Culture

Comets Part 3.   Cometary Impacts

by Duncan Lunan

Comet-Swift-Tuttle return, NASA 1992

When the orbit of Comet 1862 III Swift-Tuttle was calculated, it was found to coincide with the Perseid meteor shower through which the Earth passes every August;  the biggest display takes place between August 12th and 14th.  The Swiss astronomer Plantamour lectured on the link with the Perseid meteors in the winter of 1871-72, mentioning that the Earth would next encounter the meteors on August 12th, 1872.  Newspapers reported that the Earth would collide with the comet on that date, causing considerable public alarm.

19th century astronomers thought such fears were groundless.  Comets had passed close to Mercury, and between the moons of Jupiter, without producing any noticeable perturbations;  so their masses had to be low.  The head of Donati’s Comet passed in front of the star Arcturus in 1858 without dimming it significantly, and when Earth passed through the tail of the great comet of 1861 without any noticeable effect, that seemed to clinch the matter – comets were entirely gaseous, and the gases were very tenuous.  In the 20th century it came to be accepted that there must be something inside the great heads of the comets, but whatever it was, no doubt it was too small and flimsy to be dangerous. 

Science-fiction writers, of course, ignore the astronomers when it suits them.  In Hector Servadac and its sequel, Jules Verne described a comet made of solid gold telluride, so that it could knock lumps off the Earth for the purposes of his story.  H.G. Wells stuck to prevailing theory for In the Days of the Comet, in which there was no actual impact and the gases mix peacefully with the Earth’s atmosphere.  In Arthur C. Clarke’s short story ‘Into the Comet’, reprinted in his collection Tales of Ten Worlds  (1964), the core was a loose cluster of icebergs, giving off jets of methane and ammonia.

Perseid meteor shower, orbital path of Comet Swift-Tuttle, Popular Science Monthly, 1872

Clarke claimed to be quoting F.L. Whipple, but Whipple himself envisaged the ‘dirty ice’ as a single mass typically 1-10 km in diameter.  In my story ‘The Comet, the Cairn and the Capsule’, based on my own amateur observations of Comet Bennett in April 1970, the nucleus was one loosely compacted mass of ice and rock.  Lucifer’s Hammer, the comet which hits the Earth in the 1977 novel by Larry Niven and Jerry Pournelle, has a nucleus initially like the one in my story.  Trying to visualise the impact, the characters use the analogy of ‘hot fudge sundae’ – with the ice cream representing the ‘foamy ice’, overall density considerably less than water, and embedded crushed nuts representing the rocks.  But there’s so much vaporisation as the comet rounds the Sun that what approaches Earth is a boulder field embedded in gas, much like Clarke’s description.  We’ve all used the model which best fits the needs of our plots, but after the return of Comet Swift-Tuttle in 1992, it became apparent that with a nucleus 26 km in diameter, bigger than the one which caused the extinction of the dinosaurs, Swift-Tuttle poses a major threat for the future – one which would be very hard to deal with at our current level of technology.

The nearest star we know of is the red dwarf Proxima Centauri, 4.2 light-years away, far enough out from the twin suns of Alpha Centauri at 4.3 l.y. that it has only recently been proved to be gravitationally bound to them.  There could be smaller, cooler red dwarves closer to us, emitting mainly in the infrared, as in G. Gurevich’s story ‘Infra Draconis’  (Violet L. Dutt, trans., A Visitor from Outer Space:  Science-Fiction Stories by Soviet Writers, Foreign Languages Publishing House, c.1961).  They’d probably have been registered in the all-sky infrared surveys by satellites such as IRAS, IRS and WISE, but those have turned up thousands of sources which have still to be identified.  If such a dwarf was gravitationally bound to the Sun, passages through the Oort Cloud every 26-30 million years could send large number of comets sunwards, accounting for the period mass extinctions in the history of life on Earth.   (Dr. Richard Muller, Nemesis the Death Star, Weidenfeld & Nicolson, 1988.)

Whether Nemesis exists or not, both geological and biological evolution have indeed been punctuated by catastrophes, mass extinctions, followed by the rapid evolution of new forms;  and the dinosaurs did indeed get their coup de grace at the time of a comet, or possibly an asteroid, which struck the sea off the coast of Yucatan.

Chicxulub-Yucutan Cratermap, 65 my b.p.

We don’t yet know whether the object that killed the dinosaurs 65 million years ago was an asteroid or a comet, but if it was a comet, it was no larger than Halley’s, with a nucleus about ten miles across.  Its crater was ten times that size  (100 miles for the main feature, 300 miles to the outer edge). The shockwave of its impact threw everything on Earth that wasn’t rooted to the ground ten feet up in the air.   Our tiny mammalian ancestors fell unharmed and scuttled away, but the dinosaurs broke every bone in their bodies.  Twenty minutes after the impact, the rain of molten ejecta had killed every living thing in North America;  the mile-high tsunami had barely started across the Atlantic, and the pyroclastic flow riding on the air-blast was still blasting through Central America, but within the hour everything on the land surface of the Earth would be ablaze as the ejecta enveloped the planet.  The crater was punched right through the crust into the magma, and as the magma rose, it met the sea falling in from above and vaporised it.  The sea won, as it always does, but not before so much vapour and energy had been pumped into the atmosphere that the storm covered a hemisphere and the darkness was total worldwide for two years, during which the acid rain came down everywhere.  90% of all living species went extinct;  but the little mammals in the burrows lived, because they could hibernate, they were nocturnal, and there was lots of carrion above ground to sustain them. 

The dinosaur impact as described by the trees, by Sydney Jordan, Lance McLane strip, Daily Record (1983-84), story by Duncan Lunan

In 1998 it was discovered that the mass extinction at the end of the Triassic period, 214 million years ago, was caused by a chain of five impacts in the Ukraine, France, Minnesota and Canada – North America being joined to Europe at the time.  The impacts could have occurred in as little as four hours, or been spread over several days, in which case there might be more of them on the sea  floor.  The largest of them formed the Manicougan impact structure, over 100 kilometres in diameter and spectacularly visible from orbit. More recently, seven or more impacts may have occurred around 7640 BC, interrupting the melting of the glaciers at the end of the Ice Age.

Manicouagan crater in Canada from Space Shuttle mission STS-9

Drs. Victor Clube and Bill Napier suggested that around 3000 BC a ‘super-comet’ broke up in the inner Solar System, filling the sky with smaller comets and meteor showers, which stimulated the dramatic large-scale astronomically aligned structures all over the world in the next thousand years – perhaps in attempts to get advance warning of impact events and their environmental consequences.  In their view the present population of short-term comets and meteor showers is larger even today than can be explained by capture and break-up of smaller comets from the Oort Cloud on the fringes of the Solar System.  Dr. Duncan Steel has further suggested that so much dust was released in the break-up that the two cones of the Zodiacal Light might extend up the sky to join at the Gegenschein, so making the Ecliptic visible as a glowing arc across the night sky.   Present-day ‘Earth-grazing’ asteroids and meteor showers may be remnants of the object, including the Tunguska airburst meteor of 1908 and the Taurid meteors  (see below).

In 1178 Gervase of Canterbury reported a major event on the Moon, though he didn’t see it himself.   Gervase, who carefully recorded events such as aurorae and mock-suns, wrote:

“In this year, on the Lord’s day before the nativity of John the Baptist, after sunset, on the first day of the new Moon, there appeared a wonderful sign, five or more men witnessing it.   For the new Moon was bright, extending the horns of its new form to the east, and behold suddenly the upper horn was divided in two.   From the middle of this division there rose up a burning torch, hurling flame, burning coals and sparks for a long way.   Meanwhile the body of the Moon which was below it was twisted as if anxiously, and as it is borne by the words of those who retold it to me and saw it with their own eyes, the Moon waved like a wounded snake.   After this it returned to its normal state.”   (William Stubbs, ed., Gervase of Canterbury, The Chronicle of the Reigns of Stephen, Henry II and Richard I, Rolls Series No.73, Longman & Co., 1879.)

1178 lunar impacts, Sydney Jordan, for ‘Children from the Sky’

The ‘doubling’ would be caused by a fountain-like spray of dust in vacuum and low gravity, like the plumes of the volcanoes on Jupiter’s moon Io.  It’s been calculated that debris could have been thrown over 1200 km, so the impacting body was travelling at 20 km per second or more;  about 1% of material ejected would have escaped from the Moon altogether.  (Derral Mulholland & Odile Calame, ‘Lunar Crater Giordano Bruno’, Science 199, 875-877  (24th Feb. 1978).  The plumes prove that the sighting wasn’t a meteor in Earth’s atmosphere, in line of sight with the Moon, although that idea has been resurrected lately.  But the story goes on to rule that out in any case, with details which didn’t make sense till March 1993.

“It repeated this change twelve times and more – [my emphasis] – namely the various torments of fire, as if it endured again what had already happened, and returned to its former state.  And after these and such changes, it was made as if darkened from one horn all the way to the other.  The very men who saw these things with their own eyes retold them to myself who writes them, willing to give their oath or to swear to it, that to the above they have added nothing false.”

In 2001 the accuracy of this account was challenged by Paul Withers, who suggested that the Moon wasn’t visible on the 18th.  Analysis by Graeme Waddington showed that it was, and Peter Nockolds showed that it was visible even in the Holy Land, where its very thin crescent would have marked the beginning of the Moslem month.  (Paul Withers, ‘Meteor Storm Evidence against the Recent Formation of Lunar Crater Giordano Bruno’, Meteoritics & Planetary Science, 36, 525-529;  Peter Nockolds, ‘Reply to Paul Withers’ and ‘The Date of Gervase’s Event of June 1178’, Cambridge Conference Net, Dec. 20th 2001.)   Because it occurred in June, the impactor could have been a member of the Beta Taurid meteor stream, the intense shower of daylight meteors first detected by radar in the second World War, associated with Encke’s Comet.  During it the seismometers left on the Moon by the Apollo astronauts detected several large impacts, and the Tunguska impact in Siberia, in June 1908, may have been another example.  Precise calculations, allowing for the Julian calendar then used, put the Moon’s setting at Canterbury only 45 minutes after sunset.  So the twelve impacts were in rapid succession – and we know now what sort of event that was.

Impact off Yucutan, 65 million years b.p., Carsten Egestal Thuesen, GEUS

Comet Shoemaker-Levy 9 passed close by Jupiter in July 1992, and split into 22 fragments which were discovered in March the following year.  They hit Jupiter in July 1994, on the far side of the planet from Earth, but the flares were seen by the Galileo space probe and over the rim of Jupiter by the Hubble Space Telescope.  As Jupiter’s rotation brought the impact points into view, optical and infra-red telescopes saw features the size of the Earth forming in Jupiter’s bitterly cold cloud layers.  Even at Airdrie Observatory, where I was Assistant Curator at the time, with a five-inch refracting telescope we could clearly see the impact scars a week later.  The whole sequence was much more spectacular than expected – with correspondingly scary implications about past and future impacts on Earth.

Crater chains on two of Jupiter’s moons, Callisto and Ganymede, may be due to SL-9-type events.   (On Io impact craters are erased by active volcanoes;  Europa is covered by ice, overlying liquid water, and only small, recent craters are visible.)  Callisto has 12 or 13 of these and Ganymede three.  On the Moon, Mars and Mercury, chains are formed by secondary impacts of debris from major collisions, but on Ganymede and Callisto there are 15 chains for which no parent impacts can be found.  The biggest is 620 kilometres long, with 25 craters roughly 25 km. across, formed by bodies less than 4 km. across.  Our Moon has secondary chains, e.g. from the craters Davy and Humboldt, but they begin next to their ‘parents’.  There are at least two ‘parentless’ crater chains on the lunar Farside, north of the crater Tsiolkovsky but not pointing to it.  One is right on the lunar equator, the other at 5o Lunar North, and perhaps they were SL-9-type events.   But they look old, not showing bright against the lunar landscape.  

The Gervase ones apparently formed the 20-km crater Giordano Bruno, discovered by the USSR’s Luna III probe which first photographed the lunar Farside in 1959.  Its existence had been deduced from the rays of impact debris spreading out from it:  they reach the hemisphere facing Earth  [Patrick Moore, ‘What We Know About the Moon’  (with tentative Farside map), in L.J. Carter, ed., Realities of Space Travel, Putnam, 1957].   A bright feature extending from it was named ‘the Soviet Mountains’, but by 1961 was plotted as a crater chain.  It’s actually a pattern formed by rays from Giordano Bruno crossing another ray system, and this was confirmed by the first manned mission to orbit the Moon, Apollo 8.  The mountains persisted on Soviet maps until 1978, and after their deletion by the International Astronomical Union, the Russians substituted an equally non-existent crater Lipskiy, named after one of the original map-makers.  Western experts didn’t get the joke.  The Bruno rays are as extensive as the much larger crater Tycho’s, thought to be 60 million years old, and since ray systems fade with time, Giordano Bruno is much more recent.  If we take literally Gervase’s statement that the flare divided the upper horn of the Moon in two, that puts the impacts around 45o Lunar North, which fits.  The very recent dating of Giordano Bruno has been challenged, with the amount of cratering on the surrounding ejecta blanket suggested an age of 4 million years – but then, if there were 12 or more impacts there in succession, the ejecta blanket would have been peppered by secondary impacts from the last of them.

One major discovery of the Apollo missions was that the Moon’s crust was apparently lacking in ‘volatiles’, such as carbon, hydrogen and nitrogen compounds.  For this and other reasons it’s thought that the proto-Earth collided with another protoplanet whose ripped-off, superheated crust formed the Moon.  Smaller impacts might create a temporary atmosphere, darkening the Moon by the end of the event as described at Canterbury.  In 1960 it was suggested that released volatiles could collect in ‘cold traps’ on the permanently shadowed floors of craters near the lunar poles.  

The Clementine probe in 1994 found signs that there was indeed ice at the south pole, in small craters within the giant impact basin Aitken, which is 2500 km. across and at least 12 km. deep.   First results from the Lunar Prospector probe in 1998 supported that, but then suggested that at both poles there were much larger deposits of ice, very thinly mixed with dust.  Later results cast doubt on the ‘frost’ interpretation, however, indicating that the ice was concentrated in deep subsurface deposits.   Arguments about whether there was ice, in what form and where from continued till 2009, when the L-Cross probe impacted near the south pole and raised a plume of water vapour whose isotopic composition showed it was definitely from comets;  and at the same time India’s Chandrayaan-1 lunar orbiter discovered at least 600 million metric tons of water ice in shadowed craters at the north pole.  The more details of Gervase’s account come to make sense, the less likely it is that the story is invented.

In 1991 radar scanning by the Very Large Array in New Mexico found evidence that there is ice at Mercury’s north pole.  It shouldn’t have caused that much surprise because the Scottish astronomer V.A. Firsoff had calculated that ice caps on Mercury were possible  (letter to The Observatory, April 1971).  Patrick Moore wrote, “I admit to being profoundly sceptical.  It does not seem likely that there has ever been water on Mercury, and without water there can be no ice.”  (New Guide to the Planets, Sidgwick & Jackson, 1993).  Its existence was decisively confirmed by the Messenger probe orbiting Mercury, but Messenger also found evidence of volcanic activity on the crust, so the water may after all be ‘home-grown’.

Comet Bennet 1970 (NASA)

A comet nucleus ten miles across, or an asteroid eight miles across, would be as bad as the Chicxulub impact 65 million years ago which not only wiped out the dinosaurs, but made 90% of all species extinct.  The standard Hollywood answer of tossing an H-bomb at it isn’t likely to work.  My colleague Gordon Ross, of the Industrial Design Unit at Glasgow School of Art, came up with the design for Solaris, a ‘Comet-chaser’ which Sydney Jordan illustrated for our article ‘Keep Watching the Skies!’, Analog, October 1994  (extended versions, Asgard, May 1995, reprinted March 2002, Cambridge Conference Net, 2002, and in my book Incoming Asteroid!  What could we do about it?  (Springer, 2013).  It would use a parabolic solar sail and an adaptive optic system, or in more recent versions a laser, to burn  ‘hot spots’ on the nucleus, forming controllable jets by which to steer it.  To prevent a Shoemaker-Levy event would take a small fleet of these craft, but they could be manufactured cheaply and easily in space, if there was time available.  In Incoming Asteroid!, our discussion group went on to consider other options including use of mass drivers  (requiring a large crewed mission), impactors  (suggested by Prof. Colin McInnes), nuclear explosives and high-energy lasers.  All of them might work on a small asteroid in the 10-year timescale we allowed ourselves, but for a comet – probably larger, more fragile, composition and structure uncertain – only a ‘gravity tractor’, using the mass of the spacecraft itself to alter the impactor’s orbit, would have any chance and would take a lot longer.  For Comet Swift-Tuttle, 26 km across, a similar deflector might need 20 years;  but for Hale-Bopp, with its 30-km radius, we could need 90 years – or 90 times the spacecraft mass, about 108,000 tons, which is taking us into Star Trek technology. even without the phasers and tractor beams.

(Incoming Asteroid!  What could we do about it?  by Duncan Lunan  (Springer, 2013) is available through bookshops or on Amazon.  Full details are on Duncan’s website, www.duncanlunan.com.)

See also: Comets and Comets Part 2