Based on the press release of the Swedish Institute of Space Physics summarising the results of the Rosetta Plasma Consortium’s (RPC) Ion Composition Analyser (ICA) that are presented in the journal Science today, and on follow-up discussion with Hans Nilsson, RPC-ICA principal investigator.
The RPC-ICA instrument onboard Rosetta has been watching the early stages of how a magnetosphere forms around Comet 67P Churyumov-Gerasimenko as it moves closer to the Sun along its orbit and begins to interact with the solar wind.
As the comet gets warmer, volatile substances, mainly water, evaporate from the surface and form an atmosphere around the comet. The Sun's ultraviolet radiation and collisions with the solar wind ionizes some of the comet's atmosphere. The newly formed ions are affected by the solar wind electric and magnetic fields and can be accelerated to high speeds. When the comet gets close enough to the Sun, its atmosphere becomes so dense and ionized that it becomes electrically conductive. When this happens, the atmosphere starts to resist the solar wind and a comet's magnetosphere is born – a region surrounding the comet that is shielded from the solar wind.
"The comet environment is a laboratory for scientists; we can see what happens when the solar wind streams through an atmosphere," says Associate Professor Hans Nilsson at the Swedish Institute of Space Physics (IRF) in Kiruna. Hans Nilsson is responsible for the instrument Ion Composition Analyser, ICA, developed and built in Kiruna. The instrument measures positively charged ions and is one of five instruments in the Rosetta Plasma Consortium.
Hans explained that studying the interaction of the solar wind with a comet's atmosphere in this way could shed light on how the solar wind interacted with the planets during their early formation. "In the young Solar System, the planet’s upper atmospheres were strongly heated and extended far into space. The solar wind should have streamed through the outer part of these extended atmospheres in a way that resembles its passage through the comet atmosphere for this low activity stage," he explains.
At 67P/C-G, Rosetta’s RPC-ICA instrument detected low velocity water ions in data collected on 7 August 2014, one day after arriving at the comet at a distance of 100 km.
"It was an unambiguous signature of the comet, a clear detection of ions from the comet's atmosphere," confirmed Hans.
Rosetta is a unique space mission. Previous spacecraft that have studied comets have rushed past them at a great distance and at speeds of tens of kilometres per second. At the time of these previous spacecraft-comet encounters, the comet magnetosphere had been fully developed. By comparison, Rosetta is currently orbiting around 67P/C-G at a distance of just a few tens of kilometres, and at low speed. Importantly, the mission arrived at the comet in time to watch the very early stages of the magnetosphere formation.
“For the first time, we can see what happens before the comet atmosphere resists the solar wind," says Hans. "We discovered that the comet atmosphere affects the solar wind more than we thought it would at this early stage. We are also surprised how much structure we see in our data – the comet atmosphere appears to be very unevenly distributed around the nucleus.
"We are still in the early stages of analysing and modelling our data, but perhaps the energy transfer from the solar wind to the atmosphere is less efficient in removing an atmosphere than we originally thought.
As the comet moves closer to the Sun along its orbit, Hans and his colleagues will see the transition from this early phase to the growth of a well established comet magnetosphere.
"That transition is likely the most exciting part," he says. "It can happen any day now. Every morning we keep looking at the new data which have just arrived. What will we find today?"
"Birth of a comet magnetosphere: a spring of water ions," by Hans Nilsson et al., is published in 23 January issue of Science
Editor's note: On previous blog entries, some of you have been discussing about the formation of the comet's bow shock – a question I put to Hans, who says:
"We do not know for sure at what distance the bow shock is formed. At the distance we are right now [30 km from the comet] we can see that the solar wind is not shocked, and it does still permeate the comet atmosphere down to where Rosetta is located. So we do know that the magnetosphere has not formed yet, and we would at least indirectly see when it is formed. According to the models, the bow shock should form quite far away, so we would need rather large excursions away from the comet to observe it in situ."
GETTING TO KNOW ROSETTA’S COMET – SCIENCE SPECIAL EDITION
In a special edition of the journal Science, published online tonight, results are presented from seven of Rosetta’s 11 science instruments made during the approach to and soon after arriving at Comet 67P/Churyumov-Gerasimenko in August 2014.
Our overview article is published on the main ESA Portal and also posted below. A number of supporting blog posts provided by the different instrument teams to give more details of their specific results will also soon be published on the blog. In addition, we are pleased to point you to our “Comet close-ups” image slideshow that focuses on the OSIRIS images presented in the papers – enjoy!
Comet regional maps. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Getting to know Rosetta’s comet
Rosetta is revealing its host comet as having a remarkable array of surface features and with many processes contributing to its activity, painting a complex picture of its evolution.
The familiar shape of the dual-lobed comet has now had many of its vital statistics measured: the small lobe measures 2.6 × 2.3 × 1.8 km and the large lobe 4.1 × 3.3 × 1.8 km. The total volume of the comet is 21.4 km^3 and the Radio Science Instrument has measured its mass to be 10 billion tonnes, yielding a density of 470 kg/m^3.
By assuming an overall composition dominated by water ice and dust with a density of 1500–2000 kg/m3, the Rosetta scientists show that the comet has a very high porosity of 70–80%, with the interior structure likely comprising weakly bonded ice-dust clumps with small void spaces between them.
The OSIRIS scientific camera, has imaged some 70% of the surface to date: the remaining unseen area lies in the southern hemisphere that has not yet been fully illuminated since Rosetta’s arrival.
The scientists have so far identified 19 regions separated by distinct boundaries and, following the ancient Egyptian theme of the Rosetta mission, these regions are named for Egyptian deities, and are grouped according to the type of terrain dominant within.
Five basic – but diverse – categories of terrain type have been determined: dust-covered; brittle materials with pits and circular structures; large-scale depressions; smooth terrains; and exposed more consolidated (‘rock-like’) surfaces.
Much of the northern hemisphere is covered in dust. As the comet is heated, ice turns directly into gas that escapes to form the atmosphere or coma. Dust is dragged along with the gas at slower speeds, and particles that are not travelling fast enough to overcome the weak gravity fall back to the surface instead.
Some sources of discrete jets of activity have also been identified. While a significant proportion of activity emanates from the smooth neck region, jets have also been spotted rising from pits.
The gases that escape from the surface have also been seen to play an important role in transporting dust across the surface, producing dune-like ripples, and boulders with ‘wind-tails’ – the boulders act as natural obstacles to the direction of the gas flow, creating streaks of material ‘downwind’ of them.
The dusty covering of the comet may be several metres thick in places and measurements of the surface and subsurface temperature by the Microwave Instrument on the Rosetta Orbiter, or MIRO, suggest that the dust plays a key role in insulating the comet interior, helping to protect the ices thought to exist below the surface.
Small patches of ice may also be present on the surface. At scales of 15–25 m, Rosetta’s Visible, InfraRed and Thermal Imaging Spectrometer, or VIRTIS, finds the surface to be compositionally very homogenous and dominated by dust and carbon-rich molecules, but largely devoid of ice. But smaller, bright areas seen in images are likely to be ice-rich. Typically, they are associated with exposed surfaces or debris piles where collapse of weaker material has occurred, uncovering fresher material.
On larger scales, many of the exposed cliff walls are covered in randomly oriented fractures. Their formation is linked to the rapid heating–cooling cycles that are experienced over the course of the comet’s 12.4-hour day and over its 6.5-year elliptical orbit around the Sun. One prominent and intriguing feature is a 500 m-long crack seen roughly parallel to the neck between the two lobes, although it is not yet known if it results from stresses in this region.
Some very steep regions of the exposed cliff faces are textured on scales of roughly 3 m with features that have been nicknamed ‘goosebumps’. Their origin is yet to be explained, but their characteristic size may yield clues as to the processes at work when the comet formed.
And on the very largest scale, the origin of the comet’s overall double-lobed shape remains a mystery. The two parts seem very similar compositionally, potentially favouring the erosion of a larger, single body. But the current data cannot yet rule out the alternative scenario: two separate comets formed in the same part of the Solar System and then merged together at a later date. This key question will be studied further over the coming year as Rosetta accompanies the comet around the Sun.
How to grow an atmosphere
Their closest approach to the Sun occurs on 13 August at a distance of 186 million kilometres, between the orbits of Earth and Mars. As the comet continues to move closer to the Sun, an important focus for Rosetta’s instruments is to monitor the development of the comet’s activity, in terms of the amount and composition of gas and dust emitted by the nucleus to form the coma.
Images from the scientific and navigation cameras have shown an increase in the amount of dust flowing away from the comet over the past six months, and MIRO showed a general rise in the comet’s global water vapour production rate, from 0.3 litres per second in early June 2014 to 1.2 litres per second by late August. MIRO also found that a substantial portion of the water seen during this phase originated from the comet’s neck.
Water is accompanied by other outgassing species, including carbon monoxide and carbon dioxide. The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, ROSINA, is finding large fluctuations in the composition of the coma, representing daily and perhaps seasonal variations in the major outgassing species. Water is typically the dominant outgassing molecule, but not always.
By combining measurements from MIRO, ROSINA and GIADA (Rosetta’s Grain Impact Analyzer and Dust Accumulator) taken between July and September, the Rosetta scientists have made a first estimate of the comet’s dust-to-gas ratio, with around four times as much mass in dust being emitted than in gas, averaged over the sunlit nucleus surface.
However, this value is expected to change once the comet warms up further and ice grains – rather than pure dust grains – are ejected from the surface.
GIADA has also been tracking the movement of dust grains around the comet, and, together with images from OSIRIS, two distinct populations of dust grains have been identified. One set is outflowing and is detected close to the spacecraft, while the other family is orbiting the comet no closer than 130 km from the spacecraft.
It is thought that the more distant grains are left over from the comet’s last closest approach to the Sun. As the comet moved away from the Sun, the gas flow from the comet decreased and was no longer able to perturb the bound orbits. But as the gas production rate increases again over the coming months, it is expected that this bound cloud will dissipate. However, Rosetta will only be able to confirm this when it is further away from the comet again – it is currently in a 30 km orbit.
As the gas–dust coma continues to grow, interactions with charged particles of the solar wind and with the Sun’s ultraviolet light will lead to the development of the comet’s ionosphere and, eventually, its magnetosphere. The Rosetta Plasma Consortium, or RPC, instruments have been studying the gradual evolution of these components close to the comet.
“Rosetta is essentially living with the comet as it moves towards the Sun along its orbit, learning how its behaviour changes on a daily basis and, over longer timescales, how its activity increases, how its surface may evolve, and how it interacts with the solar wind,” says Matt Taylor, ESA’s Rosetta project scientist.
“We have already learned a lot in the few months we have been alongside the comet, but as more and more data are collected and analysed from this close study of the comet we hope to answer many key questions about its origin and evolution.”
These are among the very first scientific results from Rosetta and there is much more to come as the scientists work through the data and as the comet continues to evolve during its closest approach to the Sun. They are described in more detail in accompanying posts (to be published after this post and tomorrow) and in the 23 January 2015 Science special edition (reference below)