diff --git a/src/services/CardDeck.ts b/src/services/CardDeck.ts
index 3027a40..91f3d30 100644
--- a/src/services/CardDeck.ts
+++ b/src/services/CardDeck.ts
@@ -14,13 +14,13 @@ import Slot from './enum/Slot'
 export default class CardDeck {
 
   private _pile : Card[]
-  private _grade2 : Card[]
+  private readonly _grade2 : Card[]
   private _leftMajoritySlot? : Card
   private _rightMajoritySlot? : Card
   private _slots : CardSlot[]
   private _discard : Card[]
-  private _availableSlots : Slot[]
-  private _difficultyLevel : DifficultyLevel
+  private readonly _availableSlots : Slot[]
+  private readonly _difficultyLevel : DifficultyLevel
 
   public constructor(pile : Card[], grade2 : Card[], leftMajoritySlot : Card|undefined, rightMajoritySlot : Card|undefined,
       slots : CardSlot[], discard : Card[], availableSlots : Slot[], difficultyLevel : DifficultyLevel) {
diff --git a/src/services/LunaState.ts b/src/services/LunaState.ts
index ec0939d..8526841 100644
--- a/src/services/LunaState.ts
+++ b/src/services/LunaState.ts
@@ -15,11 +15,11 @@ export default class LunaState {
   private static readonly HELIUM_SLOT_A_TRESHOLD = 8
   private static readonly RESEARCH_SLOT_E_TRESHOLD = 4
 
-  private _cardDeck : CardDeck
+  private readonly _cardDeck : CardDeck
   private _heliumCount : number
   private _researchSteps : number
-  private _heliumCardTresholds : number[]
-  private _researchCardTresholds : number[]
+  private readonly _heliumCardTresholds : number[]
+  private readonly _researchCardTresholds : number[]
 
   public constructor(cardDeck : CardDeck, heliumCount : number, researchSteps : number, difficultyLevel : DifficultyLevel) {
     this._cardDeck = cardDeck