Helium on Mars and Venus : EUVE observations and modeling
Helium on Mars and Venus : EUVE observations and modeling
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DOI：
10.1016/j.icarus.2005.02.005
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发表时间：
2005-08
期刊：
影响因子：
3.2
通讯作者：
V. Krasnopolsky;G. Gladstone
中科院分区：
文献类型：
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作者：
V. Krasnopolsky;G. Gladstone
使用极紫外探测器 (EUVE) 对火星和金星进行的长曝光光谱显示，两颗行星上都有 He 584 Å 的发射，金星上有 He 537 Å/O+539 Å 和 He+304 Å 的发射。自从我们对火星的第一次 EUVE 观测进行分析以来，我们对 584 Å 的太阳辐射、火星高层大气中的涡流扩散、火星电离层上方的电子能量分布以及火星外逸层中的热氧密度的了解已得到显着提高 [Krasnopolsky, Gladstone, 1996, Helium on Mars: EUVE and Phobos data and suggesting for Mars' evophys, J. Geophys.资源。 101, 15,765–15,772]。这些新结果和最近的 EUVE 对火星的观测是我们在本文中重新审视这个问题的动机。我们发现，主要损失过程发生的高层大气中的氦丰度与之前论文中的相似，尽管现在更好地估计中层和中层大气中的混合比为 10±6 ppm。我们对氦气总损失的估计几乎没有变化，为 8×1023s−1，因为电离顶以上电子碰撞电离导致的损失显着减少，而与热氧碰撞时的较高损失得到了补偿。由于当前不存在火山活动且火山气体渗漏上限非常低，我们忽略了 U 和 Th 放射性衰变产生的氦气逸出。捕获太阳风α粒子是目前火星上唯一的氦气实质性来源，其效率仍保持在0.3。对金星 EUV 发射的类似分析结果表明，高层大气中的氦丰度等于之前使用两台光学仪和两台质谱仪测量的丰度的平均值，并且得出中层和低层大气中的氦混合比为 9±6 ppm。电离顶上的太阳风通过电离和清除氦离子而逸出的氦气比 Prather 和 McElroy [1983, Helium on Venus:对铀和钍的影响, Science 220, 410–411]计算的小 3 倍。然而，He+离子与 CO2 和 N2 在外碱基和电离顶之间的电荷交换以及与热氧的碰撞Prather 和 McElroy [1983, Science 220, 410–411] 预测，总损失似乎达到 106cm−2s−1 的水平。氦气的损失通过 U 和 Th 放射性衰变产生的氦气脱气以及捕获效率为 0.1 的太阳风 α 粒子来补偿。我们还将得出的火星和金星的 α 粒子捕获效率与观察到的 X 射线发射进行了比较，这些 X 射线发射是由太阳风重离子与两颗行星上的扩展大气层的电荷交换产生的 [Dennerl 等人，2002 年，Discovery of X-rays from Venus with Chandra, Astron。天体物理学。 386、319–330； Dennerl，2002 年，与 Astron 钱德拉一起发现火星 X 射线。天体物理学。 394, 1119–1128]。火星上的圆盘和光晕的排放量与我们的计算值一致；然而，我们没有看到金星上 X 射线晕发射的合理解释。火星和金星的盘状 X 射线发射得出的电荷交换效率之比与这些行星的捕获效率之比相似。 EUVE、Venera 11 和 12 观测到的 He+ 在 304 Å 处的惊人明亮发射表明，电离层周围太阳风 α 粒子流中的电荷交换比进入电离层的 α 粒子流中的电荷交换强得多。
Long-exposure spectroscopy of Mars and Venus with the Extreme Ultraviolet Explorer (EUVE) has revealed emissions of He 584 Å on both planets and He 537 Å/O+539 Å and He+304 Å on Venus. Our knowledge of the solar emission at 584 Å, eddy diffusion in Mars' upper atmosphere, electron energy distributions above Mars' ionopause, and hot oxygen densities in Mars' exosphere has been significantly improved since our analysis of the first EUVE observation of Mars [Krasnopolsky, Gladstone, 1996, Helium on Mars: EUVE and Phobos data and implications for Mars' evolution, J. Geophys. Res. 101, 15,765–15,772]. These new results and a more recent EUVE observation of Mars are the motivation for us to revisit the problem in this paper. We find that the abundance of helium in the upper atmosphere, where the main loss processes occur, is similar to that in the previous paper, though the mixing ratio in the lower and middle atmosphere is now better estimated at 10±6 ppm. Our estimate of the total loss of helium is almost unchanged at 8×1023s−1, because a significant decrease in the loss by electron impact ionization above the ionopause is compensated by a higher loss in collisions with hot oxygen. We neglect the outgassing of helium produced by radioactive decay of U and Th because of the absence of current volcanism and a very low upper limit to the seepage of volcanic gases. The capture of solar wind α-particles is currently the only substantial source of helium on Mars, and its efficiency remains at 0.3. A similar analysis of EUV emissions from Venus results in a helium abundance in the upper atmosphere which is equal to the mean of the abundances measured previously with two optical and two mass spectrometers, and a derived helium mixing ratio in the middle and lower atmosphere of 9±6 ppm. Helium escape by ionization and sweeping out of helium ions by the solar wind above the ionopause is smaller than that calculated by Prather and McElroy [1983, Helium on Venus: implications for uranium and thorium, Science 220, 410–411] by a factor of 3. However, charge exchange of He+ions with CO2and N2between the exobase and ionopause and collisions with hot oxygen ignored previously add to the total loss which appears to be at the level of 106cm−2s−1predicted by Prather and McElroy [1983, Science 220, 410–411]. The loss of helium is compensated by outgassing of helium produced by radioactive decay of U and Th and by the capture of the solar wind α-particles with an efficiency of 0.1. We also compare our derived α-particle capture efficiencies for Mars and Venus with observed X-ray emissions resulting from the charge exchange of solar wind heavy ions with the extended atmospheres on both planets [Dennerl et al., 2002, Discovery of X-rays from Venus with Chandra, Astron. Astrophys. 386, 319–330; Dennerl, 2002, Discovery of X-rays from Mars with Chandra, Astron. Astrophys. 394, 1119–1128]. The emissions from both disk and halo on Mars agree with our calculated values; however, we do not see a reasonable explanation for the X-ray halo emission on Venus. The ratio of the charge exchange efficiencies derived from the disk X-ray emissions of Mars and Venus is similar to the ratio of the capture efficiencies for these planets. The surprisingly bright emission of He+at 304 Å observed by EUVE and Venera 11 and 12 suggests that charge exchange in the flow of the solar wind α-particles around the ionopause is much stronger than in the flow of α-particles into the ionosphere.