Final answer:
A likely explanation for Planet A having a lower mass and higher deuterium-to-hydrogen ratio than Planet B is that Planet B has lost a significant amount of deuterium to space, likely because it has a heavier mass and thicker atmosphere allowing for greater retention of light gases.
Step-by-step explanation:
If Planet A has a lower overall mass and a higher deuterium-to-hydrogen ratio (D/H) than Planet B, a likely explanation is that Planet B has lost a lot of deuterium to space. This can occur because lighter hydrogen gas more easily escapes a planet's gravity than the marginally heavier deuterium, an effect that would be more pronounced on a planet with lower mass, like Planet A. However, since Planet A has a higher D/H ratio, this would suggest that it has retained more of its deuterium compared to Planet B. Differences in planetary mass and atmospheric composition contribute to a variation in the planets' ability to retain these gases.
Planet B having the thicker atmosphere could lead to a higher retention of hydrogen and deuterium, hence lower D/H ratios seen in heavier planets. Over time, lighter planets like Planet A, with weaker gravity, are more prone to lose light gases such as hydrogen, while retaining the heavier isotopes, hence having a higher D/H ratio.
Both planets are affected by the same radiation budget due to orbiting the same star, which might influence atmospheric loss processes, but the key difference in the D/H ratio and mass rests primarily on the physical characteristics of the planets, such as mass and atmospheric composition.