came from a second instrument on the Viking orbiters, one that measured water vapour in the atmosphere. After the crocus date at the edge of the permanent cap, when the lower layer of ice was revealed to the summer sunshine, the amount of water vapour in the atmosphere above the cap shot up -- exactly what you would expect if the newly revealed water ice itself was starting to sublime into water vapour, as its temperature suggested it was.
This clever piece of work led to a rash of headlines saying things like "Water on Mars". And quite rightly. As far as I'm aware, no one has ever since suggested that the residual north polar cap left behind when the seasonal carbon-dioxide cap has vanished is not largely composed of water ice. Ice at the north pole is an accepted fact of Martian life, and has been for decades.
It was, of course, conceivable that while there was ice at the north pole there was none at the south, which is the area that the Mars Express results deal with. The Vikings saw no evidence of water ice on the surface in the south, nor any tell tale increases in atmospheric water vapour over the course of southern summers. But a water-free southern cap has seemed unlikely since the results from NASA's Mars Odyssey started to come back a few years ago. The neutron and gamma ray spectrometers on Odyssey showed strong evidence of hydrogen in the top metre or so of the crust in the high latitudes of both hemispheres, which was evidence very hard to square with anything other than ice frozen into the soil. This conformed neatly with the geomorphological evidence -- things like the slumping of slopes and the shapes of craters -- that there was abundant ground ice in the high latitudes of both hemispheres. (The fact that the first Mars Odyssey data basically confirmed what the geomorphology was already accepted as showing didn't stop NASA hyping that data as a great discovery, either.)
As a way of making their evidence seem more groundbreaking, though, European scientists pointed out that Mars Odyssey's neutron spectroscopy was indirect evidence of water (since it actually detected hydrogen) whereas Mars Express's infra-red spectroscopy was direct evidence. (This made it somewhat ironic that the Evening Standard illustrated its front page splash with a plot of the Mars Odyssey hydrogen data. But one shouldn't complain - anything that makes the evening Standard see a European achievement as something "we" did should be welcomed.)
However, the hydrogen spectra are not the only evidence of water from Mars Odyssey. The redoubtable Kieffer, working with Phil Christensen and Tim Titus, used the infra-red instruments on Mars Global Surveyor and Mars Odyssey to find a region within the south polar cap that behaved just as the north polar cap does, and which had thermal properties just like those of water ice. Their work was published online by Science a bit more than a year ago under the title "Exposed Water Ice Discovered near the South Pole of Mars". Various other people looking at similar data have come to the same conclusions.
To describe the data from Mars Express as the discovery of water on Mars is thus more than a stretch. No one has found liquid water. Water ice is old news with respect to the northern cap. And its discovery in the southern cap was published in a peer reviewed journal a year or so ago.
People will undoubtedly argue that this doesn't really matter much in the context of how great it is that the public is interested in Mars, and they could be right. But its still basically wrong to call this the discovery of water, or water ice, on Mars. Its disrespectful of Kieffers work and the work of others. It carries a tone of European triumphalism which, in an area where we hear a lot of American triumphalism, is understandable but still a little unpalatable. And it could be counterproductive. There's a chance that later on in its mission, Mars Express might really find water - liquid water - with its ground penetrating radar. And what will its spokes-people say then?
The real news yesterday was that Europe's first mission to another planet is performing brilliantly, and that its instruments are spectacular, capable of rediscovering in a day or so secrets that had to be teased out through months or years of work in the past. And those instruments, god willing, are now going to send back data for at least two earth years and maybe more. That is truly great news, and it needed no further icing to make it more exciting or palatable.