Life Express

The once-thought quiet history of the Solar System has, and will continue to be, punctuated by violent collisions. Wayward comets and asteroids continually scuttle through the inner Solar System, and every now and then a collision must inevitably occur. The circular pockmarks of such impacts abound, and no old surface, whether on a planet or a moon, is free from the blemishes of past encounters.

In the previous section we examined the evidence for the past existence of microbial life on Mars via the study of Martian meteorites recovered on Earth. The impact processes that resulted in the ejection of material from Mars into space, however, is not unique to that planet, and there is every reason to suppose that there are terrestrial, Venusian, and even Mercurian meteoroids orbiting (both now and in the past) the Sun as a result of ancient impacts. There is not just a Martian invasion of Earth going on; there is also a terrestrial invasion of Mars, Venus, and Mercury. Indeed, there is a veritable communal interchange of surface material between all of the planets and moons within the Solar System.

Canadian researcher Brett Gladman (University of British Columbia) has developed a number of detailed numerical models that follow the orbital evolution of material ejected from the planets, and he finds that the interexchange of material can occur on relatively short timescales. In a recent study,5 for example, Glad-man and co-workers found that material lofted from the Earth during a large crater-forming event can reach and impact upon either the surface of Mars or Venus within 30,000 years of being ejected. Something like 0.1% of the material ejected from the Earth will, in fact, reach Venus, and about 0.001% will reach Mars.

Figure 3.6. Titan, Saturn's largest moon, is the only satellite within the entire Solar System to have an extensive atmosphere. The atmosphere is primarily composed of molecular nitrogen (N2) and methane (CH4). Image courtesy of esa/nasa.

Figure 3.6. Titan, Saturn's largest moon, is the only satellite within the entire Solar System to have an extensive atmosphere. The atmosphere is primarily composed of molecular nitrogen (N2) and methane (CH4). Image courtesy of esa/nasa.

Certainly, the amount of material exchange is small, but it raises the possibility that past life on Mars was, in fact, seeded from the Earth, the microbes being carried to their new home within the cracks and fissures of terrestrial meteorites. Gladman and co-workers6 also find that small quantities of terrestrial material can find their way to the moons Titan (Figure 3.6) and Europa (Figure 3.7), the latter moon being one of the strongest candidate worlds for supporting life at the present time (see later).

The concept of escape velocity will be discussed in Chapter 5, but for the moment we need only note that the escape velocity for material ejected from Mars is less than half of that required for material to escape from the Earth. This condition dictates that it is easier for material to be ejected from the surface of Mars than it is from the surface of Earth,7 and accordingly, Gladman finds that on a timescale of 15 million years perhaps, as much as 5% of the material ejected from Mars will ultimately find its way to the Earth's

Figure 3.7. Europa, the second largest of Jupiter's four Galilean moons. This satellite has an extensive outer icy mantle, but the conditions in the subsurface regions are suitable for the existence of liquid water. The clear presence of an interior global ocean was first demonstrated through magnetic anomaly measurements made by instruments carried aboard the Galileo spacecraft. Image courtesy of NASA.

Figure 3.7. Europa, the second largest of Jupiter's four Galilean moons. This satellite has an extensive outer icy mantle, but the conditions in the subsurface regions are suitable for the existence of liquid water. The clear presence of an interior global ocean was first demonstrated through magnetic anomaly measurements made by instruments carried aboard the Galileo spacecraft. Image courtesy of NASA.

surface in the form of meteorites. In fact, about one in every 100 meteorites that falls is estimated to be from Mars! This turns the life-transport table upside-down, and there is a small possibility that life on the Earth was actually seeded from Mars.

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