Potential landing sites on the comet nucleus

The lander Philae, currently attached to the European Space Agency spacecraft Rosetta, will make its descent towards the surface of the nucleus of Comet 67P/Churyumov-Gerasimenko in early November. Five potential landing sites have now been selected, based on information collected during the first two weeks of observation from a distance of roughly 100 kilometers. This selection is based on flight dynamics constraints (both for orbiter and lander), communication opportunities (between Rosetta and Philae), illumination conditions (battery charging, overheating), surface characteristics (we do not want to land in pits, on boulders, on steep slopes or rough terrain), and last but not least – scientific relevance.


 

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Overview of the nucleus of Comet 67P/Churyumov-Gerasimenko and location of the five potential landing sites. The nucleus is about 4 kilometers across. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


The potential landing sites have been labeled A, B, C, I, and J. Two of the sites, A and C, are located on the larger of the two lobes. The other three, B, I and J, are located on the smaller lobe. The labeling does not reflect any particular order of importance at this stage. The task now is to study the five sites in more detail, as Rosetta gradually approaches to within 30 kilometers of the nucleus. A decision on a primary landing site, as well as a backup, will be made on September 14. Whether to go for the primary or the secondary site will be determined whilst moving to within 20 kilometers of the nucleus, and a final decision will be made on October 14, roughly a month before the actual landing.


 

As a comet scientist, deeply involved in the Rosetta mission, this is a time of adventure, fascination, and the sense of discovery of something fundamentally important about our Solar System – but it also means long working hours, and not too much sleep. Which is why I have not been able to update this blog as often as I would like to – but with images like these, who can complain!


Rosetta_OSIRIS_NAC_comet_67P_20140816_SiteA

Site A is located on the larger lobe, with a good view of the smaller lobe. The terrain between the two lobes is likely the source of some outgassing. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


Rosetta_OSIRIS_NAC_comet_67P_20140816_SiteB

Site B, within the crater-like structure on the smaller lobe, has a flat terrain and is thus considered relatively safe for landing. However, boulders and illumination conditions may pose a problem. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


 

Rosetta_OSIRIS_NAC_comet_67P_20140816_SiteC

Site C is located on the larger lobe and is well illuminated but rich in surface features that potentially can make a landing risky. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


Rosetta_OSIRIS_NAC_comet_67P_20140816_SiteI

Site I is a relatively flat area on the smaller lobe, but higher-resolution imaging is needed to assess the extent of the rough terrain. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


 

Rosetta_OSIRIS_NAC_comet_67P_20140816_SiteJ

Site J is similar to site I, and also on the smaller lobe, offering interesting surface features and good illumination. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


The scientific imaging system OSIRIS was built by a consortium led by the Max Planck Institute for Solar System Research (Germany) in collaboration with CISAS, University of Padova (Italy), the Laboratoire d’Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Spain), the Scientific Support Office of the European Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), the Universidad Politéchnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institute of Computer and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially supported by the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.

Rosetta’s comet in 3D!

The colorful image of Comet 67P/Churyumov-Gerasimenko is an anaglyph – by looking at it through glasses with a red and a green filter, a three-dimensional image is seen. This is a good way to get a feel for how irregular the terrain actually is.


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The two images used to create this anaglyph were taken by our camera OSIRIS on ESA’s spacecraft Rosetta on August 7, 2014. The images were taken 17 minutes apart to change the viewing geometry, through Rosetta’s motion and the nucleus rotation, which is necessary to create the 3D sensation. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


For those who do not have such glasses – enjoy one of the original images below.


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An original image from OSIRIS used to create the anaglyph above. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


The scientific imaging system OSIRIS was built by a consortium led by the Max Planck Institute for Solar System Research (Germany) in collaboration with CISAS, University of Padova (Italy), the Laboratoire d’Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Spain), the Scientific Support Office of the European Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), the Universidad Politéchnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institute of Computer and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially supported by the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.

 

Meet the heart of Comet 67P/Churyumov-Gerasimenko!

Our new images from the camera system OSIRIS on ESA’s spacecraft Rosetta shows that Comet 67P/Churyumov-Gerasimenko has a spectacularly shaped nucleus! The nucleus consists of two large pieces with different shape, connected at a small contact surface.


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A sequence of 36 processed images of Comet 67P/Churyumov-Gerasimenko taken 20 minutes apart on July 14, 2014, from a distance of about 12,000 kilometers. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


 

One should not look too closely for details in a movie like this. The reasons is clear once we look at one of the original images below. The camera has a limited resolution and the original image consists of a number of squares, or “pixels”, that each have recorded a certain light intensity. We do not know how the nucleus looks like within each pixel – but we can guess!


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An original picture from OSIRIS before image processing. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


 

During so-called image processing, mathematical algorithms are used in an attempt to re-create how an object really looks like, before it got smeared out by the pixels. Such algorithms are good at re-creating lost information, but they are not perfect. Real surface structures may have been lost completely, while false features that do not exist in reality may have been added.


Abbildung 1 b

A processed image from OSIRIS. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


 

The only way to find out how the comet surface really looks like is to get closer – and in a short while Rosetta will be much closer to the comet!


The scientific imaging system OSIRIS was built by a consortium led by the Max Planck Institute for Solar System Research (Germany) in collaboration with CISAS, University of Padova (Italy), the Laboratoire d’Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Spain), the Scientific Support Office of the European Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), the Universidad Politéchnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institute of Computer and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially supported by the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.

OSIRIS: First glimpse of the comet nucleus!

Up to just a few days ago, the target of ESA’s Rosetta mission, Comet 67P/Churyumov-Gerasimenko, was just a dot in the sky, barely distinguishing itself from the stars by displaying a small temporary dust coma. But now Rosetta is getting so close to the comet, 40 000 kilometers – about a tenth of the distance between Earth and the Moon – that the OSIRIS camera starts to resolve the nucleus. The comet nucleus is still just a couple of camera pixels across, but as seen in the movie below, there is a hint of nucleus irregularity. The nucleus size and shape changes slightly, while it is rotating with its 12.4 hour period. From now on, the comet nucleus will just grow in size until it fills the entire field of view of the Narrow Angle Camera (NAC) in mid August. Stay tuned for more cool pictures from OSIRIS!


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First resolved images of comet 67P/Churyumov-Gerasimenko show the nucleus rotating with a rotation period of 12.4 hours. This set of 36 images was obtained by OSIRIS’ narrow angle camera (NAC) on June 27th and June 28th and covers one such period. © ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA


The scientific imaging system OSIRIS was built by a consortium led by the Max Planck Institute for Solar System Research (Germany) in collaboration with CISAS, University of Padova (Italy), the Laboratoire d’Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Spain), the Scientific Support Office of the European Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), the Universidad Politéchnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institute of Computer and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially supported by the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.

 

Organics in space

This article was originally posted on the blog Dinner Table Science were I was guest blogging – please visit and follow Rachel’s blog, it is great!


Life, as we know it, would not have been possible without the carbon atom. The scientific discipline dealing with all the molecular relationships that the carbon atom is getting itself into – organic chemistry – is essential for understanding how living organisms function and evolve. We take it for granted that organic chemistry is flourishing on Earth, because it is teeming of life, but only in the last few decades have we come to realize that such processes also are common far beyond our own planet – in the depths of space. No form of extraterrestrial life has been discovered so far, but a surprisingly rich variety of organic compounds has been found within the Solar System, as well as in the interstellar medium. None of these organic molecules have a biological origin but some may, or may not, have been prebiotic, i.e., involved in the processes that eventually led to the emergence of life on our planet.


Organic molecules are found in many different Solar System objects or space environments, but here we will focus on two of them – a type of meteorite called a carbonaceous chondrite, and the interstellar medium. After reviewing what we know about these organic substances, we will also discuss the processes believed to have created them.

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The Murchison meteorite is a carbonaceous chondrite. It is one of our most important sources of information on organics from space due to its large mass (more than 100 kg) and the fact that it was recovered right after it fell, and has suffered a minimum of terrestrial contamination. This picture shows a piece of Murchison at the The National Museum of Natural History (Washington). Original image: http://en.wikipedia.org/wiki/File:Murchison_crop.jpg

 


Carbonaceous chondrites

Most of the meteorites that impact Earth, almost 90%, are so-called chondrites. They got their name because they are rich in chondrules, a type of millimeter-sized grain formed in huge numbers 4.57 billion years ago during the earliest phase of Solar System history. They still contain these ancient particles because the chondritic meteorite parent bodies never melted (these 10-100 km parent bodies later broke up in devastating collisions, and the chondritic meteorites are tiny fragments from such collisions). This distinguishes the chondritic meteorites from achondritic meteorites (8% of the falls) and iron meteorites (2% of the falls) that both originate from parent bodies that once were heated by radioactive decay to the point that they actually melted and differentiated, e.g. separated into a rocky mantle and a metallic core. Such melting erased all memory of previous history, and that is why chondritic meteorites are so valuable – they can tell us about the time before the formation of the parent bodies (most of which eventually were merging to form the planets).


Most chondritic meteorites are so-called ordinary chondrites that are related to the rocky S-type asteroids in the inner main asteroid belt. This region was generally too warm to allow molecular compounds with a low boiling point to condense into solids. However, some of the meteorites are carbonaceous chondrites, believed to be related to the more distant C-type asteroids from the outer main asteroid belt. They formed in conditions sufficiently cold to allow condensation of very volatile species – they are therefore rich in water (up to 20% by mass) and they also contain organic molecules that are rare or absent in the inner Solar System.


The carbonaceous chondrites got their name because they are unusually dark – they look like charcoal. The name is actually misleading, because the dark color is not due to carbon, but due to iron, an atom that is very efficient in absorbing light. In ordinary chondrites, the iron is gathered into small metallic particles mixed with the chondrules (we say that the iron is reduced), leaving the rocky material virtually iron-free and thus fairly bright in color. In carbonaceous chondrites, the iron is finely distributed throughout the rocky material on an atomic level (we say that the iron is oxidized), which makes the entire rock an efficient light absorber, thus being dark.


The typical carbonaceous chondrite contains only about 2-5% carbon by mass. However, this organic material is amazing, particularly the 25% that is referred to as “extractable organic matter” that is either liquid or solid at room temperature. In decreasing order of abundance the extractable organic matter consists of the following cocktail, of which a selection is described below – carboxylic acids, sulfonic acids, amino acids, sugars, urea, aliphatic and aromatic hydrocarbons, ketones, ammonia, alcohols, purines and pyrimidines. The remaining 75% is called “macromolecular material” and is a solid substance with an extremely complex composition. In some cases, the organics in meteorites is partially terrestrial contaminations. However, the risk of contamination is low if meteorites are recovered directly after a fall, and in most cases the meteoritic organics have such unusual concentrations of the isotopes carbon-13, nitrogen-15 and hydrogen-2 (deuterium), that a terrestrial origin can be excluded. Thus, the compounds discussed below are definitively from space.


In order to understand how the macromolecular material looks like, we need to know how carbon atoms interact with their own kind and with other atoms. A carbon atom can bind itself to up to four other atoms simultaneously. Often, carbon atoms form linear chains of various length. A given carbon atom within the chain then connects with two of its carbon neighbors, meaning that it can afford to connect also with two other atoms, typically hydrogen (but sometimes oxygen, nitrogen or other atoms). The carbon atom sitting at the end of a chain only connects with one other carbon, allowing it to attach three hydrogen atoms to it. These chain-like structures are called aliphatic hydrocarbons. However, it is also common that six (and sometimes five) carbon atoms form a ring. Here, a given carbon atom uses two of its connections to bind to the first of its carbon neighbors, and one for the second, leaving space for a single foreign atom (often hydrogen) to attach itself externally to the ring. Such ring-like structures are called aromatic hydrocarbons. Rings can also attach to each other, so that two carbon atoms simultaneously are members of two rings. Molecules that contain several such rings are called polycyclic aromatic hydrocarbons (or PAHs).


The macromolecular material in carbonaceous chondrites has proven to be a gigantic web, where aromatic rings use aliphatic chains to connect to each other. Often there are single rings that have replaced one or several of their external hydrogen atoms with an aliphatic chain – typically with 2-4 carbon atom links – in order to connect to another aromatic ring located farther away. However, it is not unusual that two, three or four rings form a little aromatic island, that connects itself to other islands through the aliphatic bridges. The fraction of macromolecular carbon that is aromatic is about 60-70% for Murchison, about 70-80% for Orgueil and almost 100% for Tagish Lake (these are three different carbonaceous meteorites). A typical elemental composition of the macromolecular material is about 70 hydrogen atoms (H), 12 oxygen atoms (O), three nitrogen atoms (N) and two sulphur atoms (S), for every 100 carbon atoms (C).


The extractable organic matter is dominated by carboxylic acids – these are basically aliphatic chains where one carbon atom at the end of the chain has replaced two of its hydrogen atoms with a single oxygen atom, and replaced the last hydrogen atom with a hydroxyle group consisting of an oxygen atom with a hydrogen attached to it (thus there is a COOH group). If both ends of the chain has COOH groups, the molecule is called a dicarboxylic acid. Examples include formic acid (HCOOH) used by ants as a venom, acetic acid (CH3COOH) used in cooking in diluted from under the name vinegar, butyric acid (C3H7COOH) that gives rancid butter its unpleasant smell, and valeric acid (C4H9COOH) that is named after the valerian herb (Valeriana officinalis) that produces this molecule. They are all found in carbonaceous meteorites, that often contain carboxylic acids with up to ten carbon atoms.


If a carboxylic acid has one of its hydrogen atoms in the aliphatic chain replaced by the amino group NH2, it is called an amino acid. Amino acids are fundamental to life, because they are the building blocks of proteins. Humans and other animals need to eat proteins, and the body break them down into their amino acid constituents, that are then used by cells to build other proteins (a process called translation) that we need to function. To find amino acids in meteorites is extremely fascinating, just because they play such a central role in the chemistry of living organisms.


More than 70 different amino acids have been identified in carbonaceous meteorites. Some of these meteorites, like Murchison, Orgueil and Ivuna are rather rich in amino acids. Others, like Tagish Lake, have extremely low abundances of amino acids. Murchison contains eight of the protein amino acids, eleven that are biologically common, and several others that are not used by terrestrial organisms. The five most common, in decreasing order of abundance, is glycine (NH2[CH2]COOH), alpha-aminoisobutyric acid (NH2[C3H6]COOH), D-alanine and beta-alanine (NH2[C2H4]COOH), and isovaline (NH2[C4H8]COOH), where the chemical formulae have been written to highlight the amino and carboxyl groups. Here, the prefixes “alpha” and “beta” is a way to tell which carbon atom the amino group is attached to.


Most amino acids have so-called chirality, which means that there are two variants of each molecule (called enantiomers), that have the same chemical composition but geometrically are mirror images of each other. They are distinguished through the prefixes L and D such as in L-alanine and D-alanine. All proteins built by translation contain L enantiomers. In carbonaceous chondrites both enantiomers appear to be equally common, although some controversial studies show that there may be a slight L-excess for certain amino acids like alanine, proline and leucine.


Aromatic hydrocarbons are not only found in the macromolecular material, but also in the extractable organic material. The most common compounds are the PAHs fluoranthene and pyrene. They both have the formula C16H10 but are structurally different – flouranthene consists of three standard rings with six carbon atoms, joined by a ring containing just five carbon atoms, while pyrene is made of four standard rings.

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The structural formula of pyrimidine, showing how carbon (C) and nitrogen (N) atoms form a ring, to which a number of hydrogen (H) atoms are attached. Original image: http://en.wikipedia.org/wiki/File:Pyrimidine_2D_aromatic_full.svg

 

The really interesting thing starts when the carbon atoms in aromatic rings are replaced by nitrogen. If the starting point is a single six-atom ring (called benzene if all atoms are carbon), and two specific carbon atoms are exchanged with nitrogen, a substance called pyrimidine (C4H4N2) is formed. By replacing hydrogen atoms by amino groups or oxygen atoms, a variety of molecules can be formed, including the nucleobases cytosine (C), thymine (T) and uracil (U), that are basic building blocks in DNA and RNA. The pyrimidine uracil has been found in carbonaceous chondrites.

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The structural formula of uracil. The aromatic ring is here somewhat abstract since corners are meant to indicate the location of a carbon atom (sometimes with a hydrogen atom attached to it), while only nitrogen and oxygen atoms, with associated hydrogen atoms, are shown explicitly. Original image: http://en.wikipedia.org/wiki/File:Uracil.svg

 

If the starting point is a six-atom ring joined to a five-atom ring, and two specific carbon atoms in each ring are replaced with nitrogen, a compound called purine (C5H4N4) is obtained. As before, the replacement of hydrogen atoms with amino groups and oxygen atoms gives rise to a variety of molecules, including two other nucleobases that are found in DNA – adenine (A) and guanine (G). Both adenine and guanine has been found in carbonaceous chondrites, along with other purines, such as xanthine and hypoxanthine.

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The structural formula of various kinds of purines. Adenine, guanine, hypoxanthine and xanthine have been found in carbonaceous chondrites. Original image: http://en.wikipedia.org/wiki/File:Purines.svg

 

It is therefore clear that carbonaceous chondrites contain a variety of fairly complex organic molecules. Comet nuclei are likely to be as rich or even richer in such compounds, although we know much less about comets than meteorites since actual samples of comet material is restricted to very small amounts, collected by the Stardust spacecraft in the coma of Comet 81P/Wild 2 and brought back to Earth. Both carbonaceous chondrites and comets bombarded the young Earth, thereby bringing organic substances to the inner region of the Solar System, where such compounds initially may have been rare. The possibility that these organics were involved in the formation of life on Earth is a fascinating thought. But what is the origin of these compounds – where did they form and how? To answer that question, we must leave the Solar System and head out into interstellar space.


The interstellar medium

The space between the stars in our galaxy, the Milky Way, is not empty – about 90% of the galactic mass is bound in stars, while the rest forms an extremely thin mixture of gas and solid dust particles that fill the space between the stars – the interstellar medium. About 98-99% of the mass is hydrogen and helium, while the remaining fraction is shared between all heavier elements. Among these, oxygen is the most common element by number, followed by carbon, neon, and nitrogen. These elements mainly exist as unbound atoms in the gas phase, while the solid grains primarily consist of silicates and sulfides rich in oxygen, magnesium, silicon, iron and sulphur (these are the ten most common chemical elements in the universe, by number).


In places where the interstellar medium is particularly dense – the molecular clouds – atoms in the gas phase form small molecules, primarily molecular hydrogen (H2), carbon monoxide (CO), and molecular nitrogen (N2), but also water (H2O), carbon dioxide (CO2), ammonia (NH3), methane (CH4), and methanol (CH3OH). Over time, some of these gases will condense on top of the grains, thus forming mantles of ice that surround the rocky cores.


When such ice is exposed to ultraviolet radiation from nearby stars, it gets damaged. Molecules are cut into small pieces called radicals. These are extremely reactive, but due to the extreme cold (typically 10K or -260C) the radicals have little mobility and do not manage to get in physical contact with each other. Over time, large deposits of radicals are built up within the ice. Only a small amount of heating, perhaps due to rare collisions between icy grains, is sufficient to trigger an explosive chain reaction, were radicals unite to form a variety of complex organic molecules. Many of these leave the grain surfaces and can be observed as free molecules in the interstellar gas, while others remain on the grain surfaces.


This process is at least partially responsible for the rich variety of organic molecules that has been observed in interstellar space. Observations are made with radio telescopes, that pick up the long-wavelength radiation that the molecules emit when they change their rotation or vibration rates. More than 150 molecular compounds have been identified in the interstellar medium, of which a third contain six atoms or more. Some large molecules that have been identified include propylene (CH3CHCH2), methyltriacetylene (CH3C6H), vinyl alcohol (C2H3OH), acetic acid (CH3COOH), ethylene glycol (HOCH2CH2OH), cyanopentaacetylene (HC11N), and acetamide (CH3CONH2).


The processes taking place in the interstellar medium can be reproduced in laboratories. A mixture of ice (for example, water, carbon monoxide, carbon dioxide, ammonia, and methanol in proportions expected in interstellar ice) is deposited on a 10K substrate, and the mixture is irradiated by ultraviolet radiation and then slowly heated to trigger the chain reaction. An organic residue is thus formed, sometimes called “yellow stuff”. The material is particularly rich in carboxylic acids and hexamethylenetetramine (C6H12N4). In one particular experiment, no less than 16 amino acids where identified in this yellow stuff. These included glycine, alanine, sarcosine, valine and serine. In these samples both enantiomers of each amino acid were equally common, to within measurement uncertainties.


Stellar and planetary systems form when interstellar gas and dust collapse due to its self-gravity, in regions where the interstellar medium has become exceptionally dense. According to the “interstellar parent-body hypothesis” the organic compounds seen in carbonaceous chondrites are interstellar organics that survived the turmoil of Solar System formation. That is to say, these substances did not primarily form here, and at least a fraction of the material must have avoided heating to the point where they would have disintegrated into their atomic constituents. The material may have been processed or altered in various ways, but the key idea is that the carbonaceous chondrite parent bodies contained complex organics already when they formed. The very unusual isotopic composition that characterize these organics are often interpreted as evidence of an interstellar origin.


Although the carbonaceous chondrite parent bodies formed sufficiently late not to melt (perhaps 1-2 million years after differentiated bodies, at a time when the short-lived radioactive heat source aluminum-26 almost had vanished), they still experienced mild warming. The temperature was sufficiently high to melt ice within the parent body, and allowing liquid water to percolate through the granular interior. This has led to various levels of so-called aqueous alteration – characteristic changes of the mineralogical composition of the meteorite caused by liquid water. The presence of liquid water has also modified the composition of the organics. It appears increasingly unlikely that the organics we see today in carbonaceous chondrites were formed from scratch during aqueous alteration – they must have had a long and complex previous evolutionary history that started in interstellar space.


The exploration of organics in space has merely begun. The presence of complex organic molecules in distant asteroids and comet nuclei are strong reasons for performing sample return missions to such bodies. The answers to some of our most profound questions about our existence may lay buried within these ancient survivors of planetary formation – how did the Solar System form, what kind of material rained down on the young Earth, and is it possible that life got a jump start thanks to organics from space?

We just have to go and have a look.


Literature

Gilmour, I. (2003). Structural and isotopic analysis of organic matter in carbonaceous chondrites. Treatise on Geochemistry, 1, 269-290.

Herbst, E., van Dishoeck, E. F. (2009). Complex organic interstellar molecules. Annual Review of Astronomy and Astrophysics, 47, 427-480.

Muñoz Caro, G. M., Meierhenrich, U. J., Schutte, W. A., Barbier, B., Arcones Segovia, A., Rosenbauer, H., Thiemann, W. H.-P., Brack, A., Greenberg, J. M. (2002). Amino acids from ultraviolet irradiation of interstellar ice analogues. Nature 416, 403-406.

Comet 67P/Churyumov-Gerasimenko wakes up as ESA’s Rosetta spacecraft approaches!

I was at the OSIRIS Full Team meeting held at the Max Planck Institut für Sonnensystemforschung in Göttingen, Germany, last week. We had a great meeting, and the good news are piling up – the spacecraft Rosetta performs well, our imaging camera system OSIRIS is fully operational (as are all the other instruments), orbit manoeuvres are successfully executed to enable Rosetta to rendezvous with the comet in early August, and we have already started to do science.

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Between March 24th and May 4th, Rosetta approached comet 67P/Churyumov-Gerasimenko from a distance of around 5 million to 2 million kilometers. This sequence of images shows the comet’s movement against the background star field during this time. Rosetta (and the comet) are between 640 and 610 million km from the Sun. The comet is seen to develop a dust coma as the sequence progresses, with clear activity already visible in late-April. Exposure times are 720s for each image, taken with the OSIRIS/NAC through the Orange filter. credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

 

First of all, we have detected the nucleus of Comet 67P/Churyumov-Gerasimenko and are tracking its motion. Secondly, the lightcurve is being monitored regularly, which has allowed us to measure a 12.4 hour rotation period of the nucleus. The lightcurve is a periodic variation in the observed brightness of the nucleus. The variations arise since the nucleus is not spherical but irregular, so that the amount of solar light that is reflected by the nucleus towards the spacecraft is changing with time as the nucleus rotates. The third discovery is that the comet nucleus – which was dormant and quiet at our first observations in late March – now has become active.

Comet activity means that the ice in the nucleus surface layers has become heated sufficiently by sunlight to sublimate, i.e., turn directly to vapor without first becoming liquid. At these distances, at the time of writing 4.03 AU from the Sun, the temperature is too low to allow water ice to sublimate. Instead, more volatile substances like carbon monoxide and carbon dioxide are responsible for the activity. OSIRIS do not see these gases directly. However, the sublimation also liberates a large amount of micrometer-sized dust grains that are entrained in the gas as it rushed into space. OSIRIS detects the solar light that is reflected by this dusty coma, that currently measures about 2600 kilometers across.

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The OSIRIS Team. Yours truly is marked with the arrow. Credits: MPS

OSIRIS on Rosetta has imaged Comet 67P/Churyumov-Gerasimenko!

As a member of the OSIRIS Science Team I am happy to announce that our camera OSIRIS, that flies on ESA’s spacecraft Rosetta, now has imaged the target of its ten year long journey – Comet 67P/Churyumov-Gerasimenko!

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Comet 67P/Churyumov-Gerasimenko in constellation Ophiuchus. This image taken with the Wide Angle Camera on March 20 shows a wide field 25 times larger than the diameter of the full moon. The color composite shows a background of hydrogen gas and dust clouds in the constellation Ophiuchus. The white box indicates the position of the close-up taken with the Narrow Angle Camera (below). The images were taken from about 0.03 AU distance to the comet. Rosetta was at a distance of approx. 4.4 AU from Earth. Image credit: ESA ©2014 MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Rosetta was launched in March 2004. The purpose of the spacecraft is to explore, in situ, what happens to a comet nucleus when it approaches the Sun from a very large distance, gradually is heated and therefore becomes active. Therefore, Rosetta first had to get very far out in the Solar System. The spacecraft swung by Earth three times, and Mars on one occasion, so that the gravitational perturbations from these planets gradually could make Rosetta’s orbit around the Sun wider. On its way, the spacecraft also passed near to two asteroids – (2867) Steins in September 2008 and (21) Lutetia in July 2010. I June the following year, Rosetta had come so far from the Sun that its solar panels no longer managed to generate the electric power necessary to keep the entire spacecraft up and running. Therefore, Rosetta was put in hibernation and all available power was used to heat the instruments to prevent them from break by freezing. The ground control had no contact with Rosetta at all.

In October 2012 Rosetta was farthest from the Sun, no less than 5.3 AU (one astronomical unit, 1 AU, is the mean distance between Sun and Earth, and corresponds to 150 million kilometers). It means that Rosetta was beyond the orbit of Jupiter, that is located 5.2 AU from the Sun. Two and a half years after Rosetta entered hibernation, on January 20, 2014 to be precise, it was time for the spacecraft to wake up. It was an enormous relief when the signals from Rosetta reached the ground control! After the wake-up, careful checks were made to make sure Rosetta was feeling well after its long sleep. We are now at a stage where the scientific instruments are switched on one by one, to see how they have coped with the hibernation. OSIRIS was switched on last week, and has now taken its first images of the comet – the camera works beautifully! We will therefore be ready when Rosetta reaches the comet in August this year, at a distance of about 4.5 AU from the Sun.

OSIRIS WAC

OSIRIS is the camera system on Rosetta. It actually consists of two different telescopes. One of them is called the Wide Angle Camera (WAC) and has a rather large field of view since it will be used to image the comet coma, the cloud of gas and dust that the comet nucleus surrounds itself with (see a previous post on comets). The camera has 14 different filters – glass plates with a special composition and surface coating that makes them transparent to light only at specific wavelengths. These filters are manufactured in Sweden and is the Swedish hardware contribution to OSIRIS. Seven of these filters are so-called narrowband filters – they are transparent only at very strict wavelength regions corresponding to the wavelength were seven different molecular fragments (radicals) emit light when they are illuminated by the Sun. These radicals are CS (a compound consisting of carbon and sulphur), the hydroxyle radical OH and the oxygen atom O (these are formed when the ultraviolet light of the Sun break down water molecules), NH and NH2 (compounds of nitrogen and hydrogen), CN (the cyano radical, consisting of carbon and nitrogen), and the sodium atom (Na), that can be outgassed by dust grains that are strongly heated by sunlight.

The dust grains in the comet coma will reflect sunlight, and some of this light will find its way through the narrowband filters. This is not good, since we will use the intensity of the light to calculate the abundances of radicals and atoms in the coma. Since the dust grains contribute with light, that does not originate from within the gas at all, the risk is that we overestimate the abundance of gas. Therefore, the WAC also has seven filters that is transparent to light just next to the wavelength regions of the narrowband filters. In this way, the contribution of the dust grains to the measured light can be estimated, and compensated for when determining the gas abundance. Four of these filters are transparent in the ultraviolet wavelength region (for example, a filter called UV375), while the others are located in the green, yellow and red wavelength regions.

The image above is really three different WAC images, taken through different filters. The red filter was used during an exposure that lasted one minute. The green filter was used during an equally long exposure. Finally, the UV375 filter was used three times with a total exposure time of nine minutes. By combining these images, the color photo above could be constructed.

OSIRIS NAC

The second camera is called the Narrow Angle Camera (NAC). It has a smaller field of view than the WAC, but is capable of resolving objects that are five times smaller than the ones the WAC manages to resolve. This camera will primarily be used to study the comet nucleus. This camera also has Swedish filters, but with quite different properties – a mixture of broadband filters in different parts of the visible wavelength region to make a rough characterization of the comet spectrum, and a number of filters that will be used to search for specific minerals, like pyroxene, hematite and hydrated silicates.

The figure below shows a picture taken with the NAC, and corresponds to the white square in the picture above. The strongly magnified picture shows a globular cluster called Messier 107 (or M107), as well as the comet nucleus within the small circle. It is still far too distant to be seen in detail, and is only a dot in the sky. But day by day Rosetta is closing in on the comet and soon we will be able to see how it looks like up close!

 

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Comet 67P/Churyumov-Gerasimenko in constellation Ophiuchus. A zoom into an image taken with the Narrow Angle Camera on March 21. The comet is indicated by the small circle, next to the bright globular star cluster M107. The images were taken from about 0.03 AU distance to the comet. Rosetta was at a distance of approx. 4.4 AU from Earth. Image credit: ESA ©2014 MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

For ESAs press release click here.