Google Earth is supposed to configure updates during installation. From the Google Linux Software Repositories site:
The Linux Package Signing Keys
Google’s Linux packages are signed with GNU Privacy Guard (GPG) keys. Google’s packages will automatically configure your package manager to verify product updates with the public signing key, but you may also install it separately if, for instance, you want to verify the integrity of an initial package download.
However, in Debian I receive this error running apt-get update:
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://dl.google.com/linux/earth/deb stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 78BD65473CB3BD13 W: Failed to fetch http://dl.google.com/linux/earth/deb/dists/stable/Release.gpg The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 78BD65473CB3BD13 W: Some index files failed to download. They have been ignored, or old ones used instead.
To fix this error you need to manually install the Google key:
wget -q -O - https://dl.google.com/linux/linux_signing_key.pub | sudo apt-key add -