You'll need to unlock Mars Obelisk as part of Destiny 2's Season of Dawn. An important part of the Keeping Time quest, Mars Obelisk must be located and activated to advance the quest.
The first thing you will need to do is talk to Osiris after completing the Question of Time quest. He will have two new quests for you, and you can accept them both. He will also have a third, smaller quest called Mars: Obelisk Harmonization. If you can't get this from Osiris, take the available quests, quit them, then interact with him again, and that should fix the issue.
You can find Mars Obelisk near the BrayTech spawn point on Mars. The first part of the quest, and the first step in making the obelisk work, is to adjust the obelisk.
Stage 1 - Mars: harmonization of the obelisk
For the first step, you must complete the quest Mars: Harmonizing the Obelisk. You will need to collect 50 recovered BrayTech The Hive Enemies on Mars. This is very easy to do just outside of BrayTech, as there is a large amount of beehive spawning in the area. There is also a good chance that people will participate in the escalation protocol, which makes it even easier to do. If you need to, you can venture into the Lost Sector of Core Terminus, which is teeming with Hive.
Once you have all 50, go to the Obelisk near the BrayTech spawn point and interact with it.
Stage 2 - Mars: Collection of light
Now that the Obelisk has been repaired and modified, you need to collect Light. To do this, you will kill The enemies with your abilities, your Super, and collect The Orbs of Light. Equip one or two Mastered Weapons to make sure plenty of Orbs of Light will drop, then kill any enemies you can find. Keep using Abilities and Supers to get your kills until you've fully charged your light charge to 100%. Again, it won't take long.
When this is complete, return to the Obelisk to activate it and link it to the sundial. And that's it, Mars Obelisk is now operational.
