Three aspects concerning the habitability of giant planets’ icy satellites and Venus are examined in this thesis: (1) the degradation of amino acids in liquid water oceans of Enceladus, Europa, and other icy ocean worlds and implications to future missions characterizing amino acids as potential biosignatures, (2) the abiotic production of Venusian phosphine from volcanically extruded phosphides, (3) the outgassing history and age of Enceladus’s ocean inferred from noble gases. While the distribution of amino acids has been often referred to as a potential biosignature, it has been known whether and which species would be actively produced rather than the relics of primordial reservoirs. Chapter 2 examines the degradation patterns of fourteen amino acids using the published kinetic rate constants. Many of them decompose to a large extent (>99.9%), even in a short hydrothermally active ocean (20 Myr). Therefore, if amino acids are detected in-situ, they should have been formed recently in geological timescales. The tentative detection of phosphine in the Venusian atmosphere led to speculation that they might indicate the presence of life. Chapter 3 re-examines an abiotic pathway to produce Venusian phosphine from volcanism. If a trace amount of phosphides could be ejected into the atmosphere in the form of volcanic dusts by explosive volcanic eruptions (which were invoked before by others to explain the episodic changes of sulfur dioxide observed in the atmosphere), then react with hydrochloric acid or sulfuric acid to form phosphine, the required volcanism is sufficiently high but not implausible and consistent with the tentative detection of phosphine by Pioneer Venus. In chapter 4, a model of 36Ar outgassing from the ocean is developed to derive a lower limit for the outgassing timescale of Enceladus. Owing to the relatively small volume of Enceladus’s ocean and low solubility of argon as well as their low tendency to be incorporated in clathrate (at least for structure I clathrate), 36Ar quickly outgases from the ocean to be below the detection limit constrained from Cassini’s Ion and Neutral Mass Spectrometer in ~ 10 Myr. However, a firmer conclusion could only be reached once considering other species, such as 84Kr, as they may have different outgassing histories. Thus, further modellings are needed to gain more insights into the outgassing history of Enceladus and its potential to host life.