Rosetta’s views of Comet 67P

Mauro Pirarba BSc, a Planetary Sciences Graduate Certificate student and Fellow of the Royal Astronomical Society, discusses Dr Ramy El-Maarry’s recent talk on the geology of Comet 67P/Churyumov-Gerasimenko.

Figure 1 – Five comets have been photographed at close range during flyby missions, but only one, comet 67P, has been studied closely for an extensive period of time (image credit: El-Maarry et al., 2019, https://link.springer.com/article/10.1007%2Fs11214-019-0602-1)

Comets are temperamental, often breaking all rules, suddenly appearing out of nowhere, occasionally getting close enough to the Sun and the Earth to display long tails that make us marvel at their beauty and diversity. They have been studied by astronomers for centuries and yet we still do not understand them fully.

What can Dr. Ramy El-Maarry, a geologist at Birkbeck College, possibly tell about one of them, Comet 67P/Churyumov-Gerasimenko, at a special meeting organized by the Royal Astronomical Society?

Does that C-something – G-something sound vaguely familiar? Perhaps you are more likely to remember another name, Rosetta, the probe of the European Space Agency (ESA) tasked with deciphering its mysteries, a few years ago. Launched in 2004, Rosetta reached Comet 67P in August 2014 and spent two years observing it closely.

“I have 80,000 images from that mission and 20 minutes to go through them…”, Ramy started his talk, to everybody’s laughter.

In fact, such a big figure hints at an unprecedented and extraordinary achievement. Twelve spacecrafts have sent back to Earth data about eight different comets and images of six of them (see figure.1). What makes Rosetta stand out is the length of the observation, two years, and its closeness, on average a few tens of kilometres. You may also remember that the mission included a lander, Philae, which failed to anchor itself to the ground and bounced a few times, before settling down and sending back images and data. The greatest feat though was achieved by the “mothership”, Rosetta, which accompanied the comet for most of its orbit around the Sun, taking images that show details as small as a fraction of a meter. These images have allowed scientists for the first time to observe geological processes, as they happened, on the surface of a comet.

We are all familiar with the effects of water, ice, temperature excursions and wind in weathering, transporting and depositing sediments, reshaping the landscape on the Earth. We’ve seen images of craters and the effects of earthquakes and volcanic eruptions on our planet. What geological processes has ESA’s spacecraft uncovered on Comet Churyumov-Gerasimenko?

This is a tiny world with a miniscule gravity and an irregular shape (approximately 4 x 4 x 2 km), made of porous and light material rich in different ices. The agent driving most geological processes on P67 is solar radiation. As the comet orbits the Sun along a very elongated orbit, which takes it further away from our star than Jupiter and then brings it not much closer to the Sun than Mars, insolation varies dramatically and seasons become extreme. Autumn and winter last about 5.5 years in the northern hemisphere, while the southern spring and summer last nearly a year and are relatively hot, causing the icy surface to sublimate copiously, creating a coma, a tenuous atmosphere. Activity is patchy, occasionally “violent”, jets of gas burst into space taking dust and larger particles with them. Significant amounts of gas and some of the dust are lost, but part of the solid material is transported by “winds” to the northern hemisphere, blanketing it with dust and coarser grains. Over time, the material moves down gentle and steep slopes, “pushed” by the weak gravity, forming a variety of terrains. These give the northern hemisphere a very different look from that of the southern half, which is quite rugged (see figure.2). One of the most striking images shown by Ramy was one where aeolian ripples appeared on an otherwise smooth terrain in the neck of the comet, a narrow region connecting the two main “lumps” that make up 67P. No one had ever thought such features could occur on a comet.

Figure 2 -Dr. El-Maarry shows how different the northern and southern hemispheres of comet 67P appear (image credit: El-Maarry et al., 2019, https://link.springer.com/article/10.1007%2Fs11214-019-0602-1 and El-Maarry et al., 2016, https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/201628634).

The rate of ices’ sublimation is so high in some areas that several meters of materials are removed during the summer, uncovering a variety of features. Some of these are circular and resemble craters, others look like depressions that wax and wane over time. Their origin is baffling geologists, as that of other so-called transient surface features. Somewhere else on the surface of the comet new pits (probably sinkholes) appear and cliffs collapse. Outbursts of activity, driven by the Sun’s heat, propel jets that like rockets push the ground in the opposite direction, varying the speed of rotation of the comet. The resulting forces cause tension, leading to the formation of tectonic fractures, as several photos shown by Dr. El-Maarry clearly prove. Picture after picture a tiny complex word emerges, revealing to us in detail what we had been able to observe only from very far or for very brief instants, in the case of other comets.

In August 2016, Rosetta ended its mission and landed softly on Comet Churyumov-Gerasimenko. The legacy of the mission is not just what it has taught us about Comet P67, but it goes beyond. As Ramy summed up at the end of his talk, the number of images and other data collected in situ is helping scientists not only to understand 67P, but also interpret previous and future mission to other comets. A long time is likely to pass, before a probe like Rosetta will be launched.

In the meantime, Comet Interceptor, a new exciting cometary mission is taking shape at ESA. We’ll ask Ramy to bring a few thousand pictures next time…

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Selecting the landing site for 2018 ExoMars Rover

This post was contributed by Birkbeck student, Anja Lanin. Anja attended Dr Peter Grindrod’s lecture, ‘Selecting the landing site for 2018 ExoMars Rover’ during Birkbeck Science Week 2016.

Mars and a rover

Over the last two years, specialist teams in Europe have been working on helping ESA select the landing site for the first European rover on Mars. This is not just a project for PhD-holders, even individual research findings from undergraduate students contribute to the realisation of this mission. Birkbeck students and staff, including the presenter of this talk, Dr Peter Grindrod, have been closely involved. In his talk ‘Selecting the landing site for the ESA 2018 ExoMars rover’, Dr. Grindrod not only brought to light the difficulties in finding an appropriate site in unexplored regions of Mars but also emphasised the problem of balancing safety, and scientific output.

Birkbeck on Mars!

Birkbeck scientists have been directly involved in developing the PanCam stereo-camera system, which is part of the ExoMars instrument payload, led by UCL’s Mullard Space Science Laboratory. This camera will be crucial for understanding the context of rocks and samples investigated by ExoMars.

Risky goals

The ‘search for signs of past and present life on Mars’ is the main goal of this mission, according to Dr Grindrod. The one billion Euro investment will initially rest on the Russian Proton rocket, with an 89% success rate, on the scheduled May 2018 launch date. The precious 300kg solar-powered rover payload is to land on Mars about 7 months later, via parachute and retro-rockets.

Dr Peter Grindrod

Dr Peter Grindrod

Looking for life, but where?

We learn from Dr Grindrod that the rover needs to land on some of the oldest Martian rocks (>3.6 billion years). Why? Geochemical analysis using orbital-based technology indicates that rocks in some of these oldest regions are sedimentary and characterised by hydrated (clay) minerals.

We know that on Earth clay minerals form from the interaction of rocks with water that is neutral in pH and suits terrestrial life. Some of the geologically younger rocks on Mars contain sulfate minerals formed under water-poor conditions and around life-unsuitable acidic water. So we need a site, as Dr Grindrod says, that at one point had some life-friendly water flowing through it depositing soft sedimentary rocks that the rover can get to and drill into.

Access forbidden – the contamination problem

According to Dr Grindrod, the rover is prohibited from landing in what are called ‘Mars Special Regions,’ areas where any terrestrial organisms unintentionally carried by the rover may survive. Thus areas potentially containing terrestrial life-supporting liquid water at present are a no-go. So scientists have been looking for a rover touch-down location in areas where there may have been life in the past but probably not at present.

Zooming in on a landing target

Mars isn’t small, but temperature constraints rule out areas near the cold poles and the seasonally warm southern hemisphere. Equally very hilly areas are also rover unsuitable due to the steep slopes. Keeping these constraints in mind, the most promising geological outcrops scientists were left with covered just 2% of Mars’ surface area. Finally, they had to consider that the rover could land anywhere within a still considerably large ca. 104 x 19 km elliptical area, rather than a particular spot.

Watch Dr Grindrod’s lecture

The chosen few

Two of the eight landing site proposals submitted throughout Europe came from the UK and both made it into the final four.

In the end a site called Oxia Planum was chosen as the destination for the 2018 launch. The location appears to be near the end of an ancient delta-like river drainage network and there may even be different layers present containing different types of clay minerals, possibly suggesting groundwater interaction. However, one problem with this landing site is that some of the surface is covered in small wind ripples – could the small 20 cm ExoMars rover wheels get stuck here??

For the back-up launch date in 2020 two back-up sites have been selected, Mawrth Vallis and Aram Dorsum. The latter location would place the rover near deposits of one of the oldest river systems (now inverted) on Mars and any landing spot would be conveniently located no more than 100 m from a relevant deposit.

Mars, the red planetWhat’s next?

If Oxia Planum for some reason is proved to be not to be safe enough more work is needed to decide which of the back-up sites is best for the alternative 2020 launch. In the end this decision will be up to ESA and the Russian Space Agency.

Let’s remember though, as pointed out by Dr Grindrod, once the rover touches down the scientific results are for us, for every scientist involved, for everyone whose money has contributed to the mission, not just the investors but also the public.

 

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The use of extraterrestrial resources to facilitate space science and exploration

This post was contributed by Professor Ian Crawford, from Birkbeck’s Department of Earth and Planetary Sciences. It was originally posted on the Centauri Dreams blog by Paul Gilster on 10 June 2016. On 8 April, Professor Crawford organised a Royal Astronomical Society Specialist Discussion Meeting. Here, he discusses themes explored at the event.

Ian Crawford blog

(Centauri Dreams introduction)

We get to the stars one step at a time, or as the ever insightful Lao Tzu put it long ago, ”You accomplish the great task by a series of small acts.” Right now, of course, many of the necessary ‘acts’ seem anything but small, but as Ian Crawford explains below, they’re a necessary part of building up the kind of space economy that will result in a true infrastructure, one that can sustain the exploration of space at the outskirts of our own system and beyond. Dr. Crawford is Professor of Planetary Science and Astrobiology in the Department of Earth and Planetary Sciences, Birkbeck College, University of London. Today he brings us a report on a discussion of these matters at the Royal Astronomical Society earlier this year.

There is increasing interest in the possibility of using the energy and material resources of the solar system to build a space economy, and in recent years a number of private companies have been established with the stated aim of developing extraterrestrial resources with this aim in mind (see, for example, the websites ofPlanetary Resources, Deep Space Industries, Shackleton Energy, andMoon Express). Although many aspects of this economic activity will likely be pursued for purely commercial reasons (e.g. space tourism, and the mining of the Moon and asteroids for economically valuable materials), science will nevertheless be a major beneficiary.

The potential scientific benefits of utilising space resources were considered at a Specialist Discussion Meeting organised by the UK’s Royal Astronomical Society on 8 April. This meeting, which was attended by over 60 participants, demonstrated widespread interest in the potential scientific benefits of space resource utilisation. A report of the meeting has now been accepted for publication in the RAS journalAstronomy & Geophysics and videos of the talks are available on the RAS website.

The participants agreed that multiple (and non-mutually exclusive) scientific benefits will result from the development of a space economy, including:

  • Scientific discoveries made during prospecting for, and extraction of, space resources;
  • Using space resources to build, provision and maintain scientific instruments and outposts (i.e. in situ resource utilisation, or ISRU);
  • Leveraging economic wealth generated by commercial space activities to help pay for space science activities (e.g. by taxing profits from asteroid mining, space tourism, etc);
  • Scientific utilisation of the transportation and other infrastructure developed to support commercial space activities.

Specific examples of scientific activities that would be facilitated by the development of a space economy include the construction of large space telescopes to study planets orbiting other stars, ambitious space missions (including human missions) to the outer Solar System, and the establishment of scientific research stations on the Moon and Mars (and perhaps elsewhere).

In the more distant future, and of special interest to readers of Centauri Dreams, an important scientific application of a well-developed space infrastructure may be the construction of interstellar space probes for the exploration of planets around nearby stars. The history of planetary exploration clearly shows that in situ investigations by space probes are required if we are to learn about the interior structures, geological evolution, and possible habitability of the planets in our own solar system, and so it seems clear that spacecraft will eventually be needed for the investigation of other planetary systems as well.

Crawford-300x252

Professor Ian Crawford

For example, if future astronomical observations from the solar system (perhaps using large space telescopes themselves built and paid for using space resources) find evidence suggesting that life might exist on a planet orbiting a nearby star, in situmeasurements will probably be required to get definitive proof of its existence and to learn more about its underlying biochemistry, ecology, and evolutionary history. This in turn will eventually require transporting sophisticated scientific instruments across interstellar space.

However, the scale of such an undertaking should not be underestimated. Although very low-mass laser-pushed nano-craft, such as are being considered by Project Starshot, could conceivably be launched directly from Earth, the scientific capabilities of such small payloads will surely be very limited. Initiatives like Starshot will certainly help to develop useful technology that will enable more capable interstellar missions later on, and are therefore greatly to be welcomed, but ultimately much more massive interstellar payloads will be required if detailed scientific studies of nearby exoplanet systems are to be conducted.

Even allowing for future progress in miniaturisation, a scientifically useful interstellar payload will probably need to have a mass of at least several tonnes, and perhaps much more (as I have discussed in this recent paper in the Journal of the British Interplanetary Society). Moreover, in order to get this to even the nearest stars within a scientifically useful timescale (say ≤100 years) then spacecraft velocities of order 10% of the speed of light will be required. This will likely require vehicles of such a size, with such highly energetic (and thus potentially dangerous) propulsion systems that their construction and launch will surely have to take place in space.

The potential long-term scientific benefits of an interstellar spacefaring capability are hard to exaggerate, but it seems certain that it is a capability that will only become possible in the context of a well-developed space economy with access to the material and energy resources of our own solar system.

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Lost planetary worlds: Evidence of the unquiet early Solar System

This post was contributed by Birkbeck student, Anja Lanin. Anja attended Professor Hilary Downes’ lecture, ‘Lost Planetary Worlds’ during Birkbeck Science Week 2016.

Solar System

Professor Hilary Downes has been a research scientist in the Department of Earth and Planetary Sciences at Birkbeck for 30 years. Insights from her own and other workers’ research have left her with a strong interest in the evolution of the Solar System. As it turns out, the orderly Solar System we observe today in fact started out as everything but quiet and orderly. In its early days it was a place of violent collisions between planetary bodies. Many of these have been almost completely lost. Almost! We have evidence of their existence, ranging from the macroscopic to the elemental, and this was the subject of Professor Downes’ enthusiastic talk ‘Lost Worlds of the Solar System.’

Theory: Computer models

Starting her talk by showing a real image of a planet-forming region around stars, as observed by telescope, as well as computer models which together may suggest the organised formation of planets within an accretionary disk, Professor Downes moved on to theoretical considerations of a very different-looking chaotic early Solar System.

Computer simulations of Jupiter’s growth, for example, indicate that many planetary embryos were sent onto wildly eccentric orbits. Other models show planets such as Jupiter and Saturn moving repeatedly closer and then away from the sun causing gravitational chaos in the inner Solar System before the system became more settled.

Evidence from our Solar System planets – shaken and stirred!

The audience were then presented with some very odd and interesting facts about our planets. For one thing, they do not orbit the sun in the original plane (the location of the previous accretionary disk) of the Solar System. Some seem to defy the laws of physics by floating above and some below. Furthermore, some of the planets’ axial tilts have gone ‘wonky.’

While Jupiter and Mercury spin textbook-style perpendicular to the plane, all other planets have been knocked around to the extreme that Uranus has been completely knocked over and is now spinning parallel to the plane. Venus is even more special – it is rotating in the opposite direction to all other planets! These characteristics, according to Professor Downes, strongly suggest violent collisions of the planets with other planetary material.

‘Tangible’ evidence: meteorites within meteorites

So what happened to the impactors? We can actually study collisional space debris which comes to us in the form of meteorites. For many of these meteorites the parent body, for example a planet or an asteroid, is known, but, as Professor Downes emphasises, there are many parentless ungrouped meteorites. Perhaps the most interesting of these are brecciated meteorites which contain fragments of other meteorites. What do we learn from these fragments?

 

Real science reveals real mysteries….

As indicated in the talk, planetary science students at Birkbeck are actively accessing technology (e.g. electron microprobe) that allows them to study the mineralogy and basic chemical make-up of meteorites. This is one way that allows them to determine whether or not meteoritic material comes from a classified or unclassified parent body.

Something that cannot be analysed at Birkbeck yet!, but also yields very important clues, are oxygen isotope ratios. Each known planetary body has a unique oxygen fingerprint, so that previously unregistered ratios hint at lost parent bodies. Professor Downes, relating to her own group’s research, points out a particularly interesting brecciated meteorite fragment, which, surprisingly, turned out to be granitic, i.e. it is mineralogically and texturally similar to granites found on Earth (some of us recognise the rock from kitchen counter tops!).

However! – its oxygen chemistry indicates that it comes neither from Earth nor is it related to the other meteoritic material in which it was included as a fragment. It is therefore not related to the asteroid from which the rest of the meteorite is derived. In addition, a strange associated glass is high in sulfur (S) and chlorine (Cl), and no planet in the Solar System except Mars contains sulfur and chlorine. But the oxygen chemistry again suggests it is not from Mars. Thus, this glass may represent another lost planetary body or planet possibly disintegrated during the early collisional chaos!

There are many examples of odd, unexplained finds in meteorites. Even opal, which we recognise as a semi-precious stone, has been found by Profesor Downes and her colleagues, although its extraterrestrial origin is still unclear. Perhaps a water or ice-rich meteorite crashed into an asteroid and all that is left of this ice or water world is this little piece of opal?

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