RESEARCH NEWS & VIEWS
Sink holes and dust jets on comet 67P Analyses of images taken by the Rosetta spacecraft reveal the complex landscape of a comet in rich detail. Close-up views of the surface indicate that some dust jets are being emitted from active pits undergoing sublimation. See Letter p.63 PA U L W E I S S M A N
W
hen do 18 holes not make for a pleasant afternoon playing golf? When the 18 holes are located on the surface of a comet speeding through the Solar System. On page 63 of this issue, Vincent et al.1 describe the holes, also called pits, that comprise one of the many discoveries of the European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P). The Rosetta spacecraft went into orbit around 67P in August 2014, and the surprises have been coming fast since then. Vincent et al. propose a mechanism for the formation of the pits and identify them as one of the sources of active dust jets. Comets are the most primitive bodies in the Solar System; they are the remnants of its formation process. Comets therefore retain a physical and chemical record of the condi tions and materials in the solar nebula — the gas and dust cloud out of which the Sun and planets formed 4.56 billion years ago. Con veniently, comets have spent most of that
time in two very cold storage locations: the Kuiper belt beyond the orbit of Neptune and the spherical Oort cloud outside the planetary region, stretching halfway to the nearest stars. The distant Oort cloud is the source of the long-period comets that have orbital periods ranging up to millions of years. The Kuiper belt is the source of the Jupiter-family comets, such as 67P, which typically have periods of less than 20 years and orbital dynamics that are strongly affected by Jupiter. As a comet approaches the Sun and warms up, the central solid part, known as the cometary nucleus (comprised of volatile ices and primitive meteoritic material), begins to sublimate and becomes enveloped by a freely outflowing atmosphere called the coma. One of the first surprises for Rosetta, the first ever comet-rendezvous mission, was the odd shape of the target comet’s nucleus (Fig. 1a)2. Although some nuclei comprised of two large pieces and looking like a bowling pin had been observed before by fly-by missions to other comets, the two lobes of 67P sit on top of each other, with a narrow ‘neck’ in between.
1 km
There is intense speculation as to how this odd configuration may have formed. Did two cometary nuclei gently collide randomly in the solar nebula, or is the nucleus a single piece that has been oddly sculpted by sublimation processes? Although the former is the more likely scenario, some scientists on the mission suspect the latter. Rosetta’s camera system, the Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS), is comprised of narrowangle and wide-angle digital cameras. As the OSIRIS team of scientists2 began to map the surface of the nucleus using the cameras, they discovered 18 pits on the surface, which Vincent et al. now describe more thoroughly. The cometary nucleus has a diameter of approximately 4 kilometres. The pits are typi cally about 200 metres in diameter and about 180 metres deep. Pit-like features have been observed on other cometary nuclei, but the morphology of the pits on 67P has not been seen before. They typically have cylindri cal shapes with circular openings and nearvertical walls (although at least one pit seems to be lying at a steep angle). And some of the pits are clearly active: images of pits that are illu minated by sunlight show dust jets emanating from their walls and/or floors (Fig. 1b). How did the pits form? Vincent et al. suggest that they are ‘sink holes’, which formed when material near the surface of the nucleus col lapsed into the low-density interior. Rosetta’s Radio Science Investigation team has found2 that the nucleus has an average bulk density of only 470 ± 45 kilograms per cubic metre, about half the density of solid water ice. But the Grain Impact Analyser and Dust Accumu lator instrument has measured3 a dust-to-ice
50 m
Figure 1 | The nucleus of comet 67P/Churyumov-Gerasimenko (67P). Vincent et al.1 analysed images of comet 67P taken by the Optical, Spectroscopic and Infrared Remote Imaging System cameras on the Rosetta spacecraft. a, The complex nucleus topography includes large, flat-floored basins (indicated by white arrows). A large, circular pit is visible just above the centre of the image (red arrow). b, A string of pits dot the surface of the
cometary nucleus. In active pits such as these, bright jets of dust are seen being emitted from the sunlit walls. The contrast of this image has been enhanced to highlight the interiors of the pits and the jets. As a result, the cometary surface looks very bright, but in reality it reflects only about 6% of the incoming sunlight — roughly the same as the black toner particles in a laser printer cartridge.
4 2 | N AT U R E | V O L 5 2 3 | 2 J U LY 2 0 1 5
© 2015 Macmillan Publishers Limited. All rights reserved
ESA/ROSETTA/MPS FOR OSIRIS TEAM MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
P L ANE TARY SCIENCE
ARTHUR GEORGES
NEWS & VIEWS RESEARCH mass ratio of 4 ± 2, suggesting that silicates and organics, rather than ices, make up about 80% of the mass of the nucleus. This in turn implies that 75–85% of the nucleus interior is empty space, a parameter known as poros ity. A high porosity is predicted by the lead ing scenarios for the internal structure of cometary nuclei, which suggest that they are aggregates4 of smaller, icy bodies that gently came together in the solar nebula. These aggre gates are also referred to as rubble piles5. This concept has provided insights into the behav iour of comets, such as random and other splitting events. The morphology of 67P’s surface is domi nated in some areas by large, flat-floored basins, similar to features seen on the nucleus of comet6 Wild 2. It has been suggested that these are sublimation basins that slowly widen as the walls sublimate, leaving large, nonvolatile particles that cover the basin floor. The basins cannot be impact craters because they have the wrong size distribution (there are too many large ones), and because not many impact craters are expected on a small cometary nucleus such as 67P. Could the pits described by Vincent et al. be the precursors of the basins, slowly wid ening as their walls sublimate? Many of the pits found by OSIRIS are located in the same region on the nucleus where many of the large sublimation basins are found. Both comet 67P and comet Wild 2 are relatively young — that is, they have only recently (within the past 60 years) been perturbed by the gravitational field of Jupiter to perihelion distances (the point in their orbit closest to the Sun) at which it is warm enough for water ice in the nucleus to sublimate, and at which the activity that manifests itself as the bright cometary coma and tails begins. If this is so, why are sublima tion basins not observed on other, perhaps older, Jupiter-family comets such as Tempel 1 and Hartley 2? Older nuclei may have accu mulated thicker layers of non-volatile mater ials that have buried the sublimation basins and substantially lowered the activity levels of those comets. Rosetta has already indicated that it has more surprises for us. On 13 June 2015, the orbiter began receiving signals from the Philae lander, which is on the surface of the comet nucleus and was last heard from in Novem ber 2014. With its batteries recharging, Philae probably has much more information to trans mit about its final landing location. Also, the activity of the nucleus is expected to reach a maximum soon after the comet passes through perihelion at 1.25 astronomical units from the Sun (a point about 25% farther from the Sun than Earth’s orbit) in mid-August 2015. Rosetta will then follow 67P away from the Sun as cometary activity begins to wane. What changes will we see on the nucleus surface? And how will this alien golf course look from Rosetta’s vantage point then? ■
Paul Weissman is at the Jet Propulsion Laboratory, NASA, Pasadena, California 91109, USA. e-mail: [email protected] 1. Vincent, J.–B. et al. Nature 523, 63–66 (2015). 2. Sierks, H. et al. Science 347, aaa1044 (2015).
3. Rotundi, A. et al. Science 347, aaa3905 (2015). 4. Donn, B. & Hughes, D. in 20th ESLAB Symp. Exploration of Halley’s Comet (eds Battrick, B. et al.) 523–524 (ESA, 1986). 5. Weissman, P. R. Nature 320, 242–244 (1986). 6. Kirk, R. et al. in 46th Lunar and Planetary Science Conf. Abstr. 2244 (2015).
EVO LU TI O N
Reptile sex determination goes wild Wild populations of an Australian lizard have sex chromosomes and also exhibit temperature-controlled sexual development, providing insight into how these two sex-determining mechanisms may evolve back and forth. See Letter p.79 JAMES J. BULL
V
ertebrate sex determination is getting interesting. On page 79 of this issue, Holleley et al.1 report elaborate field and laboratory studies on an egg-laying Aus tralian lizard, Pogona vitticeps, and reveal that its sex is determined both by its complement of chromosomes and by the temperature at which its eggs are incubated. Earlier reports2,3 had hinted at the possibility of combined geno typic and environmental sex determination in some lizards, but it had never before been con vincingly reported in the wild. Fifty years ago, the dominant view, laid out by Susumo Ohno4, was that there was an inexorable evolutionary progression from genetically labile mechanisms of sex deter mination to highly refined and differenti ated sex chromosomes. Ohno suggested that
this progression was recapitulated across the vertebrate groups, with sex chromosomes becoming increasingly more entrenched as one moved up what was then perceived as the evolutionary phylogenetic ladder — fish had labile systems, mammals and birds had entrenched sex-chromosome systems, and reptiles were in the middle of this transition. Within two decades, that view had radically changed5. By then, many reptiles were known to have full-blown sex-chromosome systems, whereas many others had sex determination by incubation temperature and no hint of inher ited sex. It was also realized that the evolution of sex determination follows basic evolution ary principles, and that chromosomal and environmental sex determination can both be highly functional, adaptive systems. In other words, they are not different steps along an
Figure 1 | Pogona vitticeps. Holleley et al.1 show that, in this lizard species, embryos that have two Z chromosomes and are thus genetically male can develop as female at warm egg-incubation temperatures. 2 J U LY 2 0 1 5 | V O L 5 2 3 | N AT U R E | 4 3
© 2015 Macmillan Publishers Limited. All rights reserved
Sink holes and dust jets on comet 67P Analyses of images taken by the Rosetta spacecraft reveal the complex landscape of a comet in rich detail. Close-up views of the surface indicate that some dust jets are being emitted from active pits undergoing sublimation. See Letter p.63 PA U L W E I S S M A N
W
hen do 18 holes not make for a pleasant afternoon playing golf? When the 18 holes are located on the surface of a comet speeding through the Solar System. On page 63 of this issue, Vincent et al.1 describe the holes, also called pits, that comprise one of the many discoveries of the European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P). The Rosetta spacecraft went into orbit around 67P in August 2014, and the surprises have been coming fast since then. Vincent et al. propose a mechanism for the formation of the pits and identify them as one of the sources of active dust jets. Comets are the most primitive bodies in the Solar System; they are the remnants of its formation process. Comets therefore retain a physical and chemical record of the condi tions and materials in the solar nebula — the gas and dust cloud out of which the Sun and planets formed 4.56 billion years ago. Con veniently, comets have spent most of that
time in two very cold storage locations: the Kuiper belt beyond the orbit of Neptune and the spherical Oort cloud outside the planetary region, stretching halfway to the nearest stars. The distant Oort cloud is the source of the long-period comets that have orbital periods ranging up to millions of years. The Kuiper belt is the source of the Jupiter-family comets, such as 67P, which typically have periods of less than 20 years and orbital dynamics that are strongly affected by Jupiter. As a comet approaches the Sun and warms up, the central solid part, known as the cometary nucleus (comprised of volatile ices and primitive meteoritic material), begins to sublimate and becomes enveloped by a freely outflowing atmosphere called the coma. One of the first surprises for Rosetta, the first ever comet-rendezvous mission, was the odd shape of the target comet’s nucleus (Fig. 1a)2. Although some nuclei comprised of two large pieces and looking like a bowling pin had been observed before by fly-by missions to other comets, the two lobes of 67P sit on top of each other, with a narrow ‘neck’ in between.
1 km
There is intense speculation as to how this odd configuration may have formed. Did two cometary nuclei gently collide randomly in the solar nebula, or is the nucleus a single piece that has been oddly sculpted by sublimation processes? Although the former is the more likely scenario, some scientists on the mission suspect the latter. Rosetta’s camera system, the Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS), is comprised of narrowangle and wide-angle digital cameras. As the OSIRIS team of scientists2 began to map the surface of the nucleus using the cameras, they discovered 18 pits on the surface, which Vincent et al. now describe more thoroughly. The cometary nucleus has a diameter of approximately 4 kilometres. The pits are typi cally about 200 metres in diameter and about 180 metres deep. Pit-like features have been observed on other cometary nuclei, but the morphology of the pits on 67P has not been seen before. They typically have cylindri cal shapes with circular openings and nearvertical walls (although at least one pit seems to be lying at a steep angle). And some of the pits are clearly active: images of pits that are illu minated by sunlight show dust jets emanating from their walls and/or floors (Fig. 1b). How did the pits form? Vincent et al. suggest that they are ‘sink holes’, which formed when material near the surface of the nucleus col lapsed into the low-density interior. Rosetta’s Radio Science Investigation team has found2 that the nucleus has an average bulk density of only 470 ± 45 kilograms per cubic metre, about half the density of solid water ice. But the Grain Impact Analyser and Dust Accumu lator instrument has measured3 a dust-to-ice
50 m
Figure 1 | The nucleus of comet 67P/Churyumov-Gerasimenko (67P). Vincent et al.1 analysed images of comet 67P taken by the Optical, Spectroscopic and Infrared Remote Imaging System cameras on the Rosetta spacecraft. a, The complex nucleus topography includes large, flat-floored basins (indicated by white arrows). A large, circular pit is visible just above the centre of the image (red arrow). b, A string of pits dot the surface of the
cometary nucleus. In active pits such as these, bright jets of dust are seen being emitted from the sunlit walls. The contrast of this image has been enhanced to highlight the interiors of the pits and the jets. As a result, the cometary surface looks very bright, but in reality it reflects only about 6% of the incoming sunlight — roughly the same as the black toner particles in a laser printer cartridge.
4 2 | N AT U R E | V O L 5 2 3 | 2 J U LY 2 0 1 5
© 2015 Macmillan Publishers Limited. All rights reserved
ESA/ROSETTA/MPS FOR OSIRIS TEAM MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
P L ANE TARY SCIENCE
ARTHUR GEORGES
NEWS & VIEWS RESEARCH mass ratio of 4 ± 2, suggesting that silicates and organics, rather than ices, make up about 80% of the mass of the nucleus. This in turn implies that 75–85% of the nucleus interior is empty space, a parameter known as poros ity. A high porosity is predicted by the lead ing scenarios for the internal structure of cometary nuclei, which suggest that they are aggregates4 of smaller, icy bodies that gently came together in the solar nebula. These aggre gates are also referred to as rubble piles5. This concept has provided insights into the behav iour of comets, such as random and other splitting events. The morphology of 67P’s surface is domi nated in some areas by large, flat-floored basins, similar to features seen on the nucleus of comet6 Wild 2. It has been suggested that these are sublimation basins that slowly widen as the walls sublimate, leaving large, nonvolatile particles that cover the basin floor. The basins cannot be impact craters because they have the wrong size distribution (there are too many large ones), and because not many impact craters are expected on a small cometary nucleus such as 67P. Could the pits described by Vincent et al. be the precursors of the basins, slowly wid ening as their walls sublimate? Many of the pits found by OSIRIS are located in the same region on the nucleus where many of the large sublimation basins are found. Both comet 67P and comet Wild 2 are relatively young — that is, they have only recently (within the past 60 years) been perturbed by the gravitational field of Jupiter to perihelion distances (the point in their orbit closest to the Sun) at which it is warm enough for water ice in the nucleus to sublimate, and at which the activity that manifests itself as the bright cometary coma and tails begins. If this is so, why are sublima tion basins not observed on other, perhaps older, Jupiter-family comets such as Tempel 1 and Hartley 2? Older nuclei may have accu mulated thicker layers of non-volatile mater ials that have buried the sublimation basins and substantially lowered the activity levels of those comets. Rosetta has already indicated that it has more surprises for us. On 13 June 2015, the orbiter began receiving signals from the Philae lander, which is on the surface of the comet nucleus and was last heard from in Novem ber 2014. With its batteries recharging, Philae probably has much more information to trans mit about its final landing location. Also, the activity of the nucleus is expected to reach a maximum soon after the comet passes through perihelion at 1.25 astronomical units from the Sun (a point about 25% farther from the Sun than Earth’s orbit) in mid-August 2015. Rosetta will then follow 67P away from the Sun as cometary activity begins to wane. What changes will we see on the nucleus surface? And how will this alien golf course look from Rosetta’s vantage point then? ■
Paul Weissman is at the Jet Propulsion Laboratory, NASA, Pasadena, California 91109, USA. e-mail: [email protected] 1. Vincent, J.–B. et al. Nature 523, 63–66 (2015). 2. Sierks, H. et al. Science 347, aaa1044 (2015).
3. Rotundi, A. et al. Science 347, aaa3905 (2015). 4. Donn, B. & Hughes, D. in 20th ESLAB Symp. Exploration of Halley’s Comet (eds Battrick, B. et al.) 523–524 (ESA, 1986). 5. Weissman, P. R. Nature 320, 242–244 (1986). 6. Kirk, R. et al. in 46th Lunar and Planetary Science Conf. Abstr. 2244 (2015).
EVO LU TI O N
Reptile sex determination goes wild Wild populations of an Australian lizard have sex chromosomes and also exhibit temperature-controlled sexual development, providing insight into how these two sex-determining mechanisms may evolve back and forth. See Letter p.79 JAMES J. BULL
V
ertebrate sex determination is getting interesting. On page 79 of this issue, Holleley et al.1 report elaborate field and laboratory studies on an egg-laying Aus tralian lizard, Pogona vitticeps, and reveal that its sex is determined both by its complement of chromosomes and by the temperature at which its eggs are incubated. Earlier reports2,3 had hinted at the possibility of combined geno typic and environmental sex determination in some lizards, but it had never before been con vincingly reported in the wild. Fifty years ago, the dominant view, laid out by Susumo Ohno4, was that there was an inexorable evolutionary progression from genetically labile mechanisms of sex deter mination to highly refined and differenti ated sex chromosomes. Ohno suggested that
this progression was recapitulated across the vertebrate groups, with sex chromosomes becoming increasingly more entrenched as one moved up what was then perceived as the evolutionary phylogenetic ladder — fish had labile systems, mammals and birds had entrenched sex-chromosome systems, and reptiles were in the middle of this transition. Within two decades, that view had radically changed5. By then, many reptiles were known to have full-blown sex-chromosome systems, whereas many others had sex determination by incubation temperature and no hint of inher ited sex. It was also realized that the evolution of sex determination follows basic evolution ary principles, and that chromosomal and environmental sex determination can both be highly functional, adaptive systems. In other words, they are not different steps along an
Figure 1 | Pogona vitticeps. Holleley et al.1 show that, in this lizard species, embryos that have two Z chromosomes and are thus genetically male can develop as female at warm egg-incubation temperatures. 2 J U LY 2 0 1 5 | V O L 5 2 3 | N AT U R E | 4 3
© 2015 Macmillan Publishers Limited. All rights reserved
