Measurement of argon on Comet 67P prompts new theory of space projectiles’ role
Asteroids probably ferried water to an infant Earth (SN: 5/16/15, p. 18), but they weren’t responsible for our planet’s entire chemical inventory. Comets might have trucked in noble gases and much of the raw material needed for life, researchers suggest online March 9 in Earth and Planetary Science Letters.
Noble gases don’t play well with the other elements. They typically shun chemical reactions, which means their abundances haven’t changed much since the formation of the solar system. Comets are thought to be frozen relics from the birth of the planets, but until recently researchers didn’t know how much of any noble gas comets carried.
That changed shortly after the Rosetta spacecraft arrived at comet 67P/Churyumov-Gerasimenko in August 2014 (SN: 9/6/14, p. 8) and made the first measurement of argon in a cometary atmosphere. It’s not much — roughly 1/100,000 the amount of water — but it’s several orders of magnitude more than the abundance of argon in icy asteroids. And that’s enough for comets to have been a major source of argon (and presumably other noble gases) to Earth, Bernard Marty, a geochemist at Petrographic and Geochemical Research Centerin Vandœuvre-lès-Nancy, France, and colleagues report.
The delivery arrived during the Late Heavy Bombardment about 600 million years after the start of the solar system, Marty and collaborators suggest. That’s when the moon (and supposedly Earth) was pummeled by debris from the outer solar system as the giant planets abruptly settled into their current orbits.
Argon might seem of little relevance to everyday life. But “if argon came from comets, we can make some inferences about how much prebiotic material came in comets too,” Marty says. And those other goodies, such as amino acids, might matter a great deal. Assuming that all the argon in Earth’s atmosphere came from comets, and that cometary levels of amino acids (which no one has measured) are similar to what’s found in meteorites known as carbonaceous chondrites, the researchers calculated how much amino acids comets could have delivered to Earth. Dust collected from comet 81P/Wild 2 by the Stardust spacecraft in 2004 showed some chemical similarities between its quarry and those meteorites.
It’s a rough calculation, Marty admits, and it assumes that the deliveries survived the impacts. But the team estimates that the amount of amino acids supplied by comets could roughly equal the total mass found in all organisms from paramecia to plants and people.
“I think it’s an interesting exercise,” says Conel Alexander, a planetary scientist at the Carnegie Institution for Science in Washington, D.C. “But it’s full of so many uncertainties. My worries are that we still know so little about comet composition.” The amount of cometary argon, for example, is based on just one comet. Other comets have shown tremendous variability, for example, in the relative amounts of water isotopes. Researchers also don’t have a good handle on the concentration of other noble gases, such as xenon, lurking in comets.
There’s also uncertainty about the Late Heavy Bombardment that presumably brought the comets to Earth. Evidence for the influx of debris comes from a spike in craters on the moon roughly 4 billion years ago. But the timing comes from lunar rocks collected by Apollo astronauts and those samples might all have come from one basin, says Alexander. Rather than revealing the ages of many craters, the moon rocks might record the date of a single run-in with a giant intruder.
Recent computer simulations also indicate that the giant planet tango that allegedly triggered the bombardment should have removed at least one of the inner planets, which doesn’t appear to have happened. To avoid that catastrophe, the gas giants had to have settled down before the rocky planets finished forming and so wouldn’t have been available to fling things at Earth 600 million years later.
COMET’S DUST TRAIL STRETCHES OVER 10 MILLION KM
Comet 67P/Churyumov-Geraimenko’s dust trail has been observed to stretch at least 10 million kilometres in the latest images taken by professional astronomers working on the ground-based observing campaign.
The image mosaic shown here is composed of four images taken using the 2.5 m Isaac Newton Telescope on La Palma on the night of 12 March.
R-band mosaic of Comet 67P/C-G on 12 March 2016, taken with the 2.5m Isaac Newton Telescope on La Palma. The brightest part of the comet was imaged for 30 minutes, while the most distant parts of the dust trail required 1 hour of exposure. Credit: Isaac Newton Group/John Davies/Alan Fitzsimmons/Colin Snodgrass.
The image shows reflected light from dust grains and highlights the comet’s various tail structures. Similar images were obtained by amateur astronomers a few months ago when the comet was closer to the Sun and much brighter.
Most striking is the long trail, stretching over two degrees from the comet – equivalent to the apparent size of four full Moons on the sky as seen from Earth (see graphic below for scale) – or greater than 10 million kilometres at the comet's distance. By comparison, Rosetta is flying beside the comet nucleus as close as just 10 km.
The long trail is made up of larger grains left behind in the orbit of the comet, probably from several previous passages of the comet around the Sun. When the Earth passes through similar dust trails from other comets it gives rise to meteor showers. Unfortunately this won’t happen with 67P/C-G, as the orbit does not approach our planet.
The shorter dust tail just below the trail is a 'neck-line' structure, made from dust grains ejected by the comet during this current orbit.
Any anti-solar tail of the comet would be to the south but not so readily seen; in any case it would be very foreshortened in this geometry as the comet was at opposition, and at about 3 degrees phase angle, so the tail is mostly 'behind' the coma from our point of view.
Comet 67P/C-G’s long dust trail, seen with a full moon for scale. Note that the Moon shows the apparent scale on the sky as seen from Earth rather than the physical scale – the Moon would be much smaller at the comet's distance. Credit: Isaac Newton Group/John Davies/Alan Fitzsimmons/Colin Snodgrass.
COMETWATCH 19 MARCH
This week’s CometWatch entry from Rosetta’s NAVCAM was taken on 19 March from a distance of just 12 km.
Lightly enhanced Rosetta NAVCAM image of Comet 67P/Churyumov-Gerasimenko taken from a distance of 12 km. The image scale is 1 m/pixel and the image measures 1.1 km across. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0.
It captures a beautiful oblique view across the plains of Imhotep, taking in the cluster of boulders to the right of the image that includes 25m high Cheops.
The rugged, fractured terrain and debris slopes in the background, where Imhotep meets Khepry, makes for a spectacular horizon.
This region can also be seen, albeit from a much different orientation but which shows the wider context of the region, in the OSIRIS wide-angle camera image below captured on 16 March.
OSIRIS wide-angle camera image taken on 16 March 2016, when Rosetta was 14.4 km from Comet 67P/Churyumov–Gerasimenko. The scale is 1.36 m/pixel. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Following the brief encounter at these close distances, Rosetta is now heading out on an anti-sunward excursion to around 1000 km to investigate the comet’s wider coma, tail and plasma environment. Today, 24 March, Rosetta is already over 200 km away from the comet. The current plan is for Rosetta to make a 30 km zero phase flyby around 9 April, before entering back into closer bound orbits by 21 April.
Follow the spacecraft’s trajectory in the coming weeks with the Rosetta NOW tool.
The original 1024 x 1024 pixel NAVCAM image is provided below: