Assessment of the probability of microbial contamination for sample return from Martian moons II: The fate of microbes on Martian moons
Assessment of the probability of microbial contamination for sample return from Martian moons II: The fate of microbes on Martian moons
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DOI：
10.1016/j.lssr.2019.07.006
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发表时间：
2019-11-01
影响因子：
2.5
通讯作者：
Fujita, Kazuhisa
中科院分区：
文献类型：
--
作者：
Kurosawa, Kosuke;Genda, Hidenori;Fujita, Kazuhisa
本文给出了一个火星-卫星系统中微生物传输的案例研究。我们通过一项配套研究(本期Fujita等人)，使用撞击物理学研究了火星卫星上潜在的撞击运输微生物的空间分布。我们使用了先前研究中的绝育数据(Patel等人，2018年；Summers，2017)。我们认为这些微生物主要来自火星上的祖尼尔陨石坑，形成于1.0-0.1 Ma之间。我们发现，70%-80%的微生物可能分散在月球表面，并由于太阳和银河系的宇宙辐射而迅速灭菌，除了那些由天然流星体产生的厚厚喷发沉积物中的微生物。另外20%-30%可能被厚厚的风化岩层屏蔽，这些厚厚的风化岩层形成于火星岩石撞击产生的陨石坑中的塌陷层。据估计，在厚厚的喷射物沉积物中潜在幸存的微生物总数比火星岩石陨石坑低3-4个数量级。由于火星岩石的轰击，微生物浓度在水平方向上是不规则的，而由于辐射灭菌，微生物浓度在很大程度上依赖于深度。在Phobos上，运输微生物的存活率仅为1ppm，在Deimos上，仅为100ppm，这表明运输过程和辐射严重影响微生物的生存。还研究了火星卫星上微生物的采样概率。我们建议，即使在我们保守的情况下，从火星卫星上进行的样本返回任务也可以分为不受限制的地球返回任务，即30克样本和10厘米深度的样本。我们还对火卫一的样本进行了全面的统计分析，以包括输入参数的不确定性对抽样概率的影响。据估计，返回样品最有可能受到微生物污染的概率比空间研究委员会(空间研委会)的行星保护政策所界定的10(-6)标准低两个数量级。
This paper presents a case study of microbe transportation in the Mars-satellites system. We examined the spatial distribution of potential impact-transported microbes on the Martian moons using impact physics by following a companion study (Fujita et al., in this issue). We used sterilization data from the precede studies (Patel et al., 2018; Summers, 2017). We considered that the microbes came mainly from the Zunil crater on Mars, which was formed during 1.0-0.1 Ma. We found that 70-80% of the microbes are likely to be dispersed all over the moon surface and are rapidly sterilized due to solar and galactic cosmic radiation except for those microbes within a thick ejecta deposit produced by natural meteoroids. The other 20-30% might be shielded from radiation by thick regolith layers that formed at collapsed layers in craters produced by Mars rock impacts. The total number of potentially surviving microbes at the thick ejecta deposits is estimated to be 3-4 orders of magnitude lower than at the Mars rock craters. The microbe concentration is irregular in the horizontal direction due to Mars rock bombardment and is largely depth-dependent due to the radiation sterilization. The surviving fraction of transported microbes would be only similar to 1 ppm on Phobos and similar to 100 ppm on Deimos, suggesting that the transport processes and radiation severely affect microbe survival. The microbe sampling probability from the Martian moons was also investigatesd. We suggest that sample return missions from the Martian moons are classified into Unrestricted Earth-Return missions for 30 g samples and 10 cm depth sampling, even in our conservative scenario. We also conducted a full statistical analysis pertaining to sampling the regolith of Phobos to include the effects of uncertainties in input parameters on the sampling probability. The most likely probability of microbial contamination for return samples is estimated to be two orders of magnitude lower than the 10(-6) criterion defined by the planetary protection policy of the Committee on Space Research (COSPAR).