The Trace Gas Orbiter (TGO) was sending signals home, but "we don't have telemetry at the moment", flight director Michel Denis of the ExoMars mission said via live webcast from mission control in Darmstadt, Germany.
Ground controllers were "working towards restoring telemetry", according to a tweet from ESA Operations.
The Schiaparelli test lander's separation from its mothership earlier today, at about one million kilometres from the Red Planet's surface, had "gone perfectly well", said spokeswoman Jocelyne Landeau-Constantin of the European Space Operations Centre.
Ground controllers are now looking at other types of data to try and determine what happened, as the cutoff approaches for an overnight "uplift" manoeuvre to remove the TGO from a collision course with Mars.
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It could be that the lander separation was "a bit more violent than expected", and that this may have caused the break in telemetry, said Landeau-Constantin.
"It's not that dramatic," she added. "At one point we will be able to get in touch with it. It's just that they need to know exactly where it was at the time of separation, in which status it was.
Earlier today, as planned, the 600-kilogramme, paddling pool-sized Schiaparelli separated from the TGO after a seven-month, 496-million-km trek from Earth.
Schiaparelli's main goal is to test entry and landing gear and technology for a subsequent rover which will mark the second phase and highlight of the ExoMars mission.
Thirteen years after its first, failed, attempt to place a rover on Mars, the high-stakes test is a key phase in Europe's fresh bid to reach our neighbouring planet's hostile surface, this time working with Russia.
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