User:Robertinventor/meteorites from Mars possibilities (OR)
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This is background research to help with the article User:Robertinventor/Concerns for an early Mars sample return backup which is now deleted, but may be useful in some way in the future for refs or to reuse the material.
At present it is OR but I expect that someone in the published research has gone through a similar line of argument. I haven't yet found any papers that present this entire argument so at present is OR and can't be included in the article.
Obviously when I find a source I will present the argument as it is in the source rather in the form presented here. Researching here as a way into the literature, and also helping with background understanding and material.
Re Zubrins argument (OR)[edit source | hide | hide all]
This is OR so can't go into the article and am trying to find any recent research into it post the NRC report. The ESF report doesn't discuss Mars meteorite transfer to Earth, just one sentence that (obviously) the possibility of Mars meteorite transfer means you can't reduce the probability to 0.
Anyway first, the Martian meteorites we have are nearly all igneous rocks. Many show sign of internal shock with internal spaces filled with a black glass, and are distinguished from lunar meteorites by being much more shock transformed. Curiously, there are almost no meteorites discovered yet that resemble any of the samples analysed on the surface. Just one that is a breccia.
"Even more intriguing is the fact that none of the Martian meteorites (possibly with one exception) seem to be very similar to the rock outcrops at any landing sites explored so far with robotic craft."
Even greater shock pressures and temperatures have caused complete melting of plagioclase in some shergottites (e.g., Allan Hills 77005, Dhofar 378, Northwest Africa 5298, Northwest Africa 6342), resulting in a distinctive "microspherulitic" texture of quenched, anisotropic plagioclase blades or even the formation of complex, vesicular feldspar-rich glass. Other common effects of shock are the presence of small interior pockets of black glass (sometimes vesicular) and/or cross-cutting thin veinlets of black glass. This evidently was injected in a molten state into the rocks as they were spalled off Mars and contains trapped Martian atmospheric gases.
Lots of links here. On cursory reading seems that a 100 km crater on Mars is needed to send large amounts of unshocked material to Earth. No idea how often those happen but seems like a starting point for deciding how long ago the meteorites we receive from Mars spent on their voyage to Earth. Perhaps also that can be measured directly by looking e.g. at effect of cosmic radiation on the meteorite?
A couple of things from the NSF report - first - that they mention that no micro-organisms have yet been discovered in any of the Martian meteorites of non terrestrial origin. Of course that would be hard to prove that any micro-organism is from Mars but does mean that ideas of transfer from Mars via the meteorites are theoretical and not based on any evidence yet.
Ejection ages here: http://curator.jsc.nasa.gov/antmet/mmc/Chap%20I.pdf - interestingly, strongly clustered with similar rock types having similar ejection ages. With ages 1 million, 3 million, 4 million, 10 million, 15 millino and 20 million years approximately. Does look very like they come from a series of distinct impacts on Mars that occur every few million years. Seem also quite reasonable time periods for creation of 100 km radius impact craters on Mars.
The thing is - so far all this is OR and SYNTHESIS. Does any author make a similar point in a discussion of back contamination of Earth by Martian meteorites?
Here is what the NRC report says for reference:
"If living microorganisms have existed on Mars, it is possible that such organisms could have been intermittently transported to Earth throughout geological time, carried by meteorites of martian origin. To date, 12 meteorites from Mars have been found on Earth. They are believed to have been ejected from Mars into heliocentric orbits by large impacts and subsequently captured by Earth. The evidence for a martian origin is compelling (McSween, 1994), and there is broad consensus in the scientific community that the meteorites indeed came from Mars.
The rate of influx of martian meteorites onto Earth can be estimated only crudely. Roughly 500 meteorites larger than 0.5 kilograms are thought to fall on Earth every year, but only about 4 are actually observed because most fall in the ocean or sparsely populated areas (Mason, 1962; Brown, 1960, 1961). Of 210 meteorites observed to fall between 1815 and 1960 in densely populated areas of Japan, India, Europe, and North America, 3 were from Mars, and so the ratio of martian meteorites to total meteorites is roughly 1:100. This number is very approximate. So far, 6 martian meteorites have been identified among the 8,000 meteorites recovered from Antarctica. However, considerable analysis is required to identify martian origin, and most of these meteorites have undergone only cursory examination. If we accept the 1:100 ratio as being representative, then of the roughly 500 meteorites that fall on Earth every year, perhaps 5 are from Mars. Because meteorites resemble terrestrial rocks, they generally are recovered only when recovery is favored by special circumstances, such as their having been observed to fall or their landing on the Antarctic ice sheet.
A question of major importance with respect to back contamination is whether putative martian organisms could survive ejection from Mars, transit to Earth, and entry into Earth's atmosphere. The Shergottites1 show significant shock metamorphism, but the Nakhlites, Chassigny, and ALH84001 show little evidence of shock damage as a result of ejection from Mars (McSween, 1994). Passage through Earth's atmosphere would heat only the outer several millimeters, and survival of organics in ALH84001 and thermally labile minerals in several other meteorites indicates that indeed only minor heating occurred during ejection from Mars and passage through Earth's atmosphere. Transit to Earth may present the greatest hazard to survival. Cosmic-ray exposure ages of the meteorites in current collections indicate transit times of 0.35 million to 16 million years (McSween, 1994). However, theoretical modeling suggests that about 1 percent of any material ejected from Mars should be captured by Earth within 16,000 years and that 0.01 percent would reach Earth within 100 years (Gladman et al., 1996). Thus, survival of organisms in a meteorite, where largely protected from radiation, appears plausible. If microorganisms could be shown to survive conditions of ejection and subsequent entry and impact, there would be little reason to doubt that natural interplanetary transfer of biota is possible.
Transport of terrestrial material from Earth to Mars, although considerably less probable than from Mars to Earth, also should have occurred throughout the history of the two planets. It is possible that viable terrestrial organisms have been delivered to Mars and that, if life ever started on Mars, viable martian organisms may have been delivered to Earth. Such exchanges would have been particularly common early in the history of the solar system when impact rates were much higher. During the present epoch, no effects have been discerned as a consequence of the frequent delivery to Earth of essentially unaltered martian rocks both from the martian surface and from well below. It cannot be inferred, however, that there have been no effects.
- Also, it's well known that bacteria can live in fractures within igneous rocks. And if they're not surface rocks, that's all the more reason to think they might harbor life, because as you well know, the thinking is that if there's life on Mars, it's most likely located well below the surface. So congratulations: you just strengthened Zubrin's argument... Warren Platts (talk) 22:01, 21 June 2013 (UTC)
- the authors present this as a reason for a MSR. But it does also work the other way around (OR of course) to serve as a reason for expecting that the meteorites on Earth are likley to be sterilized of life and not include much life from either present day sedimentary rocks or ancient life evidence, so may be a reason for believing that transfer of life from Mars to Earth is rare.
Why Martian meteorites fall short: Studies of Martian meteorites provide a complementary approach to Mars exploration ; however, their study has limits . All Martian meteorites are mafic igneous rocks, and the majority date from the younger Hesperian to the Amazonian (1.3 Ga and 200-600 Ma; ), with only ALH 84001 from the Noachian (4.1 Ga; ). They cannot provide absolute pinning points for the Martian timescale, since their source craters and corresponding surface units have not been identified . Most significantly, the impact delivery mechanism  discriminates against weaker rocks, such as sedimentary units and older rocks representing primitive Martian crust and weakened by impacts and aqueous alteration . Furthermore, impact shock may preferentially collapse void spaces , forming shock melt pockets and ‘sterilizing’ the record of low-temperature alteration and weathering.