Graham writes …
This post is the first in a series looking at the implications of life elsewhere in the Universe. We start by considering our neighbouring planet Mars.
Way back in September 2019, a couple of good friends Pat and Sharon surprised me with a ‘Boarding Pass’ for Mars 2020. At first, I had no idea what it was about but they explained that there was to be a NASA launch of a mission to Mars in July 2020. They had registered my name on a data base, and this would be transported to the surface of Mars. As you can imagine, I was quite excited and inspired by the prospect that my name (in whatever form) would forever reside on the Martian surface. I printed out my ‘pass’ (see picture) and pinned it on my office notice board, where it remained unnoticed until this week.
Indeed, this week saw the arrival of the Mars 2020 mission. On the evening of 18 February 2021, I tuned into a live broadcast on NASA TV from the JPL Perseverance Rover Mission Control room to watch the events of the rover landing strategy unfold. After its 203 day, 472 million kilometer journey, the conical capsule carrying the rover hit the upper atmosphere of Mars at 20.48 GMT travelling at about 20,000 km per hour (5.6 km per second). Seven minutes later at 20.55 GMT the rover arrived successfully on the surface. Watching the broadcast, the descent time seemed to flash by. To reduce the speed of the rover to effectively zero in such a short amount of time required a pretty brutal and complex sequence of events (see NASA descent videos – animation and actual video coverage). Just 80 seconds after atmospheric entry, peak heating occurred, with the heat shield reaching temperatures of about 1,300 degrees Celsius. Ten seconds after this, the peak deceleration of 10 g occurred. At this rate of deceleration, the capsule speed is sapped at rate of about 1 km per second for each ten seconds of flight. Finally, a combination of parachute and retro thrusters brought the rover to rest on the surface in a 1200 km diameter crater called Jezero.
This particular landing site for Perseverance, was strongly prescribed by the principal mission objective – that is, to investigate the prospect that microbial life still inhabits the surface (or more likely the subsurface) of the Red Planet. So why Jezero Crater? Over decades, an armada of Mars orbiting spacecraft have looked down on the surface and imaged the spectacular landscape. Mars has the distinction of having the largest volcano (Olympus Mons) and canyon (Valles Marineris) in the Solar System.
However, perhaps the most surprising feature of the topography is the unmistakable evidence that water once flowed freely upon the surface. Scientist have estimated that around 3.8 to 3.5 billion years ago, the climate of Mars was warm and wet – effectively, the planet was a water world not unlike Earth today. Evidence of water features – tributaries, river deltas, flood plains – are abundant. So where has all the water gone? As Mars is only about half the size of Earth, with a surface gravity of 38% of ours, the atmosphere of Mars has slowly leaked away into space over the last 3.5 billion years. Current atmospheric pressure is only 0.7% of Earth’s, and any water on the surface would quickly evaporate. So now Mars is dry and arid.
So, all those years ago it is believed that Jezero was lake, with a river inlet and outlet, and Perseverance rover’s landing site is located near the inlet. The rover has the ability therefore to look for likely sites in the lake bed and river delta where Martian microbial life might still reside. This is likely to be found in the subsurface rocks, so the rover has the capability to drill into the surface to produce core samples. These precious samples will subsequently be left on the surface to collected, and returned to Earth for detailed analysis. The methodology of this retrieval process is quite torturous, and is perhaps the subject of another blog.
What then is to be learned from all this? It would be truly remarkable if life was discovered, and it was unambiguously based upon biology unlike that found on planet Earth. This would certainly be a paradigm-changing discovery, having great significance for our understanding of life elsewhere. However, another outcome could be that Martian life is found to be based on the same DNA code that is universal on Earth. This is entirely possible due to the fact that Earth and Mars are not biologically quarantined from each other. It has been known for many decades that chunks of rock from Mars can be found on Earth, and no doubt similarly bits of Earth reside on the Martian surface. The main mechanism accounting for this surprising situation is that highly energetic comet or asteroid impacts on Mars can blast rocks into orbit around the Sun, which can subsequently encounter the Earth. Presumably the reverse route is also viable. Of course, another overall mission outcome could be that no convincing evidence of microbial life is found. Only time will tell. For more information about the prospect of life on Mars, I can recommend Paul Davies’s book The 5th Miracle*.
Finally, it is worth pointing out that there are other places in the solar system where life may reside and where biological quarantine conditions are more stringent. Orbiting spacecraft missions to Jupiter (Galileo) and Saturn (Cassini) have shown that Jupiter’s moon Europa and Saturn’s moon Enceladus have significant water oceans beneath a thick surface crust of ice. The heat required to maintain this liquid state in both moons is generated by tidal heating. The source of this heating is likely to be active thermal vents on the ocean bed, around which conditions for the nuturing of life may exist. Although this is an exciting prospect, the opportunity to investigate this hypothesis is likely to be some significant time in the future. The task of delivering a submarine, robotic probe to these distant worlds is an extremely challenging one from the perspective of rocket science and spacecraft engineering.
* Paul Davies, The 5th Miracle: the Search for the Origin and Meaning of Life, Chapters 8 & 10, Simon & Schuster, 2000.