Comet C/2013 A1 (Siding Spring) has been discovered to be flying very close to Mars in October 2014… very close. In fact its current orbital error-ellipse has a minimum distance to Mars of 0 – i.e. IMPACT! The error ellipse is large and the current best guess is ~119,000 km from Mars, which is very close to a terrestrial planet for any identified comet in recorded history. Jupiter, of course, has suffered two impacts in the last 20 years and probably several more have been recorded in the past without observers knowing what they had seen.
So what if the comet does impact? Firstly we know the relative speed – a whopping 56 km/s, much faster than the usual asteroid impactors. This packs about 1.568E+9 J/kg in kinetic energy into the mass of the comet. So how big is it? Estimates seem to vary. At first I read the nuclear magnitude to be the nuclear size – normalised to 10.3-10.4 in the current observational figures. Read naively, that means with typical cometary albedo of 0.035, a size of over 60 km. A whopper! However, even at its initial discovery distance of 7.2 AU a comet’s nucleus can be quite active. A spherical coma much bigger than the nucleus can form. The long-time comet observers are estimating that the nucleus is about 4 km wide. Not as humungous as I first thought, but significant if it hit Mars.
So what if it did? At a density of ~1,000 kg/m3 and spherical, then the mass is 33 billion tonnes. Total energy is ~5E+22 J or 12 million Mega-tonnes TNT equivalent. If the comet wasn’t active and we really do see a 60 km monster headed for Mars, then the energy is 3,375 times higher or 42 BILLION Mega-tonnes TNT equivalent. The truth is probably somewhere in between. Assuming the former figure, that’s the equivalent of vapourising about ~20,000 cubic kilometres of ice. The Martian Polar caps total about 3.2 million cubic kilometres, so the smaller estimates won’t affect them much – though enough to be interesting – but the larger estimate would vaporise the lot and then some. The crucial question for Mars, in the long term, is amount of dry ice and liquid carbon dioxide available in the subsurface. All around the planet we see signs of significant amounts of dry-ice in the sub-surface. What if the impact liberated enough to significantly change the planet?
Firstly, how much is needed to give Mars an atmospheric pressure that humans can tolerate, given an oxygen supply, without the need for pressure suits? At the right temperature that can be as low as ~150 mb, or 15 kPa. But let’s assume 300 mb, 30 kPa, the same as the Summit of Everest. Exploring Mars would then be like exploring Everest, rather than the Moon as it is presently. Oxygen would still be needed, but in the right places, plants would be able to grow on the surface and, given time, the planet could be made more habitable. At the bottom of Hellas Planitia – a vast deep depression – the pressure would be ~500 mb, so even more hospitable and open bodies of water could form. Probably akin to the hyper-saline lakes in Antarctica, at its warmest, but on a larger scale.
The surface area of Mars is nearly 145 million km2, or 145 trillion square metres. The surface gravity is 3.711 m/s2 which means 30 kPa pressure requires a column mass of P/g = 8,084 kg per square metre. Over the whole of Mars that’s a total air mass of 1.172 quadrillion tonnes, which sounds immense, but is ~1/3 the mass of the ice-caps. Vaporising CO2 is much easier than vaporising H2O so the energy required to liberate it requires a smaller impactor… though only through directly hitting the source. If a smaller amount is liberated, say 1/10 the goal of 30 kPa, then it might tip Mars out of its current cold climatic state into something warmer. Then we’d see the sub-surface release more CO2 over time (making the surface locally unstable from gas out-bursts) and a self-accelerating warming trend kicks in.
If it’s so easy to remake Mars, then why hasn’t it happened naturally in the past? Perhaps it has. Abundant signs of climate change exist all over the planet – sudden floods of water as ice melts have formed rivers all over the planet. But then the warming ends and the planet chills. The suspicion is that temporary warmings increase trapping of carbon dioxide by dissolving it in water, thus an “Impact Summer” is followed inevitably by the return of the Endless Winter. An Impact Summer might last a few millennia, creating some new erosion, then the glaciers and dry-ice return.
Except, if it happens this time, maybe we – Humanity – can make an Impact Summer last.