Why doesn’t a PNG converted from a NASA PDS IMG file show more detail than the JPEG version?

Yes, this can be completely normal. Converting to PNG does not create extra detail that isn’t already in the source data. PNG is lossless, but it only preserves what the conversion gives it.

Based on the answers, the most likely reasons are:

• the original source image has limited resolution, so zooming in just reveals its pixels/noise;
• the JPEG version may already be compressed lightly enough that visible loss is negligible;
• the conversion software may be using only a certain resolution or interpretation of the IMG data.

Also, .IMG is not itself a standard viewable photo format like JPEG or PNG; it can be a container or disk-image-style format, so what matters is what image data is actually inside and how the converter reads it.

In short: PNG is not inherently “more detailed” than JPEG. If the original data doesn’t contain more detail, or if the JPEG was made with modest compression, both can look essentially the same when viewed normally or zoomed in.