Override a Property with a Derived Type and Same Name C#

I'm trying to override a property in a base class with a different, but derived type with the same name. I think its possible by covarience or generics but am not sure how to do it?

The following code gets the error:

Error 1 'Sun.Cache': type must be 'OuterSpace.Cache' to match overridden member 'OuterSpace.Cache'

public class OuterSpace {
    public virtual OuterSpaceData Data {get; set;}
    public virtual OuterSpaceAnalysis Analysis {get; set;}
    public virtual OuterSpaceCache Cache {get; set;}


    public class OuterSpaceData {
        //Lots of basic Data Extraction routines eg
        public virtual GetData();
    }
    public class OuterSpaceAnalysis {
        //Lots of Generic Analysis on Data routines eg
        public virtual GetMean();
    }
    public class OuterSpaceCache {
        //Lots of Caches of Past Analysis Results:
        public Dictionary<AnalysisType, List<Result>> ResultCache;
    }
}

public class Sun : OuterSpace {
    public override SunData Data {get; set;}
    public override SunAnalysis Analysis {get; set;}
    public override SunCache Cache {get; set;}

    public SunData : OuterSpaceData {
        //Routines to specific get data from the sun eg
        public override GetData();
    }

    public SunAnalysis : OuterSpaceAnalysis {
        //Routines specific to analyse the sun: eg
        public double ReadTemperature();
    }
    public SunCache : OuterSpaceCache {
        //Any data cache's specific to Sun's Analysis
        public Dictionary<AnalysisType, List<Result>> TempCache;
    }
}

public class Moon : OuterSpace {} etc.

For the end result, when I address the "Data" Object of Sun I don't want there to be two Data Objects (Inherited & Base Class) but when I try override the property it requires the Sun variables to be the same type as base class. Eg:

Sun EarthSun = new Sun()
EarthSun.Analyse()  //The OuterSpace Analysis Saves results to Sun Cache:

//Now try use the result:
EarthSun.Cache[0]...

Very similar to this but with derived type instead of string-array: C# Member variable overrides used by base class method

And this answer didn't make much sense to me: How to override member of base class after inheritance in C++

Or perhaps this means its just not possible? Can I Override with derived types?

Help! :) Any work around?